EFFECT OF ION-MOLECULE COLLISIONS IN THE VACUUM CHAMBER OF AN ELECTROSPRAY TIME-OF-FLIGHT MASS-SPECTROMETER ON MASS-SPECTRA OF PROTEINS

Electrospray ionization spectra of positive multiply charged ions of s everal proteins with molecular weight from 8565 to 339,100 u were reco rded at different pressures of residual gas in the vacuum chamber of a n electrospray time-of-flight mass spectrometer. The pressure was vari ed in the range (0.2 to 5) X 10(-6) torr. The effect of pressure was f ound to be significant even for the lightest protein investigated (ubi quitin), which resulted in a decrease of both sensitivity and resoluti on. Investigations of the arrival-time distributions and the energy di stributions of ions showed that collision-induced dissociation (CID) o f the protein ions in the drift region is the main process responsible for the effect. Several CID cross sections of proteins were estimated from a series of mass spectra recorded at different pressures in the reflecting mode: 1150 Angstrom(2) for cytochrome c (averaged over char ge states 14-18), 800 Angstrom(2) for lysozyme (z = 8-10), 1840 Angstr om(2) for apomyoglobin (z = 12-25), 800 Angstrom(2) for holomyoglobin (z = 8), and 2500 Angstrom(2) for carbonic anhydrase II (z = 22-35). S everal experiments with large proteins in their native conformations a nd low charge states (m/z less than or equal to 10,000) demonstrate th at these ions are less sensitive to high residual gas pressure.