GAS-PHASE CATIONIZATION AND PROTONATION OF NEUTRALS GENERATED BY MATRIX-ASSISTED LASER DESORPTION

The ionization mechanisms involved in matrix-assisted ultraviolet lase r desorption/ionization (MALDI) were studied with a time-of-flight mas s spectrometer. When protonated or cationized quasimolecular ions gene rated by MALDI are not extracted promptly, their abundance is a functi on of the delay time between laser irradiation and ion extraction, max imizing at an optimum delay time (DTM) of a few hundred nanoseconds. T he ion abundance at DTM exceeds that of prompt extraction by a factor of 2 or more. Increasing the cation density near the sample surface re duces the DTM, whereas increasing the desorption laser irradiance has the opposite effect. The enhancement suggests extensive gas-phase ion- molecule reactions after irradiation by the desorption laser has cease d.