High-Performance Collision-Induced Dissociation of Peptide Ions Formed by Matrix-Assisted Laser Desorption/Ionization in a Quadrupole Ion Trap Mass Spectrometer

A modified ion trap detector has been utilized to obtain high-performance collision-induced dissociation (CID) mass spectra of peptide ions formed by matrix-assisted laser desorption/ionization (MALDI). MALDI ions are trapped while increasing the fundamental radio frequency field, obviating the need for elevated helium gas pressures. Molecular ion isotopic clusters are then isolated by a reverse-forward-reverse scan sequence. A single species within the isotopic cluster (generally the monoisotopic mass) is then selected for activation. Finally, modulation of the amplitude of the resonant excitation voltage on the end-cap electrodes, used previously to improve mass calibration in normal mass spectra, is now utilized to provide high mass accuracy for the product ions. The CID mass spectra of several protonated and sodium-cationized peptides are presented and are often characterized by a series of rearrangement ions that can be utilized in the determination of amino acid sequences.