Advances in atomic emission, absorption and fluorescence ...

Advances in atomic emission, absorption and ¯uorescence spectrometry, and related techniques Steve J. Hill,*a Simon Chenery,b John B. Dawson,c E. Hywel Evans,*a Andrew Fisher,a W. John Price,d Clare M. M. Smith,e Karen L. Suttonf and Julian F. Tysong a

Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, UK PL4 8AA. E-mail: [email protected]; [email protected] b Analytical Geochemistry Group, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG c Department of Instrumentation and Analytical Science, UMIST, PO Box 88, Manchester, UK M60 1QD d Ellenmoor, East Budleigh, Budleigh Salterton, Devon, UK EX9 7DQ e Department of Chemistry, University College Cork, Ireland f Procter and Gamble Technical Centres Ltd., Rusham Park, Whitehall Lane, Egham, Surrey, UK TW20 9NW g Department of Chemistry, University of Massachusetts, Box 34 510, Amherst, MA, USA 01003-4510

Received 25th April 2000 1 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.2.3 1.2.3.1 1.2.3.2 1.2.3.3 1.2.3.4 1.2.3.5 1.2.3.6 1.2.3.7 1.2.3.8 1.2.3.9 1.2.3.10 1.3 1.4 1.5 1.5.1 1.5.2 1.5.3 2 2.1 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.3 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.3 2.2.3.1 2.2.3.2 2.2.4 2.3 2.3.1 2.3.2

Sample introduction Preconcentration Flow injection Off-line preconcentration Chemical vapour generation Fundamental studies in hydride generation Generation of other volatile compounds Vapour generation of individual elements Arsenic Bismuth Cadmium Germanium Lead Antimony Selenium Tin Tellurium Mercury Nebulization Solid sampling Electrothermal vaporization ET-AAS ETV-ICP-AES In-torch vaporization ICP-AES Instrumentation Spectrometers Sources and atom cells Sources for optical emission spectroscopy Plasmas Discharge lamps Other emission sources Atom cells for atomic absorption spectrometry Flame atomizers Electrothermal atomization Other atomizers Sources for atomic absorption spectrometry Continuum source AAS Other sources Atomic ¯uorescence spectroscopy Detectors Charge coupled devices (CCDs) Charge injection devices (CIDs)

*Review co-ordinator, to whom correspondence should be addressed and from whom reprints may be obtained.

DOI: 10.1039/b003255g

2.3.3 2.4 2.5 3 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.2 3.3 4 4.1 4.1.1 4.1.1.1 4.1.1.2 4.1.2 4.1.2.1 4.1.2.2 4.1.2.3 4.2 4.2.1 4.2.1.1 4.2.1.2 4.2.2 4.2.2.1 4.2.2.2 4.2.2.3 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3 5 6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.3 6.3.1 6.3.2 6.3.3 7

Other detectors Background correction Data acquisition and control Fundamentals Plasmas Microwave induced plasmas Glow discharges Inductively coupled plasmas Other Flames Furnaces Laser-based analytical atomic spectrometry Lasers as energy sources Laser ablation General studies Atom vapour generators Laser induced plasmas Fundamental studies Instrumentation Applications Lasers as sources of intense monochromatic radiation Laser excited atomic ¯uorescence Fundamental studies Applications Lasers in atomic absorption Fundamental studies Instrumentation Applications Miscellaneous uses of lasers Laser enhanced ionization Cavity ring-down spectroscopy Coherent forward scattering Chemometrics Coupled techniques for speciation Capillary electrophoresis Gas chromatography GC-AES GC-AFS GC-AAS Liquid chromatography LC-AAS LC-AES LC-AFS References J. Anal. At. Spectrom., 2000, 15, 763±805

This journal is # The Royal Society of Chemistry 2000

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This Atomic Spectrometry Update is a new review resulting from an amalgamation of the reviews formerly appearing under the titles `Atomic Emission Spectrometry' and `Advances in Atomic Absorption and Fluorescence Spectrometry and Related Techniques'. An attempt has been made to address only novel and interesting developments so less emphasis has been placed on applications, which are covered in greater detail in the applications reviews, and more on technical developments. The structure of the review has been changed somewhat to keep pace with the evolution of the ®eld. The three primary techniques of atomic absorption, emission and ¯uorescence spectrometry are all dealt with under each individual heading, so that `Sample Introduction', for example, deals with sample introduction methods pertaining to all three techniques. There are now separate sections on `Laser-based Analytical Spectrometry' and `Coupled Techniques for Speciation Studies' to re¯ect the growing importance of these ®elds. It is hoped that these developments will result in a more integrated overview of developments in these related techniques, and any comments on the merit of this approach, or suggestions for `®ne tuning' would be welcome.

1 Sample introduction 1.1 Preconcentration The published articles or conference presentations that have been included in this section have been divided into two broad categories. The ®rst of these contains a survey of procedures in which the sample pretreatment involved either: discrete sampling; continuous ¯ow; segmented ¯ow; use of ¯uid ¯ow characteristics for mixing; controlled residence times; mass transfer between phases (solid phase extraction, precipitation or liquid±liquid extraction); or direct introduction of the liquid phase into the spectrometer. This ®rst section is simply titled `¯ow injection'. The second section contains preconcentration procedures that do not fall within this rather broad de®nition of FI. 1.1.1 Flow injection. Developments in the last two or three years in the use of FI techniques with ETAAS have been reviewed.1 Topics covered included air-segmentation and new procedures for precipitation and liquid±liquid extraction (LLE) as well as the use of the knotted tubular reactor as a sorption medium. Several other English language reviews have appeared2±5 as well as ones in Russian,6 Chinese7 and Japanese.8 The use of FI procedures with AAS in dissolution testing of oral dosage forms has been reviewed.9 Stability constants have been determined10 based on FI-ion exchange with FAAS. An FI microsampling system for the determination of Fe in soils by FAAS has been developed.11 The LOD was 0.4 mg ml21 and the throughput 300 h21. Copper was determined in beer12 at 30 samples h21. Honey samples were diluted in a continuous ¯ow manifold13 with a solution containing HCl and lanthanum for the determination of Ca, Mg and Zn by FAAS. The majority of the published papers concerning FI with atomic spectrometry in this review period are concerned with some aspect of preconcentration and/or matrix separation. Precipitated metal hydroxides were collected on zirconia spheres in a micro-column,14 re-dissolved in 70 ml of dilute HNO3 and determined directly by FAAS. Detection limits of between 0.1 and 0.5 mg l21 for Cu, Ni, Pb and Zn were obtained. Zinc was preconcentrated15 as the thiocyanate complex on polyurethane foam (packed into a microcolumn) and determined by FAAS following elution with water±acetone±nitric acid (68z30z2). For 3 min loading, the LOD was 0.8 mg l21. Liquid±liquid extraction is less popular 764

J. Anal. At. Spectrom., 2000, 15, 763±805

than solid phase extraction, probably because of the dif®culty of phase separation; however, a few LLE methods have been described. Chinese workers determined (a) Pb in biological specimens16 by extraction of the iodo-complexes into IBMK, and (b) Cr and Ni in human hair17 by extraction of the DDC complexes into the same solvent. In both cases quanti®cation was by FAAS. In a `wetting ®lm' system the oxine complex of Cu was extracted18 into a ®lm of IBMK on the walls of a PTFE reactor followed by back extraction into a slug of HNO3 solution sandwiched between two air segments. Even though the eluent was delivered to the FAA spectrometer at 0.6 ml min21, the throughput was 30 h21 with an enrichment factor of 43 for an LOD of 0.2 mg l21. Four parallel hollow ®bres were impregnated19 with bis(2-ethylhexyl)phosphate in kerosene for the preconcentration of Pb from an aqueous formate buffer solution. Again, elution was with dilute HNO3. Wang and co-workers described20 a procedure in which metals were retained by an iminodiacetate±ethylcellulose membrane mounted on either side of the ¯ow channel. The eluent was 300 ml of dilute HNO3 for the determination of Cd, Co, Cu, Mn, Ni, Pb and V by FAAS or ICP-OES with LOD between 0.2 and 1 mg l21 and enrichment factors from 7 to 18. Chelation was also used to retain Cd21 on a poly(aminophosphonic acid) resin for determination in mussels. The solution LOD was 0.6 mg l21 for a sample volume of 3.4 ml with preconcentration factors ranging from 16 to 47 for sample volumes of 3.4±10 ml. The eluent was dilute HCl. The procedure was also applied in the determination of Co in steel.22 The analyte was retained at pH 2 and eluted with dilute HNO3. Several groups of Chinese workers have devised preconcentration methods based on chelation with a solid phase extractant packed into a microcolumn: these include the determination of Pb in nickel sulfate by retention on YZ 50102,23 the determination of Ag in geological materials by retention on triphenylphosphineloaded glass beads,24 the determination of Cu, FeIII and Ni in caustic soda by retention on `D401 chelating resin',25 and the determination of copper by retention of the complex with 8hydroxyquinoline-5-sulfonic acid on an anion-exchange resin, D-290.26 Holcombe and coworkers compared27 the performance of poly(L-cysteine) with that of 8-hydroxyquinoline. Both chelating agents were immobilized on controlled pore glass. The analytes were Cd, Cu and Pb and the matrices were synthetic sea-water, cobalt and nickel and Seawater CRM. Recoveries of 50 mg l21 of Cd and Pb from synthetic sea-water were quantitative for both resins; however, for solutions containing 10 000-fold excess of cobalt and nickel, the recoveries of Cd and Pb were low with the oxine material, but quantitative for the poly(L-cysteine) material. Neither material gave acceptable recovery for Cu. For the determination of Cd and Pb spikes in the CRM, the recoveries were variable. Recoveries of Cd were high, though that of Pb was acceptable with CASS-1 but low for NASS-2. Breakthrough curves were used to investigate the nature of the various binding sites on the materials and the various binding constants and capacities were estimated. Some preliminary results28 for the behaviour of poly(L-aspartic acid) immobilized on gold-plated minigrids have been obtained. Sea-water was the matrix for one of the relatively few sorbent extraction FAAS procedures published in this review period.29 The analytes, Co, Fe, Mn, Ni and Zn, were retained as the complexes with 5,7-dichloro-8-hydroxyquinoline on C18 silica and eluted with acidi®ed (pH 2) methanol. Sensitivity enhancements of between 60 and 80 were obtained with LOD ranging from 0.5 to 4 mg l21 for a 1 min loading time at 7 ml min21. The procedure was suitable for the determination of Mn and Zn in sea-water samples, but the concentrations of Co, Fe and Ni were too low to be determined. There was no discussion of the effect of sample volume on the LOD. In the determination of CrIII and CrVI,30 the CrVI was retained as the APDC complex on the interior surface of a

PEEK tube and then eluted with IBMK. The CrIII was retained from a solution of potassium hydrogen phthalate on C18; CrVI was not retained, but could be preconcentrated downstream via the APDC±PEEK tubing procedure. The CrIII was eluted with methanol. For 5 ml sample volume the LOD were 0.9 and 0.5 mg l21 for CrIII and CrVI, respectively. Preconcentration, following ion chromatographic separation, on a solid-phase extraction cartridge31 with AAS detection allowed the quanti®cation of both cations (Ca and Mg) and anions (halides, nitrate and sulfate), apparently. Cyclamate was determined32 by an indirect continuous ¯ow FAAS procedure in which oxidation to sulfate was followed by precipitation of the lead salt. Following collection on a ®lter and washing with ethanol, the precipitated lead sulfate was dissolved in ammonium acetate solution. The Ca and Mg content of brines was determined33 by a combined low- and high-pressure system. The metals were retained as ion pairs of the xylenol orange chelates with tetrabutylammonium hydroxide on C18 in the low pressure system and eluted with methanol and delivered to the spectrometer via a hydraulic high pressure nebulizer. The Ni and Sn content of brass was determined34 by a procedure in which the metals were dissolved electrolytically in a ¯ow system with collection of the electrolyte in the autosampler cup of an AA spectrometer with ETA. Apparently a sample could be analysed every 2 min. The procedure was used to determine35 Al in aluminium alloys, and Cu and Zn in brass and bronze, and has been further developed36 for the determination (in metal alloys) of Cu, Pb and Zn by FAAS and Fe, Ni and Sn by ETAAS. Adams and co-workers have devised a number of preconcentration procedures for use with ETAAS. Blood was analysed37 for Cd and Pb; the complexes with ammonium diethylphosphorodithioate were retained on the inner walls of a knotted PTFE reactor followed by elution with 35 ml of methanol. The determinations were made with a pyrolytic graphite tube pre-treated with iridium. The LOD were 0.2 and 2 ng l21 for Cd and Pb, respectively. Platinum was determined38 in the same matrix by an analogous procedure in which the analyte was retained as the APDC complex. For a 90-s preconcentration at 8.8 ml min21, a preconcentration factor of 112 and an LOD of 10 ng l21 were obtained. The performance characteristics of the procedure were improved39 by pre-coating the reactor with the chelating agent. This avoids the need for reagent addition, there are no losses to tubing walls outside the reactor, there is no need for a pre-®ll step between samples of different concentrations and it allows for better optimization of the processes of reagent sorption and analyte preconcentration. This new procedure was applied39 to the determination of Cu and Mn in some biological RMs. The chelating reagent was 1-phenyl-3-methyl-4-benzoyl-5-one, which was ®rst pumped through the reactor as a 0.05% solution for 30 s at 2.5 ml min21. The excess was removed by air pumped at 5 ml min21 for 15 s. The sample was delivered to the reactor at 2±2.5 ml min21 for 30 s and the liquid was ¯ushed out with air. The adsorbed chelates were eluted with 30±40 ml of methanol delivered by an air carrier directly to the pyrolytic graphite-coated atomizer. The enhancement factors were 34 and 21 and the LOD were 6 and 5 ng l21, for a sampling frequency of 22 h21. Single cell protein, cod muscle and freeze-dried animal blood were accurately analysed. A microcolumn of Muromac-A-1 chelating resin was incorporated40 into the autosampler capillary for the preconcentration of Cd, Co and Ni from sea-water matrices. Sample volumes of 800±1800 ml were loaded through the resin and the retained analytes eluted with 20% HNO3; LOD were 0.1, 7 and 33 ng l21 for Cd, Co and Ni, respectively. Three saline water RMs (NASS-4, CASS-3 and SLEW-1) were accurately analysed. A somewhat similar procedure was subjected41 to a Simplex optimization procedure. The microcolumn was packed with silica gel functionalized with 1-(di-2-

pyridyl)methylene thiocarbonohydrazide, the analyte was Ni and the eluent 2 M HNO3. The optimized procedure had an LOD of 60 ng l21 and a throughput of 36 h21 for 60 s preconcentration. A solid phase extractant, PbO2, highly selective for lead consisting of a crown ether macrocyle immobilized on silica gel was used in the analysis42 of some biological RMs. The material retained Pb over a wide range of acidities (0.08± 3 mol l21 HNO3) and the only interferences were from barium, potassium and strontium, whose ionic radii are similar to that of Pb. For a loading time of 20 s at 3 ml min21, with elution into 46 ml of 0.03 mol l21 EDTA (delivered by an air carrier to the furnace) an enhancement factor of 23 and an LOD of 2 ng l21 were obtained. The retention ef®ciency was 70% and the throughput was 23 h21. For the determination of Co43 a column of fullerene (C60) was used to retain the APDC complex. An autosampler cup was used as the interface between the FI system and spectrometer. The analyte in a 25-ml sample volume was eluted with 500 ml of IBMK, delivered with a nitrogen carrier, and the LOD was 8 ng l21. A similar procedure has been applied44 for the determination of Bi in steels and aluminium alloys. The analyte was retained on activated carbon as the diethyldithiophosphoric acid complex. The aluminium or iron complexes were washed from the column with 500 ml of dilute HCl and then the analyte complex was eluted to an autosampler cup with 500 ml of ethanol. The enhancement factor for 10 ml of sample (loaded at 2.5 ml min21) was initially given as 37 and the LOD was 50 ng l21. When the work was published,45 the enhancement factor was corrected to 14. Retention of Al and Cu on a microcolumn of Chelex-10046 followed by elution with 1 ml of dilute HNO3 was used in a procedure for the analysis of dialysis concentrates. For the determination of Cd in sea-water,47 a column of C18 retained either the complex with 4-(2pyridylazo)resorcinol or 2-(2-pyridylazo)-5-dimethylaminophenol. A 50 ml portion of the methanol eluent was injected into the graphite furnace. The LOD was between 2 and 4 ng l21. In a somewhat similar procedure, a C18 column retained48 the APDC complex of Cd from a sea-water matrix. The analyte was eluted with 80 ml of methanol for a LOD of 42 pmol in 2 ml (0.2 ng l21). Chinese workers49 determined Cd, Cu and Pb retained as their 5-sulfo-8-hydroxyquinoline complexes on a C18 column. The analytes were eluted with a 180 ml mixture of HCl and HNO3 and a 30 ml sub-sample was transferred by an air carrier to the atomizer. The enrichment factors were 22, 28 and 26 and the LOD were 0.7, 4.2 and 5.4 ng l21 for Cd, Cu and Pb, respectively. Burguera et al. devised a ¯ow-based procedure50 for the determination of Bi in blood in which the sample was ®rst digested in a ¯ow-through reactor in a microwave oven, the analyte being separated by precipitation with tin(II) from alkaline solution and collected in a knotted coil in an ultrasonic bath, and ®nally the precipitate was dissolved in HNO3 and a 20 ml sub-sample was introduced into the atomizer. The LOD was 0.4 mg l21. Krug et al.51,52 preconcentrated Pb electrochemically on a W-coil atomizer, which was temporarily functioning as the cathode in a ¯owthrough electrolysis cell. For a 2-min preconcentration at 1 ml min21, an enrichment factor of 25 and an LOD of 0.2 mg l21 were obtained. Martinez and co-workers have developed FI-SPE procedures for use with ICP-OES.53,54 Bismuth was determined53 in urine by retention of the 8-hydroxyquinoline complex on an Amberlite XAD-7 column, with elution by HNO3, with an LOD of 30 ng l21 (100 ml solution) by use of an ultrasonic nebulizer. However, for the determination of Pb in tap water,54 retention of the APDC complex on a knotted reactor with dissolution in 4 mol l21 HCl was employed. For a 10-ml sample the LOD was 200 ng l21 (again ultrasonic nebulization was employed). An iminodiacetate±agarose adsorbent, IDA Novarose, was used55 to accumulate Cd, Co, Cr, Cu, Fe, Mn, J. Anal. At. Spectrom., 2000, 15, 763±805

765

Ni and Zn at pH 4±8, prior to detection by ICP-OES. The retention of the trivalent cations (Fe and Cr ) was slow, whereas the divalent cations could be loaded at ¯ow rates up to 80 ml min21. The procedure was applied to river, tap and lake waters. Chinese workers56 were able, apparently, to glue Au to a PVC nylon 6 resin microcolumn, prior to elution with thiourea and detection by ICP-OES. Following digestion in 30 ml of 50% aqua regia, 3 ml of 10 g l21 animal glue were added and, after making up to 100 ml, a sub-sample was injected in the FI manifold for direct transport to the column. The LOD was 2 mg l21. Two solid phase extraction columns in series57 allowed cationic (on Chelex 100) and anionic (on AG-1 X-8) species of Cu and Mn to be preconcentrated. Following elution with dilute HCl the elements were determined by ICP-OES. The procedure was applied to the speciation of these elements in milk. On the addition of an acetate buffer, casein and other macromolecules precipitated; analysis of the ®ltrate and precipitate allowed the casein-bound metal content to be determined. An FI system was used58 to introduce a 25-ml slurry sample in an air carrier down a glass tube inserted into an axial hole in a carbon electrode into an arc or spark. Sediment samples were ground and sieved to particle sizes of less than 38 mm and slurried in nitric acid with lanthanum oxide in an ultrasonic bath. The analyte was Al, and 40 determinations per hour were possible against aqueous standard solutions. Chinese workers59 used FI as the interface between capillary isotachophoresis and ICP-OES. Unlike capillary electrophoresis, which has a ¯ow rate of 0.02±1 ml min21, the eluent from the isotachophoresis system ¯ows at about 1 ml min21, which makes interfacing with the spectrometer relatively simple. The system was used to separate and determine Cu and Cu±EDTA. The combination of ion chromatography with plasma source spectrometric detection has been critically compared5 with other techniques such as NAA, GD-MS and ETV-ICP-MS. 1.1.2 Off-line preconcentration. Publications in this section have been divided into two broad categories: the ®rst of these contains descriptions of procedures that would be relatively easy to interface directly with an atomic absorption or emission spectrometer, and the second contains descriptions of procedures that would be dif®cult to interface with such a spectrometer. Within each of these broad categories, the material is sub-divided into sections dealing with FAAS, ETAAS and ICP-OES. Baker's yeast (Saccharomyces cerevisiae) immobilized on sepiolite (the calcareous internal shell of the cuttle®sh) was used60 to preconcentrate Fe and Ni. Sample volumes of up to 500 ml were processed at 2.5 ml min21. Elution was with 1 mol l21 HCl and the LOD were 60 and 90 mg l21 for Fe and Ni, respectively. A brass SRM was analysed. The procedure has also been applied61 to the determination of Cd, Cu and Zn in river and sea-waters. The cupferron complexes of Cd and Pb were collected62 on activated carbon in a batch procedure for the analysis of urine. After ®ltering and drying the retained metals were dissolved in dilute HNO3 and determined by FAAS with the aid of a slotted tube atom retarder. The LOD were 0.03 and 0.3 mg l21 for Cd and Pb, respectively, for a 210 ml sample and 1.5 ml HNO3 solution. A melamine±urea resin was used63 in a Cr speciation procedure. The resin only retains CrVI, and thus this species can be preconcentrated and determined after elution with sodium acetate solution. Following oxidation with H2O2, total Cr was determined by the same procedure and thus CrIII was found by difference. Chinese workers have described a number of procedures including: (a) solid phase extraction on polyaminophenol resin to accumulate iron;64 (b) solid phase extraction on dithizoneloaded silica to concentrate Cu; (c) the co-precipitation of Cd, 766

J. Anal. At. Spectrom., 2000, 15, 763±805

Co, Mn and Ni with either copper pyrrolidinedithiocarbamate (PDC) or magnesium 8-hydroxyquinoline; and (d) the collection of Pb in soy sauce on activated carbon. Vietnamese workers described65 a procedure for the determination of Au by FAAS following either precipitation with tellurium or extraction from an acid solution into IBMK. A liquid±liquid extraction procedure in which the analytes (Fe, Mo, Sn and V) were extracted from acid solution in the presence of methylenedisalicylohydroxamic acid into IBMK containing tributyl phosphate was used to analyse66 food, sediment and crude petroleum. A robotic sample pre-treatment procedure has been devised67 which not only preconcentrates the analyte (Cu) as the PDC chelate on the interior of a knotted reactor but also performs sample weighing and digestion. The procedure was applied to the analysis of environmental and biological materials by ETAAS. Preconcentration on Dowex 1X8 or Chelex-100 has been used68 to preconcentrate trace elements in high purity gallium, arsenic and arsenic oxides. The desorbed elements were determined by ETAAS, FAAS, NAA and ICPOES. A diethylaminoethylcellulose anion exchange resin was used69 to collect CrVI while other cations were collected by an iminodiacetate resin. The eluted metals (Cd, Co, Cr, Ni and Pb) were determined in some pharmaceutical substances by ETAAS or TXRF. Mineral waters were analysed70 for Cd, Co, Ni, Pb and V by retention on a cellulose material modi®ed with 8-hydroxyquinoline-5-sulfonic acid. The analytes in a 20ml sample volume were eluted into less than 1 ml and determined by ETAAS. The LODs were 4, 100, 40, 60 and 200 ng l21 for Cd, Co, Ni, Pb and V, respectively. Elution with IBMK was used71 to remove Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb from a column of immobilized sodium DDC or APDC with direct introduction of the solvent into the atomizer. The LOD ranged from 40 to 300 ng l21. The same solvent was used72 to elute organo-Sn compounds from an Amberlite XAD-2 column impregnated with tropolone. The LOD was 10 ng l21. Polyethylene powder conditioned with 1-(2-pyridylazo)-2-naphthol retained73 the complexes of Cd, Cu, Pb and Zn prior to elution with perchloric acid solution. The procedure was applied to the analysis of haemodialysis concentrates. Japanese workers (describing their results in a Chinese journal) showed: (a) that In, Rh, Ru, Sr and V could be collected74 as precipitates with chitosan and that the LOD for Ru was 60 ng l21; and (b) that Cu and Pb could be collected75 as the PDC complexes on a miniature membrane ®lter which was then dissolved in methylcellosolve prior to analysis by ETAAS. Polyaniline76 extracted Cd, Cu, Pb and Sb from acid digests to which potassium iodide had been added in a procedure whose goal was to separate the analytes from potentially interfering matrix components (the volume of eluent was greater than the sample volume). The analytes were eluted with dilute HNO3 and determined by ICP-OES; the recovery for Sb was about 75%. Coal ¯y ash (CRM) and a sea plant material were analysed. For the determination of Y, Bi, Th, U and other REE in steel,77 all the metallic components of the sample were retained from a solution containing oxyethylidenediphosphonic acid (OEDPPA) on a column of tetraphenylmethylenediphosphine oxide on macroporous styrene±divinylbenzene. The iron was removed by rinsing with acid solution (0.5 mol l21 HNO3), followed by the analytes with 0.5 mol l21 OEDPPA at pH 4.5 for determination by ICP-OES or ICP-MS. Recoveries were w80% for all analytes except Th, for which the recovery was 65%. The `metal®x chelamine resin' was used78 to retain Cd, Ni and Pb from pH 4 solutions with elution by 1 mol l21 HCl. The eluent was analysed by ICP-OES and the procedure was applied to the analysis of ®r wood. In the determination of organo-P (as phosphino-polycarboxylates) in oil production waters, silica-immobilized C18 was used79 to preconcentrate the analyte species and separate inorganic P. The analyte species were eluted with a boric acid buffer at pH 9 and determined by

ICP-OES and ICP-MS with either cross-¯ow or ultrasonic nebulization (for which the LOD was about a factor of 2.5 betterÐ20 mg l21 for ICP-OES and 0.5 mg l21 for ICP-MS). Japanese workers have devised procedures for: (a) the preconcentration,80 by a factor of 1000 at mg l21 concentrations, of As on a column consisting of the iron(III) complex of lysine polyacetic acid; and (b) the preconcentration of Cd, Co, Cu, Ni, Pb, V and Zn,81 by factors of about 100 from sea and river waters, on a column containing xanthurenic acid immobilized on silica gel. In the former procedure, As was determined by ICP-OES, and in the latter both ICP-OES and ETAAS were used. The liquid±liquid extraction of Pb82 from an acid solution containing zinc hexamethylenedithiocarbamate into 2,6dimethylheptan-4-one was used for sample pre-treatment in the analysis of tap and ground water samples by FAAS. The Pb was back-extracted into an aqueous acid solution containing copper. The LOD was 70 ng l21. For the determination of Zn in sugar by FAAS,83 the analyte was extracted as the xanthate complex into a surfactant phase and then back-extracted as the EDTA complex when the solution was cooled. A column of Amberlite XAD-484 retained the complexes of Cd, Co, Cu, Ni and Pb with 1-nitro-2-naphthol in a procedure for the analysis of chemical grade potassium salts by FAAS. Total Hg in drinking water and methylmercury in air were determined85 by an ETAAS procedure in which the complexes with 2,3dimercaptopropane-1-sulfonate was retained on Sep-Pak C18 cartridges. After elution with methanol, a sub-sample of 50 ml was introduced into the furnace. The LOD was 50 ng l21. The complexes were eluted with HNO3 in acetone, which was evaporated to near-dryness and the residue taken up in HNO3. The LOD ranged from 20 to 60 ng l21. Chinese workers developed a procedure for the indirect determination of Si in a cobalt compound by extraction of the heteropolymolybdate into IBMK. After washing the organic layer to remove excess isopolymolybdate, the molybdenum was determined by FAAS with direct introduction of the organic solvent. The interference from phosphorus was overcome by a prior extraction of the phosphomolybdate into ethyl acetate. The LOD for Si was 0.2 mg l21. Also in the Chinese literature are descriptions of: (a) a ¯otation procedure86 for the determination of Ag, Cd, Cu and Pb in which the metals were collected from a mixture of potassium iodide, methylene blue and toluene; (b) a procedure for the preconcentration of Cu on microcrystalline naphthalene loaded with dithizone;87 and (c) a procedure for the determination of Cd by co-precipitation with copper sul®de. In the ®rst procedure, the analyte was eluted with DMF, which was introduced directly into the ¯ame. Reductive precipitation with selenium as the collector88 formed the basis of a method for the determination of Au, Pd, Pt and Rh in geochemical samples by FAAS and ICP-OES. Complexation and sorption on activated carbon89 was used in a procedure for the determination of Bi and Mo by ETAAS. The complexing agent was dithiophosphoric acid O,O-diethyl ester and the analytes were retained on a bed of activated carbon on a ®lter paper. After drying, mixing with HNO3, drying and mixing with dilute HNO3, a 20 ml subsample was transferred to the atomizer. The procedure was applied to the analysis of steels, and the possible interference from the iron was overcome by reduction to iron(II) with ascorbic acid. The same collection procedure was used90 for the preconcentration of As from natural waters as the heteropolymolybdate. After collection by ®ltration, the 100 mg of carbon was suspended in 5 ml of 0.1 mol l21 acetic acid containing 0.02% of Zr and 10 ml was transferred to the furnace for measurement at 197.2 nm. The LOD was 20 ng l21 for a 1 l sample. A Chinese researcher has described91 a procedure for the determination of Tl by retention of TlIII on a tributylphosphate resin. The analyte was oxidized with a mixture of iron(III) and

H2O2 and eluted with a solution of ammonium sul®te and ascorbic acid. The LOD was 3 mg l21. Coprecipitation with dithiophosphates was used92 to preconcentrate As, Cu, Pb and Se from digests of biological materials for determination by ICP-OES. The LOD ranged from 5 to 400 mg kg21. Chinese workers93 separated Au by precipitation of the complex with malachite green on microcrystalline naphthalene which was collected, packed into a column and the Au eluted with 6 mol l21 HNO3. The eluent was evaporated to near-dryness and the residue taken up in HCl for determination by ICP-OES. A somewhat similar procedure94 has been used for the determination of Zn. The 1(2-triazolyazo)-2-naphthol complex was retained on a Sep-Pak C18 cartridge and then eluted with ethanol. The ethanol was evaporated and the residue taken up in a mixture of HNO3 and H2SO4 and the Zn determined by ICP-OES with an LOD of 20 mg l21. For the determination of Cd, Co, Cu, Fe, Ni, Pb and Zn in saline matrices (haemodialysis concentrates) the analytes were extracted as the complexes with 1-pyrrolidinedithiocarbamic acid onto 30 mm Amberlite XAD-2 resin. After ®ltering and washing, the resin was slurried with 1% Triton X-100 solution and introduced directly into the spectrometer for determination by ICP-OES. Solid phase extraction has been combined with liquid±liquid extraction95 for the determination of Sc in red mud down to an LOD of 10 mg kg21. The analyte was ®rst retained on a column of Dowex 50W-X8 from which several other components were eluted ®rst before Sc and Y and other lanthanides. The eluate was extracted with di(2ethylhexyl)phosphoric acid and the Sc back-extracted with NaOH solution for determination by ICP-OES. The As content of air was determined96 by absorption in acid and then evaporation to dryness in the presence of graphite powder, which was then analysed by AES. Some preliminary results for the determination of organohalogen compounds in waters have been presented.97,98 Both methods are based on the use of a solid phase extraction procedure (either on activated charcoal or a polyacrylate ®bre) followed by detection in a helium MIP by OES. 1.2 Chemical vapour generation Tsalev has reviewed recent developments with AAS detection.99 Applications to speciation by both chromatographic (HPLC and GC) and non-chromatographic procedures are included. Sanz-Medel et al. have reviewed100 the use of organized surfactant assemblies (micelles, vesicles) and emulsions in procedures for AAS. Topics covered include the generation of volatile hydrides and ethylides, CV Hg generation and the `synergic effects of using vesicles to improve the separation of reversed-phased HPLC and the detectability of AAS by on-line vesicular hydride generation'. 1.2.1 Fundamental studies in hydride generation. Experimental conditions for the determination of As by AFS using FI-HG were optimized101 based on mathematical modelling. The use of AFS restricted the range of parameters that could be varied, as the borohydride solution ¯ow rate and concentration were ®xed to ensure a constant generation of H2. Furthermore, in those experiments where an argon purge gas was added prior to the gas±liquid separator, the total ¯ow of argon to the atomizer was kept the same. It was concluded that the optimal sensitivity and throughput were obtained with minimal GLS headspace volume and maximum carrier solution ¯ow rate. For the HGAAS determination of Se, optimum conditions were found with the aid of two 24 experimental designs.102 Helium (400 ml min21) was used as the purge gas, and both H2 (300 ml min21) and O2 (40 ml min21) were added. The LOD was between 0.7 and 2 ng, but the sample volume was not speci®ed (nor was the nature of the GLS). In the determination of Sb by FI-HG-AFS103 it was found that a cooled GLS (10 ³C) J. Anal. At. Spectrom., 2000, 15, 763±805

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with reduced headspace gave `taller and narrower' signal peaks and also stabilized the baseline. Other innovative features of the procedure included the addition of both a surfactant (Triton X-100) and a silicon antifoam agent, which eliminated matrix effects (samples included urban particulate matter, tap and sea waters). Carrero and Tyson have further developed104 their procedure for generating hydrogen selenide from the surface of an anion-exchange resin. Sample solution containing SeIV was ®rst passed through the resin to preconcentrate the analyte, then borohydride was passed through to load a controlled amount of the reagent, and ®nally acid was passed to generate the hydrogen selenide which was subsequently trapped on the interior surface of a graphite furnace atomizer. The entire procedure was automated with a FIAS 200 FI system. For a 20 ml sample volume, the LOD was 4 ng l21. There have been several reports of the use of nebulizers and spray chambers as HG devices and GLS for ICP-OES determinations. Tao and Sturgeon105 modi®ed a Meinhard-type nebulizer by the insertion of a capillary tube into the sample introduction channel through which the acidi®ed sample was introduced. The borohydride solution was introduced via the conventional channel and the solutions mixed just prior to nebulization. A Scott-type spray chamber was used. The very short residence times in the solution phase allowed the determination of Se in the presence of 5% nickel, 2.5% cobalt and 20 mg l21 copper. The LOD was 2 mg l21 and the method was validated by the analysis of a nickel oxide CRM. The procedure was also used with ETAAS with in-atomizer collection of the hydrogen selenide. Chinese workers106 described a `multi-functional cyclone nebulizer±hydride generator' for the determination of As, Hg, Pb and Sb and107 a combined HG-ultrasonic nebulizer based on a CETAC U-5000AT device. Both procedures were used in conjunction with ICP-OES. A moving-bed HG device, also for ICP-OES, has been devised.108 Solid borohydride and an organic acid were co-immobilized on a moving strip onto which the sample solution was directed. The device was used in the determination of As, Se and Sb in a Chinese tea. It has been shown109 that H2Se can be collected (at 200 ³C) on a gold wire in a manner analogous to the amalgam trapping of mercury. When heated (600 ³C) in a stream of H2, H2Se was released and detected by AFS. Electrochemical HG has been used for the determination of As by ETAAS110 and quartz tube atomization AAS.111 In the former procedure, the generated hydride was collected for 2 min and then transported to the atomizer to give an LOD of 20 ng l21 for a 200 ml sample volume. The atomizer was pre-treated with iridium, and 400 ®rings were possible without any change in performance. For the latter procedure, an LOD of 10 ng l21 (continuous ¯ow made) was obtained with a throughput (FI mode) of 36 h21. To study the mechanism of atom formation of As in a ¯ame, Tesfalidet et al.112 constructed a miniature H2±O2 burner inside the cavity of an electron spin resonance spectrometer. Radical recombination in the ¯ame was identi®ed as the atomization mechanism. The procedure was also used to study the spatial distribution of H radicals in a quartz tube atomizer. The spatial distribution of Sb atoms in such an atomizer has been measured113 in crosssection by using a charge coupled device detector. In an unheated ¯ame-in-tube atomizer, the highest free atom density was at the centre and the distribution was not in¯uenced by the purge gas and O2 delivery rates, but was signi®cantly affected by the position of the O2 delivery capillary tip. In an externally heated tube the atom distribution was more homogeneous. At high analyte concentrations (into the roll-over region of the calibration), the atom density was higher both near the walls and at the centre. The presence of polyatomic particles on whose surface free atom decay was induced was considered responsible. Chinese workers, who studied the atomization of As114 in a low-temperature atomizer (25±200 ³C) with AFS detection, concluded, from experiments with a dual generation 768

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system (one for AsH3 and one for H2), that the atomization mechanism involved collision with hydrogen radicals. Chinese workers have also investigated115 the role of organic acids in the HG determination of Pb. In a medium containing potassium dichromate as oxidant, the interference from the resulting CrIII was overcome by the addition of lactic and malonic acids. A holed T-tube atomizer (¯ame heated) has improved116 the tolerance for interference by other hydrideforming elements in the determination of As, Bi, Sb and Se compared with those observed in a conventional ¯ame-heated quartz tube atomizer. Five 2 mm holes were evenly spaced along the length of the atomizer. Presumably the holes were in the bottom of the tube facing the ¯ame. With regard to LOD, the only signi®cant decrease was for As, for which the LOD in the holed tube of 0.2 mg l21 compared with 0.06 mg l21 for the conventional tube. The linear ranges were improved for all elements except Sb, which was the element that showed least improvement in interference tolerance, being more tolerant only to Se. Chinese workers investigated117 the interference of tellurium on the HG-AFS determination of Se and showed that the interference could be removed by the addition of FeIII. Camero and Sturgeon118 trapped the hydrides of As, Se and Sb on the interior surface of graphite furnace atomizer by electrostatic deposition. The generation/collection ef®ciencies were estimated to be 80, 71 and 62%, respectively, values which seem a little lower than those for collection on an iridiumcoated surface. Workers at Xiamen University have a number of interesting developments to report.119 First, they con®rm the formation of ZnH2 (see also ref. 120). Second, they report the generation of CoH2 and NiH2, and third they report an enhancement in sensitivity in the determination of Cd, Tl and Zn by HG-ICPOES by the presence of Co, Ni and Te. The degree of enhancement was increased as the length of tubing between the GLS and the spectrometer was increased. This is in contrast to the behaviour observed in the absence of the enhancers. Several publications have described the results of efforts to determine more than one hydride-forming analyte per sample. The problems to be overcome include: (a) the dif®culty of adjustment of the various oxidation stages, particularly the production of SeIV and AsIII following oxidative sample pretreatment; and (b) the mutual interferences in both generation and atomization stages. In a procedure entitled `simultaneous determination of As, Hg, Se and Sb',121 the various hydrides were generated batchwise and trapped cryogenically (liquid nitrogen). The trapped species were simultaneously evaporated by electrical heating and transported to a He MIP for quanti®cation by OES. The procedure was also applied to the determination of Bi, Pb and Sb.122 A similar concept has been described for the determination of As, Sb, Se and Sn by gas-phase molecular absorption spectrometry. First Sn hydride was generated at low acidity, then the hydrides of As, Sb and Se were generated at higher acidity. All hydrides were trapped in a liquid nitrogen cryotrap and then volatilized simultaneously. A diode array spectrometer was used and multiple linear regression was applied to deconvolute the spectra obtained. The wavelengths of maximum absorption were 190, 198, 22 and 194 nm, respectively, and the LOD were 50, 20, 100 and 1000 mg l21. Russian workers123 claimed to have determined As, Hg and Se simultaneously by continuous ¯ow HG-ICP-OES. Samples (skim milk powder and freeze-dried bovine liver) were decomposed by HNO3 at 160 ³C under pressure. The LOD were 0.2, 1 and 0.06 mg l21 for As, Hg and Se, respectively. For the determination of As, Bi, Ge, Sb, Se, Sn and Te in 30% zinc sulfate solution, Rigby and Brindle124 divided the analytes into two groups. In the ®rst, Se and Te were determined following heating in the presence of HCl for 25 min; in the second, As, Bi, Ge, Sb and Sn were determined after the addition of L-cysteine and L-histidine (and some HCl). The hydrides were generated in a continuous ¯ow (CF) system with determination by ICP-

OES. Detection limits in the original sample solution were 3, 3, 8, 2, 3, 2 and 38 mg l21 for As, Bi, Ge, Sb, Se, Sn and Te, respectively. For the determination of As, Bi, Sb, Se and Te in nickel, Feng and Fu125 also divided the analytes into two groups. The samples were dissolved in HNO3 and the analytes coprecipitated with lanthanum hydroxide, ®ltered and redissolved in HCl. In the ®rst group, As and Sb were determined following the addition of HCl, thiourea and ascorbic acid. In the second group, Bi, Se and Te were determined after the addition of HCl. The analysis was performed by HG-AFS, giving LOD in nickel of 0.1, 0.4, 0.2, 0.1 and 0.1 mg kg21 for As, Bi, Sb, Se and Te, respectively. A HG-ETAAS procedure with trapping on an iridium coating has been developed for the determination of As, Sb and Se.126 The hydrides were generated in a CF system in a 2.5 m titanium reaction loop. The procedure was applied to the analysis of a number of environmental and clinical materials; in the determination of 1 mg kg21 of Sb in soil, up to 600 mg kg21 of As did not interfere. A procedure for the determination of As and Se in soluble coffee by ICP-OES has been developed.127 Following oxidative pretreatment, the SeVI was reduced to SeIV with HCl; presumably the As remained as AsV. Steel was analysed for Se and Te by HG-AFS128 following acid decomposition and removal of potentially interfering cations by cation exchange. Several instrumental developments for the determination of multiple hydride forming elements have been described.107,129 As, Hg and Se were extracted130 from coal by sub-critical water prior to determination by chemical vapor generation AFS. Arsenic and Se were determined131 in foodstuffs, after microwave assisted digestion in HNO3, by AAS. Selenium was reduced with HCl (after removal of the nitric acid by heating to dryness); arsenic was reduced with hydroxyammonium chloride, potassium iodide and ascorbic acid. Chinese workers132 have developed a method for the determination of As and Hg in biological samples and patent medicines by HG-AFS. The pre-treatment conditions were strongly oxidizing. 1.2.2 Generation of other volatile compounds. Lopez-Molinero and co-workers have devised procedures for the determination of As after volatilization as the tri¯uoride,133,134 the chloride135 and the bromide.136 Both AAS135 and ICPOES133,134,136 have been used for quanti®cation. It is not clear what the advantages over HG might beÐpossibly greater freedom from matrix interferences, as the LOD would appear to be much poorer than those obtained with HG. The reactions are carried out in a batch mode in concentrated H2SO4. It has been shown that Ge can be determined via the tetrachloride136±138 by ICP-OES, this time in a continuous ¯ow procedure. The LOD was 0.2 mg l21. Chloride in water was determined139 by volatilization of the chlorine formed on reaction with permanganate in sulfuric acid medium with quanti®cation by MIP-OES. The LOD was 10 mg l21. A somewhat similar procedure was used for the determination of both chloride and organochlorine compounds in waters,140 except that the ®nal determination was by either dc or rf GDOES. The LOD were between 0.1 and 0.5 mg l21. Organochloride and organobromide compounds were determined by MIP-OES141 after conversion to hydrogen chloride or bromide and trapping in NaOH solution. Organic species were separated from inorganic species by passage through a column of activated charcoal. The LOD were 8 and 3 mg l21 for Br and Cl, respectively. The ratio of 15N : 14N in soils has been determined by OES.142 Ethylation has been used143 as the basis of the determination of Bi by FI-AAS. The LOD was 0.8 mg l21 and the procedure was successfully applied to the analysis of urine, for which only a simple 1z1 dilution was needed as sample pre-treatment. 1.2.3 Vapour generation of individual elements. 1.2.3.1 Arsenic. Both AsIII and AsV were determined in sea-

water144 after preconcentration on activated alumina as the complexes with quinolin-8-ol-5-sulfonic acid by HG-AAS. The retained species were eluted with HCl; AsV was not reduced. The LOD were 0.05 and 2 mg l21, respectively for a 2 ml sample volume. Chinese workers have reduced AsV with Lcysteine for determination by HG-AAS;145 reduced AsV with KI and ascorbic acid for the analysis of foods and beverages;146 and used target factor analysis for the determination of four As species in waters.147 Muinoz et al. devised148 a microwave-assisted distillation procedure for the separation of inorganic As species from seafood products prior to determination by HG-AAS. The concentrations found ranged between 0.05 and 1 mg kg21. The As contents of a large number of water samples from the US, China and Canada have been determined149 by HG-ETAAS with inatomizer trapping on a palladium-coated cuvette. For a 25-ml sample the LOD was 0.3 ng l21. Both AFS and AAS were evaluated for the determination of As in wine and beer by HG.150 The only pre-treatment was degassing and sample solutions were injected directly in the 6 mol l21 HCl carrier in the FI manifold. The LOD was 0.3± 0.5 mg l21. Chinese workers digested the samples under auto re¯ux for the analysis of oil by HG-AFS,151 and passed the hydrides through a solution of permanganate to selectively remove stibine in the determination of As in antimony trioxide.152 Inorganic As species were selectively determined by HGICP-OES.153 By control of the solution acidity, only AsIII was determined; AsV was then reduced with L-cysteine (for 12 h) and total As was determined. Chinese workers determined As in 67 food coal-tar dyes by HG-ICP-OES,154 and As in sediments after co-precipitation with aluminium hydroxide.155 In the former study HCl and KI were used to reduce AsV, whereas, in the latter study, only HCl was used. The generation of volatile halides has also been used as the basis for the separation of As from unwanted matrix components. The generated species include the ¯uoride,134 the bromide136 and the chloride.135 In a study of As in urine, HG was used156 though the type of spectrometry was not speci®ed. The investigators found elevated concentrations in the urine of some subjects which could not be assigned to a high consumption of seafood and they concluded `that additional factors relevant in the exposure to As are still unidenti®ed'. Arsenic species have been separated by HPLC with HGAAS detection.157 In this study, UV photolysis was found to be superior to microwave-assisted decomposition in the postcolumn reaction scheme to convert non-hydride active forms of arsenic to hydride active forms. 1.2.3.2 Bismuth. A comprehensive study of reaction media for the determination of Bi by HG-ICP-OES has been undertaken.158 Tartaric acid was found to be the most effective medium in terms of ef®ciency of generation and control of interferences. The LOD was 0.3 mg l21 and the method was validated by the accurate analysis of some RM (water and silicate) and was applied to the analysis of iron ore and coal ¯y ash. Interferences from a number of transition metals and other hydride forming elements (and mercury) were evaluated. Chinese workers159 generated bismuthine from a slurry of the sample (geological materials mixed with aqua regia) on the addition of 0.8% sodium borohydride solution. The LOD was 60 mg l21 for determination by AFS. The interference from copper was eliminated by the addition of solid thiourea. Both copper and nickel interferences were considerably reduced on masking with 1-(2-thiazolylazo)-p-cresol.160 A batch generation system was used with detection by AAS. Urine was analysed by AAS143 following ethylation of the Bi. The LOD in urine was 2 mg l21. J. Anal. At. Spectrom., 2000, 15, 763±805

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1.2.3.3 Cadmium. To remove interferences in the determination of Cd in waste waters and sewage sludge by HG-AAS with room temperature atomization Munos-Olivas and Camara161 added KCN (0.5%) to the borohydride solution. This must have produced considerable amounts of potentially hazardous HCN on merging with the acid carrier stream. The LOD was 50 ng l21. 1.2.3.4 Germanium. The effects of various acids on the determination of Ge by HG-ICP-OES have been evaluated.162 It appears that tartaric might have been the best of the various acids investigated (hydrochloric, citric, oxalic, acetic and sulfosalicylic) in terms of interference tolerance. The LOD were all in the range 0.1±0.3 mg l21 regardless of which acid was used. Chinese workers also investigated the optimum conditions for HG,163 this time for AFS detection. They found that phosphoric acid, or a mixture of phosphoric acid and a mineral acid, gave the best generation ef®ciency and freedom from interferences. The gas-phase interference from As was eliminated by passing the vapors through a column of HgCl2. The LOD was 0.4 mg l21. The technique of HG-AFS was also used in a speciation procedure164 in which inorganic Ge and carboxyethylgermanium sesquioxide were determined in health drinks. As was discussed above, Ge has been determined by ICP-OES after volatilization as the chloride.137,138 The authors found that the LOD for HG and chloride generation (CG) were similar (around 0.2 mg l21) but that CG was much more selective. 1.2.3.5 Lead. Brindle and co-workers determined Pb in calcium carbonate materials (coral) by HG-DCP-OES.165 Ferricyanide was added as oxidant to aid in the production of Pb in the z4 oxidation state. The LOD was 0.7 mg l21. A specially designed gas±liquid separator was used. Chinese workers166,167 also used ferricyanide in a procedure for the determination of Pb by HG-AFS with an LOD of 0.2 mg l21. The procedure was applied to the analysis of geochemical RM. A mixture of phenanthroline, potassium thiocyanate and oxalic acid was used to mask interferences. Maleki et al. described168 a procedure for the generation of plumbane on injection of a sample solution into a reaction vessel containing solid borohydride and solid tartaric acid in a heating block at 65 ³C. The gases evolved were swept by a stream of nitrogen into one end of a ¯ame-heated tube in an AA spectrometer. The LOD was a modest 4 mg l21. 1.2.3.6 Antimony. A FI-HG-AAS procedure was developed for the determination of Sb in plant materials169 in which the SbV produced by digestion with oxidizing acids was reduced to SbIII by KI±ascorbic acid. The LOD was 0.01 mg l21. The sample digestion procedure was further developed170 for the analysis of lipid-rich materials. The method was validated by the analysis of bovine liver and pig kidney CRM and applied to the determination of Sb in pigeon eggs, bream and deer livers. The LOD, based on the dry powder, was 7 ng kg21. Dietz et al.171 cryogenically trapped the stibine produced from a batch reactor, containing acetic acid, into which the aqueous sample and aqueous alkaline borohydride were separately introduced. The hydride was re-volatilized in a stream of N2 and the Sb detected by MIP-OES. The LOD was 0.09 mg l21. A procedure for the determination of Sb in brass has been devised.172 Potentially interfering matrix elements (cobalt, copper, iron and nickel) were precipitated as the hydroxides. 1.2.3.7 Selenium. There is still considerable interest in the development of methods for the measurement of Se in a variety of materials by HG with atomic spectrometry. As it is impossible to generate hydrogen selenide from SeVI, any strongly oxidizing sample pre-treatment has to be followed by a reduction step in which the Se is reduced to SeIV. For the 770

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determination of Se in urine a microwave-assisted procedure with nitric acid and hydrogen peroxide was developed.173 Fumes of NOx, which cause a spectral interference in the subsequent HG-AAS determination, were eliminated by the addition of urea. Meat was digested in a mixture of nitric, sulfuric and percholoric acids,174 steel was dissolved in nitric, hydrochloric and perchloric acids,128 and hair was digested in nitric and perchloric acids.175 For each of these procedures, the SeVI was reduced to SeIV by heating in the presence of HCl. Li and co-workers noted173 that organoselenium compounds were partially decomposed by this process, and concluded that a simple procedure for distinguishing inorganic selenate from inorganic selenite in the presence of organoselenium compounds was not possible. Presumably, distinction between borohydride active `inorganic' forms of Se and borohydride unreactive `organoSe' would not be possible either. A procedure for the selective determination of selenite has been developed176 in which six different extraction solutions were evaluated and the Se was preconcentrated and separated by retention on an anion-exchange resin. An anion-exchange resin was used177 to remove the interference from copper (retained as anionic chloro complexes) and to preconcentrate the Se as selenotrisul®de. Chinese workers178 devised a double HG method in which the Se was converted to hydrogen selenide from a relatively large sample volume and trapped in a relatively small volume of alkaline hydrogen peroxide solution, from which hydrogen selenide was generated and detected by AFS. The LOD was 6 ng l21. By trapping H2Se on a hot gold wire109 Se was preconcentrated to give an LOD of 5 ng l21. The trapped hydride was released at 600 ³C into a stream of H2 and detected by AFS. Soluble Se-species in agricultural drainage waters and soil sediment extracts were distinguished on the basis of resistance to oxidation.179 Selenium(IV) was determined by AAS following HG from the sample solution with no pretreatment. OrganoSe was determined by HG-AAS after oxidation with alkaline persulfate, and ®nally total Se was determined after oxidation with 30% H2O2 at 90 ³C, followed by reduction to SeIV by persulfate (20 min at 90 ³C). The reduction of SeVI by a reagent that normally behaves as a strong oxidant is an interesting ®nding. 1.2.3.8 Tin. A FI-HG-AAS procedure was developed for the determination of Sn in sea-water,180 with a LOD of 0.1 mg l21 for a 500 ml sample. 1.2.3.9 Tellurium. Steel samples were decomposed with nitric, hydrochloric and perchloric acids.128 Following precipitation with hydroxide and re-dissolution in HCl, the sample solution was passed through a strong cation-exchange resin. The eluent was mixed with HCl and boiled for a few minutes and the Ge (and Se) were determined by HG-AFS. 1.2.3.10 Mercury. Russian181 and Czech182 workers have reviewed the determination of Hg. The ®rst of these concentrates on the AA determination in soils, while the second is concerned with the determination of organoHg compounds in various environmental samples. Russian workers have also reported183 on the determination of Hg in biological materials from cadavers. A new mussel tissue SRM has been issued.184 The CV-AAS determination of Hg was used in support of studies of the ef®ciency of polymer-enhanced ultra®ltration,185 a new technique for the removal of heavy metals from aqueous solutions. Provided that 10% HCl was added, the presence of the water-soluble polymer polyethylimine did not cause any interference. A procedure for the characterization of thiol (±SH) binding groups has been developed186 in which free HgII and thiol-bound HgII were distinguished on the basis of their reactivity towards borohydride solutions of different concentrations. Quantitative reduction of HgII to Hg0 takes place with a speci®c amount of

sodium tetrahydroborate according to the `stoichiometric reaction of mercury with alkaline NaBH4', but the reduction of HgII±thiol complexes may require up to 6 orders of magnitude molar excess of borohydride. The mercury was determined by AFS. On the other hand,187 the `sul®de interference' was overcome by treating the sample solution with solutions of sodium hydroxide, sodium hypochlorite (NaClO) and copper sulfate, added in that order, followed by the addition of 10% tin chloride in 0.5 mol l21 sulfuric acid. The determination was by AAS. A sequential injection AAS procedure has been developed188 in which borohydride was used as the generating agent. Small volumes of sample in low acidity HCl solution (0.05 mol l21) and reagent (15±30 ml, 0.2± 1%) separated by air segments were ®rst drawn into the holding coil. Then vapor generation was initiated on reversing the ¯ow and ¯ushing the solutions through a ¯ow-through gas±liquid separator. The LOD was 0.1 mg l21 and the consumption of reagents, compared with the FI procedure, was decreased by a factor of 25. Preliminary information189 about a new miniaturized AFS system in which the thermally decomposed samples were introduced from a metal ETV device has been provided. An MIP-AES system with amalgam trapping preconcentration is being developed.190 Russian workers have developed191 an amalgam trapping device consisting of a column of silica or alumina granules coated with sponge gold.191 Several studies have focused on the extraction/digestion procedure for the determination of total Hg. Three different acid extraction procedures were evaluated192 for the CV-AAS analysis of soils. Some of the extracts were exposed to UV radiation for up to 4 h. Mercury vapour was generated by the addition of 3% sodium borohydride solution to the sample solution diluted 1z1 with HCl. For the analysis of sediments, mercury was solubilized by microwave-assisted extraction in a sealed vessel with nitric acid.193 Borohydride solution was again used, but only at a concentration of 0.2 %, with detection by AAS. Coal was analysed194 following microwave digestion in aqua regia by FI-CV-AAS and FI-CV-ICP-MS. The results were compared with those obtained with a LECO pyrolysis system and by NAA. The LOD were 80 ng l21 and 6 ng l21 for AAS and ICP-MS, respectively. Urine was analyzed195 by a procedure in which bromate, bromide and HCl were added prior to digestion in a ¯ow through microwave system. Quanti®cation was by AAS with the PerkinElmer FIMS (¯ow injection mercury system). The LOD was around 50 ng l21. For the determination of Hg in blood and plasma, room temperature bromination was used after overnight digestion or heating for 4 h. Elemental Hg was produced by reduction with tin(II) and quanti®cation was by AFS, with a throughput of 20 h21. The LOD were 0.5 and 0.9 nmol l21 for plasma and blood, respectively. To avoid some sources of systematic error in the analyses of biological materials, a single vessel procedure has been developed.196 The samples were digested with a mixture of nitric and sulfuric acids; the Hg vapour generated on the addition of SnII was quanti®ed by AAS. A similar procedure has been used197 for the analysis of bovine kidney. Following the acid digestion, 30% H2O2 was added. The ®nal determination was by AAS. Chinese workers198 analysed foodstuffs by ®rst mixing the solid sample with vanadium pentoxide and nitric acid and leaving overnight. Then sulfuric acid was added and the mixture heated to 140 ³C. Potassium permangante was then added until a pink coloration was observed, and ®nally the excess permanganate was removed with hydroxyammonium chloride. Quanti®cation was by AAS. A somewhat similar procedure was used199 by other Chinese workers for the determination of Hg in household waste materials A microwave assisted digestion procedure has been devised200 for the analysis of foliage by CV-AFS. Water samples201 were analysed (AAS) by direct introduction into an FI system in which the sample carrier of

3% HCl merged with 3% SnCl2 in 10% HCl. Total Hg in hydrocarbons and natural gas condensate has been determined by CV-AFS.202 Samples were vaporized at 400 ³C and all mercury species were collected on a gold trap at 200 ³C. On heating to 900 ³C metallic Hg was released. The procedure was applicable to the determination of mercury chloride, methylmercury chloride, ethylmercury chloride, phenylmercury chloride, dimethylmercury, diethylmercury, and diphenylmercury. Atmospheric particulates were analyzed203 for their mercury content by pyrolysis-gold amalgamation-thermal desorption-AFS. It was found that results for denuder-based methods were higher than those of a conventional ®lter procedure, as mercury-bearing gold particles were ¯aking off the gold-coated denuder surfaces. In an effort to improve the LOD of an ICP-OES procedure for the determination of Hg, the sample solution was preconcentrated204 by retention of the Hg on an anionexchange column loaded with 1,5-bis[(2-pyridyl)-3-sulfophenylmethylene]thiocarbonohydrazide. The Hg was eluted with 2 mol l21 nitric acid and merged with a steam of tin(II). The LOD was 4 mg l21, which is not as low as can be achieved by AAS or AFS. For the determination of Hg in cosmetics,205 an APDC complex was retained on a C18 solid phase extractant by reaction with tin(II) and detection by AFS. The LOD was single digit mg l21. Treble et al. have devised206 a batch procedure in which the mercury was retained from a solution containing iodide on a cation-exchange resin impregnated with quinine. The mercury was desorbed by thiourea in HCl and Hg vapor was generated on the addition of alkaline potassium borohydride. The LOD was 1 mg l21. Some con¯icting results have been reported for the determination of inorganic-Hg (i-Hg) and methylmercury (m-Hg) based on the reactions with borohydride and tin(II). Burguera and co-workers described a procedure for the determination of i-Hg and total-Hg in urine.207 The basis of the method is that i-Hg is determined by CVAAS after the addition of tin(II) and total-Hg is determined following the microwave-assisted oxidation of organoHg with persulfate in a FI system. The LOD was 0.1 mg l21. Thus, the authors assumed that organoHg compounds do not react with SnII to give mercury vapour. These workers also applied the procedure for the determination of the same species in ®sh egg oil.208 Samples were handled as emulsions, though this time the Hg vapor was generated with borohydride, and thus the authors assumed either that this reagent produced a species which did not absorb at 253.7 nm or that borohydride did not react with organoHg compounds. A similar concept was used by Rio-Segade and Bendicho209 for the analysis of biological and environmental samples. Persulfate in sulfuric acid was used as the oxidant and the mercury vapour was generated on the addition of SnII. Distinction between i-Hg and total-Hg was based on heating the reactants, in a CF system, to 85 ³C. At this temperature organomercury compounds were decomposed to i-Hg, whereas at room temperature only i-Hg reacted with SnII. These authors also claimed210 to be able to determine m-Hg, which had been selectively extracted (ultrasound assisted) from ®sh tissue by 2 mol l21 HCl, by forming Hg vapour on the addition of borohydride. Treatment with 5 mol l21 HCl extracted i-Hg from which Hg vapor was generated by adding SnII. They also claimed in a later publication211 that Hg0 was generated by the post-column merging of a stream of borohydride (0.01%) with the column eluent (10 mmol l21 tetrabutylammonium bromide and 25 mmol l21 sodium chloride in 60% methanol) containing separated i-Hg and m-Hg. While it appears to be agreed that SnII generates Hg0 only from i-Hg, it is not clear whether borohydride generates Hg0 from organoHg compounds (speci®cally methylmercury). Work in progress (C. D. Palmer and J. F. Tyson212) supports the hypothesis that methylmercury hydride is produced when borohydride reacts with m-Hg and that this compound is stable with respect to decomposition J. Anal. At. Spectrom., 2000, 15, 763±805

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to Hg0 at room temperature in a quartz tube atomizer and so no signal is observed when borohydride reacts with m-Hg. Chwastowska et al.213 claimed to be able to determine both iHg an alkyl-Hg following `reduction by SnII'. The species were separated from the sample matrix with the aid of resin loaded with 2-mercaptobenzothiazole. The resin retained both species, which were subsequently eluted with a solution of thiourea in HCl. Inorganic-Hg was ®rst determined, and then cadmium was added to displace Hg from organoHg compounds which reacted with SnII to give Hg0. This is a modi®cation of the procedure originally described by Magos.214 The LOD was 10 ng l21. Tao et al. used215 a TMAH sample dissolution procedure for the determination of i-Hg in biological tissues. The i-Hg was released by the addition of L-cysteine and reduced to Hg0 on the addition of SnII. The LOD was 0.1 mg l21. 1.3 Nebulization As usual, there has been a large contribution to the literature concerning sample introduction in this review period. Among the papers that have reviewed or given an overview of sample introduction is one by Montaser et al.216 containing 202 references, that gave the basic concepts of sample introduction for ICP spectrometry. A paper that challenged many of the `accepted' theories has been presented by Olesik.217 Instead of regarding the spray chamber as a droplet size ®lter that, when used in conjunction with a pneumatic nebulizer, contributes to the very low sample transport ef®ciency (often 1±2%), he claimed that the low transport is caused by many other processes including droplet±droplet collisions and coagulation and evaporation of aerosol and solution from the spray chamber walls. Since larger droplets have higher momentum, they cannot follow the gas ¯ow and hence impact on the side of the spray chamber. Therefore, by decreasing the size of the droplets by either using a lower uptake rate, by using a solvent that is more volatile than water, or by heating the aerosol to cause evaporation, analyte transport rates could approach 100%. Other factors that affect the transport rate include acid or salt matrices that alter the extent of evaporation. A variety of nebulizer±spray chamber assemblies were assessed. A more applications-based review has been produced that concentrated on biota analysis by ¯ow injection (FI) coupled with atomic spectrometry.218 The review contained 95 references and information about the type of samples analyzed and summaries of the procedures. The introduction of liquids to microwave induced plasmas (MIP) has continued to attract considerable attention. This has been achieved using a variety of sample introduction devices. An ultrasonic nebulizer (USN) was used to introduce water samples at a rate of 35 ml min21 into an argon MIP operated at 150 W.219 Figures of merit including linear range, precision, LOD and accuracy were reported. The LOD for 31 elements were between 0.3 and 1000 mg l21 and the accuracy was veri®ed by the analysis of certi®ed water samples. The same sample introduction system has been used to introduce acid digests of a reference material (pine needles) into a vertically positioned, axially viewed, aerosol-cooled plasma.220 The torch cooling system for the aerosol generation and circulation was described in detail. A hydraulic high pressure nebulizer (HHPN) used in conjunction with a desolvation unit has been used to introduce different Cr species eluting from a high-performance liquid chromatography (HPLC) column.221 The sample was introduced to a BST Rutin C18 RP column and the mobile phase was 15% methanol, 0.1 mM tetrabutylammonium acetate, 0.1 mM ammonium acetate and 1 mM phosphoric acid. The linear range covered two orders of magnitude and the absolute LODs were 13 ng for CrIII and 18 ng for CrVI. Severe interferences from easily ionizable elements were observed during the analysis of real samples. A thermospray nebulizer constructed in-house, used in conjunction with a desolvation 772

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device, also made in-house, has been described.222 The design and construction of both devices was reported in detail. Under optimum operating conditions of 100 W forward power, a support gas ¯ow rate of 0.75 l min21, carrier gas ¯ow rate of 0.8 l min21, observation height 8 mm above the top of the torch, sample uptake rate of 1.5 ml min21, capillary temperature 320 ³C, aerosol spray chamber temperature 200 ³C and condenser cooling water temperature 0 ³C, LODs were found to be in the range 0.019±426 mg l21 and precision was 0.4±2% RSD. Inclusion of up to 20% methanol in the analyte solution was found not to disturb the plasma, hence offering the potential for coupling with HPLC. Very little in terms of novelty has been reported for sample introduction to ¯ame atomic absorption spectrometers (FAAS). Two papers written in Chinese have reported the determination of Zn in milk or milk powder. One223 used an emulsifying agent and reported that the results were in agreement with those obtained using a dry-ashing procedure. The other224 used pulse nebulization with injection volumes of 100 ml. This latter paper reported a LOD of 28 ng ml21, an average recovery of 99% and precision at 3.3%. No interferences were reported. Another Chinese paper reported the determination of several analytes (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in air after passing them through a micropore membrane ®lter at a rate of 80 l min21.225 After collection, the membranes were acid digested and the resulting solutions analyzed by FAAS. Digestion was reported as being complete and recoveries using the standard additions method were 91± 110%. Total Cr and CrVI has been determined in cigarette ash and smoke using FI and HHPN.226 The FI approach involved a sorption preconcentration system. The system was described in more detail (in Hungarian) in another paper.227 The majority of novel sample introduction systems described in the literature have concerned ICP-AES detection. A wide variety of different nebulizer types have been described. A sonic spray nebulizer (SSN) has been developed that has an ori®ce diameter of 250 mm.228 The ori®ce was made of a polyamide material to prevent metal contamination and allowed a gas ¯ow rate of 1 l min21. A comparison with a conventional concentric nebulizer was made and it was found that the LOD were similar, although the sample uptake was very much reduced (1±50 ml min21 compared with 850 ml min21 for the conventional nebulizer). Absolute sensitivity was improved by a factor of 13. Direct sample insertion (DSI) is a technique that is still being used by some authors because it enables both liquid and solid samples to be analysed directly. A review of the technique containing 79 references has been made by Sing.229 The review provides an overview of the instrumentation, operating parameters, ®gures of merit and applications. An application of DSI in which several analytes (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were determined by ICP-AES in sea-water and ground water samples has also been presented.230 The presence of several salts and common solvents such as hydrochloric, nitric, sulfuric and phosphoric acids was found to decrease the emission intensity. The extent of the matrix interferences was element dependent. Direct injection high ef®ciency nebulizers (DIHEN) have been described in two presentations.231,232 The device enables a transport ef®ciency of 100% to be achieved, whilst introducing the sample at a ¯ow rate of 1±100 ml min21. The analysis of `dif®cult' samples e.g., petroleum products, was described in one paper,231 and nebulizer diagnostic techniques based on optical patternation were used to characterize the aerosol in the other.232 An assortment of other nebulizers have been used. These include a modi®ed concentric glass nebulizer capable of analysing high salt content samples.233 It is a modi®cation of both a standard Meinhard and the LB nebulizer. It is reported as being capable of aspirating a saturated solution of sodium

chloride (26.54%) for two hours without clogging and the stability is superior to that of the LB nebulizer. The analytical ®gures of merit are also favourable when compared with the Meinhard, with higher SBR and lower LOD. A comparison of several different nebulizer types [high ef®ciency nebulizer (HEN), microconcentric nebulizer (MCN), Micromist and a conventional concentric nebulizer] operating at low sample uptake rates has been made.234 Parameters investigated included the gas back pressure, the free liquid aspiration rate, primary and tertiary aerosol droplet size and the solvent and analyte transport rate. It was found that the conventional nebulizer produced a coarser aerosol and subsequently had a lower solution transport rate through the spray chamber and a lower sensitivity than the other nebulizers do. Overall, it was found that the HEN provided the lowest limits of detection. A microwave thermospray nebulizer (MWTN) has been developed that operates by breaking the liquid stream by the expansion of the solvent vapour inside a capillary when heated by a microwave ®eld.235 It was found that a certain level of dissolved ions must be present in the sample for nebulization to occur and that the analysis of pure water was not possible. An interesting characteristic of the nebulizer is that the higher the dissolved salt content, the ®ner the aerosol produced. For samples containing dissolved ions, the LODs and sensitivities obtained using this nebulizer were improved when compared with a conventional nebulizer. A modi®ed nebulizer has been described that minimizes transition element interferences with selenium hydride generation (HG) ICP-AES.105 A capillary tube is placed in the sample introduction channel of a conventional Meinhard nebulizer. This carries the acidi®ed solution into the sodium tetrahydroborate solution that is introduced via the normal sample introduction channel. Gas± liquid separation is performed by a conventional Scott doublepass spray chamber. It was found that this small modi®cation enabled Se at the 0.5 mg l21 level to be determined without interference from 50 g l21 nickel, 25 g l21 cobalt and 20 g l21 copper (all introduced as their 2z salts). The LOD was 2 mg l21, precision was typically v2% and the method was validated by the analysis of NIST certi®ed material nickel oxide. Ultrasonic nebulizers (USN) have again found wide usage for a number of applications. A USN has been used in conjunction with a long torch to improve the sensitivity in an axially viewed horizontal ICP.236 Sample transport ef®ciency was increased when compared with a cross ¯ow nebulizer and hence net signal intensity increased and the intensity and ¯uctuation of the background decreased, thereby yielding improved LODs and dynamic ranges. The device was used during the analysis of a water CRM. Since the sample transport rate using a USN is substantially higher than with conventional nebulizers, desolvation is often necessary to prevent plasma perturbation. A paper that has addressed this has been published by Allen et al.237 A microporous membrane desolvator (Cetac MDX-100) was used whilst determining the effects of sodium on the analytical signal of several analytes. The authors concluded that the enhancement of the signal caused by the sodium was greater in the presence of the desolvator but that the mechanism by which the sodium interfered was the same. A HG-USN system has been described that is capable of determining both hydride forming analytes and other analytes simultaneously.107 Using the spray chamber as the gas±liquid separator, it was found that the LODs and SBR of the HGUSN device were the same as, or superior to, HG and USN separately. A USN has been used for the determination of Bi in urine using a FI preconcentration technique.53 After digestion of the urine using nitric acid, the solution was taken to near dryness on a sand bath. After re-extraction with nitric acid, the white ash was taken up in aqua-regia, diluted, mixed with ethanolic 8-hydroxyquinoline adjusted to pH 5 and passed through a column. The Bi±8'-hydroxyquinoline complex was

eluted with 2 M nitric acid. The LOD was 0.03 mg l21, precision at 2 mg l21 was 2.5% and the recoveries for 1±8 mg l21 were in the range 93±100.5%. Matrix effects caused by the use of a USN have been discussed by Budic.238 The effects of calcium, potassium, sodium, nitric acid and other matrices during the determination of several analytes were discussed. It was found that, in the presence of 20% nitric acid, the transport of the sample decreased by 8% when compared with water and that the presence of 1 mg ml21 matrix elements decreased the sample transport ef®ciency by 10%. Consequently, this led to a reduction in emission intensity. It was also found that the presence of potassium enhanced both the electron density and excitation temperature, whereas the presence of the other ions had the opposite effect. Matrix effects have also been discussed for a number of nebulizer±spray chamber designs. An overview of nebulizer diagnostics, including a discussion of tools such as optical imaging (e.g., high speed photography), particle imaging velocimetry, optical patternation, Fraunhofer laser diffraction, phase Doppler particle analysis, rainbow refractometry, absolute intensity of scattered light and the ratiometric technique, has been made.239 Another overview of matrix effects and how they can be minimized and compensated for has been made by Mermet.240 There have been several papers that addressed the effects of acids on the determination of analytes. In one paper, the transient effects of different acids on the signals of several analytes were discussed.241 It was found that the presence of the acid changed the extent of aerosol evaporation in the spray chamber and hence the sample transport rate. The effect was dependent on the type of acid used. A similar paper by the same authors stated that, as the concentration of nitric acid was increased over the range 0± 25%, the sample transport rate decreased, but that the nebulizer gas ¯ow rate also had a large effect.242 At low ¯ow rates (0.7± 1 l min21) there was a decrease in transport ef®ciency up to a nitric acid concentration of 2%, but there was no further decrease up to a concentration of 25%. At gas ¯ows higher than 1.3 l min21 there was a continual decrease in transport ef®ciency. It was therefore concluded that the acid dependent changes in aerosol properties occurred mainly during the transport of the aerosol through the spray chamber. Acid effects for a number of low-¯ow sample introduction methods was discussed by Todoli and Mermet.243 One method used a MCN with a Scott double-pass spray chamber, another two used desolvation systems comprising two Liebig condensers in series or a porous PTFE membrane, respectively, both using MCNs, and the last system used a DIN. Liquid ¯ow rates ranged from 5 to 120 ml min21 and a number of acids including 0.9 M nitric, hydrochloric and sulfuric and 3.6 M nitric acid was tested. It was found that for the MCN with the spray chamber, the acid effects increased as the ¯ow rate decreased. The opposite was true for the desolvation systems and the DIN. At a ¯ow rate of v30 ml min21 no effects were observed if a conventional desolvation system was used at a temperature of 160 ³C. In another similar paper, Todoli compared the performances of several nebulizer designs (pneumatic concentric micronebulizer, HEN, Micromist, MCN and DIN) with respect to acid introduction.244 Aerosol characteristics and ®gures of merit, such as LOD and background equivalent concentration (BEC), were compared. It was concluded that the DIN suffered fewer matrix effects. The analysis of plant material has also led to some matrix effects that were removed if higher plasma power was used, e.g., 1450 W.245 Depressions in emission intensity from the analytes As, Cd, Co, Cr, Cu, Ni, Pb and Se were observed in matrices containing calcium, iron, magnesium, manganese, phosphorus, potassium and sodium and this depression increased with increasing concentration of the concomitant species. A correction for volatility differences between organic sample analytes and standards in organic solutions has been made.246 The method is based upon the J. Anal. At. Spectrom., 2000, 15, 763±805

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measurement of the analyte signal at two different spray chamber temperatures. The technique required no prior knowledge of the chemical structure of the analyte and tests with organosilicon and organochlorine compounds were found to reduce the error by a factor of between 2 and 30. Spray chambers are the second, but equally important, component of most sample introduction systems. These come in a variety of designs, many of which have been discussed in the literature during this review period. Several designs (a Scott double-pass made of Ryton and three cyclone spray chambers made of glass, polypropylene and PTFE) were compared for their characteristics of the tertiary aerosol, solvent transport rate, analyte transport rate and net emission intensity, LOD and BEC.247 Interferences produced by the presence of widely used acids were also evaluated. In all cases, a glass concentric nebulizer was used to introduce the sample. Results indicated that the glass and polypropylene chambers produced the most coarse tertiary aerosols, but also had the greatest solution transport rates, highest emission intensity, and lower LODs and BECs than the other chambers. The position of the nebulizer was found to be critical for the cyclone spray chambers. Acid effects were found to increase as the tertiary aerosols became more ®ne and so the Scott double-pass spray chamber exhibited the worst performance. The cyclonic spray chambers were also found to exhibit better short-term stability. A doughnut-shaped spray chamber that had an internal volume of 102 cm3 has been described by Liu et al.248 A Meinhard nebulizer mounted radially at the entrance arm produced a spray that was split into two ¯ows by a central hollow cylinder. These ¯ows were then recombined at the exit of the spray chamber and transported to the plasma. This spray chamber was found to exhibit higher sample transport ef®ciency, more stable internal pressure, higher signal intensity, lower BEC and improved precision and LODs when compared with a conventional Scott style spray chamber. Another spray chamber design that reputedly had improved washout characteristics (with a four order of magnitude change in 60 s), improved sensitivity by an order of magnitude and lower LODs has also been reported.249 Unfortunately, the design of the chamber was not discussed in the abstract. It was stated, however, that the long and short term stabilities were also improved. The performance of three different spray chambers used in conjunction with a USN has been compared.250 A conventional Fassel type, a Cetac and a spray chamber designed and constructed in-house (details given) were the types compared for the analysis of transient signals. The performance was judged in terms of the washout period required, with shorter times indicating a better performance. The spray chamber designed in-house was found to be the best by far with a washout period 1/5th that of the Fassel and 3/5th that of the Cetac. In addition, it had a similar sensitivity to the Cetac which, in turn, was superior to the Fassel. Consequently, the Cetac and the chamber designed in-house had improved LODs when compared with the Fassel. Computational and simulation papers that attempt to model the processes within the spray chambers have also been published. Two papers by Berndt et al. discussed the application of such programmes.251,252 By transferring the physical ¯ow domain (the spray chamber) to the computational domain, a grid consisting typically of 50 000±150 000 volumes was constructed, then mathematical algorithms were used to obtain numerical solutions for the individual volumes, which enabled a distribution of velocity components and pressure to be constructed. In this way it is possible to calculate the equations of motion for each droplet through the spray chamber. The authors have constructed data for both a cyclone and a Scott style spray chamber. The results of the computational model were in good agreement with those obtained experimentally, with a calculated transport ef®ciency of 2.6% compared with an ef®ciency of 2.7% obtained 774

J. Anal. At. Spectrom., 2000, 15, 763±805

experimentally. Despite these encouraging results, the authors stressed that the need to build and test prototype spray chambers still exists. A paper that discussed the simulation of atomic and ionic absorption and emission spectra for thermal plasma diagnostics has also been published.253 Doppler, van der Waals, Stark and instrumental broadenings were all taken into account and the effect of self absorption was also considered. The input parameters were pressure, temperature, electron density and the emitting species number density. The simulation method was applied to the diagnosis of volatilization phenomena in thermal plasma spraying processes and the selection of diagnostic lines and instrumental and data processing parameters. It was also used to evaluate the accuracy of the temperature and density distributions calculated from spectroscopic measurements. 1.4 Solid sampling Solids analysis utilizing both AAS and AES continues to be a signi®cant area of research. A number of instrumental con®gurations are used for the introduction of either slurried or solid samples into ¯ames, furnaces and plasmas. As in previous years, many of the publications in this area are application based and these reports are summarized in Table 1. Nolte et al.254 considered both laser ablation and slurry sampling for ICP-AES in their study of the in¯uence of simultaneous versus sequential measurement of the signal, background and internal standard. The experiments were carried out using an array spectrometer allowing collection of simultaneous spectra; this permitted the use of a single data set to estimate the differences between measurements made in simultaneous and sequential modes. Simultaneous background correction and simultaneous internal standardization gave a mean RSD of 3.6%, sequential background correction and simultaneous internal standardization produced an average RSD of 6.1% and sequential background correction and sequential internal standardization produced an average RSD of 6.4%. The use of solid sampling for ETAAS as a screening technique can provide a signi®cant time saving over conventional digestion methods. Belarra et al. carried out a theoretical evaluation using 18 000 simulated results.255 The optimum conditions for analysis were determined based on the type and frequency of outliers commonly encountered in solid sampling ETAAS. The median was found to be a more useful result than the mean due to the reduced effect of outliers. The minimum number of samples required for a screening method was determined as 5±20 for a guaranteed recall of 95% (recall~no. of correct identi®cations}no. of attempted identi®cations) when the median was used. This sampling rate corresponds to between 15 min and 1 h of work, a signi®cant reduction in time compared with the preparation of liquid samples. The same workers successfully applied solid sampling to the screening of antimony in PVC.256 Slurry sampling for ETAAS has proved to be a popular technique for the analysis of solids with easy automation. Pioneer of the technique, Miller-Ihli, discussed the advantages of slurry analysis with ETAAS and provided evidence that the method can be successfully applied to ETV-ICP-MS.257 Ultrasonic mixing, either manual or automated, is commonly the preparation method of choice for slurry sampling. The maintenance of a homogeneous slurry allows the sampling of both liquid and solid from sample cup to furnace. Amoedo et al. considered the extraction of Pb from solid samples using ultrasound with subsequent analysis of the supernatant alone.258 Probe sonication allowed quantitative recoveries of Pb from several plant and animal SRMs. Several chemometric techniques were used by Cal-Prieto et al. for the optimization of a method for the determination of Sb in geological samples using automated slurry sampling ETAAS.259 Plackett±Burman

Table 1 Solid sampling Type of atomization

Type of sampling

CV-AAS

Elem.

Matrix

Sample treatment/comments

Ref.

Slurry, prepared using ultrasonic mixing

Hg

Biological, environmental

276

ETAAS

Slurry

As, Cd, Cu, Pb

Food

ETAAS

Slurry

Ba, Cu, Fe, Pb, Zn

Tea leaves

ETAAS

Slurry

Be

Rice and ¯our

ETAAS

Slurry

Te

InSb

ETAAS

Slurry, automatic preparation using Ar bubbling Slurry, automatic preparation using ultrasonic probe Slurry, automatic preparation using ultrasonic probe, manually prepared using hand-held ultrasonic probe or ultrasonic bath Slurry, prepared by ultrasonic mixing

Co, Cu, Ni

Sediments and soils

Sb

Geological

Total Hg determined in samples using ¯ow injection. Samples suspended in 15% HNO3 (mussel tissue, aquatic plant), or 9z1 15% HNO3±15% HCl (river sediment, sewage sludge). Results were comparable with those obtained after microwave digestion Slurries prepared in 0.15% agar as a stabilizing agent. Phosphate (for Pb and Cd) and Pd±MgNO3 (for As) used as modi®ers Ba and Pb determined using standard additions, Cu, Fe and Zn determined using aqueous standards. Results from slurry analysis of SRM compared well with those obtained using ICP-AES after microwave digestion. Sensitivity and simplicity found to be the bene®ts of slurry method; however, for determination of several analytes ICP-AES may be faster LOD 1.4 pg obtained, no details of method given in abstract Palladium nitrate used as modi®er. LOD 0.4 mg g21. Results agreed well with solution ETAAS and ICPMS Ground samples suspended in HF, samples containing Cu and Ni partially digested using microwave. Fast heating programme used Chemometric techiques used for the optimization of procedure

Pb

Mussel tissue and SRMs

Preparation procedures compared for extraction of Pb. Quantitative extraction into liquid only possible using ultrasonic probe. This helps avoid sedimentation and volumetric errors. However, analytical results obtained using automatic preparation are superior

258

Co, Cu, Ni

Botanical

Coulter particle analyser used to study the effect of grinding and ultrasonic agitation on slurry analysis. 1 h grinding produced similar size distribution for all materials 6 mm discs punched from paper samples, ground in a mortar, suspended in 5 ml H2O and sonicated for 2 min. 20 ml samples taken for analysis. Calibration by aqueous standards and by standard addition to clean ®lter paper used. RSDs 0.8±10% for Fe and 0.3±7.6% for Cu. Laser ablation ICP-MS was used for metal distribution studies A Mo-tube atomizer was employed with thiourea as chemical modi®er. LOD 17 pg ml21. Results in good agreement with those obtained from acid digested samples Samples, 0.2±2 g, suspended in 5 ml solution containing 0.1% m/v Triton X-100, 1% v/v HNO3, 20% v/v H2O2, 0.3% v/v nickel nitrate. 20 ml aliquots taken for analysis during stirring. Fast furnace programme used. Method of standard additions used. LOD 23 pg Sample (10±35 mg) mixed with 1 ml 60% PTFE (as modi®er), 0.4 ml conc. HNO3 and 0.2 ml 0.5% plant glue solution. The mixture was agitated ultrasonically for 60 mins. LOD 0.12 pg, RSD (n~5) 7.8% for 1 ng ml21 Sample (150 mg) mixed with 5 ml 15% HNO3±10% H2O2 and agitated ultrasonically for 15 min. 20 ml aliquot taken for analysis. LODs 23±44 ng g21, RSDs 4±8% (n value not given) Ammonium phosphate modi®er and glycerol used. LOD 6.8 pg Direct analysis used for screening of tissues. Results obtained for direct solid analysis and for analysis of digested samples were closely correlated and were statistically no different Sample was vaporized in one graphite furnace and the generated aerosol was transported in Ar and electrostatically deposited on a platform in a second graphite furnace. Multi-element analysis carried out via continuum source coherent forward scattering spectrometer. Results obtained in agreement with certi®ed values

282

ETAAS ETAAS

ETAAS

ETAAS

Slurry, prepared by ultrasonic mixing

Cu, Fe

Old manuscripts

ETAAS

Slurry, prepared in an ultrasonic bath

Cd

Calcium drugs

ETAAS

Slurry, prepared manually and with ultrasonic mixing

As

Baby food

ETAAS

Slurry, prepared using ultrasonic mixing

Cd

Biological

ETAAS

Slurry, prepared using ultrasonic mixing

Co, Cr, Ni

Wheat ¯our

ETAAS

Pb

Biological

ETAAS

Slurry, prepared using ultrasonic mixing Solid

Cd

Equine muscle

ETAAS

Solid

Cu, Fe, Mn

Flour SRM

J. Anal. At. Spectrom., 2000, 15, 763±805

277 278

279 280 281 259

283

284

285

286

287

288 289

290

775

Table 1 Solid sampling (continued) Type of atomization

Type of sampling

Elem.

Matrix

Sample treatment/comments

Ref.

ETAAS

Solid

Pb

Muscle tissue

291

ETAAS

Solid

Sb

PVC

ETAAS

Solid

Various (11)

Tungsten trioxide and tungsten blue oxide

ETAAS

Solid

Various (11)

Tungsten powder

ETAAS

Solid

Various (15)

Titanium bars

ETAAS

Solid

Various (9)

Aluminium oxide powder

ETAAS

Solid

Zn

Geological RM

Flame AAS Flame AAS

Slurry Slurry

Ca Ca, Fe, K, Mg, Na, Zn

Maize ¯our Vegetable tissues

Flame AAS

Slurry

Cu

Chinese medicinal herbs

Flame AAS

Slurry

Mn

Flame AAS

Slurry

Zn

Chinese medicinal herbs Milk fat

Flame AAS

Slurry, prepared from sample shaken with zirconia beads, particle size v0.8 mm

Ca, Cu, K, Mg, Na, Zn

Hair

ICP-AES

Cup-in-torch

Pb

Fingernails

ICP-AES

Direct insertion

Cu, Fe, Mg, Mn, Zn

Wood pulp

ICP-AES

Direct insertion

Various

-

Analysis of samples contaminated with gun-shot residue. Solid sampling suitable for determination of Pb level in non-contaminated samples and for the discrimination between original Pb content and Pb derived from gun-shot Samples (2.5-3.5 mg) were inserted into enlarged furnace injection hole through a paper cone. RSD (n~14)~12.6%, which was considered satisfactory for screening purposes. High background due to vaporizing oxides eliminated by use of hydrogen purge gas during pyrolysis. LODs 0.07 (Mg, Na, Zn)±2 (Ni) and 0.01 (Mg, Na, Ni)±1.7 (Fe) ng g21 for tungsten trioxide and tungsten blue oxide, respectively Direct analysis of high purity tungsten powders with an ETAAS system equipped with a transversely heated atomizer, deuterium background correction and solid sampling introduction by the `boat' technique. LODs in range 0.01±4 ng g21. RSDs (n~5) 1.1±30.8%. (See also 293) Sample cut from Ti bar and etched with HNO3/ HF. Samples (v30 mg) placed onto platform and inserted into furnace. LODs from 0.02 ng g21 (Mg) to 30 ng g21 (Sn) (See also 294 and 295). Sample (0.06±6 mg) placed onto a pyrolytic graphite coated platform and inserted into furnace. Platform re-coated every 5±6 ®rings to reduce background absorbance. LODs 25, 3.5, 2, 10, 0.8, 0.25, 1.5, 12 and 0.5 ng g21 for Co, Cr, Cu, Fe, K, Mg, Mn, Ni and Zn, respectively. (See also 294 and 296) Calibration graphs prepared using different solid RMs had different slopes. To overcome errors, cluster analysis was used to select materials of similar composition and three-dimensional calibrations using intensities, masses and contents were used. The latter was the preferred approach Solution of ¯our with 0.15% gelatin, LOD 0.12 mg l21 Optimized conditions found for slurry analysis. 50 mg lyophilized vegetable material slurried for 5 min using ultrasound and diluted to 50 ml with 0.720 M HNO3. RSDs v3% for all analytes. LODs 2.2, 1.8, 3.4, 8.7, 2.1 and 1.7 mg l21 for Ca, Fe, Mg, Na, K and Zn, respectively Powdered sample (1.5 g) suspended in 25 ml 0.15% agar solution. After shaking, 4 ml aliquot mixed with 1 ml 0.15% agar and 1-5 ml Cu (II) standard solution (10 mg ml21) and H2O to 25 ml. LOD 57 ng ml21, RSD (n~6) v2.5%. Sample treatment identical to that described in 300. LOD 13.6 ng ml21 5 surfactants compared for preparation of homogeneous emulsion. Triton X-100 and NaDBS preferred Acid predigestion used as a pre-treatment stage. Glycerol and Viscalex HV30 compared as wetting agents. Wetting agents were only bene®cial for dilute slurries. LODs 5, 3.5, 3.1 and 20 mg kg21 for Ca, Cu, Fe and K, respectively, and 1.7, 0.6, and 3.5 mg kg21 for Mg, Na and Zn, respectively Re cup inserted 1.5 cm above the carrier gas inlet. Drying and ashing carried out in-situ Solid samples were introduced into ICP using pyrolytically coated graphite probe after in-situ treatment with HCl and NaF. Samples were dried and ashed by heating probe prior to plasma ignition. LODs 50±1000 pg. Method suitable for screening of samples, signi®cant time saving over dissolution methods obtained An automated direct insertion device was described. Solution, powder and slurry sampling were compared and slurry sampling was found to be the most promising. Formation of carbides was eliminated by addition of SF6 to the plasma gas

776

J. Anal. At. Spectrom., 2000, 15, 763±805

256

292

294

296

295

297

298 299

300

301 302 303

304 305

306

Table 1 Solid sampling (continued) Type of atomization

Type of sampling

ICP-AES

Elem.

Matrix

Sample treatment/comments

Ref.

Direct insertion, sample preconcentrated on activated carbon

Cd, Cu, Pb, Zn

Waters

307

MIP-AES

Slurry ETV on W coil

Ag, Cd

Waste water

ICP-AES

ETV

Various (16)

Graphite, silicon carbide

ICP-AES

Slurry

Lanthanum oxides

ICP-AES

Slurry

Simultaneous analysis of slurry samples

310

ICP-AES

Slurry ETV

Rare earths Various (9) Al, Ti, Y

Metal complexes of oxalic acid, iminodiacetic acid or 8hydroxyquinoline were collected on activated carbon. Loose particles of carbon were either loaded into direct insertion cup or complexes were adsorbed onto a direct insertion probe with an activated carbon cap. The specially machined cap produced the better results. Recoveries were poor although 8hydroxyquinoline was the most promising complexing agent Samples were vaporized on W-coil. LODs 16 mg l21 for Ag, 1 mg l21 for Cd Solid sample, 20 mm for graphite and v5 mm for SiC, placed on platform and inserted into ETV unit. Freon added to Ar carrier gas Ð

311

ICP-AES

Slurry ETV

ICP-AES

Laser ablation

ICP-AES ICP-AES

Laser ablation Slurry ETV on W-coil

ICP-AES/ ICP-MS Spark AES

Samples were analysed with and without PTFE as a ¯uorinating agent. 90% of a 100 mg sample could be vaporised with no loss of analyte. LODs from 0.11 (Al) to 0.09 mg g21 (Ti) with RSD 1.9±4.2% Samples were analysed with PTFE emulsion as a ¯uorinating agent. LODs from 2 (Yb) to 130 ng ml21 (Ce) with RSD v5% Effect of matrix, chemical and physical form of analyte and laser wavelength were studied Description of a novel bulk solid sampling system Samples were vapourized on a W-coil. LODs between 0.01 (Mg) and 8.5 mg g21 (Co) Spatial resolution of 50 mm obtained. RSDs 0.3±0.7% obtained for Al and Si with Ba as internal standard, and 1.5% for B with Si as internal standard Steel sample is embedded in an ingot of Sn allowing analysis of small samples. Results in good agreement with those obtained from bulk samples

Milk and infant formula Si3Ni4 powders

Laser ablation

Rare earths (14) Al, Fe, Mg Various Various (11) Various

Environmental samples Various Al-based ceramic powders Amorphous solids

Solid

Various

Steel

Lanthanum oxide

designs were used to assess the in¯uence of sample mass, volume, HNO3 concentration, Triton X-100 concentration, ultrasonic probe power and sonication time, and to optimize the variables. A two-way ANOVA was used to evaluate the effects of sample cups and replicates. Control charts were also used to monitor graphite tube performance. In addition, two pipetting options and three quanti®cation methods were studied. The usefulness of the optimized method was demonstrated by the accurate analysis of 5 SRMs. A review (115 references) of laser ablation (LA) sampling by Russo et al. was published during the period of this review.260 Calibration and optimization, fractionation, sensitivity enhancements, mass loading and particle transportation were discussed. A single calibration graph was successfully used for the determination of major elements in geological materials in LA-ICP-AES by Kanicky et al.261 Sc and Y were used as internal standards and limestone and silicate samples were analysed. The same authors262,263 measured the acoustic signals produced during laser ablation ICP-AES using a Nd:YAG laser at 266 nm. It was found that the acoustic signal could be used to focus the laser on the solid surface by monitoring the maximum in the acoustic signal. The intensity of the acoustic signal could also be used to determine the size of the crater produced during ablation. A systematic investigation into the effects of the matrix, chemical and physical form of the analyte and laser wavelength was carried out by Motelica-Heino et al.264 Synthetic samples were prepared from different crystalline compounds of Al, Fe and Mg spiked in SiO2 or CaCO3 pressed into pellets and analysed using LA-ICP-AES using a Nd : YAG laser at 1064 and 266 nm. The experiments demonstrated the strong dependence of the LA-ICP-AES response factor on the

308 309 Ð

312 264, 313 314 315 316 270

chemical form of the analyte and on the bulk matrix composition. These effects were also dependent on the laser wavelength, and use of the UV laser did not lead to any improvement in minimization of the effects. In contrast to chemical effects, there was no effect on the response factor from the grain size or binding pressure of the pressed pellets. Matching of mineralogical and chemical composition of the matrix and of the chemical forms of the analytes for calibration standards is recommended to avoid systematic errors in quantitative analysis. The effect of the gas environment on laser ablation characteristics was investigated by two groups of workers. The effect of ®ve noble gases (He, Ne, Ar, Kr and Xe) in the laser ablation sampling chamber was studied by Leung et al.265 An enhancement of ICP emission intensity for laser sampling in He and Ne and a decrease for Kr and Xe relative to Ar was observed. Therefore, the use of He as the sample chamber gas can signi®cantly improve sensitivity. Thareja et al.266 used an intensi®ed CCD to image the plumes produced by the laser ablation of aluminium and PTFE substrates in Ar, He, air and O2 at various pressures. The effect of ambient atmosphere on the expanding front of the plume was investigated photographically. Strati®cation of the plasma was observed at moderate laser intensities for both the metal and the polymer materials. The nature of the aerosols produced from the laser ablation of polymer materials using a UV laser was investigated by Todoli and Mermet.267 Ca, Sn and Ti were determined in PVC and PE samples and the aerosols compared with those obtained from glass samples. For Ca, but not Sn or Ti, the aerosol particle size was dependent on the chemical form of the analyte. A second PVC sample, containing 11 elements, was also studied, and Al, C and Na exhibited different behaviour with J. Anal. At. Spectrom., 2000, 15, 763±805

777

particle size with respect to the remaining elements. Carbon was mainly present in gaseous form and in particles of less that 3 mm in diameter; this observation precludes its use as an internal standard to compensate for variations in the ablation process for polymers. Individual ¯uid inclusions were analysed using LA-ICPAES.268 The laser radiation is used to drill the solid sample until the ¯uid inclusion is reached. Emission from the excited atoms and ions in the ¯uid was analysed using a spectrometer equipped with a pulsed and gated multichannel detector. Na : K, Na : Ca and Na : Li intensity ratios were measured. Calibrations were established using a range of standards, including glasses and ¯uid inclusions. LODs achieved were suitable for the determination of ions in inclusions. A review (79 references) of direct sample insertion for ICPAES by Sing appeared this year.229 A useful overview of the technique including instrumentation, operating parameters, system response, analytical ®gures of merit and applications is provided. A critical evaluation of dc arc spectrometry for the analysis of solid samples by Florian et al.269 concluded that the modernized dc arc AES method, combining ®bre optics and a multi-channel spectrometer, is a ready alternative to existing solid sampling methods for AES and AAS. Grientschnig et al. proposed a procedure for the analysis of small samples of steel using spark discharge AES.270 The procedure involved embedding the sample in an ingot of pure Sn. Results from the analysis of three steel samples prepared by the embedding method were in agreement with those obtained on bulk samples of the same steels. The method is suitable for samples having a diameter of greater than 6 mm. A review (20 references) in Japanese by Shimizu271 considered the use of glow discharge (GD) for multielement surface analysis. Shimizu et al.272 assessed the resolution of GD-AES for the depth pro®ling of chemical species in anodic alumina ®lms. The ®lms, acting as cathodes, were sputtered in Ar at 3±5 Torr. Emission associated with the sputtered species (Al, B, Cr, H, P, W) was monitored throughout the experiment with a sampling interval of 0.01 s. The depth resolution was reported as being comparable to SIMS pro®ling of similar ®lms. The use of plasma etching for the characterization of nonconducting materials by GD-AES was investigated by Barshick et al.273 Studies were performed in Ar containing 0.01% by weight of CF4. When a conducting cathode such as Cu was used, the sputtering rate was decreased by a factor of ®ve compared with the use of pure Ar as support gas. When a nonconducting glass was analysed, however, ¯uoride radicals formed in the discharge reacted with the substrate to form volatile SiF4, which was spontaneously released into the gas phase, carrying Cu and U with it. As a result, enhancements of 50 and 30% were observed for the determination of Cu and U, respectively, in glass. Workers in Marcus' group274 conducted studies to determine the practical bene®ts of mixed discharge gases (Ar and He) in the bulk and depth-resolved analysis of solids using an RF GDAES source. A number of parameters were assessed for both conductive and non-conductive sample matrices. It was observed that the addition of He to Ar did not improve detection limits in the bulk analysis of conductive materials. However, in the analysis of non-conductive materials, the addition of He was found to enhance analyte emission intensity without signi®cantly in¯uencing the sputtering characteristics. For depth resolution, optimized conditions, especially gas pressure, derived for pure Ar can be improved through the addition of He to the discharge. Marshall275 evaluated the impact of lamp control parameters on the quality of both bulk and depth pro®le analysis by GDAES. The lack of current and voltage stability was shown to have a larger in¯uence on the quality of calibration curves, bulk 778

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check standard results and quantitative depth pro®le analysis than do similar ¯uctuations in pressure. Therefore, control modes that achieve constant current and voltage at the expense of constant pressure are recommended. 1.5 Electrothermal vaporization 1.5.1 ET-AAS. Modi®cation of the graphite tube surface with metals, or use of metallic atomizers, continues to be of interest in ET-AAS. Rademeyer and de Jager317 discussed the use of `permanent' metallic modi®ers in ETAAS. The process of sputtering the metal onto the graphite surface was investigated. Physical characteristics of the graphite surface after sputtering with noble metals were evaluated and reported. Fundamental studies on the stabilizing effect of sputtered modi®ers and solution modi®cation were carried out. A permanent iridium modi®er deposited on tungsten and zirconium-treated platforms was evaluated by Slaveykova et al.318 using XRF, ESCA and SEM. Two atomization processes were observed and explained by the differences between the Ir±W and Ir±Zr interactions and surface distribution. A similar study on the behaviour of As on tubes coated with a range of metals was carried out using XRD and SEM.319 The performance of tube treatments in the determination of As with a palladium modi®er were compared. Optimum sensitivity and precision were found with a Ta coating. Portable AAS systems based on tungsten-coil atomizers continue to be the subject of several presentations and publications. Temperature measurements of gas and coil surface temperatures have been made320 and descriptions of systems are given.321,322 Several papers comparing the performance of chemical modi®ers for ETAAS have provided some useful information for users of this technique. For the determination of Sr in biological ¯uids, La was found to provide optimal performance in an evaluation of 12 modi®ers, including Pd.323 Pd was found to be the best modi®er for the determination of Cd in sewage sludge and Sn in PVC after an evaluation of background effects.324 Mixed modi®ers for particular applications also received some attention.325±327 1.5.2 ETV-ICP-AES. Few papers have been published during the period of this review reporting fundamental studies with ETV-ICP-AES. A number of publications on the use of the technique for the direct analysis of solid samples have appeared and they are discussed in the previous section of this review. Work from Jones' group on the use of tungsten coils for vaporization was reviewed as conference presentations last year328 and has subsequently been published.329 The ef®ciency of sample introduction into an ICP via electrothermal vaporization was investigated using three methods by Kantor and Gucer.330 The methods used were: (i) a direct method based on deposition of aerosol particles and analyte vapour by mixing the vaporization product with supersaturated steam and subsequently condensing the mixture followed by the determination of the amount of analyte collected; (ii) indirectly by dissolution and analysis of material deposited on interface components; and (iii) by calculation from line intensities when using ETV and pneumatic nebulization using Hg as a reference element. Using a 200 ml min21 Ar carrier ¯ow, the transport ef®ciencies calculated were 67±76% for medium volatility elements such as Cu, Mg and Mn and 32±38% for volatile elements such as Cd and Zn. However, the addition of CCl4 vapour to the internal Ar ¯ow resulted in transport ef®ciencies of 67±73% for all ®ve elements studied. 1.5.3 In-torch vaporization ICP-AES. Karanassios et al. evaluated six vaporization chambers for the introduction of micro-samples to an ICP.331 The chambers ranged in shape from conical to oval, had internal volumes ranging from 3.4±

18 cm3 and were tested using in-situ generated smoke. The best results were obtained with a 6.5 cm3 internal volume chamber. The sample was deposited onto a Re ®lament forming a threecoil loop and inserted into the vaporization chamber attached to the ICP torch. Good analytical performance was obtained with the optimized system for both liquid and slurried samples.

chambers on either side of a central six-lamp changer compartment. It has an eÂchelle monochromator and facilities for both deuterium and Zeeman background correction. A choice of software is available according to the experience of the user. 2.2 Sources and atom cells

2 Instrumentation 332

The authors of a comprehensive review of recent advances in plasma emission spectroscopy project a promising future for this technique. AE detectors, optics and spectrometers, solid sampling, nebulization plasma sources, computer programming for instrument control, multi-component analysis, intelligent instrumentation and ICP-MS detection are all covered in some detail. The paper is well worth reading by all interested in this ®eld. Another `general' article, by Kuss and Bayraktar,333 described the development and general principles underlying simultaneous AAS. Some of the advantages of this method are discussed, in particular the simplex optimization of associated heating programmes. 2.1 Spectrometers The con®guration of a new transmission grating spectrograph, composed of a grazing-incidence optical system with a large area transmission grating, was described by Chinese workers.213,334 High ef®ciency and relatively high resolution are claimed. From Japan came news of a `cavity ringdown' spectrometer335 but no technical details were given in the English abstract. Compact eÂchelle spectrographs with multi-channel timeresolved recording capabilities, utilizing a gated interline CCD camera, come from Multichannel Instruments, Sweden.336 The system records 6000 time-resolved channels in the 200±1100 nm region with a shortest gate time of 100 ns. Some applications are described. Instrumental developments at Thermo Jarrell Ash and Baird were the subject of a conference paper.337 Developments in digital micro-mirror spectrometry are described by Batchelor et al.338 and de Verse et al.339 This technique achieves rapid sequential read-out using a single detector. In the former paper, a 2-D array (6406480) of individually addressable 16616 mm Al micro-mirrors enables an image of a burner to be projected on to the focal plane of a ¯at ®eld monochromator. By this means, wavelength, slit width, height and position can be programmed, thus enabling wavelength scanning. In the second paper, emphasis is placed on the application of the instrument to Hadamard transform spectrometry. Applications included use with He MIP-AES. The use of two German-manufactured mobile AES instruments with a dc arc for semi-quantitative analysis and arc-inair or spark-in-argon for quantitative analysis of metals, was described by Russian workers.340 U isotope measurement and measurement of U and rare earth elements are among applications described for a high resolution ICP spectrometer341 speci®cally designed for these purposes. A sequential ICP spectrometer with an intelligent wavelength calibrating device has been described.342 This does not require either thermal equilibrium to be attained or the line-pro®le method to be used to eliminate peak drift. This instrument is also used in a metallurgical context. For simultaneous measurement of four elements by GFAAS, Wagner et al.343 positioned the appropriate HCLs on the Rowland circle of a concave holographic grating and measured the signals simultaneously with a CCD detector. Detection limits for Cd, Cr, Cu and Pb were 4, 12, 14 and 12 pg, respectively. A new concept in AAS is claimed by Unicam GmbH.344 The SOLAAR M instrument is compact but has ¯ame and furnace

This is the ®rst time that this review has covered all the atomic spectroscopies together and it may be worth while again to clarify the above terms. `Sources' are the generators of radiation (narrow line or continuum) which is eventually to be measured by the detector. `Atom cells' (or `atomizers') are the devices for producing the population of free atoms (or sometimes ions) from an analytical sample. In AAS and AFS these two devices are separate. In AES the source is also the atom cell. 2.2.1 Sources for optical emission spectroscopy. 2.2.1.1 Plasmas. Emphasis this year appears to be again on ICP and glow discharge sources. However, there is one paper from Prudnikov et al., St Petersburg,345 on the arc-in-¯ame plasma. The cathode was positioned on the burner and the anode about 30±40 mm away, the arc being maintained through the length of the ¯ame. Arc current was 15±20 A at 220 V. The sample solution was introduced with a direct injection nebulizer and an easily ionizable element such as K was added to stabilize the discharge. Any of the usual ¯ame gases (air±C2H2, N2O±C2H2, air±H2±Ar, etc.) may be used. A feature of the discharge is a `calm' region where temperatures of up to 6000 K are reached. Stability is similar to that of the ¯ame and detection limits are said to be as good as those in ICPs. Helium plasmas are the subject of a conference paper by Montaser et al.,346 in which a number of advantages over ICPs are claimed, including better selectivity, sensitivity, precision, reliability and ease of operation, lower cost and smaller size of instrumentation. The new sources were described along with appropriate sample introduction systems for both AES and MS. Green®eld347 has recalled the reasons behind the invention of the annular ICP some 30 years ago. An optical interface was developed by Rutzke348 to reduce the matrix effects observed in an axially viewed ICP and improve recoveries of trace elements in a complex matrix. The components are mounted on a sliding platform in order to focus on different locations along the axis of the plasma. A new, compact, free running solid state generator system for ICP-OES was described by Hensman.349 Discussion of characteristics and capabilities was promised in this conference paper abstract. Barnes has co-authored two papers concerned with enclosed or sealed ICPs.350,351 Flow-through ICP systems utilize only 10±50% of atoms present. A sealed system has a much lower gas consumption and also allows the introduction of hazardous auxiliary gases like Cl2 or more expensive noble gases than Ar. The system is also used to measure hydride-forming elements direct from the aqueous solution in the hydride generation process. The general conference paper350 described all these possibilities and the second paper reported on the measurement of Fe and Ni in electronic grade chlorine, which is made to form a 50% Cl2±Ar discharge mixture. Calibration was achieved by mixing in known quantities of iron carbonyl, ferrocene or nickelocene. Problems in the measurement of volatile elements by ICPOES and ICP-MS have recently been claimed to have been solved by reducing sample aerosol interactions with smaller spray chambers or even with no spray chamber at all. Eastgate352 reported on results obtained with cyclonic and tulip-shaped chambers in conjunction with low-uptake nebulizers. Improvements in accuracy for Hg and I, reduction in J. Anal. At. Spectrom., 2000, 15, 763±805

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matrix effects and accelerated sample washout are claimed. An in-torch vaporization system for introduction of liquid and solid microsamples was described and characterized by Badiel et al.353 In a clip-on chamber, the sample is dried, charred and vaporized in a rhenium cup and the vapour carried to the plasma in a stream of Ar±H2. A 22 mm torch for ICP-AES was characterized by Horner and Hieftje.354 This could be operated at powers and ¯ow rates similar to those for a conventional 18 mm torch. Improvements in analytical performance were not claimed but the plasma in the new torch showed less long-term drift as well as other properties which made it more suitable for use in ICP-MS. A paper in Chinese described the use of an air-cooled ICP for measuring As, Bi, Ge, P, Pb, Sb, Si and Sn.355 Comparison of results showed that elements with atom lines of long wavelength and low excitation potential have as good or better detection limits than with the argon ICP. Developments in MIP technology came from Japan and Poland. Oishi et al.356 have designed and constructed a nitrogen MIP which was linked to MS for measuring rare earths, and Jankowski et al.220 described a vertically positioned axially viewed argon MIP, the design being based upon an improved rectangular cavity and strong coupling between the plasma load and the magnetron generator. Sample introduction was by means of a peristaltic pump. Excitation temperatures and analytical characteristics were determined for a number of elements and spatial distribution within the resonator was also studied. Figures of merit compared well with those generally accepted. Hieftje et al.357 described the use of an air±Ar MIP torch for the detection of tetraethyl lead. The best detection limit (as Pb) was 0.012 ppb with linearity over three orders of magnitude. Three papers on the capacitively coupled rf Ar plasma torch have appeared from the team led by Cordos in Cluj, Romania. This was developed358 for analysing non-conductive samples such as oxides and silicates. It operates at 13.56 MHz, rf input power of 20±50 W and Ar ¯ow of v11 min21. The design is modi®ed359 to have a tubular central electrode and outer ring, this time with double the frequency and with 275 W of power, resulting in improved detection limits. The performance of this plasma was also described360 at atmospheric pressure in `tipring electrode' geometry for both nebulized liquid and sputtered conductive solid samples. Excitation and ionization temperatures were given along with performance ®gures for a number of elements in liquids and in low and medium alloyed steel. 2.2.1.2 Discharge lamps. A comparison of fundamental characteristics has been made between rf and dc powering of single GD sources.361 Although the dc mode gave higher emission intensities, rf powering gave better S/N ratios, became stable more quickly and was more stable over extended periods, thus giving better detection limits. The effect of bias voltage modulation on the analytical performance of rf powered GDs was investigated by Wagatsuma.362 A `lock-in' detection method was used for measuring Cu in nickel plates and iron alloys which contributed to an overall improvement in detection limits. An rf powered Grimm-style Ar plasma was controlled by introducing a new dc channel network.363 This is claimed to improve detection sensitivity in ICP AES and to enhance emission lines from He rf plasmas, which then excite nonmetallic elements such as F and so can be applied to a wide range of elements. Wagatsuma also found,364 after a study of singly ionized lines of Cu and V, that ionic lines are usually more sensitive than atomic lines and are almost free from selfabsorption in GD-OES. The same author365 made a comparison of emission characteristics of the lanthanide elements in 780

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GD-OES and ICP-OES, ®nding again that ionic lines are more intense in the GD. A microsecond reÂgime GD has been developed by Yang et al.,366 and its performance compared with that in the continuous mode. Pulse widths varied from 0.1±300 ms and frequency from 1 to 105 Hz. The pulsed lamp has a higher sputtering rate, greater signal intensity by several orders of magnitude and better detection limits. There is a different temporal response for atoms and ions, however, emission from the former maximizing during the period of the pulse, and that for ions a few ms after the pulse termination. Chlorine has been measured in halogenated hydrocarbon vapours by introducing the samples into a He-operated gassampling Grimm-type GD.367 Cl II emission was measured at 479.5 nm with a detection limit under optimum conditions of 20 ng s21. Spatially resolved measurements showed that the distribution in emission intensities of both Cl and He were dependent on the design of the aperture of the gas inlet capillary. Background intensities, too, were found to vary during the sample introduction period, thus the background requires careful monitoring and correction. Trace elements were measured by Park et al.368 in water ¯owing through a specially designed GD tube, the sample itself acting as cathode. The anode was a Pt rod, the end of which was positioned about 1.5 mm above the surface of the liquid. The space above the liquid was ¯ushed continuously with Ar. The discharge was stable at 1.5 kV and 80 mA and detection limits of less than 1 mg ml21 were found for Al, Cd, Cr, Cu, Hg, Mn and Pb: unfortunately, though, this is not good enough for application to drinking water. A two-cascade GD ion source, suitable for use with a double focus mass spectrometer, described by Sikharulidze,369 also operated with liquid samples. As the solution enters the hollow cathode the solvent evaporates into the vacuum, stimulated by ion bombardment, and supports the glow discharge which continuously sputters a ®lm of analyte from the inside wall of the hollow cathode. Sensitivity is said to be comparable with that of an ICP. An rf GD-OE spectrometer has been designed by Hubinois et al.,370 especially for con®ned use in a glove box in order to measure light elements in nuclear materials. Preliminary studies with the system were made on low alloy steels and optimizations were carried out to improve emission intensities and reproducibility. 2.2.1.3 Other emission sources. A previously described electrothermally heated HC emission source (Fresenius' J. Anal. Chem., 1996, 355, 272, Microchem. J., 1996, 54, 296) has been further developed.371 The measurement cycle of 10 ms is broken down into 9.6 ms of low current followed by 200 ms of high current up to 125 mA, then 200 ms of high current during which the actual measurement takes place. This cycle plus a background reading cycle is repeated 33 times s21. The full heating programme consisted of one drying, two ashing, vaporization and cup clean phases, with drying of a 10 ml sample being complete in 3 min at 2200 Pa and room temperature. The operating conditions and emission intensities for Cr, Cu, Mn, Ni and Pb were discussed. 2.2.2 Atom cells for atomic absorption spectrometry. 2.2.2.1 Flame atomizers. Shimadzu have taken out a patent for an AA spectrometer372 in which the inside wall of the burner is mirror-®nished to prevent the slot clogging. A burner head claimed by Chinese workers to give higher sensitivity by a factor of about ®ve373 appeared to have separate gas and sample inlets. Russian workers have proposed the use of a N2O±methylacetylene ¯ame374 which, they state,

gives signi®cantly higher temperatures than N2O±acetylene and is therefore advantageous in the determination of some 54 elements, particularly B, Ce, Cs, Gd, Sb and Ta. Typical ¯ow rates are: N2O, 10 1 min21; methylacetylene, 1.25± 6.25 1 min21. While testing a new design of source lamp, Willis and Sturman375 found that the sensitivity of Te depends on its oxidation state in both air±C2H2 and N2O±C2H2 ¯ames, TeIV giving a better value than TeVI by a factor of 1.66. Possible reasons for this anomaly are discussed. A micro-¯ame atomizer was developed by Sun et al.376 in order to measure organometallic compounds separated by gas chromatography. The volume of the atomizer was made to be one ®fth of that of a conventional ¯ame burner atomizer and H2 acted both as carrier and as fuel gas. A detection limit for Hg of 5.5610210 g s21 is quoted. 2.2.2.2 Electrothermal atomization. A review of graphite atomizers modi®ed with high-melting carbides was presented by Volynski.377 Such atomizers are most effective in the determination of elements such as B, Ga, Ge, In, Si and Sn which form stable oxides. If, however, they are applied to carbide-forming elements, e.g., Cr, Mo, Ti and V, there can be a signi®cant decrease in sensitivity. Chemical modi®ers are usually needed for high volatility analytes. But these atomizers show advantages for measuring some organometallics for the trapping of volatile hydrides, for the analysis of organic materials and for samples with high concentrations of mineral acids. A WC±Rh coating on the platform of a transversely heated graphite atomizer has been used by Lima et al. for measuring Cd378 and Cd, Pb and Se379 by ETA. A standard THGA was pre-heated with W and then with Rh. The permanent W±Rh modi®er remains stable for 300±350 ®rings and increases tube lifetime by 50±100% as compared with pyrolytic platforms. Analytical accuracy was shown to be at least as good as with the more conventional chemical modi®ers. Wagner et al. described380 a simple, low-cost multi-element spectrometer with a tungsten coil atomizer made from a standard 150 W projector lamp. Outputs from four HCLs are aligned by means of three 60³ beam combiners. The purge gas is Ar±10%H2. 2.2.2.3 Other atomizers. Absalam, Chakrabarti and others381 discussed the geometry of an rf glow discharge for AAS. The location of the gas inlet port has an important in¯uence on the observed signal. The best location is on the sampling ori®ce disc so that the mass ¯ow of sputtered atoms is entrained in the ¯ow of discharge gas and ef®ciently transported into the analysis volume of the atomization chamber. Distribution of sputtered analyte atoms is highly inhomogeneous under all conditions, though near the central axis of the atomization chamber the effect is minimal. The ICP has generally been believed to be too hot for use as an AAS atomizer. Hensman and Rayson382 developed a new optical con®guration in an attempt to improve its performance in this context. An enlarged sample introduction tube was found to give lower Fe and Ar excitation temperatures than predicted but the response is affected by the presence of water vapour and by the analyte molecular bond strengths. Many of the problems have been accurately identi®ed and investigated but no satisfactory overall solution seems yet to have been found. 2.2.3 Sources for atomic absorption spectrometry. 2.2.3.1 Continuum source AAS. Harnly's plenary review lecture to the 9th Biennial National Atomic Spectroscopy Symposium, Bath, UK, in July 1998, referred to in last year's update328 (para. 1.4.2) has now been published.383 In

CSAAS, he claims, absorbances are more accurate, detection limits are better by an average factor of 26, calibration ranges are 10006 greater and, of course, multi-element detection is possibleÐthese are among its advantages over line source AAS. Multi-wavelength detection provides the equivalent of in®nitely fast wavelength scanning, which is at the heart of CSAAS, and which has been made possible only in recent years by the use of CCD arrays. These and all other aspects of the subject are discussed in great detail in the paper. The characterization of calibration curves, discussed in the above paper, was the subject of a further paper by Wichems, Fields and Harnly.384 Log±log plots of calibration curves, both in theory and in practice, have a slope of unity at low concentrations and 0.5 at high concentrations. This was predicted as long ago as 1961 by Mitchell and Zemansky.385 The same hyperbolic curve, with appropriate X- and Y-axis offsets, can therefore be used to ®t every calibration plot, its position for any one plot being characterized by means of just two standards, although, according to this paper, better S/N ratios are obtained if four are used. Becker-Ross et al.386 reported further on their work with CSAAS using array detectors. These enabled them to make background correction with greater accuracy than that given by the methods normally employed in line source AAS. Two eÂchelle spectrometers with focal lengths of 0.3 and 1.0 m were used in conjunction with linear CCD arrays of pixel size 0.02360.48 mm, giving very small band widths per pixel. Optimum instrumental parameters for CSAAS were established and algorithms for computer aided background correction were tested. 2.2.3.2 Other sources. A German patent387 describes a laser diode which, it is claimed, incorporates structures making it suitable for use as a light source in AAS. A long lifetime gas discharge light source,388 tested with air, N2 and Ar, has been constructed using automobile sparking plugs as electrodes. Stable light pulses of 0.8±4 ns width were generated though there was appreciable heterogeneity of emission intensity distribution along the analysis gap. 2.2.4 Atomic ¯uorescence spectroscopy. Several papers describing a system for both atomic and ionic ¯uorescence measurements originated from Huang's group at the University of Xiamen in China. The excitation source is a high current, microsecond pulsed HCL and the atomizer a short torch ICP. Fluorescence techniques are said to minimize the spectral overlap interference characteristic of OES and provide a broader working range than AAS. The work was done partly on a modi®ed Baird AFS 2000. The development and evaluation of the pulse generator system were described389 and performance ®gures were quoted for 15 elements. The application of the system to the measurement of Eu using desolvated ultrasonic nebulization was described in another paper390 where the HCL pulsing is at 1000 Hz for 0.8 ms and the current 9 A. Both atomic and ionic lines were investigated and the latter gave better detection limits. Ion lines were used exclusively for the measurement of rare earth and alkaline earth elements391,392 and it seems that, in this case, a small monochromator was employed. Results were claimed to be `much better' than those to be expected with the original Baird system. 2.3 Detectors As detectors in atomic spectroscopy, PMTs have held sway for over half a century, but modern requirements such as multielement operation, instantaneous wavelength modulation and computer compatibility have led to the adoption and development of solid state arrays in both the research and commercial sectors. A manufacturer's overall view of the present state of J. Anal. At. Spectrom., 2000, 15, 763±805

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the art in the use of such detectors was given by Yates of PerkinElmer,393 who emphasised the need to use the detector most suited to a particular purpose. Limitations and possible future development, particularly of charge injection devices, were discussed. 2.3.1 Charged-coupled devices (CCDs). De Goy et al.394 described the operational features of CCDs in an attempt to answer the question `Which CCD is right for you?' Important factors requiring consideration include spectral response, operating temperature, spatial and spectral resolution, dynamic measurement range, camera gain and data acquisition speed. Members of the American Association of Physics Teachers suggest395 that CCD detectors from scanners and computer cameras offer convenient low cost means of converting a commercial monochromator to a spectrograph or of constructing a home-made spectrometer. Pointing out that these CCDs are computer interfaced, they go on to describe experiments which demonstrate their usefulness in teaching laboratories. In investigating its response through a very wide spectrum range, a CCD has been characterized from 1100 nm through the near IR, visible, UV and vacuum UV regions down to 0.14 nm by workers in Italy.396 Their conclusion was that `the CCD represents an almost optimal detector of radiation in the extended optical domain'. A conference paper from Bohle et al. of Spectro397 also explored an extended spectral region from 130 to 766 nm with an ICP-OES system using CCDs. This is said to give many more possibilities of simultaneous element measurement with uninterfered lines, especially between 130 and 200 nm. CCDs have been backed by manufacturers Varian and their choice is supported by several papers this year. Knowles et al.398 described an `innovative' CCD for AES and compared its performance with that of a circular array of PMs and of charge injection devices in a simultaneous ICP AES instrument. Conference papers399,400 describe the detector itself and two more401,402 the complete instrument in more detail. The application of the new `CCD axially viewed ICP-AE spectrometer' to the determination of major and minor elements in silicate rocks is described by Brenner et al.403 of Varian USA. 2.3.2 Charge injection devices (CIDs). Thermo Jarrell Ash/ Baird have used CIDs since 1992 and claim in a conference paper404 that these overcome `several major shortcomings' of CCDs. Their new CID gives a higher yield with reduced ®xed pattern noise while dark current and read-out noise are reduced and UV quantum ef®ciency is improved. Newly introduced ICP instruments with radial-view plasmas are claimed to exhibit a level of sensitivity approaching that of axial-view plasmas without their increased susceptibility to chemical and spectroscopic interferences. There were two conference papers405,406 describing the application of these CID-eÂchelle ICP instruments to, respectively, the analysis of oils and the measurement of Br, Cl and I. 2.3.3 Other detectors. A conference paper407 and a Chinese publication408 with identical abstracts herald the `HDD' detector system from Jobin±Yvon, France. No technical details were given but features claimed are: ultra-fast high resolution scanning (220 000 acquisitions in steps of 0.015 nm), giving a resolution of 0.005 nm, and less than 2 min total recording time of the whole spectrum; high dynamic range by `exclusive real time gain control'; sensitivity as a result of combining this detector with `excellent optics'. 782

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2.4 Background correction Bengtson and HaÈnstroÈm409 have attempted to improve accuracy at ppm levels for many elements in GC-OES by improving the accuracy of background measurement. This involves use of a matrix-dependent background term in the calibration rather than assuming interference overlaps from other analytes. Careful assessment of instrumental drift is also essential. True simultaneous measurements of background in analyte and internal standard have been compared with sequential measurements by Noelte et al.254 using the same data in both methods. Simultaneous readings of BG and IS gave an overall RSD of 3.6% while sequential readings of both gave 6.4% (simultaneous IS and sequential BG gave 6.1%). The results applied to laser ablation and to slurry sampling and these two procedures were also discussed. Three background correction methods, deuterium lamp, high speed self reversal (SR) and Zeeman, were compared by Oppermann and Hohmann410 for a range of sample types which characteristically give high background values. The SR method appears to have been preferred though the reasons were not given in this conference abstract. Zeeman background correction in GF-AAS is evidently still able to cause problems. Over-correction in the determination of Pb in bone has been reduced by Zong, Parsons and Slavin411 by substituting an end-capped transversely heated graphite atomizer for the standard version, though the rationale for this is not given in the abstract. 2.5 Data acquisition and control Two papers from Sun's group in Baoding, China,412,413 report the use of derivative absorbance measurements, obtained with a home-made differentiating unit. The output signal is directly proportional to the rate of change of the input signal, but both signals were recorded with a double-pen recorder. The papers describe the method as applied to the measurement of trace levels of Cd in vegetables and Pb in copper. Improvements in detection limit and sensitivity by about three orders of magnitude were claimed. Lopez Garcia et al.414 are using a sample introduction system for AAS in which two analyte solutions are being pumped at different rates and merged just before reaching the nebulizer. The selectivity of this set-up has been improved by Fourier transformation of the output signals which allows the two solutions to be entirely distinguished. Another suggested application of the system is that if n different solutions of the same analyte are pumped simultaneously and the concentration of one is known, then the concentrations of the other n-1 can be calculated. The data acquisition and transfer arrangements for in-situ AES described by Yang et al.415 are based on the `intranet' structure and work on the Windows NT operation system. Designed for the critical control of steel making, the system allows remote access and Internet connection so that analysis data can be shared, proceeded with and developed by appropriate staff throughout the organization. A practical assessment of multivariate calibration for a Thermo Jarrel Ash eÂchelle ICP-AES has been made by Madden et al.416 who attempted to utilize as much as possible of the large quantity of data provided by such an instrument. Rapid semi-quantitation improves throughput and allows better choice of standards for those samples chosen for more accurate analysis. Use of many atomic lines per element can increase precision, and improve identi®cation and interference correction. An iterative approach to classical least squares ®tting improved results for samples which contained high levels of interfering elements. Varian has evolved a fast automated curve-®t technique417 to

provide automatic and on-line deconvolution of overlapped spectra in ICP-OES. Even strongly overlapped peaks can be evaluated and small peaks can be extracted from complex backgrounds, giving greater accuracy than with conventional background correction methods. Several strategies were compared by Noelte418 for correcting analytical signals for overlapping lines in the measurement of Cd by ICP-AES. These included (i) parabolic peak ®t calculation, (ii) matrix matching, (iii) Boumans' inter-element correction method, (iv) on-peak measurement and (v) multicomponent spectral ®tting.419 Best accuracy, reproducibility and detection limits were all found for method (v), followed by (iv) and (ii).

3 Fundamentals 3.1 Plasmas 3.1.1 Microwave induced plasmas. The number of fundamental studies of the microwave-induced plasma has been small this year. MIP-OES was used to study the in¯uence of molecular gases and analytes on excitation mechanisms of an atmospheric Ar MIP.420 Spectroscopic measurements gave information regarding association and dissociation processes while power interruption experiments provided information on the dominant population mechanisms of radiative levels. Nitrogen-containing molecular emission bands were observed when the MIP was expanded into air. The same group421 used MIP-OES for the analysis of N2, CO2, SF6, H2O and SO2 in gaseous mixtures such as ¯ue gases. Only N2 yielded observable emission from the non-dissociated molecule. The spectra of the other molecular gases were dominated by atomic fragments and association products, indicating that the atmospheric like plasmas are likely to be unsuitable for the analysis of gaseous mixtures under the experimental conditions used in the study. A modi®ed plasma torch was also used by the group to determine electron number densities and electron temperatures by means of spatially resolved Thomson scattering measurements and photographic records at different working conditions.422 3.1.2 Glow discharges. In recent years, research into fundamental studies of glow discharges (GD) has been particularly active and this year the trend has continued. Depth pro®ling using radiofrequency GD-OES has become an analytical method of growing importance for the analysis of bulk materials and for pro®ling layers too.423 A rf GD has been studied with the aim to optimize the depth resolution by varying the rf-powered guiding parameters such as the peak-to-peak voltage and the argon ¯ow rate.424 By applying a constant peak-to-peak voltage of 2000 V as well as low argon ¯ow rates, the optimal discharge conditions were obtained. For nonconducting materials, the wavelengths used for emission intensity measurements are often prone to interferences and this problem has been addressed and explained for three nonconducting materials.425 In addition, surface roughness, which approaches the dimensions of the ®lm being analysed, may lead to almost total degradation of depth resolution.426 Helium±argon working gas systems in rf GD-OES have been described.427,428 The optimal emission and sputtering electrical characteristics were studied with varying He concentrations in a mixed gas system. Further, Langmuir probe studies have been used to study the effects of the rf power and the discharge pressure on the electron and ion number densities, the average electron energy and the electron temperature for aluminium and Macor in a rf glow discharge in He, Ar and He±Ar mixed gases. Measured electron and ion number densities decreased with increased He pressure and the sputter rates also decreased, resulting in enhancement of the Al I and Cu I emission intensities.

A collisional-radiative model was developed for sputtered copper atoms and ions in a dc argon GD under conditions normally employed in glow discharge AES.429 The populations of the various excitation levels were calculated in two dimensions for Cu ions and atoms and the relative contributions of the various populating and depopulating processes were determined for all levels. The same authors also developed a set of three-dimensional models to describe a GD in argon.430 In general, satisfactory agreement with experimental observations, for density pro®les of various plasma species, the ¯ux energy distributions of ions bombarding the cathode, the crater pro®les and the Ar atomic emission spectrum, was achieved. The majority of glow discharge systems currently operate in a continuous direct current (dc) or radiofrequency (rf) mode so that a steady state discharge is obtained. The amount of power normally applied to a glow discharge is 1±3 orders of magnitude smaller when compared to an ICP source.431 Increasing the power of the GD can easily lead to sample overheating: therefore, a pulsed high current mode is used so that high instantaneous power may be applied to yield an intensi®ed transient signal. Primary studies of a microsecond pulsed glow discharge as an emission source have been performed using hollow cathode lamps operated under different pulse conditions.431 Passing large currents of short duration repetitively through the cathode lamp and use of a gated detection system resulted in light intensity enhancement of up to 3±4 orders of magnitude with respect to dc operation. Signal to noise ratios were also improved by up to three orders of magnitude for some ionic lines. The advantages of microsecond pulse operation over millisecond pulsing have also been demonstrated.432 The pulsed GD plasma using both helium and argon gases has been studied for the analysis of three metals, copper, iron and tantalum, by OES.433 The reactivity of the metals determined the spectral species formed. During the pulsed operation, the metal surface has more time to react with trace oxygen or water that may be in the discharge chamber resulting in the formation of increased oxides. The Marcus design chamber was used to compare the excitation ability of both a rf and a dc GD using OES detection.434 Sn, Zn and Cu cathodes were used to investigate the behaviour of atomic and ionic emission lines of these analytes under different operating conditions. Lower sputtering rates were observed for the rf glow discharge, resulting in lower emission intensities. 3.1.3 Inductively coupled plasmas. The study of matrix effects on emission intensities using ICP-OES has again received much attention this year. The effect of acid concentration on emission intensity has been studied using a minitorch sample introduction system.435 The emission intensity, sample uptake, ionic/atomic emission intensity ratio and excitation temperature were found to decrease with increasing acid concentration. The degree of decrease depends upon the type of mineral acid used. A multi-component quasi-stationary thermodynamic model for ICP thermochemical processes was used to calculate the changes in electron concentration in the analytical zone caused by the presence of matrix elements with different ionization potentials.436 The effects of plasma temperature and Na concentration on the electron concentration, ionization suppression and partial pressures of matrix ions and electrons were also measured. The effects were studied for different nebulization systems. Matrix effects can be reduced in ICP spectrometry by applying robust operating conditions such as a high rf power level and a low nebuliser gas ¯ow.437 The use of power adjustments as an alternative to matrix matching or standard additions is described. In another study, the variation of the excitation temperature with carrier gas ¯ow rate is described.438 J. Anal. At. Spectrom., 2000, 15, 763±805

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An axially viewed ICP has been used to study calcium and sodium interference effects using low and high aerosol loadings439 achieved by using a conventional cross ¯ow nebulizer and an ultrasonic nebulizer. The use of high aerosol loadings caused detection limit degradation in the presence of large amounts of Ca and Na. The optimization of the instrument, in particular the adjustment of rf power to reduce matrix effects, was described. Similarly, the use of internal standardization to compensate for matrix effects owing to the presence of Na was evaluated under robust and non-robust conditions in both axial and radial viewing modes.440 Internal standardization was used to eliminate matrix effects due to Na when measurements were made under robust conditions in the radial viewing mode. Internal standardization, however, was not as ef®cient when measurements were made in the axial mode. Again this year, the use of simulations on the basis of droplet desolvation and solute-particle vaporization processes have been used to study the mechanism of matrix interferences in ICP spectrometry.441±445 The generation of an aerosol by pneumatic nebulization, concurrent with the production of a distribution of droplet sizes, may be studied in order to examine the effects of different matrices. Spatial time-integrated and space±time resolved pro®les of excited atoms of oxygen in the ICP have been measured by optical emission spectroscopy.446 The spatial emission pro®les of the discharge were required to understand the basic kinetics of the ICP. Optical emission data were supplemented by Langmuir probe measurements of electron densities and plasma potentials. A sample gas mixture of N2 and O2 was used to study oxygen/nitrogen spectral line ratios. The in¯uence of the thermal disequilibrium on the composition and the volumic enthalpy of the plasma produced in an ICP torch was studied. It was found that, if the temperature was suf®ciently low, the thermal disequilibrium had no in¯uence on the intensity of the spectral line ratios. The effects of natural moisture and of argon addition on the plasma temperature and detection limits for an air inductively coupled plasma were investigated.447 The in¯uence of H2O and of alkali metal salts were studied in terms of their effect on the discharge parameters such as the local excitation temperature in the analytical zone and on the thermal equilibrium of the plasma. The presence of moisture and alkali metal salts both led to decreases in analytical sensitivities and worse detection limits. Detection limits could, however, be improved by adding Ar (up to 50% v/v) to the air ICP. Work was presented to demonstrate that a stepwise series model can approximate the atomic excitation process in ICP-AES. The non-Boltzmann and nonSaha factors of Ca were calculated for 48 levels corresponding to radiative decay, radiative recombination, Penning ionization and absorption processes, in the axial channel of the ICP. Two conference proceedings discussed the use of low power reduced pressure ICPs.448,449 Electron number densities of helium and mixed helium±argon ICPs were measured with the aim to optimize the analytical capabilities and ionization characteristics of the mixed gas source. 3.1.4 Other. Temporally and spatially integrated measurements of the intensities of ionic and atomic lines were used to calculate the degree of ionization in a He, Ar and He±Ar mixed gas plasma in a rf furnace atomization plasma emission spectrometry source.450 The emission intensities of both ionic and atomic lines and analyte ionization were enhanced as the Ar composition increased in the He±Ar plasma. The maximum degree of ionization was obtained in a pure Ar plasma. 784

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3.2 Flames The mechanism of hydride atomization and the fate of free atoms in a miniature diffusion ¯ame have been described.451 Macroscopic movements and free atom diffusion were studied and spectroscopic temperature measurements were made based on atomic absorption. It was found that the temperature distribution was highly inhomogeneous and ranged from 150 ³C to 1300 ³C. A steady state kinetic model has been used to study atomization processes during ¯ame atomic absorption on the effects of Group II elements.452 Two forms of interference were identi®ed in the study, the formation of refractory oxides of the Group II elements rather than free atoms and the dissociation of AlCl3 causing collisional recombination of the Group II metal and Cl atoms. Separately, measures to prevent Zeeman splitting of AlCl, causing background absorption, has been addressed.453 3.3 Furnaces Many workers continue to be involved in the elucidation of mechanisms taking place in the heated graphite furnace. Alternative methods of calibration in ETAAS have been proposed by several groups of workers. Absolute analyte detection in ETAAS was described by Gilmutdinov and Harnly.454 The technique is based on the integration of absorbance over wavelength, height in the furnace and time. Adequate resolution in these three dimensions is possible using a continuum source, a high resolution eÂchelle monochromator and a 2-D solid-state array detector. Radziuk et al.455 discussed the use of internal standardization in ETAAS. This use of this technique is based on the supposition that the sensitivity of the analyte and the internal standard depend identically on the sample matrix and any variations of conditions occurring during the pyrolysis and atomization steps. Several clinical samples were analysed for lead content using either Bi or Tl as internal standard. Correlation of r~0.94 was found between the integrated absorbances for spikes of the analyte and internal standard when Bi was used. Tl was found to be unsuitable as an internal standard for Pb determination. RSDs (%) were found to be 7, 2.2 and 4, for urine, blood and placenta samples, respectively. The mean difference in the Pb concentration found in analysed samples by the method of additions and by using an internal standard was 10%. The acid corrosion of transversely heated graphite tubes and platforms was studied using SEM by Rohr et al.456 The effect on graphite surfaces during the atomization of V in the presence of HNO3, HF, HCl or HClO4 was compared. Increased corrosion and reduced furnace lifetimes were observed in the sequence HNO3vHFvHClvHClO4. Morphological changes as a result of corrosive attack were different in each case. Changes in the electrical resistivity of the tubes during lifetime tests were within the speci®ed range, and so the voltage controlled temperature settings were unaffected. A rf furnace atomization plasma emission spectrometry source was used to measure the intensities of atomic and ionic emission lines of 7 elements in He, Ar and He±Ar mixtures by Sun et al.450 Emission intensities and the degree of ionization increased as the Ar content of the plasma gas was increased from 0±100%. The degree of ionization was w70% in the Ar plasma for elements with an ionization potential of v8 eV and maximum degrees of ionization of 99, 80, 81, 72, 76, 12 and 38% were obtained for Cr, Mn, Mg, Co, Fe, Cd and Zn, respectively, in pure Ar. A review (40 references) in Chinese described the methods used for studying atomization mechanisms in ET-AAS457 included thermodynamic, kinetic and instrumental methods. The gaseous reduction of metal oxides by carbon (ROC) in graphite furnaces was ®rst proposed by L'vov in 1981 to

explain spikes on absorption signals of aluminium. The foundation of this mechanism is the assumption that the oxide is directly reduced by gaseous molecules of metal carbides which, in turn, form by the interaction of metal vapour with carbon. Periodicity in the spikes were observed for a number of elements in addition to aluminium, and so the temporal oscillation in the kinetics of carbothermal reduction of oxides was considered. A number of subsequent studies provided alternative theories to the ROC model for the appearance of spikes in the atomization signals for aluminium, aluminium oxides detected using MS appearing at the same time as spike formation being the most compelling evidence against the ROC theory. However, responses by L'vov kept the ROC theory alive. A recent investigation into the temporal oscillations for elements from several groups of the periodic table has been carried out in two laboratories, one by L'vov's group and the other at the National Research Council of Canada by Sturgeon.458 Microgram amounts of 10 elements as their nitrates were evaporated using slow heating rates. Periodicity of spikes was observed for 10 elements; this is interpreted as the reduction of oxides. It is recognised in the paper that the study is purely descriptive but the data suggests that the ROC theory is still open for discussion! Shadow spectral digital imaging (SSDI) with charge coupled device detection was used to study the spatial and temporal distributions of atomic, molecular and aerosol species formed during the atomization of a range of elements by Panichev et al.459 Complex, non-uniform structures of aerosol species located away from the tube axis and walls were prevalent. The observed signals were postulated as being due to the presence of metal clusters in the case of Ag, Au and Pd, and oxide aerosols in the case of elements such as Al, Ca and Mg. Scatter of source radiation observed during atomization of large masses of analyte, or during analysis of samples containing high concentrations of matrix species, is due to this aerosol formation. Langer and Holcombe used TEM and high-energy electron diffraction (HEED) to analyse the Ag particles generated in a graphite tube-type atomizer.460 The Ag particles were collected by thermophoresis with a cooled Cu probe. The HEED patterns obtained matched body centred cubic Ag with microcrystalline domains. These observations are in agreement with previous studies suggesting particle formation in the electrothermal atomizer. The appearance of Ag in the gas phase at temperatures below the vaporization temperature was explained as decomposition of oxyanion salts during thermal pre-treatment when internal gas ¯ows are high. Bozodogan proposed a method for the determination of the order of release and the activation energy for atom formation process from a single absorbance signal for Au, Cu and Ni under non-isothermal conditions.461 Proportionally spaced cubic B-spline functions were used for ®tting the experimental absorbance signals and least-squares regression was used to obtain mathematical expressions for the experimental temperature signals. From this ®t, the absorbance (A), its time derivative (dA/dt), and related parameters were obtained. The proposed method was compared with results obtained from the same experimental data set using the methods proposed by Sturgeon, Smets, McNally and Holcombe, Rojas and Olivares, and Yan et al., and to the results reported by Fonseca et al. using the same methods. Good agreement was found between the activation energies and order of release for the three analytes computed using the proposed method and the existing methods. It is claimed that the proposed method is simpler than the methods used for comparison, since the order of release and the activation energies are calculated under non-isothermal conditions. The atomization kinetics of Au in the absence and presence of a range of chemical modi®ers was investigated by Thomaidis and Piperaki.462 In the absence of modi®er, a two-precursor

atomization mechanism was observed in distinctive temperature regions. When long pyrolysis times and low analyte masses are used, atomization from dispersed particles with low activation energies is observed in the low temperature region. At high masses of Au, a fractional order atomization from Au agglomerates with high, mass dependent activation energies, when approaching the heat of vaporization, is observed in the high temperature region. When ascorbic acid is used, fast atomization from surface particles at the active sites produced by the pyrolysis of ascorbic acid is suggested by a high activation energy in the low temperature region. A low activation energy is obtained in the high temperature region, with ®rst-order kinetics indicating a desorption process through the micropores of the amorphous carbon residue of ascorbic acid. In the presence of Re, a two-precursor mechanism was also observed with a high activation energy in the low temperature region, suggesting vaporization from small clusters, and a low activation energy in the high temperature region, with ®rst order kinetics, indicating vaporization of disperse particles from the graphite surface. In the presence of Pd, again, a two-precursor mechanism was observed. A low activation energy process in the low temperature region indicating vaporization of disperse particles from the available free active sites on the graphite surface. The second process, in the high temperature region, begins at the appearance temperature of Pd and shows a high activation energy value and ®rst order kinetics, suggesting that the release of Au atoms occurs only after the vaporization of Pd has begun. In the presence of Rh, a mass dependent activation energy is observed in the low temperature region, suggesting atomization from Au clusters. Further results from the investigations into the migration of analytes from the sample deposition site on the furnace wall to cooler regions of the furnace during thermal pre-treatment from Jackson et al. have appeared.463 The method was described in last year's review. The migration temperatures for Cd, Mn, Pb and Tl (300±350 ³C, 1000 ³C, 400±500 ³C and 250 ³C, respectively) were found to be signi®cantly lower than conventional platform atomization appearance temperatures. This can lead to appreciable analyte migration to the ends of the furnace, which can lead to enhanced chemical interference and diffusional losses of analyte. The effects were observed to be less severe if the sample is deposited on a platform, if gas ¯ows towards the centre of the furnace are used, and if palladium is used to physically stabilize the analyte. Lamoureux et al.464 used synchrotron X-ray absorption ®ne structure spectroscopy to investigate the mechanism of the palladium modi®cation of Se. The formation of a Se±Pd compound was proposed, thus verifying the results of previous studies using MS and Rutherford backscattering. The stoichiometry of the compound, either SexPdy or SexPdyOz, could not be determined. SEM-EDS was used by Zachariadis et al. to investigate changes in a Pt-based (H2PtCl6) and in a W-based (Na2WO4) modi®er during pre-treatment and atomization.465 The crystalline form of the modi®ers was studied using SEM at 62000 magni®cation. At this magni®cation, the topochemical composition of the products of 50 mm scanning areas was estimated using EDS, the acquired spectra allowing the estimation of the surface chemical compositional changes. Both types of modi®er were observed to change their crystalline form by formation of small, well-de®ned crystals, at temperatures between 500 ³C and 1250 ³C. Corrosion of the graphite surface was observed during the use of H2PtCl6, which was observed to be due to the liberation of chlorides from the crystal structure at temperatures between 750 ³C and 1000 ³C. The third paper from Fischer and Rademeyer on their studies of selenium atomization considered interferences due to sulfate and the use of palladium as chemical modi®er.466 Kinetic parameters were calculated for Se with varying J. Anal. At. Spectrom., 2000, 15, 763±805

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amounts of sulfate when the modi®er was either pre-mixed with the analyte solution or pre-reduced in the furnace prior to addition of analyte. At high levels of interferent, the kinetic parameters approached those for the atomization of Se in the absence of modi®er. Higher levels of sulfate could be tolerated when pre-reduced palladium was present. The interference mechanism was postulated as being due to the formation of thermally stable palladium sulfate, thus reducing the number of active palladium sites available for Se stabilization. Sulfate only interfered in the high temperature stabilization of Se and the low temperature stabilization, linked to formation of a [Pd, Se, O] compound, was unaffected. Several papers from Katskov and co-workers describing their work using atomic and molecular absorption spectrometry to investigate the vapours evolved during electrothermal atomization have been published during the time period of this review. Molecular absorption spectra were collected using a linear array of charge coupled device detectors. Absorption by alkali halides was measured during furnace heating from 500 ³C to 2000 ³C.467 The complexity of the molecular bands, and the magnitude of the absorbance, was seen to increase from ¯uorides to iodides. The appearance of vapours was observed between 680 ³C for RbI and 1220 ³C for LiF, while the maximum absorbance was reached between 800 ³C for CsI and 1440 ³C for LiF. The characteristic temperatures of the vaporization peak were shifted towards lower values going from ¯uorides to iodides. Magnesium chloride vapours were observed during vaporization from pyrocoated and tantalum lined graphite tubes.468 A broad molecular band was observed at 210 nm and attributed to MgCl2(g). The broad signal was followed by a series of sharper bands characteristic of MgCl(g). The release of MgCl vapours was accompanied by Mg atomic absorption and by light scattering. MgCl2?6H2O partially vaporizes as MgCl(g) and partially reacts with the water of crystallization, leading to a mixture of magnesium hydroxychloride and hydroxide. Further heating of the condensed phase resulted in formation of MgO(s) and MgCl vapours. The hydrolysis reaction was favoured by long pyrolysis treatments or by stopping the gas ¯ow during pyrolysis. When He was used as the purge gas, or when tantalum lined tubes were used, the fraction of salt vaporized as MgCl2 was increased, while scattering was not observed. Using the same methods, the vaporization of sea-water and its constituent salts in a graphite furnace, platform furnace and ®lter furnace was investigated.469 The most intense molecular bands, due to NaCl, were reduced, but not completely eliminated, by the addition of HNO3. The addition of HNO3, however, promoted the appearance of O-containing species, the intensities of these bands being lower in the ®lter furnace due to reactions with carbon as a result of the increased surface area for reaction. Akman and Tekgul470 studied the effect of a mixture of sodium, magnesium, sulfate and chloride on the atomization of manganese. In the presence of the four ions, X-ray diffraction studies con®rmed that sodium chloride and magnesium sulfate were found to be the major salts formed after the drying step. Sodium chloride caused a major suppressive effect when low pyrolysis temperatures were used; this is due to both analyte expulsion with matrix and to formation of manganese chloride in the gas phase. Magnesium sulfate, however, caused no suppressive effect. The presence of magnesium sulfate was actually found to decrease the suppressive effect of sodium chloride on manganese due to some protecting effect. Grinshtein et al. used a two-step atomizer for the reduction of matrix interferences during the atomization of Pb and Cd.471 Samples were dried in a transversely heated tube and then vaporized using pulse heating and carried in a stream of Ar into a pre-heated atomizer for measurement. Signi®cant reductions in background absorbance were noted in the determination of Cd and Pb in 10-fold diluted sea-water, and in the presence of a range of salts. 786

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4 Laser-based analytical atomic spectrometry In analytical atomic spectrometry lasers are employed in one or other of two modes, they may be used either (1) primarily as an energy source where wavelength is a secondary consideration or (2) primarily as a bright radiation source of precise wavelength. This review of lasers in atomic spectroscopy will be divided into those two broad categories. Several reviews of the analytical use of lasers have appeared.135,472,473 A book on the topics covered by this present review, edited by Sneddon, Thiem and Lee, is now available.474 At a more fundamental level the theoretical origin of non-linear resonances measurable by tunable lasers has been discussed by Ghosh.475 4.1 Lasers as energy sources In this mode of use, the merit of the laser lies in its ability to deliver a high energy density on to a small area, thereby vaporizing a small volume of the sample to facilitate microsampling and, by using repeated pulses, depth pro®ling. The energy delivered by the laser may also be suf®cient to generate a plasma and excite the emission spectrum of the analyte atoms. The wavelength of the laser radiation is not usually a critical factor and wavelengths ranging from 266 nm to the infrared have been used, although matching wavelength to the sample material can be advantageous. Pulse energies are of the order of millijoules with repetition rates optimized to match analytical requirements. 4.1.1 Laser ablation. 4.1.1.1 General studies. In addition to its use in analytical atomic spectrometry, laser ablation (LA) has found applications in a number of ®elds e.g., cleaning delicate objects, surgery and chemical synthesis. The conditions required for cleaning ancient papers and parchment have been investigated by Kautek et al.604 Using a model system, they found that the laser induced plasma (LIP) spectrum was a poor indicator of the end-point of the cleaning process and that laser induced ¯uorescence (LIF) spectrometry was a more promising monitoring procedure. Pulsed LA was used to generate a plasma of Al into a pulsed electrical discharge in N2 as a means of synthesising aluminium nitride.605 The plasma reaction was studied using time- and space-resolved OES. The delay time between laser and electrical pulses was adjusted to give the maximum Nz emission to produce the purest aluminium nitride powders. 4.1.1.2 Atom vapour generators. Laser ablation is used as a means of generating an atom vapour for use in conjunction with a number of different analytical techniques. The most common of these is the ICP in which LA replaces the nebulization± vaporization stages of the conventional ICP system. In one newly patented instrument an O2-containing gas was used to transport the vaporized sample to the ICP.606 A study of LAICP-AES analysis of synthetic geological powders found that response factors were dependent on chemical form, composition of the matrix and laser wavelength (266 or 1064 nm), but the effect of grain size and binding pressure of the pressed pellets was insigni®cant.264 The test elements were Al, Ca, Fe, Mg and Si. To obtain quantitative results, matching of the sample and sample matrix properties was necessary. Another in-depth study of the in¯uence of pellet composition on the LAICP-AES/MS analysis of geological reference materials found that trace element fractionating occurred on repeated irradiation of the target and that there were signi®cant differences in sensitivity when analytes were added as oxides, sulfates or in a metallic form.607 The use of internal standardization was only partly successful but the use of a suitable binding material improved performance. The sensitivity of conventional ICPAES was enhanced by preconcentration from solution onto an APDC-polystyrene (PS) ®lm which was then sampled by LA

into the ICP.608 However, the APDC-PS system was found to have poor precision, selectivity and to suffer from chemical interference therefore other ligand-polymer systems were sought. Laser ablation has been used to inject bursts of material into a GD.609 Copper in steel was the test system and the Cu emissions from the vacuum laser and the GD plasmas were investigated separately and in combination. It was found that the GD signal was enhanced when sampling LA material while the high background of the laser plasma itself was avoided. By LA nonconducting materials could be sampled into the GD. 4.1.2 Laser induced plasmas. A review of predominantly 1997 literature with 43 references has been prepared by Rusak et al.610 4.1.2.1 Fundamental studies. The energies and number densities of ions emitted from laser induced plasmas (LIP) were measured by a time-of-¯ight technique.611 The energy and length of the Ti-sapphire mode-locked laser pulses were 1.0 mJ and 100 fs, respectively, with irradiance at the solid elemental target of 261013 W cm22. Approximately 261013 atoms were removed per pulse and ion velocities ranged from 5.0± 30.0 km s21 with average kinetic energies of 20±50 eV. The composition and evolution of the plasma produced by LA at 355 nm of a La2O3±CaO±MnO2 target in an O2 atmosphere has been studied by means of time and space resolved OES.612 Emissions of atoms and ions lines and of oxide bands were observed. Time delays between the emissions of components of ablated species were investigated as a function of distance from the target surface, gas pressure and laser ¯uence and possible mechanisms proposed. Kurniawan and co-workers have investigated the characteristics of laser-induced shock-wave plasmas (LISP).613±615 The systems, used under reduced pressure (1±10 Torr), were a TEA CO2 laser (5±80 mJ, 8 ns) with a copper target and N2 and XeCl excimer lasers with various solid targets.613 Using timeresolved spatial distribution measurement of emitted radiation they observed a primary plasma, which gives off intense continuum emission close to the surface for a short time, with a secondary plasma expanding with time around the primary plasma and emitting sharp spectral lines with low background emission. The secondary plasma was attributed to excitation resulting from the blast wave. It is claimed that the secondary plasma is very suitable for spectrochemical analysis by virtue of its high excitation temperature, low background emission, good precision and linearity of response. Studies of line pro®les in low pressure (1±10 Torr) LIP, generated by Nd:YAG LA of CaCO3 doped with Rb by atomic absorption using a narrow band CW Ti-Sapphire laser, have been reported by Gornushkin et al.616 The plasma was in nonequilibrium with a kinetic temperature of 3000 K and an excitation temperature of 8000 K. Line broadening for the trace element (Rb) was Doppler dominated but for the matrix (Ca) resonance broadening was the main contributor to line shape. Number densities of electrons and atoms were deduced. Other studies by the same authors used the `curve of growth' methodology to determine fundamental parameters of LIPs.476 The system studied was 0.007±1.3% Cr in steel using the 425.4 nm Cr line in the plasma generated by a Nd : YAG laser. The damping constant `a' (yLorentzian/Doppler line width) was estimated to be 0.20¡0.05 and the number density of neutral Cr atoms corresponding to the transition between low and high optical densities was 6.561012 cm23. This point also represents the upper limit of the linear calibration range. A computer program for calibration-free quantitative elemental analysis has been developed by Ciucci et al.477 The program is based on the following assumptions: (1) the plasma composition is representative of the sample; (2) at the moment of measurement the plasma is in LTE; (3) the radiation source

is thin; (4) all the atomic components of the sample are detected by measurement of at least one of their characteristic spectral lines. Satisfactory results were obtained when the program was applied to the analysis of atmospheric air and an aluminium alloy. Aluminium alloys were also the test material for the investigation of the plasma formed in air at atmospheric pressure using sub-ps laser pulses.478 The plasmas were characterized in terms of their appearance, spectra, space-integrated temperature and electron density. 4.1.2.2 Instrumentation. Redeposition of ablated material from high purity Fe and Zn foils has been found to make a signi®cant contribution to the measured LIPS signal.479 This effect was prevented by operating at reduced pressure (1 mbar) with depth resolution improved in an Ar atmosphere. The nonhomogeneous distribution of energy in the laser beam pro®le produces an uneven crater which has a deleterious effect when depth pro®le information is sought. To improve depth resolution a 2-lens telescope has been used to generate from an XeCl excimer laser a ¯at pro®le beam (562.5 mm) with an energy density of 107 W cm22.480,481 The beam gave ¯at ablated pro®les in Zn coated steel and a depth resolution of a few nm pulse21. Plasma emission was collected using a single quartz ®bre optic cable. When using ®bre optics as collectors of plasma emission care is necessary to avoid laser damage to the ®bres. A multi-purpose ®bre optic probe incorporating both single strand optical ®bres and an image guide has been developed for use in LIPS for delivery of laser radiation and for the simultaneous observation of atomic emission, Raman spectra and Raman images of TiO2 and Sr(NO3) particles on a soil substrate and of differing regions of a granite sample.482 A linear array of ®bres (1.5 mm spacing) in an 8-ended fused-silica optic ®bre bundle was used to observe the time-resolved emission pro®le along the axis of the plasma plume.483 The optimum location in the plasma for spectral analysis was found to depend upon the matrix and analyte concentration but a clear global optimum was observed for the analyte signal as a function of location and time. For spectral analysis of emission from the laser plasma, Czerny±Turner spectrometers are widely used. The spectral resolution of these instruments is not very great nor is their spectral coverage extensive. These limitations are not serious in the case of single element analysis but they are limiting for multi-element analysis. In traditional multi-element emission spectrometry Rowland polychromators and echelle spectrometers are long established as the preferred instrumentation. The combination of echelle spectrometers with time-gated intensi®ed charge coupled devices (ICCD) have been demonstrated to be suitable for multi-element analysis using LIP sources.617±618 The spectral range covered is determined by the dimensions of the ICCD. The Rowland polychromator with PMT detection does not suffer the same spectral range limitation but is less ¯exible in the selection of spectral lines and background measurements. The performance of an echelle-ICCD system was demonstrated by the LIPS analysis of an Al alloy using a Nd:YAG laser.618 Calibration graphs of the intensity ratios (analyte/Al) were prepared for Cr, Cu, Fe, Mg, Ni and Ti; LODs were in the range 1±10 mg g21. A computer program for processing data from a LIPS multichannel analysis system has been described and applied to the determination of Mn in steel using a XeCl excimer laser at 308 nm with a pulse energy of 120 mJ.619 4.1.2.3 Applications. Table 2 presents a summary of recent applications of LIPS. The development of a LIPS analytical method is a demanding process requiring optimization of the instrumental con®guration, its operating conditions, and minimizing matrix effects and plasma ¯uctuations. One such exercise has been described by Panne et al.620 for the analysis of glass and glass melts during the vitri®cation of ¯y and bottom J. Anal. At. Spectrom., 2000, 15, 763±805

787

ashes in a process designed to immobilize heavy metals. Spectra were averaged over 100 laser shots of 75 mJ each. Considerable pulse-to-pulse variation in the excitation temperature and electron density was observed owing to changes in the lasermaterial interaction. A normalization procedure incorporating the Saha-Boltzmann equilibrium relationship reduced the variation in line intensity ratios and led to a linear calibration of LIPS intensity ratios against concentration. In an attempt to speed up the analysis of phosphate rock single laser shot LIPS has been examined as an alternative to ICP-AES.484 When only a single laser shot is used to generate analytical data correction procedures are essential. Two sources of ¯uctuations were identi®ed: ®rstly, the heterogeneity of the sample; and secondly, plasma interactions. A correction procedure was devised which gave linear calibration curves with a precision of 5±10%. 4.2 Lasers as sources of intense monochromatic radiation In this mode of utilization the laser radiation interacts directly with the analyte atom vapour which has been generated by an independent process. The wavelength of the laser emission is tuned to meet the requirements of the analyte species. 4.2.1 Laser excited atomic ¯uorescence. In addition to its use in elemental analysis, LEAF makes a signi®cant contribution to the study of a variety of systems involving atom vapour generation. A general review of LEAF has been prepared by Hou et al.485 4.2.1.1 Fundamental studies. The axial and radial distribution of atoms in the cathode hole of an HCL has been studied by LEAF. A model of the spatial distribution of sputtered atoms and ¯uorescence signal was proposed. The ion beam inside the skimmer cone of an ICP-MS instrument has been probed by LEAF.487 Reference measurements were made outside the tip of the cone with Ba and Sc ions and Pb atoms as the test analytes. Scandium ion densities dropped the most rapidly while the transmission ef®ciencies of both Sc and Ba ions were suppressed by the addition of either Mg or Pb. The lower limit for the transmission ef®ciency of the Ba ion from the plasma to the second vacuum stage was estimated to be 0.3%. Laserexcited atomic ¯uorescence of Na has been used to measure temperature, pressure, axial velocity and species concentrations in wind tunnels, rocket engine exhaust and the upper atmosphere.621 These measurements were complicated by optical pumping leading to enhanced ¯uorescence signals. An extension of rate equations originally developed to account for features of the pumped spectrum was shown to facilitate accurate measurements of temperature. When the 2-photon laser induced ¯uorecence method is applied to the measurement of small CO concentrations in a high-temperature gas mixture containing CO2 has been shown that consideration should be given to the photodissociation at 230.08 nm of the CO2 under the action of the probing radiation.622 Time resolved LEAF has been used to study the interaction between EuIII and a sugar based diblock surfactant-cage molecule (cyclam).623 The excitation wavelength was 337 nm. The separate constituents, cyclam and surfactant, had no effect on Eu ¯uorescence but together there was a signi®cant effect. The complexation constant was calculated from the intensity ratio of the 592 and 618 nm lines. As an alternative to laser excitation the potential of pulsed HCLs continues to be investigated.624 Using microsecond high current pulses through an HCL intense radiation was generated and used to excite atomic and ionic ¯uorescence in the tail ¯ame of an extended-sleeve torch. Detection limits for Ba, Ca, Eu, Sr and Yb were improved 1±2 orders of magnitude compared with those obtained using conventional pulsed HCL operation. 788

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4.2.1.2 Applications. Where LEAF is used as an established analytical method papers on such topics will be presented in the ASU review devoted to the speci®c sample type. In this section developmental topics in LEAF will be presented grouped according to mode of atomization. Laser ablation combined with LEAF has been investigated by Winefordner's group for the determination of Li and U isotopes, respectively, in solids.625±626 In the former case, an excimer laser was used for ablation and to trigger a second excimer laser which pumped a tunable dye laser. The sample was mounted in a quartz-windowed stainless steel low-pressure chamber which translated horizontally across the laser beams at 0.5 mm min21. The output of the dye laser scanned at the rate of 25 pm min21 across the Li 670 nm doublet. The ¯uorescence signal was detected using a grating monochromator and PMT. Using y2000 laser shots the 7Li to 6Li ratio was calculated from the average of 10 scans to be 12.1¡0.5. For the measurement of U isotope ratios, ablation was by Nd : YAG laser and ¯uorescence excitation by diode laser. The latter was used in two modes: either the laser was scanned across the spectral interval including the 2 isotope peaks during each ablation pulse or the wavelength was ®xed at the peak of either isotope line on alternate pulses. In the analysis of UO2 the LOD for both 235U and 238U was y0.6 mg g21. Electrothermal atomization has been used since the early days of AFS and continues to offer the lowest reported LODs. This sensitivity is largely attributable to its ef®cient sample utilization and relatively low background emission. A graphite rod atomizer and excitation of atomic ¯uorescence by an excimer-pumped tunable dye laser has been used for the analysis of geochemical samples by LEAF.627±629 The repetition rate of the excimer laser was set at 20 Hz and the tunable output of the dye laser ranged from the infrared to the UV with pulse powers of 0.01±10 mJ and line widths~0.2 cm21. The ¯uorescence radiation was observed at 90³ to the exciting beam and measured by a monochromator±PMT±time-gated detection system. The LODs for Au, Ga, In, Pb and Pd were in the range 1024±1021 ng ml21 in the digested sample; absolute LODs for Au and In were 0.1 pg and 0.2 pg, respectively. Graphite tube furnace atomization usually involves the collection of the ¯uorescence radiation at 180³ to the excitation beam. This con®guration was used in the determination of Os in aqueous solutions.630 Excitation was at 263.7 nm with a 20 ns pulse of energy of 40 mJ and measurement at 290.9 nm with an intensi®ed diode array. The LOD was 50 pg with a linear calibration from 5 ng ml21 to 1 mg ml21. Interaction of Os with the graphite furnace produced memory effects. Palladium in marine deposits has been determined by graphite furnace LEAF using excitation at 247.6 nm and detection at 348.9 nm.631 An LOD of 50 pg ml21 was reported. A L'vov platform and a chemical modi®er were incorporated into a procedure for determining Se in serum.632 Excitation was with 196.026 nm radiation with measurement of ¯uorescence at 203.989 nm. Results for Se in SRMs were in good agreement with certi®cate values. For the determination of Ge in water and blood again the L'vov platform with chemical modi®er has been found to be a satisfactory approach.633 With excitation at 269.13 nm and detection at 275.45 or 326.95 nm LOD were 900 fg and 670 fg, respectively. A comparison has been made between atomization from the wall of non- and pyrolyticallycoated furnace tubes and from L'vov platform with various chemical modi®ers for the ETA-LEAF determination of In.634 Excitation was by a dye laser pumped by a high repetition rate Cu vapour laser. Best performance was obtained by platform atomization of In in a solution of 0.02 M HNO3 and gave an LOD of 1 fg. An alternative to either wall or platform atomization is the use of ®lter furnace systems wherein the atomic and background vapours diffuse at different rates through a porous graphite ®lter into the observation zone. For the determination of Pb in whole blood by ETA-LEAF a

Katkov ®lter allowed direct comparison between blood and water standards.635 Using a copper vapour laser for excitation and detection at 405.7 nm an LOD of 0.1 fg was achieved. 4.2.2 Lasers in atomic absorption. Diode lasers are the most frequently used lasers in AA arising from their relatively low cost, compactness, small line width, tunability and ease of operation and where the high power available from other types of lasers is not required. The use of diode lasers in AAS has been reviewed by Zybin et al.636 4.2.2.1 Fundamental studies. The tunability of the diode laser has been utilized as a means of isotope analysis by AA. The output wavelength of the laser is modulated to scan the spectral region encompassing the atomic lines of interest. The atom vapour may be contained in a low pressure cell or generated in a graphite furnace. The transmitted intensity is detected by photodiodes and the signal processed by an ampli®er locked to twice the diode modulation frequency, thereby minimizing the effect of background continua. Deconvolution and interpretation of the output signal is complex and requires knowledge of the structure and pro®les of the spectral lines. A detailed study of the theoretical and experimental aspects of the measurement of the 85Rb : 87Rb isotopes has been carried out by Gustafsson and Axner.637,638 They concluded that from the theoretical simulations they were able to predict accurately the experimental signal from Rb atom under both low pressure, room temperature conditions and atmospheric pressure, high temperature (furnace) conditions. The 7Li : 6Li and 85Rb : 87Rb isotope ratios have been measured by Wizemann639,640 using the diode laser modulation with a low pressure graphite furnace approach. Computer simulation predicted that, compared with direct absorption measurements, the relative absorption sensitivity for 6Li is considerably reduced if wavelength modulation is performed around the centre of the 6Li D1 ®ne structure but will be enhanced if centred on the maximum of the red wing of the line pro®le. Lithium isotope ratios as large as y2000 were measured. Tunable diode lasers have been used to study the hyper®ne structure of the 6s2 1So±6s6p 3P1 transitions of 135±137Ba isotopes.641 All hyper®ne peaks were seen and a resolution of 50 MHz was achieved. Spectroscopic and atomic parameters were calculated from the observations. As a step toward developing an optical isotope measurement technique the pro®le of Ba lines in a low pressure graphite furnace have been studied using a scanning laser diode.642 Spectral line widths were reduced from 2.8 GHz at atmospheric pressure to 57 MHz at low pressure. There was, however, a 60-fold increase in the LOD owing to the rapid diffusion of the atoms. A study of background correction and calibration curve linearization in the determination of Rb by modulated diode laser ETA-AAS has been published.643 The laser radiation was detected by an Si photodiode and converted into voltage by a logarithmic transducer. The absorption signal was observed at the 2nd harmonic of the laser diode modulation frequency with a lock-in ampli®er and recorded on a storage oscilloscope. The calibration curve was linear up to 400 times the characteristic concentration. Background absorbances of up to 1.4 did not in¯uence the value of the analytical signal in the graphite furnace. An absorbance of 461025 could be measured against a background absorbance of 1.2. The distribution of Rb atoms in a graphite furnace with a L'vov platform has been studied using longitudinal and transverse laser beams.644 It was concluded that 6% of the total AA signal was generated by atoms outside the tube, that the transverse distribution of atoms in the tube was homogeneous apart from a 25% higher density near the platform and that gas entered the tube convectively at its ends and exited through the injection port through which most of the analyte atoms were also lost.

4.2.2.2 Instrumentation. A signi®cant factor limiting the use of lasers in AA is the absence of a range of diode lasers to cover all wavelengths between 190±900 nm. To produce emission in the UV region complicated schemes of frequency multiplication are necessary, thereby losing some of the bene®ts of the diode laser. One such scheme to generate radiation of 283 nm for the determination of Pb required the frequency doubling of 850 nm emission from a laser diode to give 425 nm radiation followed by sum frequency generation of the harmonic radiation with a second 850 nm laser diode.645 High resolution AAS measurements at 283.3 nm were achieved by scanning the diode laser injection current. A small simple AAS instrument incorporating a W coil atomizer and a laser diode light source has been described.646,647 For the determination of Al and Cr, laser modules generated wavelengths of 396.15 and 427.48 nm, respectively. Inherent background correction and high detection power were claimed. Detection limits for Al and Cr, respectively, were reported to be 0.9 and 0.03 ng ml21 in aqueous solution, 2.5 and 0.3 ng ml21 in serum and in slurries of graphite and TiO2 between 0.02 and 0.6 ng g21. 4.2.2.3 Applications. Atomic absorption spectrometry with tunable diode lasers has been used for the monitoring and study of physical vapour deposition processes.648 Wavelength modulation and balanced detection gave high sensitivity and reliability. Direct measurement of the atom ¯ux in electronbeam-evaporated Ba and Y was demonstrated. It was concluded that for deposition rate control was necessary that atom ¯ux rather than density measurements should be made. The vapour properties of electron-beam evaporated U were measured by laser AAS using an argon ion pumped ring dye laser tuned to an absorption transition of ground states of U atom vapour.649 Wavelength scanning was used to generate data on angular distributions, density, velocity and translational temperature of the vapour ¯ow. Wavelength modulated diode laser AAS was combined with HPLC for the speciation of Cr in water.650 A double beam optical system and high pressure nebulization into an air±C2H2 ¯ame were employed. The outputs of photodiodes were ratioed, ampli®ed logarithmically and the absorbance signal extracted by a lock-in ampli®er driven at twice the laser modulation frequency. The LOD for CrVI in tap water was 30 pg ml21. 4.2.3 Miscellaneous uses of lasers. 4.2.3.1 Laser enhanced ionization. The creation of an ionic vapour may be used either directly to generate an analytical signal as an electric current (optogalvanic) or indirectly to provide an input to a mass spectrometer. By correct choice of exciting wavelengths the technique is both element and quantum state selective. The latter property has been used to investigate the continuum structure of CoI and NiI near the ®rst ionization limit using onecolour and two-colour two-step photoionization spectroscopy.651 A large number of autoionizing features were identi®ed as members of different Rydberg series. By virtue of its selectivity LEI has been used to measure isotope ratios optogalvanically. Uranium isotopes were vaporized in a GDL and excited with diode laser radiation in the wavelength range 775±835 nm.652 Using the 831.84 nm U line the ratio of 235U to 235 Uz238U was measured to be 0.0026. Lead in sea-water was determined by LEI in an air±C2H2 ¯ame after FI preconcentration.653 Two-step excitation was used and the LOD was 0.51 ppb Pb. Two-step one-colour excitation was used for the determination of Cr in a separation column ef¯uent atomized in an air±C2H2 ¯ame.654 Based on an injection volume of 100 ml the LOD was 4 ng ml21. Optogalvanic signals in Ne and Xe dc discharges were generated by an amplitude modulated CW laser.655 Oscillations occurring in the J. Anal. At. Spectrom., 2000, 15, 763±805

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Table 2 Applications of LIPS Matrix

Analyte

Air Soil Single erythrocyte Uranium/plutonium oxides Wood Aerosols on ®lter paper Minerals/metals/plastics Polymers Fluid inclusions in quartz Paper coatings On-stream ¯ue gases Colloids on ®lter Molten vitri®ed ash

C, Cl, F, S Ð Na, K 30 impurities Al, As, B, Cr, Cu, Hg, Sn As, Cd, Co, Cr, Cu, Mn,Ni, Pb, Sb, Tl, V C, H2, N2, O2 C, Cl, H Ca, K, Li, Na C, Ca, Si Be, Cd, Cr, Pb Al, Cu, Fe, Mn, Ni, Zn Al, C, Si

Titania/silica catalysts Silicon wafers/catalytic convertors

V Al, Ca, Cu, Pd, Pt

voltage drop between the electrodes were used to enhance the optogalvanic signal. 4.2.3.2 Cavity ring-down spectroscopy. In CRDS a laser pulse is injected into a re¯ective cell containing the absorbing species and the decay of the pulse intensity measured. This provides data from which the absorbance of the cell contents may be calculated. At the present time very few examples of use of CRDS in analytical atomic spectroscopy have been reported. The theoretical and practical aspects of CRDS have been presented by Provencal et al.656 The technique was used to monitor trace atmospheric particulates, radical species in ¯ames and plasmas and to characterize surface processes. The coupling of a CW laser to a ring-down cavity has been investigated.657 A piezoelectric transducer was used to switch the laser beam entering the cavity. The effects of scanning rate, laser line width and mirror re¯ectivity were examined. The noise equivalent absorption was found to be y361029 cm21. In a different CRDS system the noise-equivalent absorption for a single shot from a pulsed, frequency-stabilized optical parametric oscillator light source was 5610210 cm21.658 When a transversely heated graphite furnace was placed in a 55 cm long ring-down cavity and used for the determination of Pb, using the 283 nm absorption line without chemical modi®er, an LOD of 1 pg was obtained.659 The laser system used was a frequency doubled tunable dye laser pumped by a Nd : YAG laser; the UV line width was y0.06 cm21. The laser pulse duration was y8 ns with a repetition rate of 20 Hz. It was predicted that an LOD of 40 fg should be achievable. 4.2.3.3 Coherent forward scattering. To date no reports of the use of lasers in CFS have been received though no doubt is but a matter of time before the potential of lasers in this ®eld will be explored. A single publication has reported on the use of a Xe short-arc continuum source lamp as a means of simultaneous multi-element determination by CFS. The sample was atomized in a graphite furnace and the spectrum analysed by an in-house developed polychromator with 20 PMTs and small range wavelength scanning over the absorption peaks.660 Inevitably, furnace conditions were a compromise of those used for individual elements and two regimes were employed: one for volatile elements (Ag, Cd, K, Na, Pb, Tl and Zn) and the other for less volatile elements (Al, Ca, Co, Cu, Fe, Mn, Ni and Sr). The LODs were mostly in the mg l21 range (y20 pg).

5 Chemometrics The role of chemometrics is to turn raw data into useful information. Many chemometric methods are available to 790

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Detection limit/comments 21

Ref.

20±1500 mg g Presented for LIPS as an airborne dust 8 fg 100 mg g21 2±20 mg g21 0.3±1.3 mg m23

661 662 663 664 665,666 667

Spectrographic recording Polymer identi®cation 10±750 ppm Study of surface distribution of coatings 0.1±68 mg m23 2±40 ng Observed concentration ratios were in general agreement with reference values 38 mg g21 3-dimensional distribution studies

668 669 268 670 671 672 673 674 675,676

achieve this so a major dif®culty is to decide which is the most appropriate. Sharp et al.488 have provided some excellent case studies with tutorial style discussion on the application of chemometric procedures. The use of Baysian methods, described for multi-isotopic data produced by ICP-MS, could equally be applied to modern ICP-AES instruments with simultaneous array detectors. Of speci®c interest was the potential improvements to precision and detection limits offered by applying correlation processing to simultaneous background correction of peaks in ICP-AE spectra. Improvements in precision of an order of magnitude were predicted for cases where the measured signal was less than 20 times the detection limit. However, the improvement would be less signi®cant above this level, due to noise on the peak itself. A new algorithm has been produced for investigating and calibrating AAS data.489 Four attributes of this methodology were tested with EDTA chelates of Cu, Pb, Ni and Cd. These were maximum permissible range of calibration, maximum permissible spacing between calibration points, accuracy of standard additions extrapolation, and speed of application of the algorithm. This method allowed accurate calibration, i.e., within 1 s, for all elements above 1006 the characteristic concentration (99% absorbance). To achieve this accuracy, the concentration range between calibration points should not exceed 20 times the characteristic concentration. Importantly, the speed of the algorithm was fast enough to be implemented in practical analytical situations. The chemometric procedure known as common analyte internal standardization (CAIS) has been modi®ed and used to provide a universal calibration technique for the analysis of trace elements in organic solutions of medium and low volatility by ICP-AES.490,491 The methodology focused on the problem that, in petrochemical product analyses, an organic matrix is frequently tainted with small but variable amounts of highly volatile solvents that render accurate matrix matching of calibration solutions and samples impossible. The method depended on there being a linear change in the ratio of atom to ion line intensities of the same element as plasma conditions were perturbed by mixed solvents. This dependence allowed the calculation of factors that could be used to extrapolate line intensities for a single pure solvent. For elements without a pair of suitable atom/ion lines, the procedure was extended to utilize factors derived from an appropriate pair of lines of different elements. The effectiveness of the method was demonstrated for 7 elements, by either the addition of increasing concentrations of a second solvent or the same concentration of several different solvents with varying volatilities. Neural networks have also been used to develop a system for semi-quantitative analysis using an ICP-AE spectrometer equipped with an array detector.492

Qualitative analysis was achieved using 10 `index lines' to calculate the probability of element occurrence, and semiquantitative analysis was performed by ratioing the line intensities to reference intensities. Spectral interference correction is a major application for chemometrics in ICP-AES. Li and Fan493 have reviewed appropriate methods for ICP-AES, including Kalman ®ltering (19 references). Cross-correlation of sample spectra from ICPAES using an array detector, with reference spectra of reference elements, has been used to identify peaks.494 The problem of interferences was addressed by deleting mutually interfering lines using analogue software masks. The cross correlation was carried out using a shift-multiply-add algorithm, avoiding the use of FFT. Improved methods of peak ®tting and spectrum stripping for correction of interferences in ICP-AES are of continuing interest and readers should consider several publications in this area.495±498 A tutorial guide on the use of wavelet transforms to determine peak shape parameters as a method of interference detection in GFAAS has been published.499 The absorbance versus time peak-shape of the GFAAS was recorded and transformed, using wavelets, into a set of coef®cients called Lipschitz regularities. The number of these, and their values with respect to time, accurately characterized the peak pro®le so it was possible to demonstrate changes to this pro®le resulting from chemical interference. The ability of Lipschitz regularities to detect interferences was then compared with previous measures of peak shape such as time parameter and the phase angle array procedure and found to be more sensitive.500 This methodology could equally well be applied to peak shape data resulting from other spectroscopic techniques. The search for an ideal chemometric method to correct matrix effects and sensitivity changes in atomic spectrometry appears never ending. Simple internal standardization has been regularly shown to provide signi®cant improvements in data quality, but researchers continue to look for something better. The PRISM (parameter related internal standard method)501 has been resurrected for the determination of Ni in high salt and acid matrices by sequential ICP-AES.502 The method relied on there being two underlying factors causing variations in sensitivity, namely depression of the excitation temperature of the plasma and changes in the ef®ciency of sample introduction. Two internal standards were then chosen that represented the variations caused by these factors, with variation in all other elements being accounted for by convolution. The method was found to provide signi®cant improvements in accuracy, precision and a decrease in `drift'. Many ®gures of merit have been developed for ICP-AES systems, each tending to focus on a particular need or diagnosis. Many of these have now been brought together,503 and it is suggested that: repeatability should be measured by the RSD of the Mg I 285 nm line; resolution by the line pro®le of Ba II 233 nm; robustness by the Mg II 280 nm : Mg I 285 nm line intensity ratio; long term stability by the intensities of the Ar I 404 nm, Ba II 455 nm, and Zn II 206 nm lines; and limit of detection using the Ni II 231 nm line. To enable these ®gures of merit to be applied over the maximum range of elements, spectral coverage should be 120±770 nm. The preponderance of new ICP-AE spectrometers with multi-element array detectors has allowed the evolution of new approaches to quality control. It has been suggested504 that variations in instrument-reported concentrations for multiple lines of a given element be used as a measure of the con®dence interval for the determined concentration. It will be interesting to see whether this proves to be a robust concept because of the large variations in line sensitivity and background signals. A major use of chemometrics is for classi®cation of samples into groups. Principal components analysis is a particularly powerful tool which Klemenc et al.505 have used for a detailed evaluation of multi-element data produced by ICP-AES for the

simultaneous determination of many major, minor and trace elements in archaeological copper ingots. When coupled with data processing by PCA, this allowed a full archaeological interpretation of the data in terms of a multi-stage smelting process. This was an excellent case study for those interested in interpreting a large amount of data. The use of multivariate techniques, such as cluster analysis, PCA and discriminant analysis, has also been popular in categorizing wines by their trace element content.506,507 In particular, PCA was used to determine the geographical origin of tea with some success.505 Correspondence analysis has been used to study the correlation between sex/age and a range of 16 trace elements determined in hair by ICP-AES.508 It was found, for example, that essential elements Ca, Cu, Mg and Zn were higher in women, and Fe, Hg and Pb accumulate with age. A sophisticated approach using neural networks was used to classify normal patients and those with lung cancer by determining the metal content of serum and hair by ICPAES.509 In a similar study, AAS was used to determine Ca, Cr, Cu, Fe, Mg, Mn, Sr and Zn in serum and associate their concentrations with coronary disease.510

6 Coupled techniques for speciation 6.1 Capillary electrophoresis There were no publications in this area during the review period; however, two interesting conference proceedings described a new approach to interfacing CE with ICPAES.511,512 This was accomplished by incorporating the CE capillary into a modi®ed nebulizer and by, at least in one case,512 inserting a ®ne platinum wire into the end of the capillary to complete the circuit. In this way the uptake of the capillary was matched to the electro-osmotic ¯ow, thereby avoiding dilution of the sample; however, no mention was made of the problems that might be encountered when the electro-osmotic ¯ow is negative. 6.2 Gas chromatography 6.2.1 GC-AES. Gas chromatography coupled with AES was a popular technique for speciation studies, due to the excellent element selectivity of the detection system. The predominant technique was GC-MIP-AES, so the discussion below refers to this technique unless otherwise stated. A reasonable degree of activity has been maintained in the speciation of organometallic compounds, though there were fewer publications than in previous review periods. There were publications on the speciation of organotin,513±516 organolead,517 organomercury,518,519 organoplatinum,520 and organoselenium521 compounds. Of particular note were two related papers on the speciation of organotin compounds in environmental samples and reference materials.515,516 The former paper summarized the outcomes of a feasibility study for the certi®cation of butyl- and phenyltin compounds in freshwater sediment, using a variety of techniques. The major ®ndings were that the long-term stability varied quite considerably for different compounds and a storage temperature of ±70 ³C may be necessary to ensure long-term stability for some reference materials. The latter paper described the results of a similar study, speci®cally for tributyltin (TBT) and triphenyltin (TPhT) in water, sediment, oysters and cockles. The compounds were stable in un®ltered, non-acidi®ed seawater in polycarbonate bottles, stored at 4 ³C in the dark, for between 3±7 months, which allows suf®cient time for analysis. Comparable preservation time was achieved by extraction onto C18 cartridges. Both TPhT and TBT were stable for longer than 18 months in sediments stored at ±20 ³C; however, losses were observed due to air drying or pasteurization. Organolead compounds have been determined522 by MIP-AES after GC J. Anal. At. Spectrom., 2000, 15, 763±805

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separation of the propyl derivatives on reaction with tetrapropylborate. The derivatives were simultaneously extracted into hexane. Detection limits were around 0.1 ng kg21. Multicapillary GC has been used to speciate organotins in sediment after microwave extraction in acetic acid and derivatization with sodium tetraethylborate.514 A multicapillary column with 919 capillaries (1 m60.04 mm id, 0.2 mm SE30) was used to achieve separation in 2 min compared with 12 min using a single capillary column (30 m60.32 mm id, 0.25 mm BP-5). Detection limits of 0.2 ng ml21 as Sn at the 303.42 nm line were achieved. A further paper in a similar vein described a purge and trap multicapillary GC accessory for speciation of organomercury by MIP-AES.518 Mercury species were derivatized and cryogenically trapped on a fused silica capillary prior to ¯ash isothermal separation in less than 30 s. Detection limits of 0.01 pg ml21 for methylmercury and Hg2zwere achieved. These compounds were determined in urine and biological at 5 and 100 pg g21, respectively. Impressive detection limits of 10, 0.3, and 0.7 pg for dioctyltin, tetrabutyltin and tributyltin, respectively, were achieved using GC coupled with ¯ame photometric detection,513 after derivatization and preconcentration of water and beverages by 200±400 fold, though, somewhat worryingly, up to 9.5 ng ml21 of dioctyltin was found in some of the beverages. Other papers worth mentioning describe the determination of a volatile PtII complex formed with 1,2-bis(5,5-dimethyl-4oxohexane-2-imino)ethylene and its determination by GCMIP-AES,520 and the derivatization of selenoamino acids with ethylchloroformate to make them amenable to GC separation and detection by MIP-AES for the speciation of these compounds in selenium enriched yeast-based nutritional supplements. Finally, one paper described the determination of the gasoline additive methylcyclopentadienylmanganese in highway runoff and sewage.523 The analyte was extracted by headspace solid phase micro-extraction, with detection limits of between 0.3 and 0.5 pg l21 using the 259 nm Mn line. The use of GC-MIP-AES for element selective detection of compounds containing hetero-atoms also attracted some attention, with papers describing applications for the determination of organohalide,524±527 arsenic,528,529 sulfur530±533 phosphorus,534 and nitrogen535 compounds. Applications of note include: the monitoring of diazepam and chlorpromazine in human plasma533 by element selective detection for Cl (837.6 nm), S (921.3 nm) and C (940.5 nm); the determination of lewsite in soil529 after derivatization with 1,3-dimercaptopropane and detection for Cl (480.192 nm), S (181.275 nm) and As (189.042 nm); the veri®cation of lewsite exposure by determination of 2-chlorovinyl arsenous acid, which is a metabolic product of lewsite, in the urine of lewsite-injected guinea pigs, after solid-phase extraction with C18 and derivatization with ethanedithiol;528 determination of impurities in phosphine, arsine and ammonia gases used for semiconductor manufacture.536 Two papers addressed the issue of compound-independent calibration.535,537 In particular, one study found that the relative responses for a range of organic compounds depended on retention time.537 The responses, relative to nonane, for compounds containing equimolar amounts of carbon, at 247.9 nm line, were v1 for earlier eluting compounds and w1 for later eluting compounds. In a study of empirical formulae,526 it was found that Cl : C and H : C ratios were close to expected values whereas F : C ratios were not (C at 495 nm, H at 486 nm, Cl at 479 nm, F at 690 nm), and the ¯uorine response was compound dependent. This was partly ascribed to the interaction of F with the silica discharge tube. An interesting application of simultaneous C and H determination has been described,538 where the C-selective and H-selective chromatograms for a gasoline sample were combined to yield a third chromatogram of relative hydrogen 792

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de®ciency versus boiling point. These so called aromatograms were used to identify aromatic and other hydrogen de®cient compounds in the sample. Bromination, using 1% Br2, has been employed for the selective determination of alkenes in gasoline samples.527 This enabled discrimination of the alkenes from aromatic and saturated compounds (which did not react) by Br-selective detection at 478 nm. On-line supercritical ¯uid extraction coupled with ICP-AES has been used for the determination of total petroleum hydrocarbons in soil,539 with element selective detection for C, S, P and Si. A low power ICP-AES source operated at 15 W power has been coupled with a portable spectrometer via a ®bre optic probe for the determination of organohalide compounds.524 Detection limits of 240, 49, 117, and 52 pg s21 were obtained for ¯uorobenzene, carbon tetrachloride, 1,2dibromoethane and ethyl iodide. 6.2.2 GC-AFS. Two papers were published during the review period. In the ®rst,540 a comparison of AFS and ICPMS detection, coupled with GC, for the determination of methylmercury chloride in marine samples, was undertaken, with detection limits of 0.25 and 0.9 pg, respectively. Similar performance was obtained for the analysis of IAEA 142 and NIST 8044 mussel homogenate reference materials. In the second,541 three extraction methods for methyl- and ethylmercury in soil, sediment and ®sh were compared, namely acidic KBr/CuSO4 isolation±CH2Cl2 extraction with and without alkaline pre-digestion and a milder extraction with citrate buffer and dithizone in CHCl3. The ®rst two methods gave best results. 6.2.3 GC-AAS. Very few publications appeared in this area, with the emphasis placed ®rmly on element-selective detection. Organotin compounds were determined by GC coupled with quartz furnace AAS.542,543 In the former case the hydrides were generated with NaBH4 subsequent to preconcentration on a cation exchange column and prior to extraction into hexane, and in the latter the compounds were ethylated with NaBEt4. GC-QF-AAS has also been used for the determination of methylmercury in biological samples544 and a variant of this, which utilized a stainless steel pyrolyser tube maintained at 700 ³C, was used for the determination of methyl-, ethyl- and phenylmercury chlorides in the skin and fur of mink.545 One notable development was the immobilization of dithizone on polymeric microbeads, by the suspension polymerization of ethylene glycol dimethacrylate and hydroxyethyl methacrylate, and their use for the preconcentration of methyl- and ethylmercury chloride.546 6.3 Liquid chromatography 6.3.1 LC-AAS. Several reviews have been written in this review period, but many of these are on speci®c topics. An example includes a review containing 12 references of the speciation of metallothionein and metal-binding proteins547 that covered early HPLC-AAS determinations through to HPLC-ICP-MS. Future applications and improvements to the technique were also hypothesised. Reviews of speciation of Sb in waters containing 98 references548 and in environmental samples (in Czech) containing 113 references549 have also been published. The former did not concentrate solely on atomic spectroscopic determination and covered topics such as detection using spectrophotometry with cationic dyes through to ICP-MS. The latter review was more speci®c, concentrating on detection by ETAAS and HG AAS. In addition, separation techniques, e.g., HPLC, GC and FI, used in conjunction with atomic spectrometry, were also discussed. Inorganic and methyl-Hg speciation in environmental samples has been covered in a review that described methods for sample

preservation, extraction, preconcentration and derivatization.550 This review contained 162 references. Other topics reviewed include both chromatographic and non-chromatographic separations and different methods of detection. The use of organised surfactant assemblies in analytical atomic spectrometry has been reviewed.100 Topics covered included the improved nebulization ef®ciency derived from the introduction of surfactants, improved particle dispersion when nebulizing slurries and the introduction of vesicles to aqueous mobile phases for reversed phase HPLC speciation determinations. Several analytes have been speciated using liquid chromatography coupled with AAS. The determination of CrIII and CrVI has proved to be a popular analysis in this review period. These species have been determined in polyethylene plastics used as mineral water containers.551 The CrIII was separated by coprecipitation on an alumina carrier. `Satisfactory' results were obtained although it was noted that the speciation analysis of the plastic was impossible because of speciation changes during mineralization of the plastic. Total Cr and CrVI has been determined in UHT milk.552 For total Cr determination, the milk was mixed with Triton X-100 and diluted with water. For CrVI determination, the milk was mixed with acetate buffer (pH 3.5) to precipitate proteins and the supernatant solution passed through a column of Chromabond NH2 that had previously been conditioned with 1 M nitric acid and water. Elution of the CrVI was with 2 M nitric acid. Both total Cr and CrVI were determined by ETAAS at the 357.9 nm line and with a modi®er of palladium nitrate±magnesium nitrate. Calibrations were linear from 0.15 mg l21 to 50 mg l21 for both species and recoveries were w93%. On-line preconcentration and determination of CrIII and CrVI has been reported in two papers. In one,553 CrIII was complexed with Chromazurol S and CrVI with NaDDC and the complexes adsorbed onto a C18 column. Elution was by methanol and detection was at 357.9 nm. The LODs were 2.5 mg l21 and 0.2 mg l21 for CrVI and CrIII, respectively. Tap water and soil extracts were the samples analysed. In the other,554 on line preconcentration of CrVI and CrIII from sea-water was performed at pH 1.5 and at pH 7 respectively, in the presence of manganese(II). Using a C18 microcolumn, the diethylammonium salt of diethyldithiocarbamic acid as the preconcentration reagent and methanol as the eluent, a preconcentration factor of 500 was achieved. Calibrations were linear between 0.2 and 200 mg l21, recoveries were 95±105% and the LOD was 0.02 mg l21. Various natural waters and a liquid waste have been analysed to determine TeIV and TeVI.555 Sample (100 ml) was adjusted to pH 4 and then mixed with 2 ml of 0.1% 5-mercapto3-phenyl-1,3,4-thiadazole-2(3H)-thione (bismuthiol II) and 30 mg of cobalt(III) oxide. After sonication and ®ltration through an 8 mm nitrocellulose ®lter, the residue was washed with water and then placed with the ®lter in water, thus forming a slurry. Aliquots of the slurry were then introduced into a tungsten atomizer, dried, ashed and then atomized in the presence of hydrogen to determine total Te. A second 100 ml aliquot of sample was prepared at pH 1, enabling the collection and determination of TeIV only. Hexavalent Te was determined by difference. The LOD was 0.1 ng. Antimony species in tap water, snow and urine have been determined.556 Selective sorption of the SbIII chelate of APDC on a C18-bonded silica gel microcolumn was used to determine SbIII and total Sb after the SbV had been reduced to SbIII by L-cysteine. Elution of the Sb was by ethanol. Preconcentration enabled a LOD of 0.007 mg l21 to be obtained, which was substantially superior to 1.7 mg l21 obtained by direct injection into ETAAS. The method was used to study the stability of Sb species in the matrices of interest. Trivalent Sb was found to be unstable in tap water. Magnesium speciation in serum has been achieved by liquid chromatography with an anion exchange column and ETAAS

detection.557 Serum was diluted 2-fold with Tris buffer and 0.5 mol l21 sodium acetate was used as the mobile phase. It was found that the Mg was associated mainly with albumin and globulin fractions, but not with transferrin. Mercury speciation from water samples has been reported in two papers.558,559 In one,558 water was ®ltered, acidi®ed to pH 2 and passed through a column of chelating resin± thionalide Bio-beads. After rinsing with water, total Hg was desorbed with 10 ml of 5% thiourea solution in 0.05 M hydrochloric acid. A 5 ml aliquot was then analysed for inorganic Hg by reacting it with tin(II) chloride and potassium hydroxide (7 ml, 30%). The organic-bound mercury was reacted with cadmium(II) chloride, the cadmium replacing the mercury that was then determined in the normal way. Studies were made on the stability of the mercury species in samples and during the processing. Another paper that determined mercury speciation in waters has also been published.559 These authors passed 1 l of water through a column of activated charcoal at a rate of 4 ml min21. Transferring the charcoal to a vessel and reacting it with sulfuric acid (5 ml), 50 mg tin(II) chloride and then 5 ml of 45% sodium hydroxide produced Hg vapour that was determined by AAS. The organic mercury was determined by mixing the charcoal with 1 ml of 5% cysteine, sulfuric acid (16 N, 5 ml), tin(II) chloride solution (1 ml) and then sodium hydroxide (5 ml, 45%). Mercury speciation in ®sh and urine has been reported by Yin et al.560 On-line solid phase extraction (SPE), HPLC using a C18 column and a mobile phase of 1.5 mM APDC in methanol±water±acetonitrile (19z16z15) combined with a derivatization mixture of 2 mM APDC in 10 mM ammonium acetate and cold vapour generation using 0.264 M sodium tetrahydroborate in 0.1 M sodium hydroxide enabled mercury species to be detected by AAS at the 254 nm line. Linear ranges extended to 5000 ng l21 and the LODs for methyl-, ethyl-, phenyl- and inorganic Hg were 9, 6, 10 and 5 ng l21 respectively. Inorganic and organic mercury has been determined in ®sh eggs oil using Tween 20 (0.008% v/v) to avoid the formation of microemulsions.208 Inorganic Hg was determined by reaction with sodium tetrahydroborate whilst total Hg was determined after persulfate oxidation. Calibrations were linear to 20 mg l21, LODs were 0.11 and 0.12 mg l21 for total and inorganic Hg and recoveries were in the range 93±94.8% and 100±106% for inorganic and organic Hg. Aluminium speciation has also received some attention, especially in clinical samples. Sanz-Medel has reviewed his own work (10 references) and has covered topics such as off-line ultra®ltration and AAS detection to HPLC-ICP-MS.561 Fast protein liquid chromatography (FPLC)±ETAAS has been used to speciate Al in serum in 2 papers by Bantan et al.562,563 In the former paper, Al citrate was spiked into serum and this was analysed on a Mono Q HR 5/5 anion exchange column using a gradient elution from aqueous to 4 M ammonium nitrate at a ¯ow rate of 1 ml min21 in 10 min. Fractions (0.5 ml) were collected and analysed by ETAAS at the 309 nm line. In the second paper,563 the same system was used to determine several low molecular weight Al complexes. Detection was by ETAAS and by electrospray-MS-MS. It was found that the main Al constituents were Al citrate, Al phosphate and ternary Al citrate±phosphate complexes. Selenium compounds (trimethylselenonium iodide, selenomethionine, selenious acid and selenic acid) were separated in 8 min using an ESA anion III exchange column and a 5.5 mM ammonium citrate eluent that had been adjusted to pH 5.5.564,565 Detection was by either AAS or ICP-MS. The AAS LODs were approximately 1 mg l21 for an injection volume of 100 ml. A comparison of extraction methods for Se species from biological samples has been published.566 Enzymes were used to digest CRM 186 pig kidney, CRM 402 white clover and a Se enriched yeast. Determination was by HPLC-ETAAS and selenomethionine and selenocystine were J. Anal. At. Spectrom., 2000, 15, 763±805

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found to be the major constituents. Clinical samples have again proved to be a popular matrix for Se speciation. After cleaning urine through a C18 cartridge, 100 ml of urine was passed through a Hamilton PRP-X100 column using a phosphate buffer.567 The column ef¯uent was mixed with 3% potassium persulfate and sodium hydroxide and was then irradiated in a microwave at 90 W. After cooling the hydride was formed and this was detected by quartz tube AAS at 196 nm. Detection limits were 3, 5, 4, 8 and 3 mg l21 for SeIV, SeVI, selenocystine, selenomethionine and the trimethylselenonium ion, respectively. Selenocysteine and selenomethionine have been identi®ed in protein hydrolysates by HPLC-AAS.568 The detection limit was 0.3 pmol. A comparison was made between AAS detection and ¯uorescence detection of the o-phthaldialdehyde derivatives (LOD 30 pmol and 170 pmol for selenomethionine and selenocysteine). An interesting method of microbore HPLC and ultra-low volume sample fraction collection followed by ETAAS detection has been used to separate and detect 5 Se species in 6 min.569 A ¯ow rate of 80 ml min21 through an anion exchange column enabled collection of 20 ml aliquots in sample cups and these were then analysed using ETAAS. Relative LODs of 2.8±4.1 ng ml21 were obtained for trimethylselenonium, selenomethionine, selenite, selenate and selenocystine and the method was validated by analysis of extracts from CRM 402 white clover. Four selenium species have been separated in 14 min on a Hamilton PRP X-100 anion exchange column using a phosphate buffer and the eluate was then passed through a PTFE coil wound around a water cooled high pressure mercury lamp.570 The photolysed Se compounds were then mixed with concentrated hydrochloric acid (1 ml min21) and 1% tetrahydroborate in sodium hydroxide. The Se hydrides were then ¯ushed to detection in a quartz furnace AAS by a stream of argon. Calibration was linear to at least 800 mg l21 for selenite, selenate, selenomethionine and selenocystine and LODs were 2.5±46.5 mg l21. A paper has reported the use of a boiling water bath to accelerate the reduction of SeVI to SeIV by 6 M hydrochloric acid prior to HGAAS.571 The procedure was reportedly less expensive than microwave assisted reduction and took only 2 min. Arsenic has been by far the most popular element that has undergone speciation analysis using AAS as the detection system. Matrices analysed have included sediments.572,573 In the former paper, arsenic species associated with iron oxide were determined after the sediment had been extracted with hydroxyammonium chloride. The extracts were separated using an anion exchange column and a gradient elution using pH 5.8 and pH 10 phosphate (100 mM). Detection was by HGAAS using a quartz T-piece heated in an air±acetylene ¯ame. In the latter paper,573 marine sediments were extracted and the relationships between As species and redox potential and iron and manganese load were studied. It was concluded that most samples had higher AsV than AsIII and that the speciation depended on reducing conditions and manganese concentration. Natural waters were analysed for As species by hydride generation followed by trapping of the hydride on zirconium coated tubes at 1000 ³C and ETAAS detection.574 The hydrides of different species were generated using different reducing media. Total As was reduced using 1% tetrahydroborate in 0.02 M thioglycollic acid, AsIII and AsV were reduced by 2% tetrahydroborate and 4 M hydrochloric acid, 0.2% tetrahydroborate and citric acid±sodium hydroxide buffer at pH 5 for AsIII and 0.2% tetrahydroborate and 0.14 M acetic acid for AsIII and monomethylarsonic acid (MMAA). The AsV and MMAA were calculated by difference. Detection limits were typically 50±80 ng l21. Water soluble As species in the brown alga Fucus distichus were determined using gel permeation and ion exchange chromatography and ETAAS detection.575 Further work coupling HPLC and ICP-MS was also performed. Four major species were identi®ed but one minor species remained unidenti®ed. Inorganic arsenic species were 794

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determined in seafood samples using a 9 M hydrochloric acid extraction followed by reduction using hydrobromic acid and hydrazine sulfate, extraction into chloroform, back extraction into 1 M hydrochloric acid, dry ashing and ®nally detection using HG-AAS.576 Recoveries of AsIII and AsV were 99% and 96%, respectively, and the LOD was 3.07 ng g21. Under optimum conditions other As species were reportedly not extracted. Analysis of the CRM DORM-1 yielded results in close agreement with those obtained by other workers using HPLC-ICP-MS. Clinical samples have also received attention. Arsenic speciation in serum has been reported in two papers.577,578 In the ®rst example low molecular weight As compounds were determined directly using HPLC coupled with HGAAS. Higher molecular weight species were ®rst separated by FPLC (either size exclusion, ion exchange or af®nity chromatography) and then digested before measuring off-line by HGAAS. The main species present in uremic patients was found to be dimethylarsinic acid (DMA) and arsenobetaine. The second paper used a similar approach and found that only inorganic As bound to serum proteins. Arsenic speciation in urine has been reported in which 8 species (AsIII, AsV, DMA, MMAA, arsenobetaine, arsenocholine trimethylarsine oxide and tetramethylarsonium) were separated using anion exchange resins followed by on-line photo-oxidation and digestion with persulfate and HGAAS detection.579 Between 32 and 36 determinations could be performed in 8 h and LODs were reported to be between 2 and 6 mg l21 depending on the species. The analysis of 3 urine candidate reference materials has been reported.580 Cation exchange chromatography was coupled on-line with alkaline persulfate digestion and HGAAS to determine levels of DMA and arsenobetaine, although other species (MMAA and arsenocholine) were also monitored. It was concluded that arsenobetaine concentrations in urine depend solely on food intake and that the species were stable at room temperature in a matrix of dilute perchloric acid. A membrane gas±liquid separator has been used during the FIHGAAS determination of As.581 Arsenic(III) was determined by injection (100 ml) into 0.5 M hydrochloric acid and total As was determined by injection into 15% potassium iodide and 6 M hydrochloric acid. For both determinations, the ¯ow was merged with 1% tetrahydroborate and the arsine generated passed through the membrane separator and swept to detection using a ¯ow of nitrogen. Detection limits were 150 pg for AsIII and 80 pg for total As. Arsenic(V) was calculated by difference. The sampling rate was 25 h21 and the results of the analyses of surface waters and soil extracts were compared with those obtained by ETAAS. The only other publication concerning chromatography coupled with AAS in this review period has been one that separated interfering ions such as chloride during the determination of Cd and Pb.582 An anion exchange column separated the chloride ions whilst fractions of eluate were collected for off-line determination of the analytes by ETAAS. One of the more interesting methods of speciation has been reported by Aller.583,584 In these papers, living bacterial cells (Pseudomonas putida) were used to adsorb Se species (selenomethionine and selenourea) and then the biomass was analysed directly by ETAAS. It was possible to discriminate between the different chemical species by combining the optimization of both the growth conditions and the relative rates of the retention from the sample solution. Detection limits of approximately 1 ng ml21 were achieved with a precision of typically 2±5.6% RSD. 6.3.2 LC-AES. A review of the determination of metallothioneins containing 88 references covering separation techniques such as capillary zone electrophoresis (CZE), microbore and capillary HPLC and their interfacing with AAS, AES and ICP-MS has been published.585 Applications of the hyphenated techniques to `real world' samples were also reviewed.

A number of analytes (Cu, Fe, Mn and Zn) in blood fractions were determined by size exclusion chromatography (SEC)-ICP-AES.586 The column used was a Superformance Fractogel EMD BioSEC 650(S) column that was calibrated using 5 standard proteins. The optimum mobile phase was 0.3 M sodium chloride in 0.02 M phosphate buffer at pH 6.8. Detection limits were 30, 26, 0.8 and 90 ng ml21 for Cu, Fe, Mn and Zn, respectively. The system could be used for protein identi®cation, but not for quanti®cation. Three analytical techniques for the speciation of Al in environmental samples have been used.587 The methods used were FPLC-ICP-AES, a microcolumn of chelating resin±ETAAS and 8-hydroxyquinoline±spectrophotometry. The FPLC-ICP-AES method successfully separated Al3z, Al(OH)2z and Al(OH)2z, whilst AlF2z co-eluted with Al(OH)2z and Al(SO4)z, AlF2z and negatively charged Al organocomplexes co-eluted with Al(OH)2z. The ETAAS technique separated most of the labile organic complexes, the sulfato and the hydroxy species. A combination of all three techniques could be used to speciate Al in soil extracts and percolating waters. Silicon speciation is also beginning to receive some attention. Silicate, 1,3-tetramethylsilanediol and hexamethyldisiloxane have been separated on an end-capped C18 column using 30% methanol as mobile phase in 32 min.588 The ICP utilized was used in both axial and radial modes at the 251.6 nm line, but the LODs were found to be similar (0.1±0.5 mg ml21) for both modes. This was attributed to the noise arising from the organic solvent. In another paper589 a C18 column was used with acetonitrile (20%) to separate dimethylsilanediol, tetramethyldisiloxane-1,3-diol and trimethylsilanol. A change to 5% acetonitrile was necessary to separate the dimethyldisilanediol from silicate. Detection was at 288.158 nm and LODs were at the 3±50 ng range. Selenium speciation has been achieved by a number of workers. A number of seleno-amino acids have been separated after esteri®cation using 3% sulfuric acid in methanol or ethanol followed by separation on a C8 column using a mobile phase containing water±methanol (49z1) and 0.1% tri¯uoroacetic acid (TFA).521 Alternatively, the selenoamino acids were derivatized using water±ethanol±pyridine (15z8z2), extracted into chloroform and then separated using gas chromatography with a column of HP-1 (0.17 mm, 2560.32 m) using a thermal gradient of 100 ³C to 200 ³C at a ramp rate of 5 ³C min21. Detection was by both ICP-AES and ICP-MS. The method was applied to a number of nutritional supplements and vegetables. Numerous Se species have been separated using microbore HPLC anion exchange columns followed by on-line coupling with an ICP-AES instrument via a direct injection nebulizer (DIN).590 The ¯ow rate was 80 ml min21 and the Se species selenomethionine, selenite, selenate and selenocystine were separated in less than 5 min. Detection limits were between 20 and 38 ng ml21, precision was typically 2±6 % for peak area and the method was applied to the analysis of CRM 402. Four extraction methods were compared and some evidence of selenocystine breakdown during extraction was also presented. Antimony speciation has been reported in two papers by one set of authors. In one paper591 SbIII and SbV were determined by a HGAES method that relied on the reduction rate of SbV to SbIII using cysteine. Calibration was achieved at two different times (t~2 and 8 min). The LODs were 1.2 ng ml21 for SbIII and 4.5 ng ml21 for SbV. The method was applied to spiked water samples. The second paper was not dissimilar but used potassium iodide as reductant, and relied on the preferential determination of SbIII in the presence of citric acid.592 The use of a membrane desolvation device that enabled organic solvents and liquid chromatographic samples to be introduced at a ¯ow rate of 1.5 l min21 to a low power (120 W) helium microwave plasma has been reported.593 By increasing the counter-¯ow of gas to 8 l min21, the aerosol could be

desolvated almost completely (the plasma returned to pink and showed no sign of green carbon bands). Interfacing reversed phase HPLC and the He MIP enabled 2,6-dichlorobenzene and 5,7-dichlorhydroxyquinoline to be separated and determined by monitoring the Cl signal. Direct elemental and molecular species detection for liquid chromatography by glow discharge atomic emission and mass spectrometries has been discussed.594 The system was reported as being capable of determining trace metals at the ppb level whilst C and H (from the ligands) were detectable at low ppm levels. 6.3.3 LC-AFS. An overview of the possibilities of AFS for speciation has been presented by Fodor.595 It described the relatively cheap cost of the instrumentation as well as systems used to speciate As and Se. A comparison between AFS and ICP-MS as detectors for speciation analysis of As in environmental samples has also been made.596 In this work, HPLC using an anion exchange column was used to separate arsenite, arsenate, MMAA and DMA prior to HG-AFS. An on-line photo-oxidation step was also described that was used for determining arsenobetaine. A similar paper by the same authors quoted LODs of 0.17, 0.45, 0.3 and 0.38 mg l21 using a 20 ml sample loop for AsIII, DMA, MMAA and AsV, respectively.597 The technique was used to analyse a urine reference material and a volunteer's sample. The As species AsIII, DMA, MMAA and AsV have been determined in seawater.598 The hydrides generated were cooled in liquid nitrogen and then a Variac was used to heat the tube, releasing the species at their respective boiling points. Precision was usually better than 3.5%, and LODs were in the range 0.9±3.7 ng l21. Solid-phase extraction cartridges have been used as low pressure chromatographic columns to separate AsIII and AsV prior to HG-AFS detection.599 Separation of the species occurred within 1.5 min, yielding LODs of 0.2 and 0.4 ng ml21 for the tri- and pentavalent species, respectively. The method was applied to the analysis of water samples, with results obtained from the analysis of NIST CRM 1643d being in good agreement with the certi®ed value. Eight As species (plus several unidenti®ed As species) have been separated and determined using HPLC-UV-HG-AFS.600 A total of six reference materials of biological origin were analysed for AsIII, AsV, DMA, MMAA, arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium. As well as photolysis, the authors also utilized a chemical oxidation using alkaline persulfate to ensure that non-hydride forming species were converted into a reducible form. Both cation and anion chromatographic separations were developed. Selenium in sea-water has been determined using a FI-HGAFS system.601 After ®ltration and acidi®cation, SeIV was determined by mixing with hydrochloric acid (1.5 M), potassium dihydrogenphosphate (5%) and tetrahydroborate (1.5% in 0.5% sodium hydroxide). For total Se determination, the sample was mixed with 30% sodium bromide and then irradiated in a microwave oven set at 210 W for 60 s and then passed through a cooling coil. After this reduction stage, the hydride was formed in the same way as described above. Selenium(VI) was calculated by subtraction. Calibrations were linear from 15 to 100 ng l21 for SeIV and 15±150 ng l21 for total Se and LODs were 5 and 4 ng l21, respectively. Recoveries from spiked sea-water were 93±104%. Selenomethionine, selenoethionine and selenocystine have been separated on a C18 column using 0.1% TFA±2% methanol as mobile phase and then fed directly to a hydraulic high pressure nebulizer.602 The aerosol thus generated was then passed through a desolvation system before the dry gas was passed into a H2±Ar diffusion ¯ame where the Se was determined at 196 nm. The separation was complete within 7 min and LODs were 42, 71 and 50 ng ml21 for selenomethionine, selenoethionine and selenocystine, respectively. The method was applied to the analysis of J. Anal. At. Spectrom., 2000, 15, 763±805

795

garlic. One other Se speciation application using AFS detection has been reported.603 Few details were given, but the species separated were selenomethionine, selenoethionine, selenocystine and the trimethylselenonium ion.

7 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

29 30 31 32 33

34 35 36

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229 R. Sing, Spectrochim. Acta, Part B, 1999, 54(B), 411. 230 W.-T. Chan and Z.-B. Gong, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 68. 231 A. Montaser, J. A. McLean, M. G. Minnich, Q. Jin, S.-A. E. O'Brien and J. Chirinos, (Dept. Chem., George Washington Univ., Washington, DC 20052, USA). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 232 J. A. McLean, M. G. Minnich and A. Montaser, (Dept. Chem., George Washington Univ., Washington, DC 20052, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 233 S.-Y. Chen, J.-C. Zou and X.-J. Guan, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 63. 234 J.-L. Todoli, V. Hernandis, A. Canals and J.-M. Mermet, J. Anal. At. Spectrom., 1999, 14(9), 1289. 235 L. Bordera, J. L. Todoli, J. Mora, A. Canals and V. Hernandis, (Dept. Anal. Chem., Univ. Alicante, 03080 Alicante, Spain). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 236 Y. Nakamura, K. Ide and R. Hasegawa, Bunseki Kagaku, 1999, 48(3), 339. 237 L. B. Allen, P. H. Siitonen and H. C. Thompson Jr., J. Anal. At. Spectrom., 1998, 13(8), 735. 238 B. Budic, J. Anal. At. Spectrom., 1998, 13(9), 869. 239 J. A. McLean, M. G. Minnich, L. A. Iacone, H. Liu and A. Montaser, J. Anal. At. Spectrom., 1998, 13(9), 829. 240 J.-M. Mermet, (Lab. Sci. Anal., Univ. Claude Bernard-Lyon I, 69622 Villeurbanne, France). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 241 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998, 13(9), 843. 242 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998, 13(11), 1249. 243 J.-L. Todoli and J.-M. Mermet, J. Anal. At. Spectrom., 1998, 13(8), 727. 244 J. L. Todoli (Dept. Anal. Chem., Univ. Alicante, 03080 Alicante, Spain). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 245 P. Masson, Spectrochim. Acta, Part B, 1999, 54, 603. 246 A. S. Al-Ammar, R. K. Gupta and R. M. Barnes, J. Anal. At. Spectrom., 1999, 14(5), 793. 247 S. Maestre, J. Mora, J.-L. Todoli and A. Canals, J. Anal. At. Spectrom., 1999, 14(1), 61. 248 J. Liu, B. Huang and X. Zeng, Spectrochim. Acta, Part B, 1998, 53, 1469. 249 E. Debrah and G. Legere, (Perkin Elmer-SCIEX Instruments, Concord, ON, Canada L4K 4V8). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 250 B. W. Pack, S. J. Ray, R. A. Potyrailo and G. M. Hieftje, Appl. Spectrosc., 1998, 52(12), 1515. 251 G. Schaldach, L. Berger and H. Berndt, (Inst. Spektrochem. und Angewandte Spektroskopie, 44139 Dortmund, Germany). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 252 L. Berger, G. Schaldach and H. Berndt, (Inst. Spektrochem. und Angewandte Spektroskopie, 44139 Dortmund, Germany). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 253 C. Trassy and A. Tazeem, Spectrochim. Acta, Part B, 1999, 54, 581. 254 J. Noelte, F. Schef¯er, S. Mann and M. Paul, J. Anal. At. Spectrom., 1999, 14(4), 597. 255 M. A. Belarra, M. Resano and J. R. Castillo, J. Anal. At. Spectrom., 1999, 14(4), 547. 256 M. A. Belarra, I. Belategui, I. Lavilla, J. M. Anzano and J. R. Castillo, Talanta, 1998, 46(6), 1265. 257 N. Miller-Ihli, (Beltsville Human Nutrition Res. Center, Food Composition Lab., USDA, Beltsville, MD 20705, USA). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 258 L. Amoedo, J. L. Capelo, I. Lavilla and C. Bendicho, J. Anal. At. Spectrom., 1999, 14(8), 1221. 259 M. Jesus Cal-Prieto, A. Carlosena, J. M. Andrade, S. Muniategui, P. Lopez-Mahia, E. Fernandez and D. Prada, J. Anal. At. Spectrom., 1999, 14(4), 703. 260 R. E. Russo, X. L. Mao and O. V. Borisov, Trends Anal. Chem., 1998, 17(8±9), 461. 261 V. Kanicky and J.-M. Mermet, Fresenius' J. Anal. Chem., 1999, 363(3), 294.

262 V. Kanicky, V. Otruba and J.-M. Mermet, Fresenius' J. Anal. Chem., 1999, 363(4), 339. 263 V. Kanicki, V. Otruba and J.-M. Mermet, (Lab. Plasma Sources Chem. Anal., Masaryk Univ. Brno, 611 37 Brno, Czech Republic). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 264 M. Motelica-Heino, O. F. X. Donard and J. M. Mermet, J. Anal. At. Spectrom., 1999, 14(4), 675. 265 A. P. K. Leung, W. T. Chan, X. L. Mao and R. E. Russon, Anal. Chem., 1998, 70(22), 4709. 266 R. K. Thareja, A. Misra and S. R. Franklin, Spectrochim. Acta, Part B, 1998, 53, 1919. 267 J.-L. Todoli and J.-M. Mermet, Spectrochim. Acta, Part B, 1998, 53, 1645. 268 C. Fabre, M.-C. Boiron, J. Dubessy and A. Moisette, J. Anal. At. Spectrom., 1999, 14(6), 913. 269 K. Florian, J. Hassler and E. Surova, J. Anal. At. Spectrom., 1999, 14(4), 559. 270 D. Grientschnig, H. Huber and R. Leitgeb, Spectrochim. Acta, Part B, 1998, 53B, 1601. 271 S. Suzuki, Bunseki, 1998(8), 576. 272 K. Shimizu, G. M. Brown, H. Habazaki, K. Kobayashi, P. Skeldon, G. E. Thompson and G. C. Wood, Surf. Interface Anal., 1999, 27(1), 24. 273 C. M. Barshick, K. R. Hess, A. L. Zook, R. E. Steiner and F. L. King, Appl. Spectrosc., 1999, 53(1), 65. 274 M. L. Hartenstein, S. J. Christopher and R. K. Marcus, J. Anal. At. Spectrom., 1999, 14(7), 1039. 275 K. A. Marshall, J. Anal. At. Spectrom., 1999, 14(6), 923. 276 S. Rio-Segade and C. Bendicho, J. Anal. At. Spectrom., 1999, 14(12), 1907. 277 G.-H. He, S.-P. Feng, R.-L. Chen and L.-R. Zeng, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 29. 278 J. Mierzwa, Y. C. Sun, Y. T. Chung and M. H. Yang, Talanta, 1998, 47, 1263. 279 S. Huang, Guangpu Shiyanshi, 1998, 15(1), 31. 280 M. Y. Shiue, Y. C. Chan, J. Mierzwa and M. H. Yang, J. Anal. At. Spectrom., 1999, 14(1), 69. 281 I. Lopez-Garcia, M. Sanchez-Merlos and M. HernandezCordoba, Mikrochim. Acta, 1999, 130(4), 295. 282 G. Wibetoe, D. T. Takuwa, W. Lund and G. Sawula, Fresenius' J. Anal. Chem., 1999, 363(1), 46. 283 B. Wagner, S. Garbos, E. Bulska and A. Hulanicki, Spectrochim. Acta, Part B, 1999, 54(5), 797. 284 S. Ahsan, S. Kaneco, K. Ohta, T. Mizuno and Y. Taniguchi, Talanta, 1999, 48, 63. 285 P. Vinas, M. Pardo-Martinez and M. Hernandez Cordoba, J. Anal. At. Spectrom., 1999, 14(8), 1215. 286 W. Fuyi and J. Zucheng, Anal. Chim. Acta, 1999, 391(1), 89. 287 M. Gonzalez, M. Gallego and M. Valcarcel, Talanta, 1999, 48(5), 1051. 288 H. Chen, H. Liu, Z. Tang, S. Zhang and Z. Jin, Guangpu Shiyanshi, 1998, 15(1), 44. 289 E. Luecker, J. Anal. At. Spectrom., 1999, 14(4), 583. 290 T. Buchkamp and G. Hermann, Spectrochim. Acta, Part B, 1999, 54, 657. 291 E. Lucker, J. Anal. At. Spectrom., 1999, 14(11), 1731. 292 M. Hornung and V. Krivan, Spectrochim. Acta, Part B, 1999, 54, 1177. 293 M. Lucic and V. Krivan, J. Anal. At. Spectrom., 1998, 13(10), 1133. 294 M. Hornung and V. Krivan, Anal. Chem., 1998, 70(16), 3444. 295 A. Ishida, K. Matsuda, S. Chu, F. Tanoue, S. Sakakibara, K. Ishino, S. Fuke and H. Fujiyasu, Mater. Sci. Eng., B, 1999, 59(1±3), 230. 296 V. Krivan and H. M. Dong, Anal. Chem., 1998, 70(24), 5312. 297 K. Danzer, W. Schroen, B. Dressler and K.-U. Jagemann, Fresenius' J. Anal. Chem., 1998, 361(6±7), 710. 298 L. H. Liu and S. B. Luan, Fenxi Huaxue, 1998, 26(3), 372. 299 J. C. Torres, M. C. Rodriguez and V. A. Granadillo, (Lab. Instrumentacion Anal., Dept. Quim., Fac. Exprimental Ciencias, Univ. Zulia, Zulia 4011, Venezuela). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 300 L. H. Liu, F. Yan and L. X. Wu, Fenxi Shiyanshi, 1999, 18(3), 65. 301 L. H. Liu, Q. K. Zhang, M. Y. Wang and J. L. Lei, Yaowu Fenxi Zazhi, 1999, 19(3), 181. 302 P. Bermejo, R. Dominguez, A. Reboiro, A. Bermejo, J. M. Fraga and J. A. Cocho de Juan, At. Spectrosc., 1999, 20(4), 161. 303 P. Bermejo-Barrera, A. Moreda-Pineiro, J. Moreda-Pineiro and A. Bermejo-Barrera, Fresenius' J. Anal. Chem., 1998, 360(6), 707.

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304 H. R. Badiei and V. Karanassios, J. Anal. At. Spectrom., 1999, 14(4), 603. 305 M. E. Rybak, P. Hatsis, K. Thurbide and E. D. Salin, J. Anal. At. Spectrom., 1999, 14(11), 1715. 306 V. Karanassios and T. J. Wood, Appl. Spectrosc., 1999, 53(2), 197. 307 C. D. Skinner, M. Cazagou, J. Blaise and E. D. Salin, Appl. Spectrosc., 1999, 53(2), 191. 308 X. G. Du, Y. X. Duan and Q. H. Jin, Fenxi Shiyanshi, 1999, 18(4), 10. 309 U. Schaeffer and V. Krivan, Anal. Chem., 1999, 71(4), 849. 310 P. J. McKinstry, H. E. Indyk and N. D. Kim, Food Chem., 1999, 65(2), 245. 311 P. Tianyou, J. Zucheng and Q. Yongchao, J. Anal. At. Spectrom., 1999, 14(7), 1049. 312 C. Shizhong, P. Tianyou, J. Zucheng, L. Zhenhuan and H. Bin, J. Anal. At. Spectrom., 1999, 14(11), 1723. 313 M. Motelica-Heino, J.-M. Mermet and O. F. X. Donard, (Lab. Chim. Bio-inorganique Environ., EP CNRS 132, 64000 Pau, France). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 314 T. Howe, P. Krause, A. Cox and R. C. Hutton, (CETAC Technologies Inc., Crewe, Cheshire, UK CW1 6UZ). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 315 M. Lucic and V. Krivan, Fresenius' J. Anal. Chem., 1999, 363(1), 64. 316 S. Rings, R. Sievers and M. Jansen, Fresenius' J. Anal. Chem., 1999, 363(2), 165. 317 C. J. Rademeyer and L. de Jager, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 48. 318 V. I. Slaveykova, L. Lampugnani, D. L. Tsalev, L. Sabbatini and E. De Giglio, Spectrochim. Acta, Part B, 1999, 54, 455. 319 S. Q. Lin, S. Y. Chen and M. H. Bi, Guangpuxue Yu Guangpu Fenxi, 1999, 19(1), 81. 320 P. V. Oliveira, Z. F. Queiroz, F. J. Krug, E. C. Lima and J. A. Nobrega, (Dept. Quim., Univ. Federal Sao Carlos, Sao Carlos 13.560-970, Brazil). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 321 T. Narukawa, W. Yoshimura and A. Uzawa, Bull. Chem. Soc. Jpn., 1999, 72(4), 701. 322 K. P. Kureichik, Zh. Prikl. Spektrosk., 1998, 65(4), 604. 323 M. Burguera, J. L. Burguera, C. Rondon, M. L. di Bernardo, M. Gallignani, E. Nieto and J. Salinas, Spectrochim. Acta, Part B, 1999, 54, 805. 324 M. A. Belarra, M. Resano, S. Rodriguez, J. Urchaga and J. R. Castillo, Spectrochim. Acta, Part B, 1999, 54, 787. 325 Z. Kowalewska, E. Bulska and A. Hulanicki, Spectrochim. Acta, Part B, 1999, 54, 835. 326 S. Saracoglu and L. Elci, Anal. Sci., 1999, 15(6), 569. 327 C. G. Bruhn, J. Y. Neira, M. I. Guzman, M. M. Darder and J. A. Nobrega, Fresenius' J. Anal. Chem., 1999, 364(3), 273. 328 E. H. Evans, S. Chenery, A. Fisher, J. Marshall and K. Sutton, J. Anal. At. Spectrom., 1999, 14(6), 977. 329 K. Levine, K. A. Wagner and B. T. Jones, Appl. Spectrosc., 1998, 52(9), 1165. 330 T. Kantor and S. Gucer, Spectrochim. Acta, Part B, 1999, 54, 763. 331 V. Karanassios, V. Grishko and G. G. Reynolds, J. Anal. At. Spectrom., 1999, 14(4), 565. 332 F. M. Pennebaker, D. A. Jones, C. A. Gresham, R. H. Williams, R. E. Simon, M. F. Schappert and M. B. Denton, J. Anal. At. Spectrom., 1998, 13(9), 821. 333 H.-M. Kuss and B. S. Bayraktar, CLB Chem. Labor Biotech., 1999, 50(5), 164. 334 Y. Q. Liu, R. X. Li, P. Z. Fan and Z. Z. Xu, Inst. Phys. Conf. Ser., 1996, 151(X-ray Lasers 1996), 364. 335 K. Tsuji, Bunko Kenkyu, 1998, 47(6), 280. 336 P. Lindblom, Anal. Chim. Acta, 1999, 380(2±3), 353. 337 G. N. Coleman, R. W. Foster, R. J. Krupa, S. Luan, M. J. Pilon and R. L. Stux, (Thermo Jarrell Ash/Baird, Franklin, MA 02038, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 338 J. D. Batchelor and B. T. Jones, Anal. Chem., 1998, 70(23), 4907. 339 R. A. De Verse, R. M. Hammaker, W. G. Fately, J. A. Graham and J. D. Tate, Am. Lab. (Shelton, Conn.), 1998, 20(21), 112S. 340 V. G. Lazareva, V. A. Grigorkina, L. N. Orlova, G. V. Zayats, I. D. Ismailov and V. V. Mandrygin, Zavod. Lab., 1997, 63(12), 57. 341 J. J. Giglio, P. S. Goodall and S. G. Johnson, Spectroscopy (Eugene, Oreg.), 1997, 12(7), 26. 342 K. Liu, H. Tan, J. Wei, Y. Pan, T. Huang and Z. Huang, Spectrochim. Acta, Part B, 1998, 53, 1455.

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343 K. A. Wagner, J. D. Batchelor and B. T. Jones, Spectrochim. Acta, Part B, 1998, 53, 1805. 344 U. Jerono, LaborPraxis, 1999, 23(5), 76. 345 E. D. Prudnikov and Y. S. Shapkina, (Earth's Crust Inst., State Univ., St Petersburg 199034, Russia). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 346 A. Montaser, J. A. McLean, M. G. Minnich, L. A. Lacone, Q. Jin, S.-A. E. O'Brien and A. Okino, (George Washington Univ., Washington, DC 20052, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 347 S. Green®eld, (Albright and Wilson Mfg. Ltd., UK). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 348 M. A. Rutzke, (USDA Plant Soil Nutr. Lab., Cornell Univ., Ithaca, NY 14853, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 349 C. Hensman and J. W. Olesik, (Lab. Plasma Spectrochem., Laser Spectrosc. and Mass Spectrom., Dept. Geol. Sci., Ohio State Univ., Columbus, OH 43210, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 350 C. Breer, W. Buscher, R. M. Barnes and K. Cammann, (Inst. Chemo- und Biosensorik e.V., 49149 Muenster, Germany). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 351 D. S. Bollinger and A. J. Schleisman, At. Spectrosc., 1999, 20(2), 60. 352 A. Eastgate and M. Haldimann, (Glass Expansion, 1323 Romainmotier, Switzerland). Presented at Pittcon.'99, Orlando, FL, USA, March 7±12, 1999. 353 H. R. Badiel, V. Grishko and V. Karanassios, (Guelph-Waterloo Center Graduate Work Chem., Dept. Chem., Univ. Waterloo, Waterloo, ON, Canada N2L 3G1). Presented at Pittcon.'99, Orlando, FL, USA, March 7±12, 1999. 354 J. A. Horner and G. M. Hieftje, Appl. Spectrosc., 1999, 53(6), 713. 355 Y. J. Li, Y. F. Liu, X. P. Zeng, S. L. Wang and Y. M. Ni, Lihua Jianyan, Huaxue Fence, 1998, 34(10), 440. 356 K. Oishi, T. Okumoto and T. Shirasaki, Bunseki, 1999(1), 81. 357 B. W. Pack, G. M. Hieftje and Q. Jin, Anal. Chim. Acta, 1999, 383(3), 231. 358 S. D. Anghel, T. Frentiu, E. A. Cordos, A. Simon and A. Popescu, J. Anal. At. Spectrom., 1999, 14(4), 541. 359 T. Frentiu, A.-M. Rusu, S. D. Anghel, S. Negoescu, A. Popescu, A. Simon and E. M. Cordos, ACH ± Models Chem., 1999, 137(1± 2), 119. 360 T. Frentiu, S. D. Anghel, A. Simon, A. Popescu, A.-M. Rusu and E. A. Cordos, ACH ± Models Chem., 1999, 136(1±2), 131. 361 X. Pan, B. Hu, Y. Ye and R. K. Marcus, J. Anal. At. Spectrom., 1998, 13(10), 1159. 362 K. Wagatsuma, Fresenius' J. Anal. Chem., 1999, 363(4), 333. 363 H. Matsuta and K. Wagatsuma, Anal. Sci., 1999, 15(4), 319. 364 K. Wagatsuma, Bunseki Kagaku, 1999, 48(4), 457. 365 K. Wagatsuma, Y. Danzaki and N. Yamashita, Bunseki Kagaku, 1999, 48(3), 349. 366 C. Yang, K. Ingeneri and W. W. Harrison, J. Anal. At. Spectrom., 1999, 14(4), 693. 367 J. A. C. Broekaert, T. K. Starn, L. J. Wright and G. M. Hieftje, Spectrochim. Acta, Part B, 1998, 53, 1723. 368 Y. S. Park, S. H. Ku, S. H. Hong, H. J. Kim and E. H. Piepmeier, Spectrochim. Acta, Part B, 1998, 53, 1167. 369 G. G. Sikharulidze and A. E. Lezhnev, J. Anal. At. Spectrom., 1999, 14(1), 45. 370 J.-C. Hubinois, A. Morin, P. Marty, J.-P. Larpin and M. Perdereau, J. Anal. At. Spectrom., 1999, 14(9), 1405. 371 O. Banhidi and L. Papp, ACHÐModels Chem., 1999, 136(1±2), 75. 372 A. Honda, Jpn. Kokai Tokkyo Koho JP 10 160,667 [98 160,667] (Cl. G01N21/31), 19 Jun 1998, Appl. 96/319,702, 29 Nov 1996; 3 pp. 373 X. W. Feng and Y. Q. Yang, Guangpuxue Yu Guangpu Fenxi, 1998, 18(6), 731. 374 A. A. Pupyshev and I. G. Evdokimova, Zh. Prikl. Spektrosk., 1998, 65(1), 14. 375 J. B. Willis and B. T. Sturman, J. Anal. At. Spectrom., 1999, 14(5), 895. 376 J.-M. Sun, Z. Yan, L.-P. Liu, D.-Q. Zhang, L.-L. Yang and H.W. Sun, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 6. 377 A. B. Volynsky, Spectrochim. Acta, Part B, 1998, 53, 1607. 378 E. C. Lima, F. J. Kurg, A. T. Ferreira and F. Barbosa Jr., J. Anal. At. Spectrom., 1999, 14(2), 269. 379 E. C. Lima, F. J. Krug and K. W. Jackson, Spectrochim. Acta, Part B, 1998, 53, 1791.

380 K. A. Wagner, K. E. Levine and B. T. Jones, Spectrochim. Acta, Part B, 1998, 53B, 1507. 381 G. Absalan, C. L. Chakrabarti, K. L. Headrick, M. Parker and R. K. Marcus, Anal. Chem., 1998, 70(16), 3434. 382 C. E. Hensman and G. D. Rayson, J. Anal. At. Spectrom., 1999, 14(7), 1025. 383 J. M. Harnly, J. Anal. At. Spectrom., 1999, 14(2), 137. 384 D. N. Wichems, R. E. Fields and J. M. Harnly, J. Anal. At. Spectrom., 1998, 13(11), 1277. 385 Mitchell and Zemansky, Resonance Radiation and Excited Atoms, Cambridge University Press, Cambridge, UK, 1961. 386 H. Becker-Ross, S. Florek, U. Heitmann and M. Schuetz, (ISAS, Inst. Berlin, 12489 Berlin, Germany). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 387 Ger. Offen. DE 19,725,809 (Cl. H01S3/19), 24 Dec 1998, appl. 19,725,809, 18 Jun 1997; 4 pp. 388 T. Komatsu, T. Araki and A. Scheeline, Appl. Opt., 1999, 53(1), 108. 389 L. Da, S. Y. Zhang, B. L. Huang, Z. B. Gong and P. Y. Yang, Guangpuxue Yu Guangpu Fenxi, 1999, 19(3), 352. 390 Z. B. Gong, F. Liang, P. Y. Yang, Q. H. Jin and B. L. Huang, Guangpuxue Yu Guangpu Fenxi, 1999, 19(3), 356. 391 S.-Y. Zhang, B.-L. Huang, Z.-B. Gong and P.-Y. Yang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 75. 392 S.-Y. Zhang, B.-L. Huang, Z.-B. Gong and P.-Y. Yang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 76. 393 D. Yates, C. Schneider, I. Shuttler and K. Fredeen, (PerkinElmer Corp., Norwalk, CT, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 394 S. De Goy, D. Lanzisera, K. Lopez, J. Noonan and J. Rebello, Spectroscopy (Eugene, Oreg.), 1998, 13(10), 36. 395 C. Lengacher, S. Macklin, D. Hite and M. F. Masters, Am. J. Phys., 1998, 66(11), 1025. 396 L. Poletto, A. Boscolo and G. Tondello, Appl. Opt., 1999, 38(1), 29. 397 W. Bohle, T. Brandt, H. F. Falk, H. J. Graf, U. Heynen, P. Heitland, K. Krengel-Rothensee, U. Richter and R. Zeyen, (SPECTRO Analytical Instruments GmbH, 47533 Kleve, Germany). Presented at Pittcon.'99, Orlando, FL, USA, March 7±12, 1999. 398 M. B. Knowles, T. T. Nham and S. J. Carter, Int. Lab., 1998, 28(6), 25S. 399 C. Webb, A. T. Zander, H. Visse, P. V. Wilson, G. Russell and M. Knowles, (Ginzton Res. Center, Varian Assoc., Palo Alto, CA 94304, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 400 A. T. Zander, C. B. Copper, R.-L. Chien, M. Knowles, G. Russell and A. Wiseman, (Varian Res. Center, Varian Assoc. Inc., Palo Alto, CA, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 401 M. B. Knowles, T. Nham, G. Russell and H. Visser, (Varian Australia Pty Ltd, Mulgrave, 3170 Victoria, Australia). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 402 D. Shrader and C. Rivera, (Varian Assoc., Wood Dale, IL 60191, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 403 I. B. Brenner, S. Vats and A. T. Zander, J. Anal. At. Spectrom., 1999, 14(8), 1231. 404 R. W. Foster, M. J. Pilon and R. L. Stux, (Thermo Jarrell Ash/ Baird, Franklin, MA 02038, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 405 G. N. Coleman, R. W. Foster, R. J. Pascucci and R. L. Stux, (Thermo Jarrell Ash, Franklin, MA 02038, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 406 G. N. Coleman, G. R. Dulude, R. W. Starek and R. L. Stux, (Thermo Jarrell Ash, Franklin, MA 02038, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 407 P. Am Ayasse and A. Le Marchand, (Jobin Yvon Emission, ). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 408 P. A. Ayasse and A. Le Marchand, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 64. 409 A. Bengtson and S. Hanstrom, (Swedish Inst. Metals Res., 1114 28 Stockholm, Sweden). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10± 15, 1999. 410 U. Oppermann and H. Hohmann, (Shimadzu Deutschland

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GmbH, 47269 Duisburg, Germany). Presented at Instrumental Methods of Analysis. Modern Trends and Applications (IMA '99), 19±22 September, 1999, Chalkidiki, Greece. Y. Y. Zong, P. J. Parsons and W. Slavin, Spectrochim. Acta, Part B, 1998, 53, 1031. D.-q. Zhang, C.-m. Li, L.-l. Yang and H.-w. Sun, J. Anal. At. Spectrom., 1998, 13(10), 1155. H. W. Sun, L. J. Chen and J. M. Sun, Guangpuxue Yu Guangpu Fenxi, 1998, 18(5), 597. I. Lopez Garcia, M. Sanchez-Merlos and M. Hernandez Cordoba, J. Anal. At. Spectrom., 1998, 13(10), 1151. Z.-J. Yang, Z.-Y. Zhang, Z.-M. Li, Y.-H. Jia and H.-Z. Wang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 25. S. P. Madden, M. B. Denton and M. J. Pilon, (Dept. Chem., Univ. Arizona, Tucson, AZ 85721, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. C. Webb, A. T. Zander, P. V. Wilson and G. Perlis, (Ginzton Res. Center, Varian Assoc., Palo Alto, CA 94304, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. J. Noelte, At. Spectrosc., 1999, 20(3), 103. C. Ivaldi and T. W. J. Barnard, Spectrochim. Acta, Part B, 1993, 48, 1265. E. A. H. Timmermans, I. A. J. Thomas, J. Jonkers, E. Hartgers, J. A. M. van der Mullen and D. C. Schram, Fresenius' J. Anal. Chem., 1998, 362(5), 440. E. A. H. Timmermans, J. Jonkers, I. A. J. Thomas, A. Rodero, M. C. Quintero, A. Sola, A. Gamero and J. A. M. van der Mullen, Spectrochim. Acta, Part B, 1998, 53B, 1553. C. Prokisch, A. M. Bilgic, E. Voges, J. A. C. Broekaert, J. Jonkers, M. van Sande and J. A. M. van der Mullen, Spectrochim. Acta, Part B, 1999, 54, 1253. C. Yang, K. Ingeneri, M. Mohill and W. W. Harrison, (Dept. Chem., Univ. Florida, Gainesville, FL 32611, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. V.-D. Hodoroaba and T. Wirth, J. Anal. At. Spectrom., 1999, 14(9), 1533. V. Hoffmann, R. Kurt, K. Kammer, R. Thielsch, Th. Wirth and U. Beck, Appl. Spectrosc., 1999, 53(8), 987. K. Shimizu, G. M. Brown, H. Habazaki, K. Kobayashi, P. Skeldon, G. E. Thompson and G. C. Wood, Surf. Interface Anal., 1999, 27(3), 153. S. J. Christopher, M. L. Hartenstein, R. K. Marcus, M. Belkin and J. A. Caruso, Spectrochim. Acta, Part B, 1998, 53, 1181. M. Belkin, J. A. Caruso, S. J. Christopher and R. K. Marcus, Spectrochim. Acta, Part B, 1998, 53, 1197. A. Bogaerts, R. Gijbels and R. J. Carman, Spectrochim. Acta, Part B, 1998, 53, 1679. A. Bogaerts and R. Gijbels, J. Anal. At. Spectrom., 1998, 13(9), 945. X. Yan, W. Hang, B. W. Smith, J. D. Winefordner and W. W. Harrison, J. Anal. At. Spectrom., 1998, 13(9), 1033. Y.-X. Su, P.-Y. Yang, Z. Zhou, F.-M. Li, X.-R. Wang and B.-L. Huang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 17. M. Mohill and W. W. Harrison, (Dept. Chem., Univ. Florida, Gainesville, FL 32611, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. C. Perez, R. Pereiro, N. Bordel and A. Sanz-Medel, J. Anal. At. Spectrom., 1999, 14(9), 1413. P. Halmos, J. Borszeki and L. Szita, (Res. Group Hungarian Acad. Sci., Univ. Veszprem, 8201 Veszprem, Hungary). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. A. A. Pupyshev and A. K. Lutsak, J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 1998, 53(11), 987. E. H. van Veen and M. T. C. de Loos-Vollebregt, J. Anal. At. Spectrom., 1999, 14(5), 831. S. Velichkov, E. Kostadinova and N. Daskalova, Spectrochim. Acta, Part B, 1998, 53, 1863. I. B. Brenner, M. Zischka, B. Maichin and G. Knapp, J. Anal. At. Spectrom., 1998, 13(11), 1257. C. Dubuisson, E. Poussel and J. M. Mermet, J. Anal. At. Spectrom., 1998, 13(11), 1265. J. A. Horner and G. M. Hieftje, Spectrochim. Acta, Part B, 1998, 53, 1235. A. Kaneko, Bunseki, 1999(6), 504. A. C. Lazar and P. B. Farnsworth, Appl. Spectrosc., 1999, 53(4), 465. Q. Xu, G. Mattu and G. R. Agnes, Appl. Spectrosc., 1999, 53(8), 965.

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445 A. C. Lazar and P. B. Farnsworth, Appl. Spectrosc., 1999, 53(4), 457. 446 T. Masahiro, I. Akihiko, P. Nobuhiko, L. Zoran and T. Makabe, IEEE Trans. Plasma Sci., 1998, 26(6), 1724. 447 A.-M. Gomes, A. Almi, P. Teulet and J.-P. Sarrette, Spectrochim. Acta, Part B, 1998, 53B, 1567. 448 L. S. Milstein, K. L. Sutton, J. W. Waggoner and J. A. Caruso, (Dept. Chem., Univ. Cincinnati, Cincinnati, OH 45221-0172, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 449 L. S. Milstein, J. W. Waggoner, K. L. Sutton and J. A. Caruso, (Dept. Chem., Univ. Cincinnati, Cincinnati, OH 45221-0172, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 450 F. Sun and R. E. Sturgeon, J. Anal. At. Spectrom., 1999, 14(6), 901. 451 J. Dedina, A. D'Ulivo, L. Lampugnani, T. Matousek and R. Zamboni, Spectrochim. Acta, Part B, 1998, 53, 1777. 452 M. F. Zaranyika and A. Chirenje, Fresenius' J. Anal. Chem., 1999, 364(3), 208. 453 P. Parvinen and L. H. J. Lajunen, Spectrosc. Lett., 1998, 31(8), 1761. 454 A. Kh. Gilmutdinov and J. M. Harnly, Spectrochim. Acta, Part B, 1998, 53, 1003. 455 B. Radziuk, N. P. Romanova and Y. Thomassen, Anal. Commun., 1999, 36, 13. 456 U. Rohr, H. M. Ortner, G. Schlemmer, S. Weinbruck and B. Welz, Spectrochim. Acta, Part B, 1999, 54, 699. 457 J. Y. Yao, Q. S. Dai, W. H. Xie and G. Ma, Guangpuxue Yu Guangpu Fenxi, 1998, 18(6), 696. 458 B. V. L'vov, A. A. Vasilevich, A. O. Dyakov, J. W. H. Lam and R. E. Sturgeon, J. Anal. At. Spectrom., 1999, 14(7), 1019. 459 N. A. Panichev, Q. Ma, R. E. Sturgeon, C. L. Chakrabarti and V. Pavski, Spectrochim. Acta, Part B, 1999, 54, 719. 460 D. Langer and J. Holcombe, Anal. Chem., 1999, 71(3), 582. 461 A. E. Bozdogan, Spectrochim. Acta, Part B, 1999, 54, 557. 462 N. S. Thomaidis and E. A. Piperaki, Spectrochim. Acta, Part B, 1999, 54, 1303. 463 G. Chen and K. W. Jackson, Spectrochim. Acta, Part B, 1998, 53, 981. 464 M. M. Lamoureux, J. C. Hutton and D. L. Styris, Spectrochim. Acta, Part B, 1998, 53, 993. 465 G. A. Zachariadis, S. Sklavounos, P. Mandjukov and J. Stratis, (Lab. Anal. Chem., Dept. Chem., Aristotle Univ., Thessaloniki, Greece). Presented at Instrumental Methods of Analysis. Modern Trends and Applications (IMA '99), 19±22 September, 1999, Chalkidiki, Greece. 466 J. L. Fischer and C. J. Rademeyer, Spectrochim. Acta, Part B, 1999, 54, 975. 467 G. Daminelli, D. A. Katskov, R. M. Mofolo and P. Tittarelli, Spectrochim. Acta, Part B, 1999, 54, 669. 468 G. Daminelli, D. A. Katskov, R. M. Mofolo and T. Kantor, Spectrochim. Acta, Part B, 1999, 54, 683. 469 G. Daminelli, D. A. Katskov, P. J. J. G. Marais and P. Tittarelli, Spectrochim. Acta, Part B, 1998, 53, 945. 470 S. Akman and H. Ince Tekgul, Spectrochim. Acta, Part B, 1999, 54, 505. 471 I. L. Grinshtein, Y. A. Vilpan, L. A. Vasilieva and V. A. Kopeikin, Spectrochim. Acta, Part B, 1999, 54, 745. 472 G. M. Hieftje, (Dept. Chem., Indiana Univ., Bloomington, IN 47405, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 473 X. F. Li and S. L. Deng, Fenxi Ceshi Xuebao, 1999, 18(2), 56. 474 J. Sneddon, T. L. Thiem and Y. I. Lee, Lasers in Analytical Atomic Spectroscopy, John Wiley and Sons, New York, NY, USA, 1997, 0 471 18623 6, 288 pp. 475 P. N. Ghosh, Indian J. Theor. Phys., 1996, 44(4), 45. 476 I. B. Gornushkin, J. M. Anzano, L. A. King, B. W. Smith, N. Omenetto and J. D. Winefordner, Spectrochim. Acta, Part B, 1999, 54, 491. 477 A. Cuicci, M. Corsi, V. Palleschi, S. Rastelli, A. Salvetti and E. Tognoni, Appl. Spectrosc., 1999, 53(8), 960. 478 M. Sabsabi, M. Chaker and Y. V. Kaenel, (Ind. Materials Inst., Natl. Res. Council Canada, Boucherville, PQ, Canada J4B 6Y4). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 479 J. M. Vadillo, J. M. Fernandez Romero, C. Rodriguez and J. J. Laserna, Surf. Interface Anal., 1998, 26(13), 995. 480 J. M. Vadillo, C. C. Garcia, S. Palanco and J. J. Laserna, J. Anal. At. Spectrom., 1998, 13(8), 793. 481 J. M. Vadillo, M. Corral, C. C. Garcia and J. J. Laserna, (Dept.

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513

Anal. Chem., Fac. Sci., Univ. Malaga, 29071 Malaga, Spain). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. B. J. Marquardt, D. N. Stratis, D. A. Cremers and S. M. Angel, Appl. Spectrosc., 1998, 52(9), 1148. V. Bulatov, R. Krasniker and I. Schechter, Anal. Chem., 1998, 70(24), 5302. G. E. Potts, I. Gornushkin, S. Claggett, H. Nasjpour, B. W. Smith and J. D. Winefordner, (Dept. Chem., Univ. Florida, Gainesville, FL 32611-7200, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. X. Hou, P. Stchur, K. X. Yang and R. G. Michel, Trends Anal. Chem., 1998, 17(8±9), 532. G. Gergelyi and Z. Szilvassy-Vamos, ACHÐModels Chem., 1999, 136(1±2), 69. B. S. Duersch and P. B. Farnsworth, Spectrochim. Acta, Part B, 1999, 54(3±4), 545. B. L. Sharp, J. Batey, I. S. Begley, D. Gregson, J. Skilling, A. B. Sulaiman and G. Verbogt, J. Anal. At. Spectrom., 1999, 14(2), 99. W. Prewett and M. Promphutha, Spectrochim. Acta, Part B, 1999, 54, 571. A. A. Al-Ammar, R. K. Gupta and R. M. Barnes, J. Anal. At. Spectrom., 1999, 14(5), 801. R. M. Barnes, A. A. Al-Ammar and R. K. Gupta, (Dept. Chem. Lederle Graduate Res. Center Tower, Univ. Massachusetts, Amherst, MA 01003-4510, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. M. R. Anderson, A. T. Zander, P. V. Wilson and S. G. Hamilton, (Varian Assoc., Palo Alto, CA 94304, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. F. Li and J. Fan, Guangpuxue Yu Guangpu Fenxi, 1998, 18(5), 587. V. Karanassios, P. J. Drouin and G. A. Spiers, Spectrochim. Acta, Part B, 1998, 53, 1149. C. Webb, A. T. Zander, P. V. Wilson, G. Perlis and D. Shrader, (Varian Assoc., Palo Alto, CA 94304, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. J. Noelte, D. Yates, P. Krampitz and C. Schneider, (Environ. and Applied Inorg. Anal., Perkin Elmer Corp., Norwalk, CT 06859, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. Y.-S. Wang, T.-J. Huang and K.-L. Liu, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 49. D. Merten, J. A. C. Broekaert and A. Le Marchand, Spectrochim. Acta, Part B, 1999, 54, 1377. D. A. Sadler, P. R. Boulo, J. S. Soraghan and D. Littlejohn, Spectrochim. Acta, Part B, 1998, 53, 821. D. A. Sadler, D. Littlejohn, P. R. Boulo and J. S. Soraghan, Spectrochim. Acta, Part B, 1998, 53, 1015. M. H. Ramsey and M. Thompson, Analyst (London), 1985, 110, 519. H. Kola and P. Peramaki, At. Spectrosc., 1999, 20(4), 142. J. M. Mermet, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 46. K. Thaxton, J. W. Olesik and S. V. Olesik, (Lab. Plasma Spectrochem., Laser Spectrosc. and Mass Spectrom., Dept. Geol. Sci., Ohio State Univ., Columbus, OH 43210, USA). Presented at Pittcon.'99, Orlando, FL, USA, March 7±12, 1999. S. Klemenc, B. Budic and J. Zupan, Anal. Chim. Acta, 1999, 389(1±3), 141. G. Thiel, K. Danzer and M. Anke, (Ed.), ICP-emission spectrometric and multivariate statistical investigations of wine origin. Mengen-Spurenelem., Arbeitstag., 17th, Verlag Harald Schubert, Leipzig, Germany, 1997, 635±643. V. Lizama, J. L. Aleixandre, I. Alvarez and M. J. Garcia, Riv. Vitic. Enol., 1997, 50(4), 29. V. Zakrgynska-Fontaine, J.-C. Dore, T. Ojasoo, F. PoirierDuchene and C. Viel, Biol. Trace Elem. Res., 1998, 61(2), 151. Z.-Y. Zhang, S.-D. Liu, B.-J. Ding, Y.-L. Ren, H.-T. Chen and X.-J. Zeng, Gaodeng Xuexiao Huaxue Xuebao, 1998, 19(4), 530. S. M. Jiang, Y. K. Shi, X. Z. Chi and H. Z. Zhou, Fenxi Shiyanshi, 1999, 18(2), 42. B.-Y. Deng, Y. Y. Chan Flora, K. H. Choi Anton and W.T. Chan, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 66. F. Y. Y. Chan and W. T. Chan, (Univ. Hong Kong, Hong Kong). Presented at 12th International Symposium on High Performance Capillary Electrophoresis, Palm Springs, CA, USA, January 24±28, 1999. G. B. Jiang, F. Z. Xu and F. J. Zhang, Fresenius' J. Anal. Chem., 1999, 363(3), 256.

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554 T. P. Rao, S. Karthikeyan, B. Vijayalekshmy and C. S. P. Iyer, Anal. Chim. Acta, 1998, 369(1±2), 69. 555 T. Narukawa, J. Anal. At. Spectrom., 1999, 14(1), 75. 556 S. Garbos, M. Rzepecka, E. Bulska and A. Hulanicki, Spectrochim. Acta, Part B, 1999, 54, 873. 557 B. Godlewska-Zylkiewicz, B. Lesniewska and A. Hulanicki, Anal. Chim. Acta, 1998, 358(2), 185. 558 J. Chwastowska, W. Skwara, E. Sterlinska, J. Dudek and L. Pszonicki, Chem. Anal. (Warsaw), 1998, 43(6), 995. 559 K. Seshaiah and K. R. Mohan, J. Indian Chem. Soc., 1998, 75(6), 387. 560 X. F. Yin, W. Frech, E. Hoffmann, C. Luedke and J. Skole, Fresenius' J. Anal. Chem., 1998, 361(8), 761. 561 A. Sanz-Medel, Analusis, 1998, 26(6), M76. 562 T. Bantan, R. Milacic and B. Pihlar, Talanta, 1998, 47, 929. 563 T. Bantan, R. Milacic, B. Mitrovic and B. Pihlar, J. Anal. At. Spectrom., 1999, 14(11), 1743. 564 F. Li, W. Goessler and K. J. Irgolic, J. Chromatogr., A, 1999, 830(2), 337. 565 F. Li, W. Goessler and K. J. Irgolic, Sepu, 1999, 17(3), 240. 566 M. Potin-Gautier, N. Gilon, M. Astruc, I. De Gregori and H. Pinochet, Int. J. Environ. Anal. Chem., 1997, 67(1±4), 15. 567 M. M. Gomez, T. Gasparic, M. A. Palacios and C. Camara, Anal. Chim. Acta, 1998, 347(2±3), 241. 568 C. Hammel, A. Kyriakopoulos, U. Rosick and D. Behne, Analyst (Cambridge, U.K.), 1997, 122(11), 1359. 569 H. Emteborg, G. Bordin and A. R. Rodriguez, Analyst (Cambridge, U.K.), 1998, 123(5), 893. 570 D. A. Dantz, H.-J. Buschmann and E. Schollmeyer, GIT LaborFachz., 1998, 42(9), 909. 571 P. Costa, J. Stripeikis, M. Tudino and O. Troccoli, (Lab. Anal. Trazas, Dept. Quim. Inorg., Anal. y Quim. Fis., Fac. Ciencias Exactas y Naturales, Univ. Buenos Aires, Buenos Aires, Argentina). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4±10, 1998. 572 J. L. Gomez-Ariza, D. Sanchez-Rodas and I. Giraldez, J. Anal. At. Spectrom., 1998, 13(12), 1375. 573 M. C. Villa Lojo, E. Beceiro Gonzalez, E. Alonso Rodriguez and D. Prada Rodriguez, Int. J. Environ. Anal. Chem., 1997, 68(3), 377. 574 P. Bermejo-Barrera, J. Moreda-Pineiro, A. Moreda-Pineiro and A. Bermejo-Barrera, Anal. Chim. Acta, 1998, 374(2±3), 231. 575 C. F. Harrington, A. A. Ojo, V. W.-M. Lai, K. J. Reimer and W. R. Cullen, Appl. Organomet. Chem., 1997, 11(12), 931. 576 O. Munoz, D. Velez and R. Montoro, Analyst (Cambridge, U.K.), 1999, 124(4), 601. 577 X. Zhang, R. Cornelis, L. Mees, R. Vanholder and N. Lameire, Analyst (Cambridge, U.K.), 1998, 123(1), 13. 578 X. Zhang, R. Cornelis, J. De Kimpe, L. Mees and N. Lameire, Clin. Chem. (Washington, D.C.), 1998, 44(1), 141. 579 D. L. Tsalev, M. Sperling and B. Welz, Analyst (Cambridge, U.K.), 1998, 123(8), 1703. 580 R. Cornelis, X. Zhang, L. Mees, J. M. Christensen, K. Byrialsen and C. Dryschel, Analyst (Cambridge, U.K.), 1998, 123, 2883. 581 M. Krenzelok and P. Rychlovsky, Collect. Czech. Chem. Commun., 1998, 63(12), 2027. 582 A. Viitak and V. Lepane, (Tallinn Tech. Univ., Tallinn 0026, Estonia). Presented at Instrumental Methods of Analysis. Modern Trends and Applications (IMA '99), 19±22 September, 1999, Chalkidiki, Greece. 583 L. C. Robles, B. de Celis, J. M. Lumbreras and A. J. Aller, Anal. Commun., 1997, 34(12), 409. 584 A. J. Aller and L. C. Robles, Analyst (Cambridge, U.K.), 1998, 123(5), 919. 585 R. Lobinski, H. Chassaigne and J. Szpunar, Talanta, 1998, 46(2), 271. 586 K. Pomazal, C. Prohaska, I. Steffan, G. Reich and J. F. K. Huber, Analyst (Cambridge, U.K.), 1999, 124(5), 657. 587 R. Milacic, N. Kozuh and B. Mitrovic, Mikrochim. Acta, 1998, 129(1±2), 139. 588 L. Ebdon, M. Foulkes, K. Fredeen, C. Hanna and K. Sutton, Spectrochim. Acta, Part B, 1998, 53, 859. 589 R. Grumping and A. V. Hirner, Fresenius' J. Anal. Chem., 1999, 363(4), 347. 590 H. Emteborg, G. Bordin and A. R. Rodriguez, Analyst (Cambridge, U.K.), 1998, 123(2), 245. 591 Y. L. Feng, H. Narasaki, H. Y. Chen and L. C. Tian, Anal. Chim. Acta, 1999, 386(3), 297. 592 H.-B. Hou and H. Narasaki, Anal. Sci., 1998, 14(6), 1161. 593 O. T. Akinbo and J. W. Carnahan, Anal. Chim. Acta, 1999, 390(1±2), 217.

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594 R. K. Marcus, (Clemson University, Clemson, SC, USA). Presented at Pittcon '99, Orlando, FL, USA, March 7±12, 1999. 595 A. M. Dobney, H. Klinkenberg, C. de Koster and A. Mank, (Philips CFT, 5656 AA Eindhoven, Netherlands). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 596 J. L. Gomez-Ariza, E. Morales, D. Sanchez-Rodas and I. Giraldez, (Dept. Quim. Ciencia Materiales, Escuela Politecnica Superior, Univ. Huelva, 21819 Palos de la Frontera, Spain). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 597 J. L. Gomez-Ariza, D. Sanchez-Rodas, R. Beltran, W. Corns and P. Stockwell, Appl. Organomet. Chem., 1998, 12(6), 439. 598 A. M. Featherstone, E. C. V. Butler, B. V. O'Grady and P. Michel, J. Anal. At. Spectrom., 1998, 13(12), 1355. 599 S. Yalcin and X. C. Le, Talanta, 1998, 47, 787. 600 Z. Slejkovec, J. T. van Elteren and A. R. Byrne, Talanta, 1999, 49, 619. 601 Y. He, H. El Azouzi, M. L. Cervera and M. de la Guardia, J. Anal. At. Spectrom., 1998, 13(11), 1291. 602 E. Puskel, Z. Mester and P. Fodor, J. Anal. At. Spectrom., 1999, 14(6), 973. 603 J. L. Gomez-Aria, D. Sanchez-Rodas, I. Giraldez and E. Morales, (Dept. Quim. y Cienc. Materiales, Escuela Politecnica Superior, Univ. Huelva, Huelva, Spain). Presented at Instrumental Methods of Analysis. Modern Trends and Applications (IMA '99), 19±22 September, 1999, Chalkidiki, Greece. 604 W. Kautek, S. Pentzien, P. Rudolph, J. Kruger and E. Konig, Appl. Surf. Sci., 1998, 127, 746. 605 P.-N. Wang, Q. Pan, N. H. Cheung and S.-C. Chen, Appl. Spectrosc., 1999, 53(2), 205. 606 J. Nishimura and I. Fukui Jpn. Kokai Tokkyo Koho JP 11 101,748 [99 101,748] (Cl. G01N21/73), 13 Apr 1999, Appl. 97/ 259,680, 25 Sep 1997; 4 pp. 607 R. Wennrich, P. Morgenstern, G. Liebergeld and G. Werner, Chem. Anal. (Warsaw), 1999, 44(3B), 523. 608 C.-F. Chau Wilson, W.-T. Chan and H. H. C. Yip, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 67. 609 K. Ingeneri and W. W. Harrison, (Dept. Chem., Univ. Florida, Gainesville, FL 32611, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7±12, 1999. 610 D. A. Rusak, B. C. Castle, B. W. Smith and J. D. Winefordner, Trends Anal. Chem., 1998, 17(8±9), 453. 611 A. V. Pakhomov, A. J. Roybal and M. S. Duran, Appl. Spectrosc., 1999, 53(8), 979. 612 H.-J. Dang, M.-F. Zhou and Q.-Z. Qin, Appl. Spectrosc., 1998, 52(9), 1154. 613 H. Kurniawan and K. Kagawa, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 45. 614 H. Kurniawan, M. Pardede, K. Kagawa and M. O. Tija, Bunko Kenkyu, 1998, 47(5), 220. 615 W. S. Budi, H. Suyanto, H. Kurniawan, M. O. Tjia and K. Kagawa, Appl. Spectrosc., 1999, 53(6), 719. 616 I. B. Gornushkin, L. A. King, B. W. Smith, N. Omenetto and J. D. Winefordner, Spectrochim. Acta, Part B, 1999, 54, 1207. 617 C. Haisch, R. E. Neuhauser and U. Panne, (Inst. Hydrochem., Tech. Univ. Munich, 81377 Munich, Germany). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10±15, 1999. 618 H. E. Bauer, F. Leis and K. Niemax, Spectrochim. Acta, Part B, 1998, 53, 1815. 619 V. I. Povstugar, A. A. Shakov, S. S. Mikhailov, E. V. Voronina and E. P. Elsukov, J. Anal. Chem. (Transl. of Zh. Anal. Khim.), 1998, 53(8), 697. 620 U. Panne, C. Haisch, M. Clara and R. Niessner, Spectrochim. Acta, Part B, 1998, 53, 1957. 621 C. C. Dobson, Appl. Opt., 1999, 38(18), 3924. 622 A. P. Nefedov, V. A. Sinel'shchikov, A. D. Usachev and A. V. Zobnin, Appl. Opt., 1998, 37(33), 7729. 623 C. Moulin, C. Larpent and D. Gazeau, Anal. Chim. Acta, 1999, 378(1±3), 47. 624 S.-Y. Zhang, B.-L. Huang, Z.-B. Gong and P.-Y. Yang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 76. 625 B. W. Smith, I. B. Gornushkin, L. A. King and J. D. Winefordner, Spectrochim. Acta, Part B, 1998, 53, 1131. 626 B. W. Smith, A. Quentmeier, M. Bolshov and K. Niemax, Spectrochim. Acta, Part B, 1999, 54, 943. 627 W.-Y. Ma, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 1. 628 W.-Y. Ma, M. Xue, J. Zhang and D.-Y. Chen, Spectrochim. Acta, Part B, 1998, 53, 1421. 629 G. S. Knapp, M. A. Beno and K. J. Gofron, Proc. SPIE-Int. Soc.

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