MASS-SELECTIVE ANALYSIS OF IONS IN TIME-OF-FLIGHT MASS-SPECTROMETRY USING AN ION-TRAP STORAGE DEVICE

A quadrupole ion trap was used as a front-end device for selecting spe cific ions formed via matrix-assisted laser desorption for analysis in time-of-flight mass spectrometry. Using low-voltage auxiliary RF sign als, the ion trap can be used to resonantly eject all but a selected m ass-to-charge range prior to time-of-flight mass analysis. A series of simple analog resonance ejection circuits were developed to provide t his selective ion trapping. These circuits use both single- and modula ted-frequency waveforms applied directly to the repeller endcap of the quadrupole ion-trap source in a monopolar fashion. Commercially avail able waveform generators, as well as a multichannel voltage-controlled oscillator circuit, were used for resonant ion ejection and ion isola tion prior to pulsed extraction into the time-of-flight mass spectrome ter. The analytical utility of these circuits was tested on a series o f biologically active peptides from m/z 700 to 1650 using matrix-assis ted laser desorption/ionization (MALDI) directly into the ion trap. Th ese simple analog circuits were found to provide a number of useful qu alitative improvements in the ion-trap/reflectron time-of-flight devic e including the broadband removal of matrix background ions formed via MALDI and the isolation of molecular ions from complex peptide mixtur es. The utility of this methodology for tandem mass spectrometry, incl uding collision-induced dissociation and photodissociation, is discuss ed.