Empirical Approach to Quantifying Sensitivity in Different Chemical Ionization Techniques for Organonitrates and...

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%.