Patent ID: 7615742

Claim:
A method for generating and measuring light fragment ions from selected parent ions in an ion trap of an ion trap mass spectrometer that uses an RF storage voltage to produce a pseudopotential for retaining ions and is filled with collision gas, comprising the steps of: (a) filling the ion trap with ions, (b) selecting and isolating parent ions to be fragmented, each of the parent ions having a mass, (c) adjusting the RF storage voltage so that a cut-off mass for ion trap storage is higher than one third of the mass of the parent ions, (d) deflecting the parent ions by applying a deflecting DC voltage to the ion trap for a time period sufficient for an equilibrium to occur between forces on the parent ions produced by the pseudopotential of the ion trap and by the deflecting DC voltage, (e) switching off the deflecting DC voltage so that the parent ions oscillate in the pseudopotential of the ion trap and undergo collisions with the collision gas in the trap to generate light fragment ions, and reducing, in a controlled way, the RF storage voltage amplitude to an amplitude which permits ion trap storage of the light fragment ions, and (f) measuring the light fragment ions produced.