Simultaneous Determination of Inorganic Mercury, Methylmercury, and Total Mercury Concentrations in Cryogenic Fresh-Frozen and Freeze-dried Biological Reference Materials
David Point, William C. Davis, J. I. Garcia Alonso, Mathilde Monperrus, Steven J. Christopher, O.F. X. Donard, Paul R. Becker, Stephen A. Wise
A double spike speciated isotope dilution (DS-SID) reaction model was developed to study and correct for the inadvertent transformations affecting methylmercury (MeHg), inorganic mercury (iHg) and total mercury (HgT) measurements in biological tissues, using alkaline microwave digestion and gas chromatography inductively coupled plasma mass spectrometry (GC-ICP-MS). Five natural biological standard reference materials (SRMs) representing freeze-dried (SRM 1566b, 2976 and 2977) and fresh-frozen (SRM 1974a, 1974b) mussel and/or oyster tissues were spiked with 201iHg and 202MeHg solutions. A dominant methylation reaction (9.1-11.4%) was observed in freeze-dried materials (FD) with almost no demethylation. Inversely, the apparent demethylation yield was estimated between 12.5-18.4% in fresh-frozen (FF) materials. These difference of behavior indicated that the interplay between potential methylating/demethylating agent or sites, the reactive fraction of iHg and MeHg and the liquid/solid interactions between solute species and reactive particles are important to consider. These results suggest that FF and FD materials are not always commutable using SID measurements. To evaluate the systematic analytical biases affecting the final concentrations of mercury species and HgT, the accuracy and performance between Single and Double/Corrected spike, speciated isotope dilution (SS-ID or DS-SID) was investigated and compared. The application of DS-SID indicated that the concentrations of iHg, MeHg and HgT were not statistically different from the certified and reference concentrations in the five SRMs, exemplifying the robustness of this method together with its increased commutability.