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

Abstract

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.