Data analysis and isotopic ratios measured onboard the Spacecraft Atmosphere Monitor

The Spacecraft Atmosphere Monitor (SAM.) is a scientific payload, which includes a Paul quadrupole ion trap (QIT) mass spectrometer and has been continuously operated onboard the International Space Station (ISS) between August 2019 and July 2021. S.A.M. measures the chemical composition of ISS cabin air and reports on major constituents (N-2, O-2, Ar, CO2, and CH4) once every 2 s. Each second, S.A.M. measures one hundred individual mass spectra stored in 4800 ins-long channels. This report presents the autonomous onboard data analysis software that calibrates the mass spectrum, removes the residual gas background signal, identifies species of interest, and reports relevant abundances. In addition, we describe a ground-based analysis of downloaded S.A.M.'s data to extract the H, C, N, O, and Ar isotope ratios. We also describe experiments, requested by the ISS Vehicle office, to ascertain if benzene was present in the ISS cabin air (it was not). We demonstrate that S.A.M. instrument measures isotope ratios with precision and accuracy comparable to the state-of-the-art laboratory-size magnetic sector mass spectrometers. (C) 2022 Elsevier B.V. All rights reserved.