Matrix effect investigation and accurate microanalysis of Sn isotope in cassiterite and bronze by fsLA-MC-ICP-MS

Abstract

This study investigated matrix effects during in situ Sn isotope analysis using a femtosecond (fs) laser ablation system coupled to a multi-collector ICP-MS. Contrary to the common perception of “matrix-free” analysis, matrix effects are evident during in situ analysis. The measured δ124Sn/120Sn ratios of cassiterite and bronze deviated from the true values by up to 0.7‰ and 0.5‰, respectively, when using a pure tin metal as the bracketing standard. Introducing water vapour into the carrier gas substantially reduced these matrix effects, though minor deviations, from -0.24‰ to -0.33‰, persisted for cassiterite and bronze when tin metal was used as a non-matrix-matched calibration under wet plasma conditions, indicating that ablation-related processes also contribute to matrix effects. Therefore, matrix-matched reference materials are required for high-precision and accurate Sn isotope analyses of cassiterite and bronze samples. We evaluated four candidate samples, NWU-CAS (a natural cassiterite), NWU-Sn (tin metal plate), GBW02140 and GBW02137 (two certified bronze reference material from the National Research Center for Certified Reference Material of China). Repeated analyses of the two candidates by fsLA-MC-ICP-MS yielded good external reproducibility of <0.10% (2SD) for δ124Sn/120Sn, validating their suitability as matrix-matched reference materials for in situ Sn isotope analysis in cassiterite and bronze.