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

Comprehensive mass spectrometric detection requires multiple ionization schemes. Chemical ionization at low pressure is suitable for the detection of weakly polar volatile organic compounds (VOCs). Negative mode ionization at ambient pressure delivers superior performance for polar acidic compounds. Positive mode chemical ionization has 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 have been proposed for more sensitive and soft ionization. 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 alternative for ionizing moderately oxygenated, basic, and polar neutral compounds at ambient pressure. Urea, a non-toxic solid with negligible vapor pressure, is desorbed by X-ray irradiation, forming the uronium ion. We experimentally determined the calibration factors and behavior of uronium ionization 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 complements other ionization schemes. Excellent sensitivities were achieved to several key components (including amines, dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP), verbenone and dimethylformamide (DMF)) allowing detection limits at the low to mid parts per quadrillion per volume (ppqv) level. Uronium exhibits the tendency to selectively form strong ion--molecular clusters which renders the ionization robust against sample humidity changes. X-ray desorption of solid urea enables trivial handling of the reagent supply and long-term stability of the ion production system. This represents an innovative technique with significant potential for standardization and wide applicability, demonstrating efficiency, safety, and low maintenance requirements.