Structural Characterisation of Dimeric Esters in α-Pinene Secondary Organic Aerosol Using N2 and CO2 Ion Mobility Mass Spectrometry

The atmospheric oxidation of monoterpenes leads to the formation of secondary organic aerosol (SOA). While numerous works have been carried out in the past to characterise SOA at a molecular level, the structural elucidation of SOA compounds remains challenging owing to the lack of authentic standard compounds. In this work, the structures of α-pinene originating dimeric esters in SOA with m/z 357 (C17H25O8-) and m/z 367 (C19H27O7-) were characterised using UPLC/ESI(-)IMS-TOFMS2 (ultra-performance liquid chromatography coupled to ion mobility spectrometry tandem time-of-flight mass spectrometry). The measured collision cross-section (ΩN2) values were compared to theoretically calculated ΩN2 values. Selected product ions of dimeric compounds and the authentic standard compounds of product ions were subjected to CO2-IMS-TOFMS for more detailed structural characterisation. Our results were consistent with previously reported subunits of the m/z 357 (terpenylic acid and cis-pinic acid), and the m/z 367 (10-hydroxy-cis-pinonic acid and cis-pinic acid) ions. The measured and calculated ΩN2 values of m/z 367 ions further support the conclusion of earlier structural characterisation; however, the structure of the m/z 357 ion remains vague and requires further characterisation studies with a synthesised reference compound.