SEQUENCING OF PEPTIDES IN A TIME-OF-FLIGHT MASS-SPECTROMETER - EVALUATION OF POSTSOURCE DECAY FOLLOWING MATRIX-ASSISTED LASER-DESORPTION IONIZATION (MALDI)

In matrix assisted laser desorption ionisation (MALDI) a large fraction of analyte ions undergo postsource decay (PSD) during flight in the field free drift path. By means of a modified two-stage reflectron, daughter ion time-of-flight spectra of medium sized linear peptides (up to 2800 u) were recorded containing full sequence information. Precision, accuracy and mass resolution of fragment ions were almost as good as obtained in high energy CAD studies performed in four-sector instruments. Instrumental sensitivity was better by at least one order of magnitude. In reflectron time-of-flight mass spectrometry (RETOF-MS) the cleavage pattern of PSD products is different from that obtained by high energy and low energy CAD. In our instrument, conditions which were energetically comparable to high energy and low energy CAD could easily and comparatively be studied in the same experiment by varying instrumental parameters. Activation mechanisms of PSD were found to be largely determined by collisional events (ion/neutral) induced by the acceleration field during early plume expansion. Future potentials of PSD analysis after MALDI are discussed.