Gas-phase esterification within a miniature ion trap mass spectrometer: mechanistic insight and diagnostic markers for quinolone screening

The potential use of miniature ion trap mass spectrometry (mini-IT-MS) as a reliable on-site screening tool remains largely unrealized, primarily due to the lack of compatible spectral libraries and a limited understanding of the unique ion chemistry inherent to the platform. During the analysis of quinolone antibiotics, we identified the formation of intense, solvent-dependent adduct ions during collision-induced dissociation (CID). These ions are not presented in conventional spectral databases and pose a considerable risk of misidentification. This study systematically demonstrates that this phenomenon originates from an intra-trap, gas-phase SN2 esterification reaction between the carboxyl group of quinolones and alcohol molecules present in the mobile phase. The proposed mechanism was conclusively validated through solvent-exchange experiments, CID energy-dependent analyses, and isotopic labeling. Notably, the MS³ spectrum of the adduct ion closely matched the MS² spectrum of the original precursor ion, confirming the ester structure of the adduct. Building on this mechanistic understanding, we constructed a customized tandem mass spectral library that explicitly incorporates these adduct ions as diagnostic features. This strategy enabled the unambiguous identification of all 14 quinolones in a standard mixture and demonstrated high reliability in screening spiked fish and shrimp samples, effectively transforming an analytical interferent into a robust identification tool. This work establishes a paradigm for tackling platform-specific analytical challenges through fundamental ion chemistry research, laying the groundwork for more reliable on-site analysis using miniature mass spectrometers.