PEPTIDE SEQUENCE IONS PRODUCED BY POSTIONIZATION OF NEUTRAL MOLECULES FORMED DURING RESONANT 266-NM LASER DESORPTION

A number of examples of peptide mass spectra obtained using 266-nm laser desorption (LD) followed by 255-nm multiphoton ionization (MUPI) of the laser desorbed neutral molecules are presented which show a variety of structurally significant sequence fragment ions. It is believed that most of these sequence fragment ions result from postionization of neutral fragments produced during 266-nm LD. MUPI postionization of the fragment neutrals produced during LD can provide sequence information which is complementary to the sequence information derived from direct ion desorption experiments. In addition, a variety of immonium fragment ions may be produced at higher ionizing laser power densities which give an overview of the amino acid residues contained in the peptide structure. These fragment ions are formed during MUPI positionization step by multiphoton fragmentation of larger intact ions. Finally, a comparison of the sequence fragment ions produced in these experiments with the sequence ions produced using other direct ion desorption techniques illustrates the analytical usefulness of the sequence ions produced in these experiments.