Multi-Pressure Chemical Ionization Mass Spectrometry: Comprehensive Analysis of Complex Gas Mixtures

Chemical Ionization Mass Spectrometry (CIMS) is a well-established analytical method in atmospheric research, process monitoring, forensics, breathomics and food science. Despite significant advancements in procedural techniques, several instrument configurations, especially operating at different ionization pressures, are typically needed to analyze the full range of compounds from non-functionalized parent compounds to their functionalized reaction products. For polar, functionalized compounds, very sensitive detection schemes are provided by high-pressure adduct-forming chemical ionization techniques, whereas for non-functionalized, non-polar compounds, low-pressure chemical ionization techniques have consistently demonstrated superior performance. Here, using a MION2 chemical ionization inlet and an Orbitrap Exploris^TM 120 mass spectrometer, we present multi-pressure chemical ionization mass spectrometry (MPCIMS), the combination of high- and low-pressure ionization schemes in a single instrument enabling quantification of the full distribution of precursor molecules and their oxidation reaction products from the same stream of gas without alterations. We demonstrate the performance of the new methodology in a laboratory experiment employing a-pinene, a monoterpene relevant to atmospheric particle formation, where MPCIMS allows to measure the spectrum of compounds ranging from the volatile precursor hydrocarbon to highly functionalized condensable reaction products. Furthermore, we demonstrate field applicability of the technique by measuring ambient air in automated switching sequence. MPCIMS carries the potential as an all-in-one method for the analysis of complex gas mixtures, reducing technical complexities and the need for multiple instruments without compromise of sensitivity.