FUNDAMENTAL-ASPECTS OF POSTSOURCE DECAY IN MATRIX-ASSISTED LASER DESORPTION MASS-SPECTROMETRY .1. RESIDUAL-GAS EFFECTS

Prompt ion fragmentation within the first almost-equal-to 10% of the acceleration time is generally lacking in matrix-assisted laser desorption. Decay of peptide and protein ions has, however, been found to take place to a great extent in the field-free drift region of a time-of-flight mass spectrometer, resulting from both unimolecular and bimolecular reactions. Ion stability and total fragment yield in time-of-flight mass spectrometry has been investigated as depending on the residual gas pressure. Total reaction cross sections and unimolecular decay rates have been determined for 20-keV peptide ions up to m/z 12 360. Whereas reaction cross sections reach the size of the physical cross sections for a typical N2-dominated residual gas composition, they have been found to be only a tenth of that for a He-dominated residual gas. An important factor for the resulting size of the reaction cross sections is the internal energy of the analyte ions prior to collision. The importance of ion instability on achievable mass resolution and sensitivity in time-of-flight mass spectrometry is discussed, as well as its potential for structural analysis such as peptide sequencing.