Mercury Stable Isotope Ratio Testing

Environmental forensics is the application of scientific techniques to identify the sources, distribution, and movement of pollutants in the environment. One emerging tool in this field is the use of mercury isotopes. Mercury (Hg), a toxic metal, has been historically used in various industrial and agricultural applications, and its presence in the environment can provide valuable clues about pollution sources and pathways. Mercury isotope ratios are used as a tool in an increasing number of field studies for source attribution and transport processes in stream systems contaminated by different sources such as former Hg mines, gold mining or processing activities, chlor-alkali or other industrial activities(1, 2). The determination of natural variations in mercury isotope ratios is a rapidly emerging area of research, opening new avenues for studying the fate of mercury in the environment(1). Mercury isotope signatures can be used as tracers for sources and processes, and can help understand the global mercury cycle(1).

Brooks Applied Labs currently offers mercury (Hg) stable isotope ratio testing for environmental forensic studies. Mercury has a unique isotopic pattern and small differences in the ratios of these isotopes can help in identification of the source, transport, and biogeochemical fate of mercury in atmospheric bulk deposition, soils, freshwater, and seawater matrices.

Isotopic analysis of mercury provides the following forensic insights:

Brooks Applied Labs utilizes a Nu 1700 or a Nu Plasma HR Multi Collector ICPMS to achieve very high accuracy and precision for mercury isotopes. Our results include all appropriate QA/QC which is usually not available from academic institutions.

NIST RM 8610 (UM-Almaden) is used by Brooks Applied Labs as a secondary standard. Mercury isotopic composition of this standard is measured using cold vapor multiple collector inductively coupled plasma mass spectrometry (CV-MC-ICP-MS). Uncertainty in the average isotope ratios measured by Brooks Applied Labs is represented by 2SD across all analysis replicates. Certified values are provided for comparison. 

Mass-dependent isotope fractionation (MDF) is reported as the permil (‰) deviation from the average of NIST SRM 3133 bracketing standards using delta notation:

δ^xxxHg (‰) = ([(^xxxHg∕¹⁹⁸Hg)_sample ∕ (^xxxHg∕¹⁹⁸Hg)_{NIST SRM 3133}] − 1) ∗ 1000

where xxx is the mass of each mercury isotope between ¹⁹⁹Hg and ²⁰⁴Hg.

Mass-independent isotope fractionation (MIF) is reported as the difference between the measured δ^xxxHg value and that which is theoretically predicted by the kinetic mass-dependent fractionation law using capital delta notation:

Δ^xxxHg (‰) ≈ δ^xxxHg − (δ²⁰²Hg ∗ β)

where xxx is the mass of each mercury isotope ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg, and ²⁰⁴Hg, and β is a constant for each isotope (0.2520, 0.5024, 0.7520, and 1.493, respectively) (Blum and Bergquist 2007).

Use of mercury stable isotope ratio testing can often decrease the scope of work for investigative projects, ultimately reducing the cost of the investigation and increasing the confidence in results due to the absolute nature of the testing procedure. This is especially important when dealing with tort and environmental forensic litigation cases. In addition, institution of proper sampling plans is a necessity to ensure all localized variables are identified to account for as many variables as possible. 

For project inquiries, applicability of methodologies, and pricing information feel free to contact us.

References:

1. https://doi.org/10.1525/california/9780520271630.003.0004 “Use of Stable Isotopes in Mercury Research” – Oxford Academic
2. https://doi.org/10.3389/fenvc.2022.1058890 “Large extent of mercury stable isotope fractionation in contaminated stream sediments induced by changes of mercury binding forms”