Sulfate and sulfide sulfur isotopes (δ34S and δ33S) measured by solution and laser ablation MC-ICP-MS: An enhanced approach using external correction

Isotope ratio measurements using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) commonly use standard-sample bracketing with a single isotope standard for mass bias correction for elements with narrow-range isotope systems measured by MC-ICP-MS, e.g. Cu, Fe, Zn, and Hg. However, sulfur (S) isotopic composition (delta S-34) in nature can range from at least-40 to + 40%, potentially exceeding the ability of standard-sample bracketing using a single sulfur isotope standard to accurately correct for mass bias. Isotopic fractionation via solution and laser ablation introduction was determined during sulfate sulfur (S-sulfate) isotope measurements. An external isotope calibration curve was constructed using in-house and National Institute of Standards and Technology (NIST) Ssulfate isotope reference materials (RM) in an attempt to correct for the difference. The ability of external isotope correction for S-sulfate isotope measurements was evaluated by analyzing NIST and United States Geological Survey (USGS) S-sulfate isotope reference materials as unknowns. Differences in delta S-34(sulfate) between standard-sample bracketing and standard-sample bracketing with external isotope correction for sulfate samples ranged from 0.72% to 2.35% over a delta S-34 range of 1.40% to 21.17%. No isotopic differences were observed when analyzing S-sulfide reference materials over a delta S-34(sulfide) range of -32.1% to 17.3% and a delta S-33 range of-16.5% to 8.9% via laser ablation (LA)-MC-ICP-MS. Here, we identify a possible plasma induced fractionation for Ssulfate and describe a new method using external isotope calibration corrections using solution and LA-MC-ICP-MS. Published by Elsevier B.V.