Interpretation of mass spectra by a Vocus proton-transfer-reaction mass spectrometer (PTR-MS) at an urban site: insights from gas chromatographic pre-separation

Abstract

Volatile organic compounds (VOCs) are important atmospheric components that contribute to air pollution, but their accurate quantification by proton-transfer-reaction mass spectrometry (PTR-MS) remains challenging. In this work, we coupled a gas chromatograph (GC) prior to PTR-MS and analyzed complex ambient air in urban Shanghai to speciate the PTR signal to identify the VOC species that were responsible for the generation of the ions detected by PTR. We classified 176 individual PTR signals with associated compounds resolved by the GC based on whether they could be used to quantify a VOC species without pre-separation. In this classification, Category I includes 45 decent signal ions that were produced from a single VOC species and thus can be used for reliable quantification, although some of the Category I ions are not the conventionally used protonated quasi-molecular ions (MH+). Category II includes 39 signal ions that were produced from a group of isomers and can be used to quantify the isomeric sum, but with an increased uncertainty if a single calibration factor for one specific isomer is used to represent all structures. Category III includes 92 signal ions that were generated from more than one non-isomeric species (e.g., through protonation, fragmentation, cluster formation) and thus merely gave an upper limit of VOC concentrations. In addition, we propose taking aromatic compounds, for instance, the quantification of selected VOCs, utilizing either non-MH+ or non-Category I ions. Our results help to achieve more comprehensive species identification and reliable VOC quantification in PTR measurements.