Mercury speciation in aquatic environments is difficult to decipher due to the ultratrace levels (~2-20 pM) at which it occurs. Analytical methods for both total Hg (THg) and monomethyl mercury (MMHg) in natural water samples are now automated and standardized. Methods for quantifying volatile Hg are also well established, albeit not often used. While a great deal about Hg biogeochemistry and ecotoxicology has been discovered via these methods and it seems reasonable to assume that divalent HgII comprises the vast majority of the 80-90% of dissolved THg that is not MMHg or Hg0, the speciation of the unidentified Hg needs further investigation. Furthermore, definitive proof that conventional MMHg methodology exhaustively extracts MMHg awaits the development of other, more potent methods of extracting the analyte from the sample matrix. For these reasons, alternative methods for investigating Hg speciation can fill an important niche. This lab has been investigating and refining a new method for Hg speciation analysis based on aqueous phase ion chromatography of the thiourea complexes of Hg species, or HgTU-IC for the past decade. Our early work was optimized to separate MMHg and HgII, assuming the speciation scheme discussed above was complete. More recently, we have coupled the HgTU-IC speciation with ICP-MS detection and greatly improved its performance. In the process of investigating differences between results of our early methodology with those from distillation/ethylation/GC, we discovered that by altering the composition of our mobile phase, other trace species can be detected in surface waters, including ethyl Hg and an unknown species of low net charge. The bulk of the non-MMHg peak does appear to be HgII, as expected, and our sample preparation yields MMHg results that compare very closely to distillation/ethylation/GC. Investigations of the unknown species suggest that it is, non-volatile and stable with respect to heating or addition of large amounts of some strong ligands, i.e., thiourea, but is less stable than MMHg during long-term (years) storage. Evidence from experiments designed to discriminate between likely candidates – including very strongly-bound HgII, HgI, and nanoparticulate Hg – will be presented.

New insights into mercury speciation in freshwaters using mercury-thiourea complex ion chromatography with ICP-MS detection

249th National Meeting (2015)

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