V. Vorsa et al., Femtosecond photoionization of ion beam desorbed aliphatic and aromatic amino acids: Fragmentation via alpha-cleavage reactions, J PHYS CH B, 103(37), 1999, pp. 7889-7895
We report the photoionization of ion beam desorbed amino acids using femtos
econd laser pulses at 195 and 260 nm. Ionization of ion-desorbed glycine, a
lanine, valine, leucine, isoleucine, and phenylalanine using 195 nm laser p
ulses is found to produce almost exclusively a decarboxylated ion fragment,
while tyrosine and tryptophan produce a functional group cation. The adiab
atic ionization potentials for the former amino acids correspond to the rem
oval of an electron from the amine nitrogen atom, while for tyrosine and tr
yptophan it corresponds to the removal of an electron from the aromatic fun
ctional group. We find that fragmentation is initiated by the formation of
a radical site which leads to an alpha-cleavage reaction and, depending upo
n which electron is removed, results in the formation of a decarboxylated i
on or a functional group ion. These results indicate that a significant fra
ction of the amino acids are desorbed intact, but are fragmented when ioniz
ed. Except for leucine, isoleucine, and phenylalanine, the mass spectra pro
duced by 260 nm irradiation are similar to the mass spectra produced by 195
nm irradiation. When using these two wavelengths, the desorbed amino acids
exhibit a 2- to 8-fold higher ion yield than is found using only secondary
ions produced directly by the incident ion beam. Tyrosine exhibits up to a
40-fold increase in signal using 195 nm irradiation.