High precision Hg isotope analysis of environmental samples using gold trap-MC-ICP-MS

Abstract

Precise and accurate analysis of Hg isotope compositions in environmental samples can be an invaluable tool in investigating Hg cycles in the environment. We have developed a method for high precision Hg isotope analysis using a gold trap sample introduction system coupled to a Thermo Finnigan MC-ICP-MS. Environmental samples were pyrolyzed at high temperature and Hg released from the samples was collected in gold traps. The Hg on the gold traps was then thermally released into a sample introduction system coupled to an MC-ICP-MS. The sample introduction system extended the Hg signal duration, which would otherwise be a short transient signal if released directly into the plasma, thus significantly improving the precision and accuracy of Hg isotope measurement. Instrumental mass bias was corrected using a Tl internal isotopic standard and a “standard–sample–standard” bracketing technique. Based on the long term measurement of NIST-2225 elemental Hg standard, the external precision ranges from 44 to 109 ppm for ²⁰¹Hg/²⁰²Hg and ¹⁹⁹Hg/²⁰²Hg ratios, respectively. The average mass bias corrected Hg isotope ratios for NIST-2225 are: ¹⁹⁸Hg/²⁰²Hg = 0.335948 ± 87 (2σ), ¹⁹⁹Hg/²⁰²Hg = 0.567423 ± 109, ²⁰⁰Hg/²⁰²Hg = 0.776552 ± 93, and ²⁰¹Hg/²⁰²Hg = 0.443 085 ± 44. These ratios agree within error with the IUPAC values issued in 1983, but differ significantly from the IUPAC values issued in 1997. Our data suggest that there are significant Hg isotope variations among different environmental samples. Specifically, organic samples containing predominantly methylmercury appear to be enriched in light isotopes compared with samples with inorganic Hg, suggesting that biomethylation may play an important role in Hg isotope fractionation in the environment.