Methylmercury Isotopic Analysis in Environmental and Biological Samples
Advanced speciation-aware isotopic analysis to trace methylmercury sources, differentiate transport pathways, and better understand mercury cycling across complex ecosystems.
Why measure methylmercury isotope ratios in Organisms and Potential Source Materials?
Measuring methylmercury isotope ratios in organisms and potential source materials provides a more direct framework for understanding how mercury enters, moves through, and transforms within food webs.
Source Attribution within Food Webs:
To distinguish among different basal resources (e.g., sediment, biofilm, leaf litter) as sources of MeHg to the food web.
Differentiating Mercury Inputs at Contaminated Sites:
To distinguish between legacy contamination and ongoing inputs as contributors of bioavailable mercury that is methylated and accumulated in biota.
Atmospheric Deposition Pathways:
To differentiate between wet (precipitation) and dry atmospheric deposition as sources of bioavailable mercury at background or remote sites.
Understanding Biogeochemical Transformations:
To infer the relative importance of microbial methylation, microbial demethylation, and photochemical demethylation in controlling MeHg cycling within ecosystems.
Limitations of Indirect Estimation Methods:
Estimating MeHg isotope ratios in organisms using mass balance or linear regression requires assumptions of single-source mercury inputs (Tsui et al., 2012; Kwon et al., 2015). However, evidence indicates that organisms within the same ecosystem may assimilate mercury from multiple isotopically distinct sources (Tsui et al., 2013; Laffont et al., 2021; Rosera et al., 2022; Crowther et al., 2023), rendering such estimation approaches unreliable in complex food webs.
Enhanced Analysis for Low MeHg Samples:
Isotopic analysis of isolated MeHg is particularly valuable for lower trophic level organisms and environmental matrices with low MeHg-to-total mercury ratios, where total mercury isotopic composition is dominated by the inorganic mercury signal.
Mass Balance Applications:
In samples where MeHg comprises less than 85% of the total mercury, concurrent measurement of MeHg and total mercury isotope ratios enables mass balance calculations to infer inorganic mercury isotope ratios. These values can be used to trace sources of inorganic mercury to the food web. In some cases, direct measurement of inorganic mercury isotope ratios is also feasible.
ANALYTICAL TECHNIQUES
Automated methylmercury analysis with MC-ICP-MS
We are utilizing an automated methylmercury analyzer (MERX-III-M, Brooks Rand Instruments) coupled to a Multicollector-ICPMS. This online method requires significantly less Hg per sample and provides uncertainties of 0.1 to 0.15‰ 2SD δ202Hg for both methylmercury and inorganic mercury.
The automated sample introduction system separates and preconcentrates mercury species from the sample matrix before their isotopic measurements by MC-ICP-MS.
Using this system, uncertainty (2SD) in MeHg & iHg isotope measurements is not very sensitive to concentration matching between samples and bracketing standards, though we still do that in our analyses.
In addition, uncertainty (2SD) in MeHg isotope measurements was roughly similar across samples containing 100%, 66%, 50%, & 33% MeHg, and likewise for iHg isotope measurements across samples containing 66%, 50%, 33%, & 0% MeHg (relative to total Hg). This is important as samples almost always have varying concentrations of each species.
This method was validated in-house and ISO 17025 accredited, adding maximum data defensibility to each project.
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