A microsecond time-resolved mass spectrometry device and method of using desorption electrospray ionization (10) was described for the kinetic study of fast reactions. The device includes a liquid jet generator (64) that is configured to emit a continuous liquid jet (50) having a length. An ambient ionization source (10) is directed toward a selected variable location along the continuous liquid jet (50) to desorb at least a portion of the continuous liquid jet (50). A mass analyzer (30) analyzes a mass-to-charge ratio of an ionized sample that is within the desorbed portion of the continuous liquid jet (50). The acquired mass spectra reflect the reaction progress in different reaction times and, therefore, may be used to derive the reaction rate.