Uronium from X-ray-Desorbed Urea Enables Sustainable Ultrasensitive Detection of Amines and Semivolatiles

Comprehensive mass spectrometric detection requires multiple ionization schemes. Chemical ionization (CI) at a low pressure is suitable for the detection of weakly polar volatile organic compounds (VOCs). Negative-mode ionization at ambient pressure delivers a superior performance for polar acidic compounds. Positive-mode CI 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 sensitive and soft ionization. These reagents are often reactive, toxic, and difficult to control, which impede their applicability and operability. Inspired by these challenges, we explored uronium, protonated urea, as an alternative for ionizing moderately oxygenated, basic, and polar neutral compounds at ambient pressure. Urea, a nontoxic solid with negligible vapor pressure, is desorbed by X-ray irradiation, forming the uronium ion. We experimentally determined the behavior of uronium ionization under different humidities for several semivolatile organic compounds (SVOCs), amines, and ammonia and explored the mechanism using theory. In laboratory measurements of α-pinene and dimethyl sulfide (DMS) oxidation systems, we characterized how uronium complements other ionization schemes. Excellent sensitivities were achieved for several key components (including amines, dimethyl sulfoxide (DMSO), <i>N</i>-methyl-2-pyrrolidone, verbenone, and dimethylformamide (DMF)), requiring sample sizes of only a few attomoles for detection in individual spectra, equivalent to detection limits at the low to mid parts per quadrillion by volume (ppqv) level. Uronium exhibits a tendency to selectively form strong ion-molecular clusters, which renders the ionization robust against sample humidity changes. X-ray desorption of solid urea simplifies reagent supply handling and ensures the long-term stability of the ion production system, providing a safe and sustainable alternative to equivalent CI methods.