Preparation of sulfur-bearing reference materials for in situ sulfur isotope measurements using laser ablation multicollector inductively coupled plasma–mass spectrometry

In this study, a method for solidifying sulfide and sulfate nanoparticulate powder to prepare bracketing standards and samples for in situ S isotopic analysis using laser ablation multicollector inductively coupled plasma–mass spectrometry (LA-MC-ICP–MS) was developed. Sulfide or sulfate powder was transformed into a stable and mechanically resistant target by mixing it with epoxy resin. Using this method, seven bracketing standards with diverse matrix minerals (pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, barite, and gypsum) were synthesized in the form of resin-preserved powders (RPPs). The effects of ablation behavior, powder particle size, and homogeneity on the S isotopic ratios in these RPPs were rigorously evaluated. The precision of the δ34SVCDT sulfur isotopic results obtained by LA-MC-ICP–MS was better than 0.47‰ (2SD uncertainty, n ≥ 20 over 4 months) for the S-bearing mineral RPPs. Finally, the in situ S isotope results of the seven RPPs measured by LA-MC-ICP–MS were consistent with the values obtained using solution nebulizer (SN)-MC-ICP–MS. The isotopic composition and homogeneity of a range of sulfide mineral RPPs (seven minerals referred to above) were characterized for use as potential reference materials. Our method provides the opportunity to prepare bracketing standards and samples for in situ S isotope analysis for a wide range of sulfur-bearing materials and is particularly appropriate for geologic samples with complex matrices and for which high spatial resolution in situ microbeam analysis is critical. More importantly, this method provides new prospects for the preparation of homogenized reference materials from natural minerals that are urgently needed for matrix-matched calibration and quality control of LA-ICP–MS geochemical and isotope data.