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 (δ³⁴S) 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) S_sulfate 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 δ³⁴S_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 δ³⁴S range of 1.40‰ to 21.17‰. No isotopic differences were observed when analyzing S_sulfide reference materials over a δ³⁴S_sulfide range of − 32.1‰ to 17.3‰ and a δ³³S range of − 16.5‰ to 8.9‰ via laser ablation (LA)-MC-ICP-MS. Here, we identify a possible plasma induced fractionation for S_sulfate and describe a new method using external isotope calibration corrections using solution and LA-MC-ICP-MS.