Secondary organic aerosol (SOA) formed through multi-phase atmospheric reactions makes up a large fraction of airborne particles. The chemical composition, molecular structures, and emission sources of SOA are complex and vary between different SOA regions, complicating their identification in complex mixtures. In this work, we utilize drift tube ion mobility (IM) with quadrupole time-of-flight mass spectrometry (Q-TOF-MS) detection for rapid separation and multi-dimensional characterization of isomers in two sets of lab-generated SOA samples (D-limonene, LSOA and α-pinene, PSOA). All samples were ionized using electrospray ionization (ESI) and acquired in both positive and negative ion modes. The IMderived collision cross-sections in nitrogen gas (DTCCSN2) for isomer components in the samples were obtained using multi-field measurements. Novel ion multiplexing/high-resolution demultiplexing strategies were used to increase the sensitivity and mobility baseline resolution, which helped distinguish conformational and isomeric structures for the measured ions. For LSOA and PSOA samples, we report significant structural differences of the isomer structures complemented by theoretical calculations. The average DTCCSN2 values for monomeric structures measured as [M+Na]+ ions are 1.2-2% higher than [M-H]- counterparts, meanwhile, dimeric and trimeric isomer structures in both SOA samples where 3.5% - 7% higher for [M-H]- than their [M+Na]+ ion structures, respectively. Therefore, characteristic DTCCSN2 values of isomers in aerosols depend strongly on the mode of polarity and ionization mechanism. Additionally PSOA monomers and dimers exhibit larger DTCCSN2 values (~1-4% deviation) than LSOA counterparts as the products are too rigid to fold in the gas-phase, whereas all LSOA products have flexible open-chain structures capable of folding into much more compact structures. Last, structural changes associated with photochemical aging of single molecules in complex PSOA are measured with IM-MS. We provide, for the first time, detailed molecular and structural descriptors for accurate detection and annotation of structural isomers in complex SOA mixtures, common in atmospheric chemistry.

Molecular and structural characterization of isomeric compounds in atmospheric organic aerosol using ion mobility: Mass spectrometry

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