Online Dual-Ionization Mass Spectrometer Coupling Photoionization and Paper Spray Ionization for Enhanced Detection of Particle Ozonolysis Kinetics

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Organic particles undergo atmospheric multiphase ozonolysis, generating secondary organic aerosols with health and environmental hazards. Unraveling this mechanism is highly dependent on advanced detection techniques, yet single-ionization methods suffer from inherent limitations in the analytical scope. Here, we report a creative online dual-ionization technique by coupling photoionization (PI) and paper spray ionization (PSI) with an ultrahigh resolution mass spectrometer (HRMS). This integrated PI-PSI-HRMS platform enables online characterization of multiphase reactions of model organic particles, i.e., oleic acid and squalene. It features a straightforward configuration, facile operation, and low cost. Monitoring particle decay kinetics via this platform allows quantitative determination of the multiphase ozonolysis rate (γ_eff). Critically, its dual-ionization design confers capability in detecting a range of fingerprint products such as secondary ozonides (SOZs), alkyl hydroperoxides, carbonyls, and acids. Tracking the evolution of these products further provides reliable quantitative evidence to reveal the multiphase reaction pathways. Relative to stand-alone PSI, the integration of PI uniquely broadens the analytical scope from polar to low-polar molecules, achieves up to 4.4-fold SOZ signal enhancement, and lowers the required PSI operating voltage. Simultaneously, compared to neat PI, PSI integration enables efficient particle ionization without external heater-assisted evaporation, an improvement that avoids thermal perturbation. In summary, this coupled PI-PSI technique minimizes interference from thermally induced side reactions and electric-field effects, highlighting its promising potential for quantifying diverse multiphase chemistries beyond ozonolysis.