Patent ID: 7399962

Claim:
A method of mass spectrometry using an ion trap, the method comprising: a) generating a plurality of precursor ions from a sample, each ion having a mass to charge ratio selected from a first range of mass to charge ratios M 1 /Z 1 , M 2 /Z 2 , M 3 /Z 3 . . . M N /Z N ; b) causing at least some of the plurality of precursor ions to dissociate, so as to generate a plurality of fragment ions, each of which has a mass to charge ratio selected from a second range of mass to charge ratios m 1 /z 1 , m 2 /z 2 , m 3 ,z 3 . . . m n /z n ; c) directing the fragment ions into an ion trap, the ion trap including means for generating an electromagnetic field which allows trapping of ions in at least one direction thereof, the ions entering the trap in groups at a time which depends upon the mass to charge ratio of the precursor ions; d) determining the mass to charge ratio of ions in at least one of the groups of ions, based upon a parameter of motion of the ions in that or those groups in the said electromagnetic field in the trap; and e) distorting the electromagnetic field in the trap so as to permit separate detection of fragment ions within the trap which have the same mass to charge ratio, but which are derived from different precursor ions.