Validation of methylmercury determinations in aquatic systems by alkyl derivatization methods for GC analysis using ICP-IDMS

Isotope dilution mass spectrometry (IDMS), using an inductively coupled plasma quadrupole mass spectrometer (ICPMS) and a species-specific methylmercury spike was applied to validate the commonly used GC method for methylmercury (MeHg+) determination, which is based on the formation of volatile methylethylmercury by derivatization with NaBEt4. The spike compound, (MeHg+)-Hg-201, was synthesized by reaction of Hg-201-enriched mercury chloride with methylcobalamin. By analyzing different environmental aquatic samples, it was found that in most cases, transformation of MeHg+ into elemental mercury (Hg-0) took place. From investigations of synthetic solutions, it could be followed that halide ions are responsible for this transformation process. Chloride and bromide converted MeHg+ into Hg-0, whereas iodide caused transformation into Hg2+ and Hg-0. It could also be shown that transformation of MeHg+ took place only during the derivatization step. In contrast to ethylation, propylation by NaBPr4 did not cause any transformation; however, accurate results of MeHg+ determinations could be obtained by propylation as well as by ethylation when GC/ICP-IDMS was applied. This work demonstrates the great power of isotopically labeled element compounds for the validation of element speciation methods and for species-specific IDMS analyses.