Application of a selective extraction method for methylmercury compound specific stable isotope analysis (MeHg-CSIA) in biological materials

Methylmercury (MeHg) is a key organometallic compound of Hg and a potent neurotoxin that bioaccumulates along food webs. Hg speciation analysis of environmental samples is essential for understanding the sources, dynamics and impact of MeHg in ecosystems. Hg stable isotope analysis is a new tool to try and answer outstanding questions on Hg cycling, but has thus far been applied essentially to total Hg fractions in biota. Hg compound specific stable isotope analysis is a new approach to understand the isotopic fraction of Hg at the molecular level. In this study, we developed a robust, accurate and precise selective extraction method for MeHg in biological samples for subsequent quantification by AFS or ID-GC-ICPMS and determination of the isotopic composition by CV-MC-ICPMS. The method consists of an optimized two-step extraction/back-extraction technique validated using a MeHg standard solution and seven certified reference materials with a large range of total Hg and MeHg values. In the final extract solution, MeHg purification yields were higher than 97%, and accurate MeHg concentrations were obtained for the different certified materials. High precision MeHg stable isotope analysis (2SD approximate to 0.1 parts per thousand) by CV-MC-ICPMS showed a four-fold improvement in precision for this method relative to previous on-line GC-MC-ICPMS measurements. Reference values for total Hg and MeHg compound specific stable isotope values are given for NRCC-TORT-2, NRCC-DOLT-4, IRMM-ERM-CE464, IRMM-BCR-463, IRMM-BCR-414, NIST-SRM-1947 and NIST-QC03LH03 reference materials.