Simultaneous multi-species determination of trimethyllead, monomethylmercury and three butyltin compounds by species-specific isotope dilution GC-ICP-MS in biological samples

An accurate and sensitive multi-species species-specific isotope dilution GC-ICP-MS method was developed for the simultaneous determination of trimethyllead (Me₃Pb⁺), monomethylmercury (MeHg⁺) and the three butyltin species Bu₃Sn⁺, Bu₂Sn²⁺, and BuSn³⁺ in biological samples. The method was validated by three biological reference materials (CRM 477, mussel tissue certified for butyltins; CRM 463, tuna fish certified for MeHg⁺; DORM 2, dogfish muscle certified for MeHg⁺). Under certain conditions, and with minor modifications of the sample pretreatment procedure, this method could also be transferred to environmental samples such as sediments, as demonstrated by analyzing sediment reference material BCR 646 (freshwater sediment, certified for butyltins). The detection limits of the multi-species GC-ICP-IDMS method for biological samples were 1.4 ng g-¹ for MeHg⁺, 0.06 ng g-¹ for Me₃Pb⁺, 0.3 ng g-¹ for BuSn³⁺ and Bu₃Sn⁺, and 1.2 ng g-¹ for Bu₂Sn²⁺. Because of the high relevance of these heavy metal alkyl species to the quality assurance of seafood, the method was also applied to corresponding samples purchased from a supermarket. The methylated lead fraction in these samples, correlated to total lead, varied over a broad range (from 0.01% to 7.6%). On the other hand, the MeHg⁺ fraction was much higher, normally in the range of 80-100%. Considering that we may expect tighter legislative limitations on MeHg⁺ levels in seafood in the future, we found the highest methylmercury contents (up to 10.6 μg g-¹) in two shark samples, an animal which is at the end of the marine food chain, whereas MeHg⁺ contents of less than 0.2 μg g-¹ were found in most other seafood samples; these results correlate with the idea that MeHg⁺ is usually of biological origin in the marine environment. The concentration of butyltins and the fraction of the total tin content that is from butyltins strongly depend on possible contamination, due to the exclusively anthropogenic character of these compounds. A broad variation in the butylated tin fraction (in the range of <0.3-49%) was therefore observed in different seafood samples. Corresponding isotope-labeled spike compounds (except for trimethyllead) are commercially available for all of these compounds, and since these can be used in the multi-species species-specific GC-ICP-IDMS method developed here, this technique shows great potential for routine analysis in the future.