Microdroplet Separation Mass Spectrometry Resolves the Charge-Dependent Interfacial Catalytic Roles of Microdroplets

Microdroplets are highly efficient microreactors, yet conventional mass spectrometry typically evaluates their reactivity as an ensemble average, obscuring the independent catalytic roles of positively and negatively charged microdroplets. In this study, we developed a microdroplet separation mass spectrometry method. By separating mixed-charge microdroplet plumes prior to MS sampling, this approach enables the independent, online analysis of positively and negatively charged microdroplets. The performance was validated by combining experiments and simulations. Application of this methodology to perfluorooctanoic acid (PFOA) degradation revealed that negatively charged microdroplets exhibit a higher degradation efficiency than positive counterparts. Mechanistic investigations, supported by radical trapping and density functional theory, reveal that negative interfacial charge effectively stabilizes the transition state, facilitating a hydroxyl radical (•OH)-driven degradation pathway. This polarity-dependent enhancement proved universal across varying reaction conditions and other perfluorinated compounds with diverse chain lengths and terminal functional groups. Moreover, we rationally modulated the microenvironment via negative voltage application and Fenton's reagent supplementation, boosting the PFOA degradation from approximately 38% to 83%. These findings demonstrate that the developed separation-MS method is a reliable tool for resolving mixed-charge microdroplet behaviors and provides insights for reasonably elucidating and optimizing charge-regulated microdroplet interfacial chemistry.