Isotopic fractionation of Sn during methylation and demethylation reactions in aqueous solution

Laboratory experiments, modeling the methylation of inorganic Sn(II) by methylcobalamin and the decomposition of methyltin under irradiation with UV light in aqueous solution, have been performed. Methyltin has been separated from inorganic Sri using anion-exchange chromatography and subjected to Sri isotope ratio measurements via solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The methylation of Sn(II) in the dark was accompanied by mass-dependent Sri isotopic fractionation, which resulted in preferential partitioning of the lighter Sri isotopes into the organic phase, with a shift of similar to 0.57 +/- 0.12%D in terms of delta(124)/Sn-116 between methylated and inorganic Sn. The methylation of Sn(II) by methylcobalamin under UV irradiation resulted in the accelerated formation of methyltin in the beginning of the process, but was followed by the photolytic degradation of methyltin until its complete mineralization. The photolytic degradation of methyltin in the presence of methylcobalamin and inorganic Sn(II) was slower than that of pure solutions of commercially obtained monomethyltin. This is attributed to the methylating action of methyl radicals produced from photolytically decomposing methylcobalamin. Both synthesis and decomposition of methyltin under UV irradiation Were accompanied by both mass-dependent and mass-independent Sri isotopic fractionation, with the latter due to the magnetic isotope effect. As a result of this, the lighter Sri isotopes preferentially partition into reaction products, while the odd isotopes, Sn-117 and Sn-119, are selectively enriched relative to the other isotopes in the starting molecules. The extent of the observed variations in the isotopic composition of Sri is larger than that documented previously for geological and archeological samples. These results indicate that Sri isotopic fractionation between various chemical forms of Sri in the natural aquatic systems may be significant and can provide new insights into the biogeochemical cycling of the element.