Patent ID: 8030612

Claim:
A method for mass spectrometry comprising: providing an excitation q value that is greater than zero and less than 0.908, and maintaining an ion trap of a mass spectrometer under vacuum pressure of 1 mTorr or less while: (a) introducing an ion population into the trap, the ion population comprising an ion of interest; (b) applying a resolving direct current to the ion trap for a time sufficient to isolate from the trapped ion population an ion subpopulation within a window of about 10 m/z or less, the ion subpopulation comprising the ion of interest; and one of (c) when the m/z of the ion of interest is above the low-mass cut-off determined by the excitation q, applying an excitation signal to the ion of interest, at an excitation amplitude (V) that is from about 1 mV to 100 mV for a time sufficient to generate fragment ions that arise from a mass window having a width of 2 m/z or less and being centered on the ion of interest, said excitation amplitude (V) being about 0.05 to about 10 mV above a minimum that is the threshold amplitude for the onset of ion-of-interest fragmentation, and said fragment ions including fragment ions of the ion of interest; or (d) when the m/z of the ion of interest is below or equal to the low mass cut-off determined by the excitation q, (1) applying an excitation signal to the ion subpopulation to remove any ions, other than the ion of interest, from the subpopulation that have a mass/charge ratio (m/z) that is within 2 m/z or less of the ion of interest, at an excitation amplitude (V) that is from about 1 mV to 100 mV for a time sufficient to generate fragment ions that arise from a mass window having a width of 2 m/z or less and being centered on the ion of interest, said excitation amplitude (V) being about 0.05 to about 10 mV above a minimum that is the threshold amplitude for the onset of fragmentation of said ions, while retaining the ion of interest unfragmented in a remaining ion subpopulation in the ion trap; and thereafter (2) decreasing the excitation q to a reduced value, greater than zero, that permits the m/z of the ion of interest to be above the low mass cut-off determined by that reduced value; and thereafter (3) applying an excitation signal to the remaining ion subpopulation, at a sufficient excitation amplitude (V) and for a time sufficient to generate fragment ions from the ion of interest, said excitation amplitude (V), said time, or both, being the same as or different from that of step (c).