Accurate determination of sulfur isotopes (delta S-33 and delta S-34) 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 (similar to 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) N-2 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 mm showed excellent agreement with the reference values (the accuracy of 0.01-0.15 parts per thousand for delta S-34 and 0.11-0.45 parts per thousand for delta S-33 and the precision of 0.16-0.40 parts per thousand (2 s) for delta S-34 and 0.35-0.78 parts per thousand (2 s) for delta S-33) and the mass-dependent fractionation line, validating the applicability of the proposed approach for providing high-quality in situ isotope data (delta S-33 and delta S-34) of sulfides and elemental sulfur at high spatial resolution using non-matrix matched analysis.