Patent ID: 7528370

Claim:
Apparatus for fragmenting ions in a mass spectrometer, comprising: an ion trap having a plurality of electrodes, the ion trap having an interior region into which ions are admitted; an RF trapping voltage source for applying an RF trapping voltage having a first amplitude to one or more of the plurality of electrodes to generate a field for trapping at least a portion of the ions admitted into the ion trap; a resonance excitation voltage source for applying a resonance excitation voltage pulse for a pulse duration to cause at least a portion of a selected set of ions to undergo collisions and break into ion fragments; and the RF trapping voltage source being configured to reduce the RF trapping voltage after application of the resonance excitation voltage pulse to a second amplitude wherein the RF trapping voltage source is configured to reduce the RF trapping voltage sufficiently rapidly after initiation of application of the resonance excitation voltage pulse to retain a substantial portion of ion fragments, formed during application of the resonance excitation voltage pulse or within a delay period thereafter, having mass-to-charge ratios below the low-mass cut off at the first amplitude of the RF trapping voltage.