Multi-Pressure Chemical Ionization Mass Spectrometry & Uronium CIMS: Novel ionization approaches to achieve comprehensive sensitivity with a single mass spectrometer

The chemical diversity among atmospheric trace gases demands different chemical ionization strategies under various pressures to enable comprehensive mass spectrometric analysis. For instance, organic acids and highly oxygenated compounds are detected sensitively via near-atmospheric pressure ionization in negative mode (e.g., using NO₃^-, Br^-, I^-). Certain low-polarity volatile organic compounds (VOCs), on the other hand, are effectively ionized by positive mode ion attachment. Meanwhile, minimally polar molecules often require non-selective positive mode ionization (e.g., proton-transfer reaction, PTR) at low pressure to suppress matrix effects (notably humidity). By combining these approaches, one can achieve a near-complete sensitivity to atmospheric trace constituents.

In the first part of the talk, I will elaborate on multi-pressure chemical ionization mass spectrometry (MPCIMS), the combination of high- and low-pressure ionization schemes within a single instrument, which enables the quantification of the full distribution of precursor molecules and their oxidation reaction products from the same stream of gas. 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.

Second, I discuss uronium as an efficient and robust reagent cation for the ionization of VOCs at atmospheric IMR pressures. Urea, a solid chemical safe to humans with a negligible vapor pressure under normal circumstances, is sublimated from the solid phase under x-ray irradiation to form the uronium ion. We determine the calibration factors for VOCs, amines, and DMSO under different humidities in calibration experiments, interpret the ionization efficiencies using theory, and show results of test measurements of different chemical systems. Beyond the favorable sensitivities allowing detection at the low ppq level - attainable due to uronium’s applicability at atmospheric IMR pressure and a tendency to form strongly bound ion-molecular clusters – and low susceptibility to humidity changes, the marked benefit of uronium CIMS lies in the trivial handling of the reagent supply and long-term stability of the ion production system. The combination of favorable performance and easy handling render uronium CIMS promising to become a go-to method for ultra-sensitive positive mode chemical ionization.