SIFT-MS Analysis of Amines: Unusually efficient O2 Addition to the radical cation product

Abstract

Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) is routinely used for real-time detection of volatile organic compounds (VOCs), including amines that are relevant in pharmaceutical, environmental, and food safety applications. As VOC monitoring in SIFT-MS depends on the knowledge of the underlying ion-molecule reactions, a comprehensive understanding of the ionisation processes is key for robust analyte identification and accurate quantitation. This study investigates the product ion formation of selected primary and secondary amines with H₃O⁺, NO⁺ and O₂^+• reagent ions. All reactions proceed at or near the collisional limit. While proton transfer from H₃O⁺ yields the intact protonated molecule, charge transfer from O₂^+• predominantly leads to fragment ions. Notably, NO⁺ reactions with all investigated amines yield an unusual product ion at nominal mass + 32, with its observed product ion ratio exhibiting substantial differences between compounds. The experimental data indicate an equilibrium reaction, initially suggestive of adduct formation between the radical cations and O2. However, quantum chemical modelling shows that simple van der Waals adducts are thermodynamically unstable at the instrument temperature (393 K). Instead, the results support a reaction mechanism in which the nascent radical cation undergoes an intramolecular hydrogen shift to form a distonic ion that subsequently binds O₂. This pathway is exergonic for all four amines studied, and the computed Gibbs energy changes agree closely with values derived from observed experimental equilibrium constants under quasi-steady state conditions, explaining the observed differences in product ion ratios. These results provide more detailed mechanistic insights into the O₂ addition of amine radical cations.