Calibration and Detection of Phosphine Using a Corrosion-Resistant Ion Trap Mass Spectrometer

We present a corrosion-resistant quadrupole ion trap mass spectrometer (QIT-MS) designed for trace detection of volatiles in sulfuric acid aerosols, with a specific focus on phosphine (PH3). Here, we detail the gas calibration methodology using permeation tube technology for generating certified ppb-level PH3/H2S/CO2 mixtures, and report results from mass spectra with sufficient resolution to distinguish isotopic envelopes that validate the detection of PH3 at a concentration of 62 ppb. Fragmentation patterns for PH3 and H2S agree with NIST data, and signal-to-noise performance confirms ppb sensitivity over 2.6 h acquisition periods. We further assess spectral interferences from oxygen isotopes and propose a detection scheme based on isolated phosphorus ions (P+) to enable specific and interference-resistant identification of PH3 and other reduced phosphorus species of astrobiological interest in Venus-like environments. This work extends the capabilities of QIT-MS for trace gas analysis in chemically aggressive atmospheric conditions.