Direct Detection of Small <i>n</i>‑Alkanes at Sub-ppbv Level by Photoelectron-Induced O₂⁺ Cation Chemical Ionization Mass Spectrometry at kPa Pressure

Direct mass spectrometric measurements of saturated hydrocarbons, especially small <i>n</i>-alkanes, remains a great challenge because of low basicity and lack of ionizable functional groups. In this work, a novel high-pressure photoelectron-induced O₂⁺ cation chemical ionization source (HPPI-OCI) at kPa based on a 10.6 eV krypton lamp was developed for a time-of-flight mass spectrometer (TOFMS). High-intensity O₂⁺ reactant ions were generated by photoelectron ionization of air molecules in the double electric field ionization region. The quasi-molecular ions, [M–H]⁺, of C3–C6 <i>n</i>-alkanes, gradually dominated in the mass spectra when the ion source pressure was elevated from 88 to 1080 Pa, with more than 3 orders of magnitude improvement in signal intensity. As a result, the achieved limits of detection were lowered to 0.14, 0.11, 0.07, and 0.1 ppbv for propane, <i>n</i>-butane, <i>n</i>-pentane, and <i>n</i>-hexane, respectively. The performance of the HPPI-OCI TOFMS was first demonstrated by analysis of exhaled small <i>n</i>-alkanes from healthy smokers and nonsmokers. Then the concentration variations of exhaled small <i>n</i>-alkanes of four healthy volunteers were analyzed after alcohol consumption to explore the alcohol-hepatoxicity-related oxidative stress. In summary, this work provides new insights for controlling the O₂⁺-participating chemical ionization by adjusting the ion source pressure and develops a novel direct mass spectrometric method for sensitive measurements of mall <i>n</i>-alkanes