Secondary Ion Chemistry Mediated by Ozone and Acidic Organic Molecules in Iodide-Adduct Chemical Ionization Mass Spectrometry

Iodide-adduct chemical ionization mass spectrometry (I-CIMS) is a widely used technique in the atmospheric chemistry community to detect oxygenated volatile organic compounds (OVOCs) in real time. In this work, we report the occurrence of secondary ion chemistry from interactions between a strong oxygen donor (such as O3 and peracids) and acidic OVOCs (such as carboxylic acids and organic hydroperoxides) in the ion-molecule reaction (IMR) region of I-CIMS. Such interactions can lead to acidic organic molecules (HA or HB) clustering with IO- (e.g., HA + IO-) and dimer adducts (A + B + I-), in addition to the well-known iodide clusters (HA + I-). This ion chemistry was probed using common chemical standards as well as the gas-phase oxidation products of α-pinene and isoprene in a flowtube reactor. The results show that secondary ion chemistry can lead to misinterpretations of molecular compositions and distributions of the gas-phase products and an overestimation of the elemental O/C ratio overall. Nevertheless, the varying degrees of signal change in response to the secondary ion chemistry might be a clue to inform OVOCs' functionalities. Specifically, in the α-pinene ozonolysis system, the extents of ion signal reduction in the presence of additional acids in the IMR suggest that C9H14O4 produced in the gas phase is a peracid, rather than the often-assumed pinic acid. Thus, we suggest that the potential application of the secondary ion chemistry to inform organic functionalities is promising, which could help better understand the molecular compositions of gas-phase OVOCs and the reaction mechanisms therein.