Anal. Chem. 1998, 70, 395-399
Determination of Methylmercury in Natural Water Samples by Steam Distillation and Gas Chromatography-Atomic Fluorescence Spectrometry Karl C. Bowles†,‡ and Simon C. Apte*,†
CSIRO Division of Coal and Energy Technology, Centre for Advanced Analytical Chemistry, Private Mail Bag 7, Bangor, NSW 2234, Australia, and CRC for Freshwater Ecology, University of Canberra, P.O. Box 1, Belconnen, ACT 2616, Australia
Steam distillation was evaluated as a technique for the separation of methylmercury from natural water samples prior to quantification by GC-atomic fluorescence spectrometry. Recoveries of methylmercury chloride spikes ranged from ∼100% in a wide variety of natural freshwater and estuarine samples to 80% in seawater. The addition of ammonium pyrrolidine dithiocarbamate (APDC) was found to improve the recovery of methylmercury chloride spikes in MilliQ water from 73 to 89% and in seawater from 80 to 85%. Codistillation of inorganic mercury was eliminated by addition of APDC to the samples. Precision in MilliQ water was 2.0% RSD at 0.2 ng‚L-1 CH3HgCl (n ) 10) and 1.6% RSD at 2.0 ng‚L-1 CH3HgCl (n ) 10). The limit of detection for the method was 0.024 ng‚L-1 (3 σ) for a 50 mL sample. The steam distillation procedure was tested for and found free of measurable artifactual formation of methylmercury. The method was compared to nitrogen-assisted distillation and found to give comparable results with the added advantage of handling sample sizes up to 100 mL. Compared to previously employed separation procedures, steam distillation offers the advantages of robustness and a considerably increased sample throughput (at least four samples per hour) without any compromise in analytical performance. Current concerns regarding the effects of mercury in the environment have led to a rapid increase in research in this field. Understanding the environmental issues is underpinned by the need for precise and sensitive techniques that are able to accurately quantify mercury at the extremely low concentrations in which it frequently occurs. The analysis of methylmercury, in particular, is necessary due to its bioconcentration from sub-partper-trillion levels in natural waters to part-per-million levels in fish and other aquatic organisms.1,2 * Corresponding author: e-mail,
[email protected]; fax, 61-2-9710 6837. † Centre for Advanced Analytical Chemistry. ‡ University of Canberra. (1) Westo¨o¨, G. Acta Chem. Scand. 1967, 21, 1790-1800. (2) Bloom, N. S.; Effler, S. W. Water Air Soil Pollut. 1990, 53, 251-265. S0003-2700(97)00826-3 CCC: $15.00 Published on Web 01/15/1998
© 1998 American Chemical Society
Gas chromatography (GC) of mercury chloride species with electron capture detection has been used to quantify mercury species in natural samples.1 This approach is problematic in that the detection method involves detecting the chloride associated with mercury species, which necessitates the application of rigorous and tedious solvent cleanup procedures. Rapsomanikis et al.3 developed a GC-atomic absorption spectrometry method to quantify mercury and lead species involving aqueous-phase derivatization of the mercury species with sodium tetraethylborate (STEB). The resulting dialkylmercury species are more easily chromatographed than the chlorides, and element-specific mercury detection was a significant improvement on electron capture detection of the associated chloride. Bloom4 developed a method using aqueous-phase ethylation and gas chromatography coupled with atomic fluorescence spectroscopy (GC-AFS) that was able to precisely quantify methylmercury and inorganic mercury at subparts-per-trillion concentrations in natural waters. This is currently the most widely used method for determining mercury species at ultratrace concentrations. Unfortunately, the ethylation step is prone to interferences, particularly from humic substances and chloride, which necessitates the separation of methylmercury from the sample matrix. Solvent extraction and nitrogen-assisted distillation have been employed as methods to separate methylmercury from natural water samples.4-6 Horvat et al.5,6 have compared the two techniques and found the distillation technique gave higher and more consistent recoveries. Unfortunately, the method suffers from a couple of shortcomings. Bloom et al.6 showed that, under some conditions, the method can be responsible for artifactual methylation of inorganic mercury added to sediments and waters. The percentage of methylmercury produced varied from 0.001% of the inorganic mercury present in clear, low organic waters to 0.1% in humic-rich waters.7 This effect is therefore not likely to be significant in most natural waters where methylmercury typically comprises 10% of total mercury; (3) Rapsomanikis, S.; Donard, O. F. X.; Weber, J. H. Anal. Chem. 1986, 58, 35-37. (4) Bloom, N. Can. J. Fish. Aquat. Sci. 1989, 46, 1131-1140. (5) Horvat, M.; Bloom, N. S.; Liang, L. Anal. Chim. Acta 1993, 281, 135-152. (6) Horvat, M.; Bloom, N. S.; Liang, L. Anal. Chim. Acta 1993, 281, 153-168. (7) Bloom, N. S.; Colman, J. A.; Barber, L. Book of Abstracts, 4th International Conference on Mercury as a Global Pollutant, Hamburg, 1996; p 51.
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however, in contaminated systems and systems with a very low proportion of methylmercury naturally present, this may lead to erroneous results. The nitrogen-assisted distillation method is also very slow with the distillation most effective at between 6-8 mL‚h-1. This results in typical sample preparation times of 6 h per sample. The long distillation time also limits the maximum sample size, which is a concern for the quantification of very low methylmercury concentrations. Steam distillation is a method that has previously been used to separate methylmercury from biological and sediment samples.8-10 This method differs from nitrogen-assisted distillation in that the carrier gas is water vapor at a flow rate of 20 mL‚min-1 or greater, which ensures the rapid and efficient distillation of methylmercury. Furthermore, added reagents and the original sample matrix are not concentrated by evaporation of the sample, which lessens the chance of matrix-analyte interactions during the latter stages of distillation. In the past, steam distillation has been used in conjunction with some form of digestion procedure to quantify total distillable mercury, which has been assumed to comprise solely methylmercury and dimethylmercury.8,9 This is of concern since it is possible that certain inorganic mercury species such as mercuric chloride may be codistilled. To our knowledge, no previous attempt has been made to quantify methylmercury in natural water samples at ultratrace levels using steam distillation as a pretreatment procedure. This paper presents the results of a study to develop and assess steam distillation in conjunction with aqueous-phase ethylation and GCAFS to determine methylmercury in natural water samples. Particular attention was paid to the assessment of artifactual methylmercury formation and the possibility of codistillation of inorganic mercury species. EXPERIMENTAL SECTION Due to the low concentrations of methylmercury and total mercury in natural waters, extreme care was taken at all stages of analysis to avoid contamination. All glassware was prepared by soaking in 10% nitric acid. Mercury(II) nitrate (HgNO3) standards were prepared by serial dilution of a certified stock solution (1000 mg‚L-1, BDH Chemicals) with 0.1% (v/v) nitric acid. It was found necessary to UV irradiate working inorganic mercury standards in quartz tubes placed around a Hanovia 1 kW mediumpressure mercury arc lamp for >0.5 h to ensure no trace methylmercury species were present. A methylmercury chloride stock solution was prepared by dissolution of the crystalline solid (K and K Chemicals, Trenton, NJ) in isopropyl alcohol. Methylmercury chloride working standards (0.1, 1.0, and 10 µg.L-1) were prepared by serial dilution of the stock solution with 0.1% (v/v) hydrochloric acid. Working standards were stored refrigerated (4 °C) in the dark for up to 4 months. No change in concentration was detected over this period. The methylmercury standards were standardized against mercury(II) nitrate standards using BrCl oxidation and cold vapor atomic fluorescence spectrometry (CV-AFS).11 Dimethylmercury (Aldrich) stock solutions were prepared in ethanol and were used within 8 h of preparation. (8) Uchida, M.; Hirakawa, K.; Inoue, T. Kumamoto Med. J. 1961, 14, 181187. (9) Floyd, M.; Sommers, L. E. Anal. Lett. 1975, 8, 525-535. (10) Collett, D. L.; Fleming, D. E.; Taylor, G. A. Analyst 1980, 105, 897-901. (11) Liang, L.; Bloom, N. S. J. Anal. Atom. Spectrom. 1993, 8, 591-594.
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STEB (Strem Chemicals, Newburyport, MA) was prepared as a 1% (m/v) solution in high-purity water (MilliQ) and purged with N2 for 10 min before adding KOH (2% (m/v) final concentration). Acetate buffer (2 M) was made up in Hg-free water and purified by equilibration with a thiol-modified silica gel, synthesized inhouse.12 Other reagents were of analytical reagent grade or better. The steam distillation apparatus comprised a 250 mL roundbottom flask fitted with a dropping funnel and a splash head leading to a water condenser. A purpose-built steam generator, able to convert up to 23 mL‚min-1 water to steam, was used to introduce steam via a side arm into the distillation flask. The steam flow rate was controlled by altering the flow of water to the steam generator using a variable-speed peristaltic pump. A refrigerated water sample (typically 50 mL) was weighed into the round-bottom flask. A 1 mL aliquot of 1.0% ammonium pyrrolidine dithiocarbamate (APDC) was added to MilliQ water or seawater samples (but not to natural freshwaters or estuarine waters). The round-bottom flask and a 250 mL collection flask were connected to the distillation apparatus. The sample was preheated for 2 min to allow it and the glassware to approach the boiling point before the steam flow (23 mL‚min-1) was connected. A 2.5 mL aliquot of a 3.0% (m/v) KCl/20% (v/v) H2SO4 mixture was added via the dropping funnel. The distillate was collected for 8.5 min (∼190 mL) through a water condenser into the collection flask and stored in the dark until analysis by GC-AFS. The derivatization with STEB was carried out in the same flask in order to minimize transfer losses. Between samples, the apparatus was cleaned by distilling ∼50 mL of MilliQ water. The nitrogen-assisted distillation procedure was adapted from Horvat et al.6 A 50 mL sample was used throughout, and the heating block was maintained at 110 °C which gave a flow rate of ∼8 mL‚h-1. The GC-AFS method of Bloom,4 modified by Liang et al.,13 was used for the analysis of the distillates. Prior to analysis, 0.4 mL of 2 M acetate buffer was added to the distillates to stabilize the pH between 4.5 and 5.0. Water samples were analysed for total mercury by CV-AFS after BrCl oxidation and single-stage gold amalgamation.11 A variety of natural waters were collected for validation of the technique (Table 1). Selected samples were filtered through Whatman GF/F glass fiber filters which had first been rigorously cleaned by heating to 450 °C in a muffle furnace followed by acid washing. All samples were stabilized by addition of 1 mL‚L-1 ultrapure HCl and stored in the dark. RESULTS AND DISCUSSION Optimization of Parameters. Distillation procedures for methylmercury5,6,9,10 have typically involved sample acidification and the addition of excess chloride. This combination ensures the dissociation of methylmercury complexes and the formation of comparatively volatile methylmercury chloride. Sample matrix modification by addition of KCl and H2SO4 was adopted in this work in preference to the addition of HCl alone, as this mixture has been found to reduce the occurrence of low pH and Cl- in distillates prepared by nitrogen-assisted distillation.6 The control of distillate pH and chloride concentration is necessary as both can inhibit the ethylation reaction.4 The pH of the steam distillates (12) Howard, A. G.; Volkan, M.; Ataman, D. Y. Analyst 1987, 112, 157-162. (13) Liang, L.; Horvat, M.; Bloom, N. S. Talanta 1994, 41, 371-379.
Table 1. Water Sample Locations and Descriptions location Woronora R., NSW L. Gordon, Tasmania Upper Gordon R., Tasmania Gordon Tail Race, Tasmania sample A sample B sample C L. Murray, Papua New Guinea Florentine R., Tasmania Georges R., NSW Shelley Beach, NSW
total Hg (ng‚L-1)
DOC (mg.L-1)
pH
2.6 2.6 2.0
1.3 8.0 7.8
6.9 6.2 7.2
small clear water creek brown water, low turbidity brown water, fast flowing river, filtered
1.6 1.7 1.3 1.1 7.0 1.6
5.3 5.3 5.3 3.5 23 4.3 1.5
5.9 5.9 5.9 6.7 4.5 7.1 8.0
brown water, release from hydroelectric power station as above as above, filtered clear water, low to moderate turbidity small seepage pool, very brown water, filtered urban estuary urban coastal seawater
Figure 1. Effect of steam flow rate on the recovery of methylmercury from 50 mL of spiked lake water (100 pg of CH3HgCl added): pump speed (0) 16 and (9) 23 mL‚min-1.
was consistently between 4.0 and 4.5. This was adjusted to the optimum pH for ethylation (4.9) by the addition of 0.4 mL of 2 M acetate buffer. Optimization experiments were carried out on an acidified lake water sample (Lake Gordon, Tasmania, Table 1). This was chosen as being more representative of real analytical samples than ultrapure water. Experiments that varied the concentrations of KCl from 0 to 10 g‚L-1 and H2SO4 from 0 to 1.5% (v/v) indicated little effect on methylmercury recovery. Even in the absence of added KCl and H2SO4, satisfactory recovery was obtained. This was most likely because the sample water had been acidified with 1.0 mL‚L-1 (v/v) HCl and the small amount of added chloride may have been sufficient to ensure high recoveries. A KCl concentration of 1.5 g‚L-1 and H2SO4 concentration of 1.0% were selected for further work. Estuarine (>5.0‰ salinity) and seawater samples were distilled without KCl addition in order to avoid matrix problems associated with high chloride concentrations (see Matrix Modification below). The distillation volume and time required to attain quantitative methylmercury recovery were optimized by collecting the consecutive distillate fractions of spiked lake water and examining the resulting distillation curves (Figure 1). The time required for quantitative recovery decreased with increasing flow rate. Distillation time decreased from 9 min at a flow rate of 16 mL‚min-1 to 7 min at 23 mL‚min-1. A flow rate of 23 mL‚min-1 was the highest attainable with the available equipment and was used in all further work. A distillation time of 8.5 min was sufficient to ensure quantitative distillation of methylmercury from a 50 mL sample. Sample volume also affected the distillation rate, with smaller sample volumes giving faster distillation times (Figure
salinity (‰)
8 35
comments
Figure 2. Effect of sample volume on the recovery of methylmercury from spiked lake water (100 pg of CH3HgCl added): volumes of (9) 20, (0) 50, and (O) 100 mL.
2). Sample sizes of 20 and 50 mL had similar cumulative recovery curves, whereas a 100 mL sample distilled somewhat slower and required 9 min for complete recovery. Using aqueous-phase ethylation and GC-AFS, 50 mL sample volumes were sufficient to determine methylmercury levels in most natural water samples (see Performance Characteristics below). Larger sample volumes may be required in the case of very low level samples. A 100 mL sample gave a distillate volume of ∼230 mL. The analysis of distillate volumes of >230 mL requires the reoptimization of reagents and purge and trap conditions employed during the aqueous-phase ethylation. Matrix Modification. Low methylmercury recoveries and poor precision for waters having low DOC concentrations (e.g., deionized water, rainfall, and low-DOC freshwaters) and seawaters have been noted by Bloom and Von Der Geest14 when using nitrogen-assisted distillation. In this study, low-DOC freshwaters gave good recoveries (Table 2), however, poorer recoveries (
