Ultralow H2O content analysis with a large-geometry secondary ion mass spectrometer†

Large-geometry secondary ion mass spectrometry (LG-SIMS) is routinely used to determine the elemental and isotopic compositions, thanks to its high sensitivity and in situ micro-analysis capability. Its large volumes of transfer and coupling column, however, might bring low vacuum and high background of volatile elements (esp. H), which hampers its application in ultralow H₂O content measurement. In this study, we report a modified analytical procedure to accurately analyze samples with ultralow-level H₂O content by using a LG-SIMS of type CAMECA IMS 1280-HR. Based on four new water content working reference materials of quartz glasses with H₂O contents ranging from ca. 0 to 39 ppm, the estimated detection limit of LG-SIMS is 0.15 ppm for H₂O. This ultralow detection limit is achieved mainly by the intrinsic high sensitivity of the instrument and the conjunctive usage of a stabilized temperature-controlled cooling trap device and a novel tin–bismuth alloy mount preparation technology, which are able to maintain a vacuum pressure at 1.7 × 10⁻⁹ mbar in the analysis chamber. Based on these new improvements, our approach can potentially become a routine choice for analyses of H₂O content and oxygen–hydrogen isotopes for (extra)terrestrial materials with ultralow-level water content.