EFFECTS OF DESORPTION METHOD AND PHOTOIONIZING LASER CHARACTERISTICS ON MOLECULAR FRAGMENTATION

The influence of desorption technique and photoionization method on the mass spctral fragmentation of L-tryptophan and serotonin has been investigated using 7-kev Ar+ static sputtering, 266- and 355-nm pulsed laser-induced desorption, and thermal desorption at 150-200 degrees C in conjunction with 118-nm (10.49 eV), 266-nm (4.66 eV), and 355-nm (3.49 eV) laser postionization. In addition, positive secondary ion data from pulsed Ar+ sputtering and 266-nm resonant laser-induced desorption were obtained for comparison with the secondary neutral data. These results have implications for the surface analysis of labile organic compounds. For both L-tryptophan and serotonin, molecular fragmentation was dominated by the effect of the desorption process rather than the photoionization method for the wavelengths studied; fragmentation was maximal with ion-stimulated desorption, intermediate with laser-induced desorption, and minimal with thermal desorption. However, single-photon ionization (SPI) with 118 nm did yield less fragmentation than multiphoton ionization (MPI). The effect of laser pulse width on the 266-nm MPI of thermally desorbed neutral species was also examined. The use of 35-ps rather than 5-ns laser pulses at 266 nm was found to cause a decrease in the molecular fragmentation of L-tryptophan, but had little effect on serotonin decomposition.