SIFT-MS analysis of amines: unusually efficient O₂ addition to the radical cation product

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.