A novel chemical ionization source for proton-transfer-reaction mass spectrometry and other chemical ionization schemes

We present the development and characterization of a novel chemical ionization source useful for sensitive detection and analysis of atmospherically-relevant species in real time. The source can be used for proton-transfer-reaction mass spectrometry (PTR-MS) as well as other chemical ionization schemes generating both positive and negative analyte ions. It features fast reagent ion switching on the timescale of seconds or better, and can be operated in a broad range of ion-molecule reactor pressures (0.1 - 10 mbar) and sample flows. We couple the novel source with the high-resolution time-of-flight mass spectrometer and demonstrate sub pptv detection limits (<<1 pptv) along with a high dynamic range. We also present two weeks of ambient measurements in Thun, Switzerland using the novel ionization source switching between H₃O⁺ and O₂⁺ reagent ions as well as flow tube experiments which demonstrate the utility of multiple reagent ion switching between positive and negative ion modes.