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Coupling of nitric acid digestion and anion-exchange resin separation for the determination of methylmercury isotopic composition within organisms

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Abstract

Isotope ratios of methylmercury (MeHg) within organisms can be used to identify sources of MeHg that have accumulated in food webs, but these isotopic compositions are masked in organisms at lower trophic levels by the presence of inorganic mercury (iHg). To facilitate measurement of MeHg isotope ratios in organisms, we developed a method of extracting and isolating MeHg from fish and aquatic invertebrates for compound-specific isotopic analysis involving nitric acid digestion, batch anion-exchange resin separation, and pre-concentration by purge and trap. Recovery of MeHg was quantified after each step in the procedure, and the average cumulative recovery of MeHg was 93.4 ± 2.9% (1 SD, n = 28) for biological reference materials and natural biota samples and 96.9 ± 1.8% (1 SD, n = 5) for aqueous MeHgCl standards. The amount of iHg impurities was also quantified after each step, and the average MeHg purity was 97.8 ± 4.3% (1 SD, n = 28) across all reference materials and natural biota samples after the final separation step. Measured MeHg isotopic compositions of reference materials agreed with literature values obtained using other MeHg separation techniques, and MeHg isotope ratios of aqueous standards, reference materials, and natural biota samples were reproducible. On average, the reproducibility associated with reference material process replicates (2 SD) was 0.10‰ for δ202MeHg and 0.04‰ for Δ199MeHg. This new method provides a streamlined, reliable technique that utilizes a single sample aliquot for MeHg concentration and isotopic analysis. This promotes a tight coupling between MeHg concentration, %MeHg, and Hg isotopic composition, which may be especially beneficial for studying complex food webs with multiple isotopically distinct sources of iHg and/or MeHg.

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Acknowledgements

We would like to thank Teresa Mathews, senior scientist and group leader of the Biodiversity and Ecosystem Health Group within the Environmental Sciences Division at Oak Ridge National Laboratory, for helping facilitate our collection of aquatic invertebrate samples from East Fork Poplar Creek and for supplying the fish tissue sample used in this study. Patrick Donovan, Spencer Washburn, and Aaron Kurz also provided valuable assistance with the collection of benthic invertebrates used in this study. We would also like to thank Paul Drevnick for the use of the Brooks Rand automated MERX MeHg analyzer. This manuscript was improved substantially thanks to the thoughtful comments of two anonymous reviewers.

This research was supported by the U.S. Department of Energy (DOE), Office of Science, Biological and Environmental Research (BER), Subsurface Biogeochemical Research (SBR) program under Award No. DE-SC0016489 and is also a product of the Critical Interfaces Science Focus Area (SFA) at Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, LLC for the DOE under Contract No. DE-AC05-00OR22725. Additional funding was provided by the Geological Society of America Graduate Student Research Grant (2019), the University of Michigan (UM) Rackham Graduate Student Research Grant (2019), the UM Scott Turner Award (2020 and 2021), the Anchor QEA Scholarship (2021), and the John D. MacArthur Professorship.

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  1. Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI, 48109-1005, USA

    Elizabeth R. Crowther, Jason D. Demers, Joel D. Blum & Marcus W. Johnson

  2. Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, 8 College Rd., Durham, NH, 03824-2600, USA

    Jason D. Demers

  3. Environmental Science Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN, 37831-6038, USA

    Scott C. Brooks

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Crowther, E.R., Demers, J.D., Blum, J.D. et al. Coupling of nitric acid digestion and anion-exchange resin separation for the determination of methylmercury isotopic composition within organisms. Anal Bioanal Chem 415, 759–774 (2023). https://doi.org/10.1007/s00216-022-04468-8

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