Determination of mercury stable isotopes in environmental samples

Analysis of Mercury Stable Isotopes using a Thermo Neptune Multicollector Inductively Coupled Mass Spectrometer (MC-ICP-MS)

The following standard operating procedure is used by the U.S. Geological Survey’s Mercury Research Laboratory (MRL) to determine the mercury (Hg) stable isotopic compositions in solid matrices (biological tissue, sediments, and soils).  This SOP describes the preparation of samples via hot acid digestions and subsequent preparation for isotope analysis. Following digestions samples are diluted to (0.5 to 2 ng mL^-1) and <15% final acid content. Samples containing excess oxidant, including bromine monochloride (BrCl), are neutralized prior to analysis with hydroxylamine hydrochloride. Samples are paired with a Hg standard solution (NIST 3133) of the same concentration and acid content, such that sample-standard bracketing (SSB) is performed. Samples and standard solutions are introduced to a custom-made gas liquid separator (GLS) using a peristaltic pump. During introduction the sample is mixed in line with stannous chloride (SnCl₂) which reduces the Hg²⁺ to the volatile Hg⁰ form, which is then introduced into the MC-ICP-MS plasma using a counter stream of argon (Ar). Thallium (Tl) is also introduced into the GLS as a mass bias correction for isotope measurements. The Neptune is tuned to optimal voltage, stability, and peak shape prior analysis. Quality assurance and control protocols are employed through sample preparation and analysis including: laboratory practices to prevent contamination, analysis of certified reference materials, analytical replication, and use of secondary standard NIST 8610 (UM) to confirm accuracy and precision of measurements.

Mercury Research Laboratory, 2021