High-precision low-level Cd isotopic analysis using MC-ICP-MS and its application to marine samples from Terra Nova Bay (Antarctica)

Abstract

This study presents the development, validation and use of an approach for precise and accurate cadmium (Cd) isotopic analysis at low concentration levels using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) equipped with a standard sample introduction system, an ICP operated under wet plasma-conditions, and Faraday cup amplifiers equipped with 10^13 Ω resistors. One-column anion exchange chromatography allowed isolation of Cd with ≥ 80% recovery and absence of potential on-column fractionation on the isotope ratio results was demonstrated. Use of both an internal (using Ag) and an external (measured in a sample-standard bracketing sequence) standard was relied on for correction for instrumental mass discrimination. A long-term precision of 0.09‰ (2SD) for δ114/110Cd was achieved at a Cd concentration of 10 ng mL-1. The method developed was validated by analysing NIST 2711a (Montana soil) and TORT-3 (Lobster hepatopancreas) reference materials, yielding results consistent with literature values. Subsequently, the method was applied to two Antarctic marine organisms Adamussium colbecki and Trematomus bernacchii, collected during both the 1990s and 2020s, to investigate potential changes in the biogeochemical cycle of Cd over time and evaluate the potential for identifying potential natural or anthropogenic sources. A preferential uptake of the lighter Cd isotopes in both species was observed, indicated by negative δ114/110Cd values ranging between -0.24 and -0.09‰. This finding is consistent with previous studies that have reported Cd fractionation during its uptake by marine organisms, with a preference for lighter isotopes. No significant differences in δ114/110Cd‰ values were observed between organs of the same species or between the same species collected in the different decades, suggesting minimal Cd isotopic fractionation during internal transfer and consistent Cd sources over time. Comparison with literature data suggests that the Cd source in Antarctic biota may be predominantly of natural origin, with δ114/110Cd‰ values indicating isotopically heavier Cd than that found in Cd-polluted areas. However, further Cd isotope ratio data from various Antarctic sample types are necessary to further evaluate the Cd sources in marine samples.