Comparison of different mass bias correction procedures for the measurement of mercury species-specific isotopic composition by gas chromatography coupled to multicollector ICP-MS†

Mercury is a toxic element that can have negative effects on the environment and human health. The study of its isotopic composition provides essential information on the origin and distribution of Hg species in the environment. The determination of precise and accurate Hg species-specific isotope ratios can be carried out by coupling chromatographic techniques with multicollector ICP-MS. The preferential transmission of heavier versus lighter isotopes at the ICP-MS interface influences the isotope ratio calculation. This so-called mass bias effect must be corrected to obtain accurate and precise Hg isotope ratios. When dealing with transient signals, such as those obtained from the coupling of gas chromatography with MC-ICP-MS, mass bias has a higher impact on isotope ratio accuracy and precision than when working with continuous signals. Accuracy and precision of compound-specific isotope ratios can be improved by calculating isotope ratios from the slope of a linear regression between two isotopic signals. However, mass bias correction using this strategy needs a careful evaluation that has not been reported thus far. We demonstrate here that the classical mass bias correction based on a point-by-point isotope ratio calculation does not correct for variations of the Tl isotope ratio during Hg elution. Better internal precision in the Hg(II)-specific isotope ratios was obtained applying the mass bias correction models of Russell and Baxter and the linear regression approach for Tl isotope ratios. However, a large range of data points for Tl isotope ratios along the chromatographic peak profile must be selected to improve the internal accuracy of the Hg(II)-specific isotope ratio measurements. The standard-sample bracketing and Baxter approaches combined with the Russell and Baxter models were tested for the calculation of Hg(II)-specific delta values and did not show significant differences in terms of accuracy and precision which were evaluated with the analysis of NIST 3133 vs. NIST 3133 and NIST 8610 vs. NIST 3133, respectively.