Methylmercury species determination

Kato, T., Uehiro, T., Yasuhara, A. and Morita, M. (1992) Determination of methylmercury species by capillary column gas chromatography with axially viewed inductively coupled plasma atomic emission spectrometric detection./. Anal. At. Spectrom., 7, 15-18. 

The method has been used for the determination of methylmercury species in fish and in drinking water. 

For the determination of the Hg species methylmercury, phenylmercury and Hg(ll), ozone was used successfully in a batch cold-vapor system [126]. The preconcentration and speciation of Cr(lll) and Cr(VI) in water sample can be performed using solid-phase extraction (SPE) [105]. An SPME (solid-phase microextraction) technique has been used as the sample preparation system for the innovative simultaneous multielement/multi-species determination of six different mercury, tin. and lead species in waters and urine with GC/MS-MS [141], [142], The determination of arsenic species [As(III), As(V), MMA. DMA, arsenocholine, arsenobetaine)] has also been shown to work with an on-line digestion step prior to hydride generation AAS [125] (see Fig. 7). 

As speciation analysis slowly enters authority s regulations (already the case for chromium, organotin and methylmercury species) and routine analysis becomes required, precise and accurate quantitative species determination, as well as access to certified reference materials, is becoming imperative. 

Cappon and Crispin-Smith [59] have described a method for the extraction, clean-up and gas chromatographic determination of alkyl and aryl mercury compounds in sediments. The organomercury compounds are converted to their chloroderivatives and solvent extracted. Inorganic mercury is then isolated as methylmercury upon reaction with tetramethyltin. The initial extract is subjected to a thiosulphate clean-up and the organomercury species are isolated as their bromoderivatives. Total mercury recovery was in the range 75-90% and down to lpg kg-1 of specific compounds can be determined. 

The addition of anticoagulents and/or preservatives, a practice that may be tolerated for total element determination, should be avoided for two reasons. First, the compounds used are usually complexing agents. They therefore could bind various trace elements. Second, they could destroy some species. For example, the addition of potassium dichromate to urine in the presence of nitric acid could destroy methylmercury If the use of such compounds is unavoidable, then there should be experimental evidence to show that the speciation of the element imder study has not been adversely affected. 

A technique for the determination of methylmercury in aqueous samples (natural and seawater) involved the conversion of methylmercury to gaseous methyl-ethylmercury by reaction with sodium tetraethylborate (Bloom, 1989 Bloom and Watras, 1989). The volatile derivative was purged from the solution and concentrated on a graphitic column at room temperature. The derivative was thermally desorbed from the column, and then analysed by cryogenic gas chromatography with cold vapour atomic fluorescence detection. In addition to methylmercury, labile Hg11 species could be determined (as diethylmercury) as well as dimethylmercury (which is not ethylated). The detection limit for... 

The formation of butyl derivatives of methylmercury and ethylmercury was studied as a means of providing stable, volatile derivatives for the gas chromatographic determination of these compounds (Quimby and Sullivan, 1990). The resulting dialkylmercury species were separated satisfactorily on a capillary... 

Uncontrolled species transformations during analysis form another source of error. For methylmercury determinations in sediments it was demonstrated that errors of up to 80% resulted from the formation of the compound from inorganic mercury during separation and analysis [28, 29], For the study of possible species transformations during analysis multiple isotope dilution could be used as a diagnostic tool for identifying the error and bias inherent in specific methods of storage, sample preparation and measurement [30, 31]. 

Z. Mester, J. Earn, R. Sturgeon, J. Pawliszyn, Determination of methylmercury by solid-phase microextraction inductively coupled plasma mass spectrometry a new sample introduction method for volatile metal species, J. Anal. Atom. Spectrom., 15 (2000), 837 D 842. 

The model simulations were in close agreement with the observed results from the distribution and metabolism studies. Physiological processes that were highlighted by the results and the discrepancies that did occur include the probable active transport into the brain (versus passive diffusion) of a methyl-mercury-cysteine complex, the bidirectional transport of methylmercury between the gut lumen and gut tissue as a more important determinant of methylmercury fecal excretion than biliary secretion, the importance for the determination of methylmercury half-life in rats of the recycling of mercury from ingested hair, and the need for better estimates of the rate constants for the demethylation of methylmercury in order to adapt the model to other species. 

A better understanding of certain physiological and biochemical processes affecting mercury kinetics may help explain these species differences. Specific processes that appear likely determinants include differences in demethylation rates affecting methylmercury fecal secretion, reabsorption, and membrane transport (Farris et al. 1993) differences in tissue glutathione content and renal -glutamyltranspeptidase activity (Tanaka et al. 1991, 1992), differences in antioxidative status (Miller and Woods 1993), differences in plasma cysteine concentrations compared with other thiol-containing amino acids (Aschner... 

Wilken, R. D., Falter, R. Determination of methylmercury by the species-specific isotope addition method using a newly developed HPLC-ICP-MS coupling technique with ultrasonic nebulization. Appl Organomet Chem 1998, 12, 551-557. 

An excellent summary of the analytical methods for determining varions species of Hg in biological specimens, including blood, urine, hair, breath, and tissnes, as well as in environmental samples can be found in Table 6-1 in Toxicological Profile for Mercury (Update) (ATSDR 1999) and in the World Health Or nization (WHO) report Methylmercury (IPCS 1990). 

The need for the determination of individual chemical species occurs especially where these species are known to be very toxic to humans and biota, e.g. As(III), Cr(VI), Se(VI), methylmercury or tributyltin. Speciation is an indispensable tool for studies on the biogeochemistry of trace elements. Consequently, the increasing concern for an improved control of environmental contamination levels and of knowledge of trace element behaviour in the environment has led to the development of new analytical techniques for the determination of a wide variety of compounds, which is also reflected by a considerable increase in the number of determinations. 

The main difficulty in preparing a sediment reference material for interlaboratory studies on chemical species is to achieve the stability of the relevant compounds [12]. With respect to methylmercury, the main source of instability is due to bacteria, either by demethylation [88,89] or formation of volatile dimethylmercury [90]. The conversion is indirectly provoked by the biological activity of various types of bacteria, such as (i) aerobic mesophilic heterotrophic microorganisms, (ii) anaerobic sulfate-reducing bacteria and (iii) anaerobic spore-forming bacteria. In order to control the remaining bacteria present after different irradiation treatments, a bacterial enumeration was performed on samples which were dehydrated after homogenization and irradiated at various 7-ray doses (0, 4, 8, 12, 25 and 50 kGy). The determination of the sulfate-... 

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