Overcoming Polyatomic Interferences in Sulfur Isotope Measurement Using MC-ICP-MS

Abstract

Accurate and precise sulfur isotope (³²S, ³³S and ³⁴S) measurement using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) is often challenged by the formation of polyatomic ions, leading to isobaric interferences that overlap with sulfur isotopes of interest. Traditionally, high-resolution mode is required to resolve sulfur isotopes from these interferences, but achieving high resolving power typically comes with a substantial reduction in analyte sensitivity. In this study, we address the challenge of spectral interferences by adjusting plasma operating conditions (quantified using the Normalised Argon Index, NAI), thereby significantly mitigating the formation of polyatomic ions. As a result, δ³⁴S and δ³³S can be accurately measured at the peak centre in low-resolution mode. The use of low-resolution mode leads to an approximately threefold increase in sensitivity compared with conventional high-resolution measurement. This sensitivity improvement is particularly advantageous for analysing samples with low sulfur mass fractions, or where sample material is limited such as ice cores. In addition, the optimised plasma condition provides a predictable pattern of instrumental mass fractionation, which is essential for accurate mass bias correction using the standard-sample bracketing technique. Overall, our new approach simplifies the analytical workflow and minimises instrument wear, offering a sensitive, user-friendly, and high-throughput method for sulfur isotope measurement with MC-ICP-MS.