Uronium CIMS: Robust high-pressure positive mode ion attachment chemical ionization mass spectrometry via X-ray-assisted sublimation of urea

The chemical diversity in the varied spectrum of atmospheric trace gases requires to combine different ionization approaches to enable comprehensive mass spectrometric analysis. Ion-molecule reactors (IMR) at high pressure generally enable better detection limits, due to a larger reaction time, but are also more prone to matrix effects (e.g., dependency on humidity). For the ionization of low polarity volatile organic compounds (VOCs), positive mode chemical ionization (e.g. PTR, low IMR pressure) has been found to be more suitable than negative mode ionization (e.g., NO₃^-, I^-, Br^-), but humidity dependency and other matrix effects of unselective reagent ions need to be constrained e.g. by reduction of IMR pressure. More selective reagent ions such as ammonium and aminium have been previously proposed for more sensitive and soft ionization. However, they are reactive, toxic, and difficult to control.

Inspired by these challenges, we demonstrate uronium as an efficient and robust reagent cation for the ionization of VOCs at high 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, which also subsequently forms 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 high IMR pressure and a tendency to form remarkably strongly bound ion-molecule 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.