The origins and the processes driving the inadvertent transformations of inorganic mercury (iHg) and methylmercury (MeHg) in cryogenically stored and homogenized fresh-frozen versus freeze-dried biological Standard Reference Materials (SRM) were investigated using alkaline digestion, derivatization and GC/ICP-MS analysis. Labile enriched 201iHg and 202MeHg isotopic standards together with their cysteine-complexed molecular analogs (201Hg(Cys)2 and 202MeHgCys) were used in a double spike speciated isotope dilution (SID) model to study the role and influence of the complexing ligands/radicals originally associated with mercury species in these materials, on the equilibration, the reactivity and the transformation processes of mercury species. The results revealed that a negligible methylation occurred in both materials, whereas a significant demethylation yield was only detected in the cryogenically stored fresh-frozen materials. Systematic investigation revealed that this apparent demethylation yield, as given by the double-SID model, resulted from the possible influence of demethylating agents, which seem to be inhibited using tetrapropylborate as an alternative derivatizating agent, compared to tetraethylborate. However, a significant fraction of the demethylation yield was found to be potentially biased and resulted from a lack of equilibration between labile spiked iHg species and their endogenous analogs, indicating probable different complexation/lability patterns in the FF material after the extraction step. This effect was not observed in the FD material. A complementary analytical step to account for and to minimize the effect of this competing-ligand complexation process is proposed, which allows the two classes of materials to be denoted as commutable.
Citation: Journal of Analytical Atomic Spectrometry
Pub Type: Journals
complexation, demethylation, double spike, GC/ICP-MS, mercury, methylation, methylmercury, speciation, SRM