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determination of mercury by cold vapour-atomic absorption spectrometry. Sandra Gil, Marta ... consumption to 2 μg LА1 in accordance with the primary drinking.
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www.rsc.org/methods | Analytical Methods

On-line UV photoreduction in a flow-injection/stopped-flow manifold for determination of mercury by cold vapour-atomic absorption spectrometry Sandra Gil, Marta Costas, Franciso Pena, Inmaculada De La Calle, Noelia Cabaleiro, Isela Lavilla and Carlos Bendicho* Received 20th May 2010, Accepted 3rd August 2010 DOI: 10.1039/c0ay00326c Photo-chemical vapour generation has been applied in a flow-injection system under stopped-flow conditions for determination of Hg by atomic absorption spectrometry. The system allows mercury vapour generation without the need for conventional reduction reactions based on sodium/potassium tetrahydroborate (III) or tin chloride in acid medium. The photo-induced reaction is accomplished by applying ultraviolet irradiation (UV) to the sample solution containing Hg(II) in the presence of an organic acid (i.e., acetic, citric, oxalic, ethylendiaminetetraacetic) as precursor of reducing species. A remarkable improvement in sensitivity is observed with acetic acid when stopped-flow is employed as compared to continuous operation, meaning that kinetics play an important role in the photo-induced reaction. A detection limit of 0.3 mg L1 can be obtained, which represents a 6-fold improvement in respect to that obtained without stopped-flow. The repeatability expressed as relative standard deviation was about 2.7% (n ¼ 15) for a 50 mg L1 Hg standard. The effect of potential interferences on the photo-generation of Hg vapour was investigated. In the UV-photo-induced CVG, both inorganic Hg and organomercury species can be reduced to elemental mercury with the same efficiency. The method was applied to determination of Hg in several enriched natural water samples and recoveries of MeHg+, Thiomersal, EtHg+ and PhHg+ were in the range 97% to 102%.

Introduction Mercury pollution is recognized as a primary environmental issue and public health problem.1 A great variety of industrial uses involve Hg and its compounds, e.g., as catalysts in the chloralkali industry, preservatives and pigments in the paint industry, pesticides, pulp and paper industry, battery production, etc.2 Additionally, toxicity of Hg can be increased upon transformations occurring in the environment.3 Many international guidelines have limited maximum allowable contents of Hg in water and food. Thus, the U.S. environmental protection agency (EPA) has limited Hg concentration in water for human consumption to 2 mg L1 in accordance with the primary drinking water standard,4 while the European Community has indicated a maximum Hg concentration of 1 mg L1 in drinking water.5 Owing to the extremely low concentration of Hg in natural water, environmental and biological samples, very sensitive analytical methods are often required for accurate determination. The most used technique worldwide for the determination of mercury is cold vapour generation (CVG) with detection by atomic absorption spectrometry (AAS) or atomic fluorescence spectrometry (AFS),6 because of its simplicity, high sensitivity, and freedom from interferences. Hg(II) can be effectively converted into Hg(0) by chemical reductants such as stannous chloride or sodium tetrahydroborate (III), and subsequently

 Departamento de Quımica Analıtica y Alimentaria, Area de Quımica Analıtica, Facultad de Quımica, Universidad de Vigo, Campus As Lagoas-Marcosende s/n, 36310 Vigo, Spain. E-mail: [email protected]; Fax: +34-986-812556; Tel: +34-986-812281

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swept by a carrier gas into the detection system for determination. Main disadvantages include chemical interferences from transition metals, unstable reductant solution, vigorous chemical reactions that can result in liquid transport to the atomization cell and significant waste production. Development of new vapour generation systems that may replace or diminish the use of chemical reagents remains an attractive research topic.7 An alternative to chemical reduction is the use of photoreduction, based on the exposure of a sample to UV radiation.8 Photo-CVG may provide a powerful alternative to conventional CVG owing to its simplicity, versatility, and cost-effectiveness. Photo-induced CVG eliminates the need for freshly prepared chemical reductant, can simplify the flowinjection manifold by removing a reductant line and should lead to a smoother, less violent reduction process. Photo-induced CVG can be carried out by exposure of Hg(II) (and MeHg+) solutions to UV irradiation in the presence of low molecular weight organic acids with or without a photocatalyst such as TiO2,9–17 although other precursors such as mercaptoethanol,18 ethanol,19 aldehydes,20 etc have also been tried. So far, UV photoreduction for total Hg determination has been performed in batch and flow systems coupled to a variety of detection systems such as atomic fluorescence spectrometry,10,16–20 inductively-coupled plasma mass spectrometry,9,15 quartz tubeatomic absorption spectrometry11,13,14 and graphite furnaceatomic absorption spectrometry.12 Advantages of flow systems include automation and increased sample throughput. So far, continuous operation has been mostly employed in flow systems for photo-CVG of mercury vapours but flow injection has been scarcely investigated. Regarding the latter approach, It is clear that the residence time of the sample zone in the flow-through This journal is ª The Royal Society of Chemistry 2010

Fig. 1 Schematic diagram of the flow-injection/stopped-flow manifold including a flow-through photoreactor for on-line generation of Hg vapour.

photoreactor should largely influence the sensitivity and detection limit achieved, since the dose of UV radiation received determines the extent of radical formation and, in turn, the efficiency of UV reduction ionic Hg species. In earlier work, the authors have demonstrated the feasibility of on-line manifolds with flow-injection/stopped-flow (FI/SF) operation for photo-CVG of Se vapours.21 In this work, UV irradiation of Hg(II) solutions containing several organic acids (i.e., acetic, oxalic, citric and ethylendiaminetetraacetic) as precursors of reducing species was investigated for the determination of Hg by FI/SF-photo-CVGAAS. Stopped-flow conditions were employed with the aim of increasing the time for which the sample zone was subjected to UV irradiation while being transported to the detector.

Experimental Apparatus A Thermo Electron Corporation series M5 atomic absorption spectrometry (Cambridge, UK) equipped with deuterium background corrector and a quartz cell located in the optical path was employed. A wavelength of 253.7 nm and a spectra bandpass of 0.5 nm were employed. Volatile mercury was generated using the system shown in Fig. 1. The photo-induced reduction process was accomplished with a flow-through photoreactor (Model 10.570) from PS Analytical (UK), consisting of a 12 m PTFE tubing of dimensions 0.8 mm i.d.  1.2 mm o.d. wrapped around a low-pressure Hg lamp (78 W power). The organic acid carrier stream was pumped through the photoreactor by a Gilson Model Minipuls 3M peristaltic pump. Flow injection operation was performed with a Chemiert model C12 eight-port external volume sample injector (Valco instruments Co., Houston, USA) incorporating a 200 mL capacity loop. Separation of volatile compounds was performed with a Perkin-Elmer PTFE € membrane gas-liquid separator (Uberlingen, Germany). High purity Ar was employed to carry the Hg vapour from the gas-liquid separator to the quartz cell. The Ar flow-rate was controlled by a flow-meter from Fisher and Porter (Warminster, PA). This journal is ª The Royal Society of Chemistry 2010

Reagents All chemicals were of analytical-reagent grade. A Hg(II) stock standard solution (1000 mg L1) was obtained from Hg(NO3)2 (Panreac). Ultrapure water from a PETLAB water purifier system (Peter Taboada) was used. Thiomersal (i.e., ethylmercurithiosalicylate) (Sigma), methyl-, ethyl- and phenylmercury chlorides (Riedel de Ha€en) were used to evaluate the FI/ SF-photo-CVG-AAS system for organomercurials. A stock solution of 500 mg L1 thiomersal was prepared in ultrapure water. Stock solutions of MeHg+, EtHg+ and PhHg+ were prepared in methanol–water mixture at 500 mg L1 (as Hg) concentration. All stock solutions were protected against light and stored at 4  C in the dark. The following chemicals were employed for the interference study: NaCl, Na2CO3, CaCl2, MgCl2$6H2O, Mg(NO3)2$6H2O, Fe(NO3)3$9H2O, CuCl2$2H2O, CrCl3$6H2O from Merck; KCl from Prolabo; KNO3 and NiCl2 from Probus; MnCl2$4H2O and CoCl2$6H2O from Scharlau; Mn(NO3)2$4H2O, Ni(NO3)2$6H2O, Cd(NO3)2$4H2O, K2CrO4, and Pb(NO3)2 from Panreac; Cr(NO3)3 and CaCO3 from Sigma-Aldrich; humic acid from Fluka Chemica. Organic acids used like precursors of reducing species in the photogeneration of Hg vapours were the following: acetic, citric and oxalic acids from Merck and ethylenediaminetetraacetic acid from Prolabo. Other chemicals used were K2Cr2O7, NaOH, HCl, and HNO3 (Prolabo). High purity Ar was employed to sweep out the Hg vapour to the quartz cell.

Procedure A schematic diagram of the FI/SF-photo-CVG-AAS system is depicted in Fig. 1. Determination of Hg by FI/SF-photo-CVGAAS was carried out according to the following procedure: the carrier was constantly pumped through the flow-through photoreactor and a 200 mL volume of sample was loaded in the injection valve to fill the loop. In the ‘inject’ position, the sample was introduced into the carrier stream. The sample zone was arrested inside the flow-through photoreactor after the sample injection for a variable time depending on the organic acid employed. An Ar stream flushed the Hg vapour into the Anal. Methods, 2010, 2, 1798–1802 | 1799

Fig. 2 Effect of the stopped-flow time on the Hg absorbance using different organic acids precursors for photogeneration of Hg vapour. (50 mg L1 Hg for all acids; the concentration of each acid was: acetic, 1.2 mol L1; EDTA, 0.125 mol L1; citric, 0.4 mol L1; oxalic, 0.4 mol L1).

gas-liquid separation unit, which was connected to the quartz cell by PTFE transfer tubing. Wastes were withdrawn from the gas-liquid separator using the same peristaltic pump. Atomic absorption profiles could be integrated within 100 s. The atomic absorption signal was recorded and the peak height was used for quantification. Spiked natural freshwater were made in order to evaluate the proposed method. The final concentration in the samples was 50 mg L1 Hg. Water samples (except mineral water and CRM NWTM-27.2) were filtrated through 0.45 mm cellulose nitrate filters (Sartorius) and stored at 4  C. Optimal conditions using acetic acid as carrier were the following: a 16.5 mL min1 acid carrier stream flow-rate, a 100 mL min1 argon flow-rate, a 150 s stopped-flow time and a 2.5 mol L1 acetic acid concentration.

Results and discussion Effect of instrumental parameters and reaction conditions on UV-photo-induced cold vapor generation When applying on-line treatments in flow-injection manifolds, stopped-flow methods can be implemented so that kinetically slow reactions can be used. Stopped-flow conditions allow the sample zone to be UV-irradiated for the required time with minimum dispersion. All optimization studies were performed with a 50 or 100 mg L1 Hg(II) aqueous standard. The stopped-flow time was studied between 0 and 300 s. Results showed that improved sensitivity is generally reached using stopped-flow (Fig. 2). Stopped-flow times of 120, 150 and 240 s are needed to obtain maximum Hg absorbance with oxalic, acetic and ethylendiaminetetraacetic acids, respectively. No effect is observed with citric acid when stopped-flow is implemented. The most outstanding effect is observed with acetic acid. Peak absorbance is increased about six times as compared to experiments without stopped-flow. However, slight improvement is observed with the rest of the acids. Photoreduction kinetics have not been taken into account in previous work with the acetic acid system regarding the photogeneration of Hg vapour in online systems, but it is clear from these results that a noticeable enhancement in absorbance is achieved when the sample zone is subjected to longer UV irradiation times. If the manifold is 1800 | Anal. Methods, 2010, 2, 1798–1802

Fig. 3 Effect of organic acid concentration used as carrier on the Hg absorbance following photogeneration of Hg vapour (50 mg L1 for acetic acid and 100 mg L1 for the remaining acids. The stopped-flow times employed with each acid were: acetic, 150 s; EDTA, 240 s; citric, no stopped-flow; oxalic, 120 s).

employed in the flow-injection mode without stopped-flow and with lower flow-rates for the carrier in an attempt to increase the residence time, wide atomic absorption signals are obtained, which are difficult to integrate within the measurement time. So, sensitivity is drastically decreased. This is due to the increased dispersion occurring in such manifold, which counteracts the positive effect of the increased residence time of the sample zone inside the photoreactor. The influence of the acid concentration is shown in Fig. 3. Differences between optimal acid concentrations required with each acid are evident. While a maximum is reached with ca. 2.5 mol L1 for acetic acid, much smaller concentrations were required with citric, oxalic and EDTA. It is remarkable to observe that only 0.05 mol L1 of EDTA, 0.2 mol L1 of citric and 0.2 mol L1 of oxalic acid concentration are needed to obtain an optimum photoreduction efficiency, although poor precision is observed for the above acids when low acid concentration is used. The effect of the quartz cell temperature on the Hg absorbance when the photoreduction is performed in acetic acid medium was studied in the range 25-900  C (Fig. 4). Best results were obtained without heating the quartz cell, meaning that Hg(0) was the Hg species reaching the measurement cell. As can be observed in Fig. 5, the effect of Ar flow-rate on peak absorbance was found to be critical. The Ar flow-rate was varied between 40 and 100 mL min1. The flow-rate chosen for further experiments was 100 mL min1. Low flow-rates provided smaller

Fig. 4 Effect of the quartz cell temperature on the Hg absorbance.

This journal is ª The Royal Society of Chemistry 2010

Fig. 6 Interference effect of HCl (B) and HNO3 (:) on the photogeneration of Hg vapour.

Fig. 5 Effect of the Ar flow-rate on Hg absorbance.

and wider peaks that could not be integrated within the measurement time. Effect of foreign substances Under stopped-flow conditions, the effects of several coexisting ions and humic acid on the photogeneration of Hg vapour with the acetic acid system was investigated. The effect of potential interferents over the absorbance corresponding to a 50 mg L1 Hg(II) solution can be observed in Table 1. An interference effect is considered to be significant when it is beyond 10%. Significant depressive interferences were observed in the presence of Na2CO3, CoCl2$6H2O, CuCl2$2H2O and humic acid. Little effect was caused by KNO3, Mg(NO3)2$6H2O, Table 1 Effect of different substances on the photogeneration of Hg vapour in the FI/SF-photo-CVG-AAS system

Interferent

Interferent concentration/ mg L1

Effect (%)

Na2CO3 NaCl KNO3 KCl Mg(NO3)2$6H2O MgCl2$6H2O CaCO3 CaCl2 CrCl3$6H2O Cr(NO3)3 K2CrO4 MnCl2$4H2O Mn(NO3)2$4H2O CoCl2$6H2O Cd(NO3)2$4H2O Pb(NO3)2 Ni(NO3)2$6H2O NiCl2 CuCl2$2H2O Fe(NO3)3$9H2O Humic acid Humic acid Humic acid

100 100 100 100 100 100 100 100 10 10 10 10 10 10 10 10 10 10 10 10 0.1 1 10

24.89 +12.06 +6.99 +15.13 +0.90 +20.63 +17.52 +16.26 +4.72 +0.89 4.00 +7.16 +6.20 22.42 0.82 6.02 +4.20 +15.96 42.72 3.12 21.24 46.90 54.87

This journal is ª The Royal Society of Chemistry 2010

CrCl3$6H2O, Cr(NO3)3, K2CrO4, MnCl2$4H2O, Mn(NO3)2$4H2O, Zn, Cd(NO3)2$4H2O, Pb(NO3)2, Ni(NO3)2$6H2O and Fe(NO3)3$9H2O, whereas an enhancement effect was observed for NaCl, KCl, MgCl2$6H2O, CaCO3, CaCl2 and NiCl2. The interference effect owing to HCl and HNO3 was examined in the range 0–5% v/v (Fig. 6). Results showed that whereas no interference effect was observed for HNO3, the absorbance signal dropped to almost 0 with HCl. HCl caused an interference effect at a concentration as low as 0.1% v/v. This could be due to the stabilization of Hg(II) in the form of chloride complexes that would prevent Hg(II) photoreduction. UV-photo-induced cold vapor generation from several Hg species In the UV-photo-induced CVG, both inorganic Hg and organomercury species can be reduced to elemental mercury. It was checked that standard solutions (50 mg L1 as Hg) of thiomersal, methyl-, ethyl- and phenyl-mercury chlorides, yielded a similar absorbance as compared to a Hg(II) solution. A recovery study showed that complete Hg recovery could be obtained for a variety of Hg species (Table 2). This is a noteworthy finding, since analytical signals for Hg using conventional reducing systems in the cold vapour technique typically depend on the Hg species. Analytical figures of merits Under optimal conditions, the calibration function was linear at least up to 100 mg L1 Hg(II) (r ¼ 0.999). The repeatability expressed as relative standard deviation for N ¼ 15 measurements of a 50 mg L1 Hg(II) solution was 2.7%. The limit of detection (LOD) (calculated as 3 s) with stopped-flow was 0.3 mg L1 of Hg. When no stopped-flow was used, an LOD of about 1.8 mg L1 of Hg was obtained. The latter LOD is similar Table 2 Recovery of Hg from solutions spiked with different Hg species

Hg species

Hg spiked/ mg L1 (as Hg)

Hg recovered/ mg L1 (as Hg)

Recovery (%)

MeHg+ Thiomersal EtHg+ PhHg+

50 50 50 50

48.5 51.2 50.9 48.9

97 102 102 98

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Table 3 Determination of Hg in spiked natural waters by FI/SF-photoCVG-AASa

Sample

Hg(II) spiked/ mg L1

Hg recovered/ mg L1

Recovery (%)

Drinking water Spring water Tea River Mineral water (Fontecelta) Dam water (Zam ans) River water (Zam ans) CRM NWTM-27.2

50 50 50 50 50 50 50

50.5 50.4 46.3 50.8 47.6 48.1 51.4

101.6 100.9 92.7 101.6 95.2 96.2 102.9

a

Each result represents the mean of three measurements.

to that reported elsewhere for the determination of mercury by photo-CVG-AAS with acetic acid using flow injection without stopped-flow.13 The LOD reported here with our FI/SF-photoCVG-AAS system is enough to meet the requirements of regulations, where maximum limits in drinking water are 1–2 mg L1 Hg. The achievement of lower LODs would require preconcentration, e.g., amalgamation, or the use of more sensitive techniques such as atomic fluorescence spectrometry. Application to Hg determination in natural waters The developed method was applied to the analysis of seven water samples: drinking water, spring water, river water (Tea River, Ponteareas), mineral water (Fontecelta), dam water (Zam ans), river water (Zamans) and CRM NWTM-27.2 (Ontario Lake) (without Hg). All samples, except the mineral water and CRM, were collected in the north-west of Spain. Blanks were run together with the samples. Analytical results for the water samples are shown in Table 3. The concentration of mercury was below the LOD in all cases. Therefore, spiking experiments were performed in order to test the usefulness of the proposed method. Results showed good recoveries, in the range of 93–103%. The low recovery found in the water from the Tea River could be due to the presence of natural organic matter. This interference effect was overcome following calibration by the standard addition method, which yielded a Hg recovery of 98% for that sample.

Conclusions Photoassisted reduction of Hg species to yield Hg (0) can be performed in a FI/SF-photo-CVG-AAS system without the need for a chemical reducing agent and a mineral acid at high concentration. Stopped-flow operation overcomes the occurrence of kinetically-slow photoreduction reactions, thus improving the LOD when flow-injection systems are employed. This approach allows larger doses of UV radiation to be

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provided to the sample zone in the flow-through photoreactor. Remarkably, the same photoreduction efficiency is observed regardless of the Hg species injected in the FI/SF manifold. This approach can be applied as a simple, low-cost and ecofriendly technique for determination of Hg(II) in natural freshwater samples in the presence of a low molecular weight organic acid. Best results were obtained using acetic acid in comparison with oxalic, citric and ethylenediaminetetraacetic acids.

Acknowledgements Financial support from the Spanish Ministry of Science and Innovation (projects CTQ2006-04111/BQU and CTQ200906956/BQU) and the Vigo University (Contract for Reference Research Groups 09VIA08) are gratefully acknowledged.

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This journal is ª The Royal Society of Chemistry 2010