Application of stable isotopes in environmental tracer studies – Measurement of monomethylmercury (CH3Hg+) by isotope dilution ICP-MS and detection of species transformation
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The monovalent cation monomethylmercury (CH3Hg+) was determined in certified reference materials by isotope dilution GC/ICP-MS and good agreement between measured and certified values has been found. The use of enriched stable isotopes with subsequent detection by ICP-MS is a powerful tracer technique to study dynamic environmental processes. For the first time, it was possible to monitor opposite processes like Hg2+ methylation and CH3Hg+ demethylation at ambient tracer levels simultaneously in the same sample. A scheme for calculating the formation of new species from stable tracers used in environmental studies is presented. The sensitivity of stable tracer methods is superior to traditional tracer or radiotracer techniques. In case of mercury methylation, where the generation of a new compound is monitored, the limit of detection depends only on the precision of the isotope ratio measurements and the concentration of the ambient CH3Hg+ already present in the sample, not on the absolute detection limit of the GC/ICP-MS technique used for analyzing CH3Hg+. A 0.25% change in concentration of CH3Hg+ is detectable. In the case of CH3Hg+ demethylation, where the decrease of the added tracer is monitored, the detection limit again depends on the precision of the isotope ratio measurement and the ambient CH3Hg+ concentration but additionally on the amount of the added tracer as well. A decrease in the CH3Hg+ level of 2% of the added tracer is detectable. The validity of the calculation scheme was tested and no difference was found between individually measured isotope concentrations and calculated concentrations from solutions with multiple stable tracers.
KeywordsStable Isotope Isotope Dilution Monovalent Cation Isotope Ratio Measurement Mercury Methylation
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