A Low-Background, High-Flatness Mounting Method for In Situ SIMS Isotopic Analysis for Fine-Grained Samples

Accurate in situ determination of water content and hydrogen isotopes in minerals is essential for understanding magmatic processes on Earth and other planetary bodies. However, such measurements are technically challenging due to low hydrogen concentrations and contamination from atmospheric water vapor. Common mounting methods─such as alloy mounts, indium mounts, and thin sections─are often unsuitable for fine, loose-grained samples. Here, we present a simple and effective mount preparation technique using quartz glass substrates with minimal epoxy resin. This method is optimized for NanoSIMS analysis, achieving ultralow hydrogen background (∼8 ppm) and high-vacuum conditions (∼1.4 × 10^-10 mbar) with the aid of a custom-designed liquid nitrogen cold trap, while maintaining excellent sample surface flatness. The mounts also proved compatible with high-precision oxygen isotope analysis by large-geometry secondary ion mass spectrometry (LG-SIMS), yielding δ¹⁸O reproducibility better than 0.30‰ (2SD). This preparation approach enables reliable, high-precision isotopic analysis of fine-grained and extraterrestrial materials and is broadly applicable in microanalytical research.