A novel sample preparation method for ultra-high vacuum (UHV) secondary ion mass spectrometry (SIMS) analysis†
Secondary ion mass spectrometry (SIMS) has been applied to analyze a wide range of materials for earth science research due to its high sensitivity, high precision and capacity for in situ micro-analysis. This technique operates under ultra-high vacuum (UHV) conditions, especially for water content measurements on nominally anhydrous minerals (NAMs). However, UHV conditions are hard to achieve when using epoxy mounts, which degas readily during the vacuum processes. In this study, we developed a novel sample preparation method using a tin-based alloy instead of epoxy resin. Using a tin alloy mount coupled with an automatic liquid nitrogen refilling system, the results show that the vacuum conditions have been significantly improved from ∼1.2 × 10−8 mbar to ∼1.9 × 10−9 mbar. The background for mineral water content, measured with a CAMECA IMS 1280-HR SIMS installed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), is lowered to <10 ppm. Moreover, tin alloy mounts are harder and easier to polish than indium mounts, which was commonly used for previous UHV analyses. A relief of <2 μm is yielded for tin alloy mounts, which is essential for high precision isotope analysis. Our results also show that the routine external precision of oxygen isotope ratios is better than 0.15‰ (1SD), comparable with that obtained using epoxy mounts.