Measurement Of Sulfur Isotope Ratios In Trace Amounts Of Dissolved Sulfate By Mc-Icp-Ms

Sulfur is a versatile element with valance states ranging from +6 to -2 and thereby involved in diverse geochemical and biological processes as an electron donor or acceptor. Since these reactions are often accompanied by fractionation of its four stable isotopes (S-32, S-33, S-34, S-36), their relative abundance has been used to trace the sulfur cycle at a range of temporal and spatial scales. While the use of Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) has emerged as a promising approach for measuring the triple sulfur isotope ratios of samples with small amounts of sulfate, MC-ICP-MS has never been used for sulfur isotope analysis in South Korea. Here we first present the results of triple sulfur isotope analysis using a MC-ICP-MS installed in the Inter-university Research Facilities at Seoul National University. To minimize isobaric interferences, sulfate is separated from matrix cations by anion exchange resin and introduced into MC-ICP-MS via desolvator, and sulfur isotopes are measured as S+ ions on the low-mass shoulder of the aligned peaks in medium resolution mode (M/Delta M* similar to 8000). Standard-sample bracketing is used to correct instrumental mass bias, and the sulfur isotope ratio of the in-house bracketing standard is evaluated by comparing it with the international reference material IAEA-SO-5. Typical reproducibility (2 sigma) for delta S-34 and Delta S-33 analyses is better than 0.44 parts per thousand and 0.57 parts per thousand, respectively, and the sulfur isotope ratios of the reference materials IAEA-SO-6 and NBS-127, determined against the in-house bracketing standard, agree well with the reported values within errors. Although purification and isotope analysis of the samples containing down to 14 nmol sulfate achieves consistent results, great care to avoid contamination is needed when the sample contains only a few nmols of sulfate. With the new methodology being available, sulfur isotope studies in South Korea could be further extended to samples with lower sulfur contents, such as meteoric water, ice cores, and aerosols.