Determination of artifactual formation of monomethylmercury (CH3Hg+) in environmental samples using stable Hg2+ isotopes with ICP-MS detection: Calculation of contents applying species specific isotope addition

Various extraction techniques, as distillation, acid and alkaline extraction, have been tested with regard to their potential to form a monomethylmercury (CH3Hg+) artifact from inorganic Hg during sample preparation. Hg2+ has been added to different reference materials in the form of enriched stable tracers and the formation of new methylmercury from that tracer has been analyzed by HPLC/ICP-MS and GC/ICP-MS. Both techniques gave comparable results. In particular; the distillation technique was prone to artifact formation. The resulting overestimation of methylmercury in sediments was as high as 80%. Artifact formation in hair, liver and algae samples was less significant, though still observable. Fish muscle tissue showed no artifact formation upon distillation, but some of the inorganic tracer was converted to methylmercury during alkaline extraction. Acid extraction of sediments resulted in low artifact formation rates. Fractionated measurements of sediment distillates revealed high methylmercury formation rates towards the end of the distillation process when acid concentrations in the solution are highest. A technique for correction of the measured apparent methylmercury content applying species specific isotope addition (SSIA) is proposed and the calculation scheme is presented.