Deciphering Photocatalytic Intermediate Kinetics via a Pump–Probe Transient Electrospray Mass Spectrometry

The precise detection of chemical reaction intermediates and their kinetic evolution are crucial for understanding the mechanisms of chemical reactions. Mass spectrometry, while capable of providing structural fingerprint information, typically lacks temporal resolution. Here, we combine the pump-probe principle from ultrafast spectroscopy with mass spectrometry to develop time-resolved electrospray mass spectrometry, named transient mass spectrometry. By introducing a time delay between the pump laser pulse and the electrospray high-voltage pulse, we have for the first time endowed electrospray mass spectrometry with a clear temporal resolution definition. This transient mass spectrometry combines millisecond temporal resolution with the molecular structural fingerprint of mass spectrometry, thus enabling tracking of the kinetic evolution of intermediates in photochemical reactions. Through the modulation of light intensity and photocatalyst concentration, we elucidated the rate-determining roles of photon absorption and kinetic concentrations of active species. This technical breakthrough endows electrospray mass spectrometry with a temporal resolution. It provides a powerful tool to identify key intermediates, study their kinetics, and guide the design of photocatalysis and photosynthesis systems in solution.