High-Precision Measurement of Sn Isotopic Compositions in Cassiterite and Igneous Rock Reference Materials by Double Spike Technique Using MC-ICP-MS/MS

Abstract

Tin (Sn) isotopic analysis offers profound insights into geochemical and cosmochemical processes, as well as into metallurgical history or archeometry. Yet the accurate determination of Sn isotopic composition is hindered by Sn volatilization, the resistance of cassiterite to dissolution, and purification-induced fractionation. We present a protocol to obtain high-precision Sn isotopic ratios using a 117Sn‒122Sn double spike technique and a Thermo Fisher Scientific Neoma multi-collector inductively coupled plasma-mass spectrometer equipped with a collision/reaction cell and pre-cell mass filter (MC‒ICP‒MS/MS). Tin was purified from sample matrices with a specific ion exchange resin, while minimizing evaporation steps and mitigating isotope loss. Evaporation‒re‒dissolution tests show that pure Sn solution suffers substantial losses at elevated temperatures, whereas mineral and rock sample matrices stabilize Sn with near-quantitative recoveries up to 120 °C. Concentration mismatch tests reveal a positive linear relationship between sample-to-standard concentration ratio and δ122/118Sn3161a, with δ122/118Sn3161a offsets limited to less than 0.02 ‰ when the ratios were in the 0.90‒1.10 range. Long-term reproducibility for δ122/118Sn3161a, yielded 0.017 ‰ (2SD, n = 71 over 10 months), while in-house Sn standards gave 0.029 ‰ (2SD, n = 22). The Sn isotope compositions of geological reference materials (BCR-2, BHVO-2, AGV-2, GSP-2, W-2, JA-2) obtained with this protocol show high reproducibility and agree well with reports in other studies. Further, we applied an NaOH fusion-based dissolution method for acid-resistant mineral cassiterite (SnO2), which enables efficient and complete digestion of this phase. The Sn isotope composition of a cassiterite dissolved by NaOH is consistent with that of samples dissolved using the traditional KCN-based digestion. Our results demonstrate that our double-spike protocol provides accurate, reproducible Sn stable isotope measurements and establish a robust framework for future applications in geochemistry, cosmochemistry, and archaeometry.