Patent ID: 8378298

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; (c) when the m/z of the ion of interest is above or equal to the low mass cut-off determined by the excitation q: 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 eject ions 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 ejection of said ions, while retaining the ion of interest in a remaining ion subpopulation in the ion trap.