Empirical Approach to Quantifying Sensitivity in Different Chemical Ionization Techniques for Organonitrates and Nitroaromatics Constrained by Ion–Molecule Reaction and Transmission Efficiency

Accurate identification and quantification of nitro-containing species are of great significance to understanding their chemical behaviors in the atmosphere. By optimizing the operational conditions of the H3O+ and NO+ ionization modes in a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) and evaluating the performance of an iodide chemical ionization mass spectrometer (I– CIMS), this study leveraged the individual advantages of each ionization mode to effectively detect a diverse array of nitroaromatics and organonitrates (ONs). The H3O+ ionization mode largely fulfilled the criteria for real-time monitoring of gas-phase alkyl-, aryl-, and hydroxy-nitrates, and nitrophenols, albeit its reduced sensitivity toward ONs due to extensive fragmentation. In contrast, the NO+ mode demonstrated enhanced sensitivity for ONs with less fragmentation than the H3O+ mode. The I– CIMS featured distinguished sensitivity toward oxidized compounds containing polar functional groups, particularly increasing with the incorporation of hydroxyl, carboxyl, or nitrate groups. Further, we developed a calibration-based semiquantitative framework to enhance the accuracy of sensitivity estimation, constrained by ion–molecule reaction, transmission efficiency, along with possible decomposition of ion-clusters, with uncertainties ranging from 21% to 41% for H3O+ and 21–43% for NO+. Given considerable discrepancies (up to 1 order of magnitude) between measured and predicted sensitivity in I– CIMS using previously reported log–linear fitting, a declustering voltage (dV50)-based categorization approach was introduced, leading to a 5-fold improvement in measurement accuracy and an overall uncertainty of I– CIMS in quantifying nitro-containing species varying from 27% to 60%.