Uronium from X-ray Desorbed Solid Urea Enables Attomole Sensitivity to Amines and Semivolatiles

The vast chemical diversity of compounds relevant for gas-phase molecular analytics necessitates the combination of complimentary ionization approaches optimized for specific classes to enable comprehensive mass spectrometric detection. For the ionization of weakly polar volatile organic compounds (VOCs), chemical ionization at low pressure (e.g. proton- or charge-transfer) is more suitable. Negative mode ionization at ambient pressure has delivered superior performance for moderately and highly polar acidic compounds. Numerous alternative positive mode ionization techniques have been explored to detect basic and polar neutral compounds, for which negative polarity and low-pressure ionization techniques have shown insufficient performance. Several ion attachment reagents, such as ammonia and amines, have been previously proposed for more sensitive and soft ionization at ambient and reduced pressures. However, these reagents are often reactive, toxic, and difficult to control, impeding their applicability and operability. Inspired by these challenges, we explored uronium as a sensitive and robust reagent cation for ionizing moderately polar, basic, and neutral compounds at ambient pressure. Urea, a solid chemical safe to humans with negligible vapor pressure under normal circumstances, is desorbed by x-ray irradiation, forming the uronium ion. We experimentally determined the calibration factors and behavior under different humidities for several semivolatile organic compounds (SVOCs), amines, and ammonia, and explored the ionization characteristics using theory. In laboratory measurements of a-pinene and dimethyl sulfide (DMS) oxidation systems we characterized how uronium chemical ionization complements other ionization. Beyond excellent sensitivities to several key components (including amines, dimethyl sulfoxide (DMSO), pyridine, N-Methyl-2-pyrrolidone (NMP), verbenone and dimethylformamide (DMF)) allowing detection at the low to mid parts per quadrillion per volume (ppqv) level - achieved due to uronium's tendency to selectively form strong ion-molecular clusters – and low susceptibility of these cluster formation properties to sample 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. Overall, uronium CIMS represents an innovative technique with significant potential for standardization and wide applicability, given its demonstrated detection efficiency that complements negative mode and low-pressure ionization techniques, safety, and low maintenance requirements.