Graphite surface-assisted laser desorption/ionization time-of-flight mass spectrometry of peptides and proteins from liquid solutions.

Laser desorption time-of-flight mass spectra of peptides and proteins, as well as of lower molecular weight analytes, have been obtained by using a pulsed nitrogen UV laser (337 nm) to irradiate mixtures of 2-150 microns graphite particles and solutions of the analytes in glycerol. Protonated analytes as well as abundant alkali cation adducts were observed. Carbon cluster ions, Cn+, typically had a low abundance but dominated the mass spectrum at elevated laser powers. In spectra of a cytochrome c tryptic digest, all but one of the tryptic peptides were easily observed. Spectra of low molecular weight analytes dissolved in glycerol are very similar to FAB spectra of the same glycerol solution with added alkali salts. However, in many peptide and protein spectra, glycerol ion abundances are very low, and the alkali ions dominate the spectra at low mass. These spectra may correspond to wet and dry surface desorption conditions, respectively. The best spectra of the larger molecules were observed under dry conditions. In these initial experiments, we have obtained a sensitivity in the pico- to nanomole range and a mass resolution of about 300. The signal intensity is as good as that in conventional MALDI, and under optimal conditions, few background peaks appear, even at low mass.