Calculation of the sensitivity of proton-transfer-reaction mass spectrometry (PTR-MS) for organic trace gases using molecular properties

Abstract Proton-transfer-reaction mass spectrometry (PTR-MS) allows the detection of a large number of trace gases in air through proton-transfer reaction with H3O+ reagent ions and detection by a mass spectrometer. Measurement sensitivities can be experimentally determined using calibration gases or calculated using the rate constant for the proton-transfer reaction, but rate constants have only been measured for a subset of compounds. Numerous theoretical approaches that describe the ion-molecule collision processes have shown how to accurately calculate capture collision rate constants between an ion and neutral molecules using the polarizability and permanent dipole moment of the molecule. Here we show that polarizability, dipole moment, and resulting capture rate constants for proton-transfer reactions of H3O+ with various different volatile organic compounds (VOCs) can be obtained using the molecular mass, elemental composition, and functionality of VOCs. The polarizabilities of a class of VOCs possessing a specific number of electronegative atoms were linearly correlated with their molecular mass. The dipole moments in a series of VOCs, in which VOCs contain a specific functional group and arbitrary residual hydrocarbon parts, can be approximated as a constant value. The capture rate constants calculated using polarizability and dipole moment, as estimated from molecular mass, elemental composition, and functional group, agreed within 10% with measured values for most VOCs. Those capture rate constants were applied to the calculation of the sensitivities of VOCs detected by our PTR-MS, taking into account the ion transmission efficiency and the degree of fragmentation of protonated VOCs observed in that instrument as well as chemical properties of the VOCs. The resulting calculated sensitivities agreed within 20–50% of those measured by PTR-MS, but several notable exceptions exist. This result shows that the neutral concentration of a VOC detected as a protonated molecule in PTR-MS can be approximated using only molecular mass, elemental composition, and functionality of the VOC. The present study is useful for all PTR-MS instruments regardless of the type of mass analyzer; however, the identification of elemental composition by high mass resolution instrumentation is important.