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

A new method has been developed for the accurate and precise measurement of sulfur isotopes (S-32, S-33, S-34) in solids on a scale down to 80-100 pin 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 Cl-37/Cl-35 standard solution. The second employs high mass resolution by sector field mass spectrometry to avoid the interfering O2+ isobars with a Si-30/Si-29 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 delta(34)S(V-CDT) and delta(33)S(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.