Experimental study of laser desorbed biological molecules using ion-trap/reflectron time-of-flight mass spectrometry.

Abstract

The chemistry of fragmentation produced by matrix-assisted laser desorption/ionization (MALDI) was studied using an ion-trap/reflectron time-of-flight mass spectrometer for large biological molecules. MALDI-generated peptide ions were introduced into a quadrupole ion trap, which is capable of storing ions with a range of masses for a time period ranging from microseconds up to seconds, followed by extraction into a time-of-flight drift tube for mass analysis. It is shown that large ions may undergo fragmentation over long periods of trapping time and that these fragments are detected as stable ions rather than as metastable ions. The ion decay was found to depend strongly on the laser intensity and the RF voltage applied. Sequence specific fragmentation of MALDI-generated peptide ions was investigated and the types of fragmentation for several target peptides were studied. The implications of this work for structural analysis are discussed. The trapping and extraction efficiency of an ion trap for MALDI-generated ions was studied and it was found to depend on a number of experimental parameters including the voltage of trapping RF and buffer gas pressure. The effect of trapping time on the ion signal intensity appears to be due to the required finite time for the ions to collapse to the center of the ion trap, where they could be efficiently extracted, through collisions with buffer gas molecules. The effects of the initial energy distribution and the buffer gas on the trapping efficiency was also investigated. Ultimately a study of a quadrupole ion trap as a front-end device for selecting the ions with specific mass range was performed, which could provide a technique for tandem mass spectrometry of MALDI-generated fragmentation.