Development of a double-spike procedure for correcting isotopic fractionation induced during sample preparation and measurement of inorganic mercury isotope ratios by gas chromatography coupled to multicollector ICP-MS.

内容摘要

Mercury stable isotope analysis is a powerful tool for tracing Hg sources and transformation processes in the environment. However, Hg species-specific isotope ratio measurements remain challenging due to low analyte concentrations, instrumental mass discrimination, and potential isotopic fractionation during sample preparation, chromatographic separation and isotope ratio measurement. In this study, we develop and evaluate, for the first time, a double-spike protocol based on enriched 199Hg and 201Hg for isotope ratio measurements in a transient signal of Hg(II) obtained by gas chromatography coupled to multicollector ICP-MS. The double spike was fully characterized against NIST SRM-3133 and its performance compared to the conventional correction approach combining Tl internal normalization and sample-standard bracketing. Several GC injection modes (Hot Splitless, Cold Splitless, and Programmed Temperature Vaporization) and plasma conditions (wet and dry) were assessed to evaluate isotopic fractionation effects and sensitivity. Dry plasma increased sensitivity by a factor of 1.5, whereas PTV injections allowed up to tenfold larger sample volumes, improving detection limits at the expense of introducing stronger isotopic fractionation. Results show that the DS + SSB approach provides more robust correction of mass discrimination, particularly under conditions where isotopic fractionation is induced in the injector (PTV and Hot Splitless at 250 °C). When no injector-related fractionation occurs, the traditional Tl + standard-sample bracketing method yields slightly better precision; however, the double spike approach offers improved accuracy under challenging conditions. Overall, the study demonstrates the feasibility and advantages of combining a199Hg and 201Hg double spike with GC-MC-ICP-MS for Hg(II)-specific isotope-ratio measurements. Due to expected low Hg (II) concentrations in real samples the developed DS procedure is the only suitable internal correction of mass discrimination to determine reliable Hg(II)-specific isotope compositions as the use of a PTV injection in which procedural isotopic fractionation is induced is mandatory to achieve the required sensitivity.