Efficient Desorption and Capture of Reactive Carbocations from Positively Charged Glass Surface Bombarded with High-Speed Water Microdroplets

Recent studies with desorption electrospray ionization mass spectrometry (DESI-MS) revealed that reactive carbocations could be captured and stabilized in water microdroplets to probe their intermediacy in chemical reactions. Here, we demonstrate how the amine functionalization (chemical modification) of the glass surface, where a reaction aliquot is bombarded with the DESI spray of water microdroplets, enhances carbocation desorption and capture in splashed secondary microdroplets for mass spectrometric interrogation. The amine groups on the glass surface remained protonated under the impinging spray of charged water microdroplets, enabling effective splashing and subsequent propulsion of the positively charged secondary microdroplets under electrostatic (repulsive) and pneumatic forces. Carbocations, being intrinsically charged, could also be repelled by this charged surface for their efficient desorption. Thus, this tailored surface chemistry improved the DESI-MS detection limit of carbocation intermediates up to five times, allowing us to monitor such species, albeit sparsely populated in the reaction medium.