Instights from Atmospheric Mercury Isotopes: Analytical Considerations

Studying atmospheric mercury (Hg) isotopic composition is crucial for understanding the complex biogeochemical cycling of mercury, a global pollutant with significant environmental and health impacts. Mercury isotopes provide valuable information about the sources, transformation processes, and deposition pathways of Hg in the environment, allowing researchers to trace its movement and identify the contributions from natural versus anthropogenic sources. Studies of atmospheric Hg isotopic composition have gained significant momentum in recent years, with over 80% of the relevant literature published within the past 7 years. While the analytical capabilities of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) for Hg isotopic measurements have been extensively investigated and reviewed, certain analytical challenges remain, particularly related to the preconcentration methods used prior to MC-ICP-MS analysis. Our study investigates these constraints, focusing on which preconcentration techniques are optimal for measuring isotopic compositions of various atmospheric Hg forms. The results indicate that direct gold sampling, which is typically used to measure gaseous elemental mercury (GEM), also captures 20-80% of gaseous oxidized mercury (GOM) unintentionally. Thereby, a fraction between GEM and total gaseous mercury (TGM) is measured. Similarly, particulate-bound mercury (PBM) membrane sampling not only measures PBM but also inadvertently captures over 50% of GOM. These findings suggest that many current datasets may not represent pure end-members but rather mixed samples of GEM/TGM or PBM/GOM, potentially leading to inaccuracies in isotopic mixing models. Based on our findings, we offer several recommendations for future analyses to improve the accuracy and reliability of atmospheric Hg isotope studies.