Molecular and Structural Characterization of Isomeric Compounds in Atmospheric Aerosols Using Ion Mobility Spectrometry – Mass Spectrometry

CHRISTOPHER P. WEST, Daniela Mesa Sanchez, Ana Morales, Yun-Jung Hsu, Jackson Ryan, Andrew Darmody, Lyudmila Slipchenko, Julia Laskin, Alexander Laskin, Purdue University

     Abstract Number: 353
     Working Group: Aerosol Chemistry

Abstract
Secondary organic aerosol (SOA) formed through multi-phase atmospheric chemistry makes up a large fraction of airborne particles. The chemical composition, molecular structures, and emission sources are complex and vary between different SOA samples, complicating their identification in complex mixtures. In this work, we utilize drift tube ion mobility spectrometry (DTIMS) with quadrupole time-of-flight mass spectrometry (Q-TOF-MS) detection for rapid gas-phase separation and multi-dimensional characterization of isomers in two sets of biogenic SOA samples (D-limonene, LIM/O₃ and α-pinene, APIN/O₃). All samples were ionized using electrospray ionization (ESI) and acquired in both positive and negative ion modes. The IMS-derived collision cross-sections in nitrogen gas (^DTCCS_N2) for isomer components in the samples were obtained using multi-field measurements. Novel ion multiplexing/high-resolution demultiplexing strategies were used to increase sensitivity and mobility baseline resolution, which helped reveal several conformational and isomeric structures for the measured ions. For LIM/O₃ and APIN/O₃ SOA samples, we report significant structural differences of the isomer structures complemented by theoretical calculations. The average ^DTCCS_N2 values for monomeric structures measured as [M+Na]⁺ ions are 1.2% 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 ^DTCCS_N2 values of isomers in aerosols depend strongly on the mode of polarity and ionization mechanism in ESI. Additionally APIN/O₃ monomers and dimers exhibit larger ^DTCCS_N2 values (~1-4% deviation) than LIM/O₃ counterparts owing to their bulky organic gas-phase ion structures, comprising cyclobutane ring with extended methyl (-CH₃) groups in APIN/O₃ compared to acyclic branched ketone groups in LIM/O₃ system. We provide, for the first time, detailed molecular and structural descriptors for accurate detection and annotation of structural isomers in complex SOA mixtures, which are common in atmospheric chemistry.