In situ determination of sulfur isotopes in sulfur-rich materials by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Abstract

A new method has been developed for the accurate and precise measurement of sulfur isotopes (³²S, ³³S, ³⁴S) in solids on a scale down to 80–100 μm by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The method was developed independently on two different sets of instrumentation, both of which give equivalent results with comparable accuracy and precision. The first instrumental set-up utilizes Xe gas in a hexapole collision and reaction cell for interference attenuation coupled with a mass discrimination correction by external normalization using a nebulised vapour of ³⁷Cl/³⁵Cl standard solution. The second employs high mass resolution by sector field mass spectrometry to avoid the interfering O₂⁺ isobars with a ³⁰Si/²⁹Si standard aerosol for external normalization. The external isotope mass discrimination correction was applied using the exponential law and was further calibrated for both sets of instrumentation by linear interpolation in a sample–standard bracketing method. Mean δ³⁴S_V-CDT and δ³³S_V-CDT show excellent agreement (within analytical error, typically 0.6 and 1.5‰, respectively) with compiled data for IAEA-S series AgS standard reference materials. Results for NIST SRM 127 (sulfate) were less accurate when calibrated against the IAEA-S series sulfides, whilst significant and consistent deviations in accuracy of up to 3‰ were observed in both sets of instrumentation for Soufre de Lacq SRM sulfur. Such generic matrix effects may be widespread in LA-ICP-MS due to differential ablation rates, particle formation, particle transport efficiency and ionization efficiency in an argon plasma.