Matrix-assisted laser desorption ion trap mass spectrometry: efficient trapping and ejection of ions

The present paper explores the coupling of a matrix-assisted laser desorption/ionization (MALDI) ion source with an ion trap mass analyzer, with particular emphasis on the development of methods for improving the efficiency of ion trapping and ejection. A technique is described for directly measuring, for the first time, the trapping efficiency of peptide ions produced in a remote external MALDI ion source. The technique was used to devise an improved scheme for trapping, which yielded efficiencies as high as 39%. An improved understanding of the resonant ejection process led us to a new resonant ejection parameter set that increased the ejection efficiency by 1 order of magnitude over more conventionally used parameter sets and allowed for marked improvements in the mass resolution of ions with m/z > 2500. The presently described improvements in the efficiencies of ion trapping and ejection together with improved methods for isolating and fragmenting ions (Qin, J.; Chait, B.T.Anal. Chem., following article in this tissue) lay the foundation for highly sensitive MALDI ion trap mass spectrometry of proteins (Qin, J.; et al. Anal. Chem. 1996, 68, 1784-1791).