A high-resolution gas-source isotope ratio mass spectrometer

Abstract

We describe a new high-resolution, multi-collector gas source mass spectrometer designed for isotopic analysis of volatile and semi-volatile molecules: the Thermo Scientific MAT253-Ultra, a prototype double-focusing isotope ratio mass spectrometer installed in the Caltech laboratories for stable isotope geochemistry. This instrument achieves mass resolving power of up to ∼27,000 (M/ΔM) and can analyze diverse gases and semi-volatile compounds using a conventional dual inlet and/or a carrier gas. It has a multi-collector array comprised of 7 detector positions with adjustable spacing, all of which can register ions through an SEM or Faraday cup and spanning up to a 1013 range in signal strength. Abundance sensitivity in the He mass range is as good as 10−12, and precision commonly approaches the counting statistics limit down to 0.1‰ (SEM) or 0.01‰ (Faraday) for a range of analytes. This instrument permits resolution of isobaric interferences arising from both contaminants and multiple isotopologues of an analyte that share a cardinal mass, enabling direct isotopic analysis of molecules with complex mass spectra such as hydrocarbons. This ability should enable the measurement of position-specific isotopic compositions, including multiple substitutions, by comparing isotope ratios of molecular ions with those of daughter fragment ions (assuming products of recombination and other source reactions are recognized and corrected for). The combination of high mass resolution with stable multicollection will provide a wide range of potential new tools for isotope geochemistry, including (but not limited to): singly and multiply substituted methane and larger hydrocarbons; position-specific 13C analysis of propane and larger hydrocarbons; precise analysis of 17O/16O and 18O/16O on fragment ions from CO2 and other molecules; analysis of a variety of N2O isotopologues (including 18O, 17O, position-specific 15N, and various ‘clumped’ species); and high precision and abundance sensitivity noble gas analyses. These capabilities greatly extend the scope of stable isotope variations that can be utilized for problems in forensics, environmental geochemistry, biochemistry, and Earth and planetary sciences.