Accurate determination of sulfur isotopes (δ33S and δ34S) in sulfides and elemental sulfur by femtosecond laser ablation MC-ICP-MS with non-matrix matched calibration

The isotopic composition of sulfur is a vital tracer used in the Earth and planetary sciences. In this study, the laser- and ICP-induced isotopic fractionation in S-rich minerals (sulfides and elemental S) with different matrices was investigated by using 257 nm femtosecond (fs) and 193 nm ArF excimer nanosecond (ns) laser ablation systems coupled to a Neptune Plus MC-ICP-MS. Compared to ns-LA-MC-ICP-MS, higher sensitivity (1.4–2.4 times) under similar instrumental conditions and better precision (∼1.6-fold) under the same signal intensity condition were achieved by fs-LA-MC-ICP-MS. In addition, a fs-laser provides less fluence and matrix dependent S isotopic fractionation, and more stable transient isotopic ratios compared to a ns-laser. Better results acquired by fs-LA-MC-ICP-MS were attributed to the smaller size of particles and less thermal effect produced by using the fs-laser, which were evidenced by the morphologies of the ablation craters and ejected aerosol particles of P-S-1 (the pressed powder pellet of IAEA-S-1) and PPP-1 (a pyrite single crystal from the Sukhoi Log deposit). The ICP-induced isotopic fractionation (matrix effect) was still found in fs-LA-MC-ICP-MS under the maximum sensitivity conditions. However, a significant reduction of the matrix effect was obtained under robust plasma conditions at a lower makeup gas flow rate (0.52–0.54 l min−1) relative to the maximum sensitivity condition (0.6 l min−1) for S isotope analysis. This could be ascribed to the particles that not only pass into the higher temperature ICP for a longer residence time at a lower makeup gas flow rate that resulted in more efficient vaporization of the particles, but also experience a more robust plasma induced by adding 4–6 ml min−1 N2 into the plasma. Furthermore, under the robust conditions, the results of six reference materials with different matrices obtained by fs-LA-MC-ICP-MS with non-matrix matched calibration with a spot size of 20–44 μm showed excellent agreement with the reference values (the accuracy of 0.01–0.15‰ for δ34S and 0.11–0.45‰ for δ33S and the precision of 0.16–0.40‰ (2 s) for δ34S and 0.35–0.78‰ (2 s) for δ33S) and the mass-dependent fractionation line, validating the applicability of the proposed approach for providing high-quality in situ isotope data (δ33S and δ34S) of sulfides and elemental sulfur at high spatial resolution using non-matrix matched analysis.