Measurement of compound-specific Hg isotopic composition in narrow transient signals by gas chromatography coupled to multicollector ICP-MS

Abstract

The study of the natural variations of Hg isotopic composition is a valuable tool to understand the Hg biogeochemical cycle. Hg mobility, toxicity and bioaccumulation depends on its chemical form, so compound-specific Hg isotope ratio measurements, by coupling chromatographic techniques to multicollector ICP-MS, provide an extra degree of information in comparison to total Hg isotope ratio measurements. We present in this work a thorough evaluation of the main parameters affecting the accuracy and precision of Hg compound-specific isotope ratio measurements by Gas chromatography (GC) coupled to MC-ICPMS. The main parameters evaluated in this work were the chromatographic peak width, integration time, number of acquisition points and data treatment strategy. A new method for the correction of the time lag between Faraday cups, responsible for isotope ratio drift during peak elution, is proposed and evaluated under different experimental conditions. A standard-sample-standard bracketing approach was applied to calculate the delta values from the mass bias corrected Hg(II)-specific isotope ratios, using NIST-3133 as the delta zero standard. The optimized conditions were obtained when working with regular GC peaks (2–5 s at the peak base), 0.131 s as integration time, 321–641 acquisition points and calculating the isotope ratios using the slope of the linear regression obtained when plotting two isotopic signals. The accuracy and precision of the optimized methodology were determined with the analysis of the secondary standard NIST RM-8610 (UM-Almaden) versus the delta zero standard NIST-3133. When injecting 2 μL of a Hg(II) solution of 250 ng g⁻¹ Hg(II)-specific delta values in agreement with the reference values were obtained with an external reproducibility (expressed as 2SD) of 0.34–0.40‰. Our results demonstrate that the measurement of Hg isotope ratios in narrow GC peaks provides the same level of accuracy and external reproducibility of Hg compound specific delta values in comparison with previously published approaches based on wide GC peaks, therefore offering shorter analysis times and higher chromatographic resolution than those previously obtained with wide chromatographic peaks.