Multiphoton ionization and photodissociation at the second-order space focus in a time-of-flight mass spectrometer: The amino acid tryptophan

Neutral IR-desorbed molecules of the amino acid tryptophan entrained in a supersonic beam were post-ionized by multiphoton ionization (MUPI) in the gas phase. Variations in the intensity of the ionization laser produced different degrees of fragmentation observed in the time-of-flight mass spectra of tryptophan according to the ladder-switching model for MUPI fragmentation. From the number of absorbed photons the upper values for the appearance energies of specific fragment ions were obtained. A complete fragmentation pathway of tryptophan was established by using photodissociation of molecular ions and fragment ions formed by MUPI. Photodissociation was successfully performed by secondary excitation with a UV laser at the second-order space focus of a reflection time-of-flight instrument. A fragmentation tree for a laser-desorbed molecule obtained in a laser tandem time-of-flight instrument is shown for the first time.