Rapid and Highly Sensitive Mass Spectrometric Detection of N-Alkanes Via a Novel Photoionization-Induced Nitrogen Dioxide Cation Chemical Ionization Source.

BACKGROUND:Soft ionization mass spectrometry is an excellent direct detection technique with advantages such as speed, simplicity, and ease of operation. Chemical ionization (CI) is a typical soft ionization method that enables targeted detection of specific compounds through reactions between reagent ions and analytes. However, current CI methods, such as proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT-MS), cannot achieve high sensitivity and low fragmentation for n-alkanes. Therefore, there is a need for a detection method tailored for n-alkanes that provides high sensitivity and minimizes fragmentation to preserve the sample's integrity. RESULTS:In this work, a novel photoionization-induced nitrogen dioxide cation chemical ionization time-of-flight mass spectrometry (PNO2CI-TOFMS) was developed by adopting photoionization-generated NO2+ as the reactant ion. A low-fragmentation hydride transfer reaction ionization process between the NO2+ ion and n-alkane molecule was identified and investigated, resulting in easily identifiable [M - H]+. The concentration of NO2 reagent gas, the electric field intensity in the ionization region, and the ion source pressure were systematically examined. Consequently, the limits of detection (LODs) ranging from 0.18 to 2.37 ppbv (parts per billion by volume) were achieved for C5-C11 n-alkanes within just 5 s. Furthermore, the degree of fragmentation was remarkably low, with the branching ratio of the dehydrogenated molecular ion peaks varying between 90.9 % and 99.9 %. The application of PNO2CI-TOFMS on the detection of n-alkanes in gas samples collected from a petroleum refinery demonstrated the satisfactory performance and potential of this system for n-alkanes in atmospheric measurement. SIGNIFICANCE:The development of the PNO2CI-TOFMS method offers a new option for detecting n-alkanes, providing fast detection, high sensitivity, and lower fragmentation. For C5-C11 n-alkanes, the branching ratio can be controlled above 90 %, with detection results obtained in just 5 s. It has already demonstrated its potential in real air samples, and in the future, it will offer more possibilities for n-alkane detection in various scenarios.