ANALYST DECEMBER 1986 VOL. 111 1371 Decomposition and Stability Studies of Methylmercury in Water Using Cold Vapour Atomic Absorption Spectrometry Riaz Ahmed" and Markus Stoepplert Institute of Applied Physical Chemistry Chemistry Department Nuclear Research Centre Julich (KFA), P.O. Box 1913 0-5170 Julich FRG The long-term stability of Hg*+ and methylmercury chloride (MeHgCI) and the decomposition of MeHgCl under various conditions were investigated. In the absence of light MeHgCl does not decompose to Hg2+, even in the presence of 25% acids during a period up to 3 d. MeHgCl is not easily decomposed by heating in the presence of 50% acid concentrations; however with UV light the decomposition of MeHgCl to Hg2+ takes place immediately depending on the intensity of UV light.In the presence of SH groups (L-cysteine) partial stability and complexation of mercury was observed. For the long-term storage of Hg and MeHgCl at low concentrations in addition to certain reagents the material of the container is important. Keywords Mercury; methylmercury; decomposition and stability studies; water; cold vapour atomic absorption spectrometry Owing to their toxicological significance much work has been carried out on the determination of mercury1 and methylmer-cury.2 Methylmercury (MeHgCl) has most often been deter-mined in fish as this is the predominant form in these organisms.3 Studies have been carried out on the decomposi-tion of MeHgCl in fish samples using high concentrations of acids,4 and there is now increasing interest in the determina-tion of total and inorganic mercury and MeHgCl in water.5 For the decomposition and determination of MeHgCl in aqueous samples acid concentrations as high as those employed for fish samples cannot be used owing to the extremely low concentrations of mercury in water.Therefore, decomposition studies of MeHgCl in water are required using lower more practical acid concentrations. It has been reported that mercury can be taken up into the cysteine disulphide bridges of proteins. 6 Sulphydryl com-pounds (L-cysteine) also act as effective sensitisers for the photochemical methylation of inorganic mercury,7 and there is a possibility of mercury entering the biosphere as an Hg -cysteine complex.8 As sulphydryl compounds can be present in several different environmental systems particularly sea water and rain water it seemed appropriate to investigate the behaviour of MeHgCl with respect to its decomposition in the presence of cysteine a typical sulphydryl compound.Numerous papers have been published concerning the stability of ionic mercury in solutions9 but very little is known about the stability of MeHgCl in aqueous systems.10There are few references available that discuss the stability of MeHgCl in aqueous systems close to natural levels of mercury and MeHgCl. This paper describes the behaviour of MeHgCl in water when exposed to light and heat in the presence of different reagents acids sulphydryl compounds (L-cysteine) and also long-term stability studies of MeHgCl in the presence of different reagents and different container materials.Experimental Chemicals All the acids used (HCl HN03 HC104 and H2S04) were of Suprapur grade from E. Merck FRG. All other chemicals (NaC1 L-cysteine SnClz - H2S04 methylmercury HgO, H202 Na2SOq NaOH sodium acetate and toluene) were of * Present address NCD-PINSTECH P.O. Nilore Islamabad, 1- To whom correspondence should be addressed. Pakistan. analytical-reagent grade from E. Merck. Labelled methyl-mercury (1 mCi = 3.7 X 107 Bq) was obtained from Amersham UK. Apparatus The UV lamp for decomposition studies was a 150 W mercury vapour lamp and the samples were irradiated from a distance. An atomic absorption spectrometer (Bodenseewerk Perkin-Elmer FRG Model 400) with a mercury vapour lamp (0.2 A/15 V) as a light source was used at a wavelength of 253.7 nm and a 2.0 nm slit width.The mercury reduction vessel (1 1) was of Pyrex glass. Additional equipment included a recorder (SE 120 Goertz Metrawatt FRG). The automatic heating and gas flow control system used was constructed in the workshop of Dr. Beerwald Bochum FRG. A gas - liquid chromatograph Model 5710 equipped with an electron-capture detector (Hewlett-Packard USA) was used for the MeHgCl determination. 11 A multichannel Series 8 Analyzer from Canberra USA with Canberra Spectron F version V2 D1 software and a PDP 11/03 computer with a 64K memory was used for the activity measurements. Procedure Approximately 50 ml of a 10% SnC12 + 20% H2S04 solution was taken in the reduction vessel. A known volume of sample, usually 1-50 ml was taken.Nitrogen was passed through this solution at a rate of 2.0-2.5 1 min-1 and Hg was pre-concentrated on gold wool. After this step the nitrogen flow-rate was decreased to 50-100 ml min-1 and the gold wool heated to 700-800 "C to volatilise the Hg which was subsequently determined in the cuvette of the atomic absorp-tion spectrometer. After the determination of the sample a known amount of the standard solution was added to the same solution in the reduction vessel and determined at the same matrix conditions as for the sample. The apparatus and procedure have been described previously12 and will be given together with all recent modifications in detail elsewhere.13 Results and Discussion Precision and Linearity The cold vapour AAS (CVAAS) procedure has a detection limit of approximately 0.1 ng 1-1 for 100-ml water samples.The relative standard deviation for the analysis of mercury at ca. 1 ng levels (absolute) is <5"h. The procedure has a wide linearity range from 0.0 to 10 ng of mercury. The mercury concentrations used for the experiments were 0.005-1 . 1372 ANALYST DECEMBER 1986 VOL. 111 pg 1-1. The reduction mixture used for ionic mercury was 10% SnCl2 + 20% H2S04 and it was found that this reduction mixture did not decompose MeHgCl during the reduction aeration period. Decomposition of MeHgCl in the Presence of NaCl and HCI Sodium chloride and HC1 are often added to environmental samples for the extraction of MeHgCl by organic solvents r I 0.6 -c 0 1 2 3 Timeidays Fig.1. Decomposition of MeHgCl (1.03 pg 1-l) in Pyrex flasks covered with A1 foil in the presence of A 15% NaCl + 0.2 M HCl; B 15% NaCl + 2.2 M HCI; C 15% NaCl + 5.0 M HCI; D 5% NaCl + 0.2 M HCI; and E 5% NaCl + 2.2 M HCl 1.0 -0.8 -c I - . 0 1 2 3 4 Timelh Fig. 2. Decomposition of MeHgCl(l.03 pg 1-l) in quartz glass flasks in the resence of A 15% NaCl + 0.2 M HCI; B 15% NaCl + 2.2 M HCI; 8 15% NaCl + 5.0 M HCI; D 15% NaCl; and E 5.0 M HCl (toluene benzene) and the subsequent determination of MeHgCl by gas chromatography. Decomposition studies of MeHgCl in the presence of these reagents in Pyrex flasks covered with A1 foil were performed (Fig. 1). High concentra-tions of HCl or of NaCl alone did not decompose MeHgCl as quickly but when 15% NaCl + 2.2 M HCl were added up to 40% of the MeHgCl was decomposed within one day.This combination has been extensively used for the extraction -separation of MeHgC1.14 It is apparent from Fig. 1 that in the presence of these reagents losses of MeHgCl may take place if B 6 0 C e D ~ 0.010 0 I 0.01 5 -0 0.5 1 .o 1.5 2.0 AcidslM Fig 3. Decomposition studies of MeHgCl (0.0206 pg 1-l: presence of H202 (1.0%) and different concentrations of ac HCI; B HN03; C H2S04; and D HClO4 0.020 0.01 5 r I 0 0 I -2 0.010 0.005 in the Is A, 0 1 2 3 4 H202 Yo Fig. 4. Decomposition studies of MeHgCl (0.0206 pg I-l) in the presence of acids and different concentrations of H202. A 1.0 M HCI; B 1.0 M HNO,; C 1.0 M H2S04 and D 1.0 M HC104 ~ ~ Table 1.Decomposition studies of MeHgCl in the presence of different reagents with and without UV irradiation. 1.03 pg I-' of MeHgCl was added to each sample Concentration of Hg found with UV irradiation Sample (20 min)/ number Reagents added After0.0d After 1.0d After2.0d After3.0d After4.0d ygl-1 Concentration of Hg found without UV irradiatiodyg 1-l 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 10% HC1+ 2.5% HN03 20.0% HCI + 5.0% HN03 5.0% HCl + 20.0% HN03 2.0% HC104 + 10.0 HN03 1 .O% HN03 5.0% HN03 50.0% HCl 4.0% HC104 + 20.0% HNO, 15.0% NaCl 15.0% NaCl + 1.0% HCl 1 .O% H2S04 5.0% H2S04 1.0% HC104 10.0% HC104 H20 0.02 0.03 0.03 0.02 0.02 0.05 0.05 0.003 0.006 0.02 0.04 0.04 0.06 0.06 0.04 0.02 0.03 0.03 0.01 0.01 0.05 0.05 -0.07 0.11 0.08 0.06 0.08 0.09 0.14 0.09 0.08 0.09 0.05 -0.05 -0.05 -0.02 -0.02 -- 0.04 - 0.04 - -1.04 1.02 0.74 0.61 0.58 1 .oo 0.75 1.03 1.04 1.05 0.98 1.01 1.02 1.03 1.0 ANALYST DECEMBER 1986 VOL.111 1373 more time is taken for the extraction. In the presence of 15% NaCl + 5 M HC1 the rate of decomposition of MeHgCl is less than that seen with 15% NaCl + 2.2 M HCl; the reason for this may be the low solubility of NaCl in the presence of 5 M HCl. In order to determine the influence of light on the decomposition of MeHgCl studies were carried out in the presence of NaCl and HC1 in quartz glass flasks kept in the laboratory. There was no direct sunlight in the laboratory, only diffused daylight.It can be seen from Fig. 2 that decomposition of MeHgCl is very rapid and that in the presence of 15% NaCl + 2.2 M HCl MeHgCl is decomposed completely in less than 4 h. Hence it is clear that if extractions of MeHgCl are carried out in quartz vessels without covers, the concentrations of MeHgCl found will be unreliable. Effect of Ultraviolet Light on the Decomposition of MeHgCl In order to study the effect of UV light on the decompo-sition of MeHgCl various solutions were prepared in quartz glass flasks and exposed to UV light (Table 1). In the presence of NaCl HC1 HC104 H2S04 1 YO HN03 and H20 MeHgCl decomposes completely after less than 20 min of UV irradiation but in the presence of higher concentrations of HN03 the decomposition of MeHgCl is complete.It is also apparent from Table 1 that MeHgCl is stable in the presence of up to 25% acids without UV light. Detailed studies have shown15 that the decomposition of MeHgCl is difficult in the presence of more than 20% HN03 even with long UV irradiation times. The reason could be the negligible optical opacity in the UV region of strong HN03 solutions. MeHgCl was also not completely decomposed by UV light in the presence of NaOH and decomposition was more difficult at lower concentrations of NaOH (0.1 M) than at 0.75 M. Hence NaOH has the opposite effect to that of HN03 on the decomposition of MeHgCl with UV irradiation. Decomposition of MeHgCl in the Presence of Acids and H202 In the presence of 1% H202 and various concentrations of acids (0.25-2.0 M HC1 HN03 H2SO4 and HC104) the percentage of MeHgCl decomposed remains the same (Fig.3). Increasing acid concentrations do not increase the decom-position of MeHgCl and the effect on the decomposition of MeHgCl decreases in the order HN03 > H2S04 3 HC104 > HCl. If the acid concentration is kept constant and the H202 concentration increased the decomposition of MeHgCl increases with increasing H202 concentration (Fig. 4). The rates of increase in MeHgCl decomposition in the presence of HC1 and H2S04 are comparable and the rates of increase in MeHgCl decomposition in the presence of HN03 and HClO4 are also similar (Fig. 4). In the presence of acids and H202 the decomposition of MeHgCl takes place immediately on the addition of H202, and no further decomposition takes place when the solutions are allowed to stand for 3 d.It is possible that H202 reacts immediately in the presence of acids and is decomposed. 0 1 2 3 4 Time/h Fig. 5. Decomposition studies of MeHgCl (1.134 pg I-') in the presence of acids and heating at 200°C under pressure. A 50% HNO,; B 40% HN03 + 10% HC104 Effect of Heating on the Decomposition of MeHgCl MeHgCl decomposes immediately in the presence of UV light but it is very stable during heating. MeHgCl did not decompose when heated for 2.5 h at 80°C in the presence of 1-5% HCl or HN03 or 5% NaCl or in the absence of any addition. MeHgCl only decomposes in the presence of 100% HN03 or 90% HN03 + 10% HC104 when heated at 200°C under slight pressure for 1 h.16 However for the decompo-sition of MeHgCl in water samples it is impossible to add 100% acid because of the low concentrations of mercury and MeHgCl in water samples.Only up to ca. 50% acids can be added and this creates large blank values and contamination problems. Studies were carried out to determine the decom-position of MeHgCl in the presence of 50% HN03 and 40% HN03 + 10% HC104 and with heating at 200 "C under slight overpressure for longer times. It was found that even on heating for 3 h in the presence of 50% HN03 only 25% of the MeHgCl was decomposed and on heating for 4 h this decreased to 20% (Fig. 5 ) . When heated for 3 h in the presence of 40% HN03 + 10% HC104 only 34% of the MeHgCl was decomposed and on heating for 4 h this decreased to 29%.This decrease in Hg when heated for more than 3 h could be due to losses from prolonged heating. In conclusion it is difficult to decompose more than one third of the total MeHgCl using 50% acids and heating under pressure at 200°C. Disappearance of Detectable Mercury in the Presence of Cysteine Humic substances and sulphydryl compounds are present in sediments sea water and other systems of the environment. Cysteine was selected for a study of the decomposition and analysis of MeHgCl as mercury in aqueous samples in the presence of this compound. Detailed studies showed15 that if water samples were exposed to light in the presence of MeHgCl and cysteine an appreciable amount of the mercury could not be detected by CVAAS.When the samples containing MeHgCl and cysteine were exposed to UV irradiation in the presence of acids NaCl or even without any addition the major fraction of the decomposed MeHgCl could be initially determined but when the time of UV irradiation was increased to more than 1 h or even if the samples were allowed to stand for this length of time the major part of the mercury that was initially detectable changed into an undetectable form. In order to investigate the influence of chlorides and cysteine on the determination of MeHgCl after its decomposi-tion using CVAAS various experiments were carried out. Chlorides were selected as sea water and other water samples contain large amounts of chlorides. The detectable concentra-tion of mercury in the presence of 0.05% cysteine and various concentrations of HC1 after UV irradiation is plotted in Fig.6. The ratio of cysteine to HC1 is 0.0025 at the minimum 0.6 I I I I J 0 1 2 3 4 5 H C h Fig. 6 . Decomposition of MeHgCl (1.03 pg I-') in the presence of different concentrations of HCI 0.05% cysteine and after UV irradiation for 90 mi 1374 ANALYST DECEMBER 1986 VOL. 111 Table 2. Percentage recovery of MeHgCl after storage in Pyrex polyethylene and PTFE containers Pyrex Polyethylene PTFE Sample number Reagent added 11 d 35 d 5d 26 d 6 d 26 d 1 1.0% HCl 91.5 81.0 53.8 1.5 97.8 83.6 2 1 .0% HN03 96.8 87.5 94.1 72.2 88.4 68.1 3 5.0% NaCl 95.6 89.5 100.0 86.1 92.0 82.6 4 H20 18.8 3.13 26.2 1.4 21.8 4.4 1.2 , I / Cysteine O h Fig. 7. Decomposition of MeHgCl (1.07 pg 1-I) in the presence of 4.0% NaCl and different concentrations of cysteine and after UV irradiation tor 3.5 h detectable concentration of mercury.For different concentra-tions of cysteine at 0.2 M HC1 the minimum detectable concentration of mercury was at 0.005% cysteine; here also the ratio of cysteine to HC1 is 0.0025.15 In the presence of 4.0% NaCl and increasing concentrations of cysteine the minimum detectable mercury concentration is at 0.01% cysteine and again the ratio of cysteine to NaCl at the minimum is 0.0025 although here the minimum is present from ratios of 0.0025 to 0.0125 (Fig. 7). It is important to point out that for the determination of mecury in sea water the water samples are usually slightly acidified and then UV irradiated and measured for total mercury.Sea waters contain ca. 4.0% NaCl. If these sea waters contain small amounts of sulphydryl compounds as is usually so then the determination of total mercury will be unreliable. However as has been seen 50% acids cannot decompose MeHgCl in water and with UV irradiation in the presence of sulphydryl compounds much of the mercury goes into an undetectable form. Also many sea waters contain particulate matter which acts as a scavenger for mercury and MeHgCl. Thus the determination of mercury and MeHgCl in sea water samples is not easy although some approaches have been reasonably successful.~7~~~ In order to find out whether the undetectable mercury was converted back to MeHgCl, experiments were carried out using MeHgCl labelled with 203Hg measuring y-radiation with the multichannel Analyzer and also by gas - liquid chromatography with an electron-capture detector.15 It was found that this undetectable mercury remained in solution and was not MeHgCl although the possibility of the formation of complexes or sulphides should not be ruled out.Long-term Stability Studies of Mercury and MeHgCl The use of 0.1 M HC1 0.1 M NaOH 0.1 M HN03 and 5.0% NaCl for the preservation and storage of ionic mercury was compared for a period of 59 d as these reagents are either present in environmental water samples or are usually added before analysis. Only HN03 and NaCl can be used for short-term preservation purposes. High concentrations of HN03 cannot be used as they prevent the decomposition of MeHgCl with UV light.Stability studies of MeHgCl in glass polyethylene and PTFE bottles are shown in Table 2. In all three types of containers MeHgCl decomposes very rapidly in the absence of any additions of other reagents although the loss of ionic mercury is faster than the decomposition of MeHgCl in polyethylene bottles whereas in glass and PTFE containers the loss of ionic mercury is less than the rate of decomposition of MeHgCl. Thus without any additions MeHgCl decomposes very quickly at all concentrations (the mercury concentrations examined were 0.412 pg 1-1 in glass 0.0412 yg 1-1 in polyethylene and 0.0206 pg 1-1 in PTFE bottles). Various concentrations were taken to see if the stability increases with concentration but the concentration of MeHgCl had no effect on its stability.The stabilisation effects of 1% HCl 1% HN03 and 5% NaCl on the storage of MeHgCl in different types of containers are compared in Table 2. As can be seen the best preservative for MeHgCl is 5% NaC1 followed by 1% HN03 and HC1 and the best container material is glass then PTFE and polyethylene. During storage of MeHgCl solutions, maximum care must be taken to avoid any exposure to light. The valuable technical assistance of Mr. K. May is gratefully acknowledged. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. References Greenwood M. R. and Von Burg R. in Merian E. Editor, “Metalle in der Umwelt,” Verlag Chemie Weinheim 1984, p. 511. Schreiber W. Sci. Total Environ.1983 31 283. Egan H. Proc. Anal. Div. Chem. SOC. 1978 15 117. Harms U. Z. Lebensm. Unters. Forsch. 1976 162 365. May K. Ahmed R. Reisinger K. Torres B . and Stoeppler, M. in Lekkas T. D. Editor “Proceedings of the 5th International Conference on Heavy Metals in the Environ-ment Athens September 1985,” Volume 2 CEP Consultants, Edinburgh 1985 p. 513. Marston A. W. and Wright H. T. J . Biochem. Biophys. Methods 1984 9 307. Zuo Y. and Pang S. Huanjing Kexue Xuebao 1985,5,239. Ponnamperuma C. “Investigation of Mercury - Amino Acid Complexes in the Aqueous Environment,” NTIS Govt. Rep. Announc. Index (U.S.) 1984 84 38. Christman D. R. and Ingle J. D. Jr. Anal. Chim. Acta, 1976 86 53. Stoeppler M. and Matthes W. Anal. Chim. Acta 1978 98, 389. Torres B. Reisinger K. Stoeppler M. and Niirnberg, H. W. in Miiller G. Editor “Proceedings of the 4th International Conference on Heavy Metals in the Environ-ment Heidelburg September 1983,” Volume 2 CEP Consul-tants Edinburgh 1983 p. 838. Stoeppler M. Spectrochim. Acta Part B 1983 38 1559. May K. and Stoeppler M. in preparation. Rodriguez-Vasquez J. A. Talanta 1978 25 299. Ahmed R. and Stoeppler H. in Stoeppler H. and Ourlech, H. W. Editors “Decomposition and Stability Studies of Methylmercury in Water,” Jii1.-Spez. No. 349 Kernforschungsanlage Jiilich Jiilich 1986 52 pp. May K. and Stoeppler M. Fresenius 2. Anal. Chem. 1984, 317 248. Farey B. J. Nelson L. A, and Rolph M. G. Analyst 1978, 103 656. Ahmed R. May K . and Stoeppler M. Sci. Total Environ., in the press. Paper A611 92 Received June I6th 1986 Accepted July 30th 198