A Pb-free Sn–Bi alloy mount preparation method for secondary ion mass spectrometry (SIMS) analyses†

Sample preparation is a critical step to achieve reliable in situ chemical analysis. Sample mounting technique with a tin-based alloy was developed in recent years, which is particularly useful for high-precision volatile analyses by secondary ion mass spectrometry (SIMS). However, the success of this technique is hindered by challenges, such as complex alloy preparation and potential Pb contamination. Herein, we introduce a new Sn–Bi alloy preparation method that may overcome these hurdles and assess its potential as a standard preparation method for in situ volatile and isotope analyses. This new alloy can be manufactured with commercially available pure tin and bismuth metal (atomic Sn : Bi = 42 : 58), and its production requires only a heating plate and clean containers. This ensures its high accessibility to laboratories worldwide. The Pb content of the alloy is dependent on the tin and bismuth used. The material (Sn and Bi) from three different manufacturers were evaluated in this study, resulting in the virtually Pb-free MAC alloy (Pb <0.2 μg g⁻¹). The SIMS U–Pb dating results of the zircon standards (Qinghu, Plešovice, and SA01) are consistent with the recommended values (within error). Furthermore, the mounted samples exhibit satisfactory relief on this alloy, suggesting that this alloy material is appropriate for the analysis of oxygen isotopes. The routine external precision of oxygen isotope ratios is better than 0.30‰ (2sd), on par with that obtained with epoxy mounts. The water background in the SIMS sample chamber can be recovered rapidly after sample transfer from the storage to the sample chamber. Hence, this tin-based alloy is suitable for sample mounting for SIMS volatile and isotope (incl. U–Pb) analyses.