A Chemical Ionization Residual Gas Analyzer for the Sensitive Detection of Atmospheric Trace Gases

We have developed a compact, chemical ionization mass spectrometer for the sensitive detection of atmospheric trace gases using a commercial residual gas analyzer (RGA). A high-pressure ion molecule reactor interface incorporating two radiofrequency (RF)-only octupoles, powered by Wisconsin Oscillator RF power supplies, is used to focus ions through three stages of pumping before analysis via a modified RGA. We operated this instrument with benzene cation cluster reagent ions generated using a ²¹⁰Po α emitter to enable the efficient ionization, transfer, and detection of analyte ions and ion-molecule clusters. Using single ion monitoring, the instrument demonstrated high sensitivity to dimethyl-1,1,1-d₃ sulfide, corresponding to a 20 s 3σ limit of detection (LOD) of 18 ppt. This detection threshold is limited by the magnitude and variability in the background determination. We characterized the impact of tuning instrument pressures and ion optics on sensitivity and ion clustering to demonstrate the potential for enhanced sensitivities and electronically controllable clustering modalities. The application of the instrument to the measurement of ambient trace gases, in scan mode, was tested by sampling continuously through a window inlet on the fourth floor of the UW-Madison chemistry building (Madison, WI) for 3 days during the summer of 2025, resulting in the detection of four unique, time-correlated ammonia clusters via peak integration. The simple, robust design and relatively low cost of the instrument highlight the utility of commercial residual gas analyzers as mass analyzers for gas-phase measurements of trace species.