A fundamental challenge of the Deep Carbon Observatory (DCO) is to distinguish methane (CH4) produced by degradation of relict organic matter ("fossil" fuel) from that produced by inorganic synthesis or from the activity of microbes ("methanotrophs") in the deep biosphere. This grant supports a project to develop and build a tandem gas-source, electron-impact mass spectrometer with sufficient mass resolving power and sensitivity to make it possible to analyze the rare isotopologues of gas molecules present in hydrocarbon deposits, deep crustal reservoirs, and other settings. The proposed instrument will be the first to combine exceptionally high mass resolving power with a gas source inlet to a mass spectrometer. The full cost of the instrument is $2 million. Proposals submitted to the National Science Foundation (NSF) and Department of Energy (DOE) for $1.15 million have won very favorable reviews, and both agencies have indicated a desire to fund the instrument, with Foundation funding completing the funding gap. The Foundation believes support for the mass spectrometer powerfully exemplifies the effective leveraging of Sloan funds, and a working instrument within 24 months could produce significant published scientific results on the provenance of deep methane in natural environments within three to four years.