An optimized protocol for high precision measurement of Hg isotopic compositions in samples with low concentrations of Hg using MC-ICP-MS

Abstract

Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) enables the precise determination of mercury (Hg) isotopic compositions in different types of samples. However, isotopic measurements in samples with low Hg concentrations and small sample mass remain a challenge. In this study, we developed an optimized protocol for high precision Hg isotope determination using a modified cone arrangement (X skimmer cone + jet cone) with Neptune Plus MC-ICP-MS. Through modification of the HGX-200 continuous-flow cold-vapor generation system, and careful optimization of the instrument gas flow rates, we obtained steady and high signal sensitivity for Hg (202Hg: 1.78 V per ng mL-1). Our method allowed the precise determination of low Hg concentration in solutions (0.10 ng mL-1), which is the lowest according to the literature. Only 0.70 ng Hg in samples was required according to our new analytical method, which enables the direct measurement of low Hg concentration in samples (down to 5 ng g-1), after direct acid digestion treatment. Using our method, the Hg isotopic compositions of four igneous rock standard reference materials (BCR-2, BHVO-2, GSP-2, and GSR-2) were reported for the first time. The standard reference materials showed large variations of δ202Hg (-1.24 to -2.47‰), indicating that mass dependent fractionation (MDF) of Hg isotopes occurred during magmatic processes.