Efficient analysis of n-alkanes by high-pressure photoionization-induced NO2+ chemical ionization time-of-flight mass spectrometry.

Normal alkanes (n-alkanes) are important constituents of anthropogenic volatile organic compounds (VOCs) and petroleum products. Their rapid, sensitive, and selective detection is crucial for environmental monitoring and industrial process control. Conventional GC-based methods are time-consuming, while existing direct MS techniques often suffer from low sensitivity or severe fragmentation. In this study, we introduce a novel high-pressure photon ionization-NO2+ chemical ionization time-of-flight mass spectrometry (HPPI-PNO2CI-TOFMS) platform for the efficient analysis of C4-C11 n-alkanes. A vacuum ultraviolet (VUV) lamp is used to photoionize NO2 reagent gas at pressures of approximately 600-700 Pa, generating NO2+ reactant ions that undergo hydride-transfer reactions with n-alkanes to produce predominantly [M - H]+ ions with minimal fragmentation, with [M - H]+ branching ratios exceeding 90-96% for C4-C11 alkanes. Under optimized conditions (1 V, ∼660 Pa), limits of detection (LODs) reach ∼8.6 ppbv for n-butane and 0.21-2.57 ppbv for C5-C11 n-alkanes with 5 s integration times. The method was successfully applied to refinery gas samples, facilitating the selective and rapid quantification of n-alkanes against complex VOC backgrounds. However, this method does not detect C1-C3 n-alkanes and cannot distinguish n-alkanes from their branched or cyclic isomers by mass spectrometry alone. Nevertheless, the HPPI-PNO2CI-TOFMS thus provides a robust, real-time analytical tool characterized by high sensitivity, clean spectra, and minimal fragmentation, making it well suitable for environmental monitoring, emissions tracking, and industrial process control.