Factors affecting the ultraviolet laser desorption of proteins

Abstract

The production of high-mass quasimolecular ions from proteins by matrix-assisted ultraviolet laser desorption is described. A simple time-of-fight system using a Q-switched frequency-quadrupled Nd-YAG laser to desorb protein molecules is shown to have a mass range of up to 116 000 u by the observation of intact, singly charged quasimolecular ions from 700 fmol of β-galactosidase subunit (mol.wt = 116 336 Da). Both positive- and negative-ion spectra of proteins are shown. Four new matrix materials, with properties as good as or better than nicotinic acid, are described. A mass resolution of approximately 500 (full width at half maximum definition) is demonstrated for proteins with mol.wt < 20 000 Da. Product species, formed by fast photochemical reactions in the matrix, are observed to form adduct ions with protein molecules. These adduct ions are a significant cause of the observed broadness of protein quasimolecular ion peaks. The practical physical considerations in detection of large-mass quasimolecular ions from laser desorption, such as detector overloading, are discussed.