MULTIPHOTON IONIZATION AND PHOTODISSOCIATION AT THE 2ND-ORDER SPACE FOCUS IN A TIME-OF-FLIGHT MASS-SPECTROMETER - THE AMINO-ACID TRYPTOPHAN

Neutral IR-desorbed molecules of the amino acid tryptophan entrained i n 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-f light mass spectra of tryptophan according to the ladder-switching mod el for MUPI fragmentation. From the number of absorbed photons the upp er values for the appearance energies of specific fragment ions were o btained. A complete fragmentation pathway of tryptophan was establishe d by using photodissociation of molecular ions and fragment ions forme d by MUPI. Photodissociation was successfully performed by secondary e xcitation with a UV laser at the second-order space focus of a reflect ion time-of-flight instrument. A fragmentation tree for a laser-desorb ed molecule obtained in a laser tandem time-of-flight instrument is sh own for the first time.