Thermal desorption and atomic emission spectrometric determination ...

Analyst, April 1998, Vol. 123 (637–640)

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Thermal desorption and atomic emission spectrometric determination of adsorbable organically bound elements for water analysis† Heike Lehnert*, Tanja Twiehaus, Dietmar Rieping, Wolfgang Buscher and Karl Cammann University of Münster, Institut für Chemo- und Biosensorik eV (ICB), Mendelstr. 7, D-48149 Münster, Germany A new analytical method and system has been developed which can be used for the simultaneous determination of adsorbable organically bound elements (AOE) contained in a water sample. This new method of water analysis, by summary detection of substance categories, will significantly extend the existing water parameter for adsorbable organically bound halides (AOX). The organic water impurities are separated quantitatively along the lines of German standards DIN 38409, H14 through adsorption on activated charcoal. Simultaneous element-specific detection can then be carried out by plasma emission spectrometry. As a special innovation, the new method allows the use of highly sensitive plasma types (e.g., microwave-induced plasma) through thermal desorption of the analyte compounds because no interfering CO2 is generated. Here other sorbents of the kind needed for other problematic separation processes can also be used. The system is equipped with the possibility of removing gases containing solvent vapours. Compared with the conventional method, the thermal desorption step has no influence on the sensitivity or the precision of the determination of adsorbable organic model substances. The first calibration of the new system resulted in a detection limit of 0.20 mg of chlorine.

contained in a water sample. Using plasma atomic emission spectrometry, the sums of adsorbable organically bound chlorine (AOCl), bromine (AOBr) and iodine (AOI) can be measured element specifically instead of the AOX parameter, and in addition other adsorbable organically bound elements such as fluorine (AOF), sulfur (AOS) and phosphorus (AOP) can be determined. The plasma emission detector (PED) developed by Cammann and co-workers4,5 uses bandpass filters with narrow bandwidths instead of expensive poly- or monochromator systems. The filters perform a tilting oscillation against the optical path, and lock-in amplifiers for detection selectively in phase and frequency allow spectral background correction.4,5 In the past, the efficiency of determinations using an atmospheric-pressure capacitively coupled helium plasma (CMP) was limited by high loads of interfering CO2 and H2O.5 Replacing the combustion of the loaded charcoal by thermal desorption, the absence of CO2 now allows very sensitive determinations using an atmospheric-pressure helium microwave-induced plasma (MIP). Water can be separated by carefully drying at moderate temperatures in the carrier gas flow. For optimized sample input into the analytical plasma, the new method uses a very effective cryofocusing unit based on gas chromatographic techniques.

Keywords: Activated charcoal; thermal desorption; organically bound halides; organically bound elements; plasma emission detector; microwave-induced plasma

Experimental

In the area of water analysis, methods for the determination of sum parameters have been established to give a comprehensive survey of the total load with organic impurities quickly, without regard to individual substances. The AOX parameter, for example, stands for the sum of the adsorbable organically bound halides chlorine, bromine and iodine. Monitoring of the AOX parameter is compulsory under the German Effluent Charges Act.1 The respective organic water impurities are enriched on activated charcoal and combusted to hydrogenous halides, which are finally determined by microcoulometric titration with silver ions along the lines of DIN 38409, H14.2 The results are calculated as chlorine in mg l21. As usual for this screening method, the informational value is low and the detection limit of 10 mg l21 is high for the benefit of a fast and simple analysis.3 For routine analysis, it would be useful to develop a new analytical method that increases the analytical performance and the informational value of the sum parameter determination while keeping the speed and simplicity of a screening method. In this paper, a new analytical method and system will be introduced which can be used for the simultaneous determination of all adsorbable organically bound elements (AOE) †

Presented at the XXX Colloquium Spectroscopicum Internationale (CSI), Melbourne, Australia, September 21–26, 1997.

Instrumentation To optimize the desorption process and to determine its efficiency, the existing DIN method for AOX determination involving adsorption on activated charcoal, combustion in an oxygen gas flow and microcoulometric detection was extended by a desorption step. For this purpose, the Coulomat 7020CL (Ströhlein, Karst, Germany) was modified by subdivision of the quartz furnace into a combustion part with an oxygen gas flow of 200 ml min21 and a preceding desorption part using an additional inert gas flow (nitrogen or helium) of 50 ml min21, as shown schematically in Fig. 1. The Coulomat 7020CL was also used as comparative device with an oxygen gas flow of 150 ml min21. For atomic emission spectrometric measurements, an MIP tangential dual flow torch with a helium main flow of 100 ml min21 and a helium tangential flow of 200 ml min21 (Analysentechnik Feuerbacher, Tübingen, Germany) was used. A model 24-502 DR microwave generator (Analysentechnik Feuerbacher) was operated with an output of 100 W. For detection of the emission lines, the PED was equipped with interference filters (Omega Optical, Brattleboro, VT, USA) for detection of the halides, sulfur and phosphorus. The characteristic data of these filters are given in Table1 together with the optimum filter parameter. The interference filters perform a tilting oscillation against the optical path to obtain background correction.4,5 The frequency is 20 Hz and the amplitude is ±6°. LIV392 lock-in amplifiers (Ithaco, New York, USA) were applied for detection selectively in phase and frequency.

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As the desorption furnace, a quartz tube (200 mm 3 14 mm id) heatable in an I-05/RP/DIG IR-furnace (Ströhlein) was applied. The construction of the new device, the AOE spectrometer, including the desorption furnace, a new cryofocusing unit (ICB, Münster, Germany), the MIP tangential dual flow torch, the PED and a new controlling unit (ICB), is shown schematically in Fig. 2. After desorption in a helium carrier gas flow of 100 ml min21, the organic substances are focused in an inert fused silica capillary (275 mm 3 0.53 mm id), which is supported in an electrically heatable steel capillary (185 mm 3 14 in id) with electric contacts at both ends. The steel capillary is supported in a PTFE tube (150 mm 3 20 mm od) that can be cooled by liquid nitrogen. The organic substances are then fed directly into the plasma by means of the fused silica capillary. The temperature of the cryofocusing unit is controlled by means of a Type 2 ABB 25 NN Ni–Cr–Ni-thermocoax (Philips, Suresnes, France). During the desorption step the organic substances are frozen out by cooling the unit down 2170 °C. In the second step the unit is heated to 300 °C to carry the now gaseous substances directly into the plasma for element-specific determination. The liquid nitrogen flow, the heating current and the desorption gas flow are controlled by the new microprocessor unit T-Desco (ICB). Procedure All measurements were made using the column method.2 Samples of 50 ml are pumped through two columns in series which are each filled with 50 mg of activated charcoal (Merck, Darmstadt, Germany) operating with a sample flow of 3 ml min21. In a second step, the columns are rinsed with a nitrate-containing rinsing solution to displace inorganic anions.

The loaded charcoal is then transferred into the desorption furnace or into the combustion furnace for comparative measurements. The combustion temperature is 950 °C. The combustion gases are dried using concentrated sulfuric acid (Merck) and then transferred into the microcoulometric titration cell where the halides are precipitated by electrolytically generated silver ions. The amount of converted halides can be calculated according to the Faraday equation:

m=

qM F

(1)

with electric charge q = ∫Idt, electric current I and time t; M is the molar mass of the halide and F is the Faraday constant. Preparation of Solutions The nitrate-containing rinsing solution was prepared by dilution of 50 ml of parent solution to 1 l. For the preparation of the parent solution, 17 g of NaNO3 (Merck) were dissolved in highpurity water. After the addition of 1.4 ml of concentrated nitric acid (Merck), the solution was diluted to 1 l. The high-purity water was produced using a PRO 90 CN system (Seralpur, Ransbach, Germany). For the preparation of the standard solutions, parent solutions were also used. The following weighed amounts of non-volatile model substances (Merck) were dissolved in and diluted to 1 l with high-purity water: p-ClC6H4OH (M = 128.56 g mol21), 725.303 mg; and CCl3COOH (M = 163.39 g mol21), 921.805 mg. Thus parent solutions were prepared containing 200.0 mg l21 of chlorine. In each case eight standard solutions containing 250, 200, 150, 100, 70, 50, 30 and 20 mg l21 of chlorine were prepared by diluting the parent solution with high-purity water. To prepare standard solutions containing 10 and 5 mg l21 of chlorine, the 200 mg l21 standard solution was diluted appropriately. For the preparation of parent solutions of volatile model substances, the use of sealed vials was advisable. To obtain a parent solution containing 1 g l21 of chlorine, 26 ml of 2-ClC3H5

Fig. 1 Coulomat 7020CI (Ströhlein, Karst, Germany) for microcoulometric measurements. 1 Combustion of loaded charcoal. 2 Thermal desorption and combustion of adsorbable organic substances.

Table 1 Interference filter data

Element Fluorine (685.60 nm) Chlorine (479.46 nm) Bromine (478.55 nm) Iodine (533.82 nm) Sulfur (921.29 nm) Phosphorus (253.56 nm)

Filter central wavelength/ nm 685.3 479.3 478.6 534.1 921.5

Filter halfbandwidth/nm 0.3 0.4 0.3 0.3 0.4

Filter maximum transmission (%) 33 22 29 21 33

Irradiation angle (°) 3.6 3.4 8.0 4.0 3.0

253.6

0.9

10

9.0

Fig. 2 AOE spectrometer (ICB, Münster, Germany) for emission spectrometric measurements of adsorbable organic substances.


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Table 2 Chlorinated model substances: physico-chemical properties and desorption behaviour Molar mass/g mol21 78.54 128.56 163.39

Compound 2-ClC3H7 p-ClC6H4OH CCl3COOH

Boilingpoint/°C

Water solubility/g l21

Dipole moment/D

Maximum drying temperature/°C

Minimum desorption temperature/°C

35 217 198

3.1 27 1200

1.65 2.11 Acid

230 350 230

400 750 800

(Merck) were injected into a sealed vial containing 10 ml of high-purity water. Subsequently the injected amount had to be determined by weighing. In this work, weighing resulted in 2-ClC3H5 (M = 78.54 g mol21), 22.155 mg. Thus, a parent solution was prepared containing 1.0001 g l21 of chlorine. Ten standard solutions containing 250, 200, 150, 100, 70, 50, 30, 20, 10 and 5 mg l21 of chlorine were prepared by diluting the parent solution with high-purity water.

Method DIN 38409

Statistics

Desorption

The results of microcoulometric detection are available as absolute masses in micrograms. In contrast, the spectrometric detection results are given as the output voltage of the amplifier unit in volts (here, peak height) and have to be calibrated using standard solutions. For the determination of the analytical performance in each case 10 standard solutions were measured and the data were fitted using linear regression. In one case quadratic regression fitted significantly better, thus the linear working range was limited by Mandel’s test.6 Function coefficients a, b and c are given for the description of the fitting functions. The residual standard deviation, sy, is given as a measure of the precision. The 95% confidence interval was calculated for all fitting functions and the detection limit xNG was calculated as

x NG =

sy b

t f;p

x2 1 + n Σ( x i − x ) 2

(2)

with the Student factor tf;p and p = 95%.7 Results and discussion Drying and thermal desorption of chlorinated model substances To characterise the desorption behaviour of the great variety of possible organic water impurities, chlorinated model substances were chosen, covering different physio-chemical properties. By means of a measuring series with a desorption time of 15 min, the recoveries of the model substances were determined as functions of desorption temperature using microcoulometric detection. The resulting maximum drying temperatures and minimum desorption temperatures are given in Table 2. As the maximum drying temperatures of 2-ClC3H7 and CCl3COOH are below room temperature, loss-free removal of interfering residual water can be achieved based on freezedrying.8 To save time and energy, drying in the helium carrier gas flow in the desorption furnace at moderate temperatures is desirable. Therefore, the desorption device is equipped with a magnetic valve for removing gases containing solvent vapours. When using the latter method, the samples must not contain volatile impurities. This can be achieved by means of prior detection of the volatile components, the purgeable organically bound halides (POX), in the same sample solution along the lines of DIN 38409, H25.9 The prior detection of the POX parameter is recommended for AOX determination anyway.2 At temperatures higher than 900 °C, crystallisation of the quartz furnace increases. Therefore, the optimum desorption

Table 3 Efficiency of the desorption process: microcoulometric determination of AOX Function coefficients Residual standard Detection b/mg mg21 deviation, limit, xNG/mg Compound a/mg (recovery) sy/mg 0.19 0.677 0.13 0.40 2-ClC3H7 p-ClC6H4OH 0.35 0.997 0.15 0.31 0.30 0.980 0.11 0.23 CCl3COOH 2-ClC3H7 0.65 0.633 0.13 0.51 p-ClC6H4OH 0.16 0.984 0.16 0.33 0.26 0.984 0.24 0.50 CCl3COOH

temperature was set at 850 °C. Higher temperatures do not result in acceleration of the desorption process. Efficiency of the desorption process A comparison of the efficiency of the method including the additional desorption step with that of the existing DIN method is given in Table 3. The data demonstrate that the thermal desorption step has no influence on the sensitivity or the precision of the determination of the adsorbable organic model substances. Efficiency of AOE determination of AOE by atomic emission spectrometry The first calibration using the new AOE spectrometer showed results demonstrating its good analytical performance. Charcoal loaded with p-ClC6H4OH was dried in the carrier gas flow at 300 °C for 5 min and the substance was then desorbed at 850 °C for another 5 min and cryofocused before being fed into the plasma. The result is a quadratic working range of 0.4–25 mg of chlorine (a = 14.8 V, b = 27.8 V mg21, c = 21.31 V mg22) with a residual standard deviation sy of 0.19 mg and a detection limit of 0.20 mg. The linear working range is limited to 2.5 mg (a = 14.2 V, b = 1.32 V mg21). Conclusion In this paper the first results obtained with a new AOE spectrometer have been presented. As a special innovation, the thermal desorption step not only allows the use of highly sensitive plasma types but moreover opens up the possibilities of using other sorbents of the kind needed for other problematic separation processes. These results form part of our dissertation. The authors thank the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) for funding the project no. 02WA9461/6. References 1

Gesetz u¨ ber Abgaben für das Einleiten von Abwasser in Gewässer (Abwasserabgabengesetz), Bundesgesetzblatt I 880, 1987. 2 Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung DEV, DIN 38409, H14, Säulenmethode, 1985.

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Gewässergütekriterien, ed. Frimmel, F. H., and Gordalla, B. C., VCH, Weinheim, 1996, Vol. 13, pp. 51–677. Camman, K., and Buscher, W., Ger. Pat. 4 309 045, 1997. Rosenkranz, B., Breer, C. B., Buscher, W., Bettmer, J., and Cammann, K., J. Anal. At. Spectrom., 1997, 12, 993. Statistik in der analytischen Chemie, ed. Doerffel, K., Deutscher Verlag für Grundstoffindustrie, Leipzig, 5th edn., 1990. Deutsche Norm DIN 32645, 1994.

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Twiehaus, T., Edel, H., Buscher, W., and Cammann, K., Wasser, 1997, 89, 49. 9 Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung DEV, DIN 38409, H25, 1989.

Paper 7/07109D Received October 1, 1997 Accepted December 4, 1997