A mass spectrometry method in which notch-filtered noise is applied to an ion trap to resonate all ions except selected reagent ions out of the region of the trapping field. Preferably, the trapping field is a quadrupole trapping field defined by a ring electrode and a pair of end electrodes positioned symmetrically along a z-axis, and the filtered noise is applied to the ring electrode to eject unwanted ions in radial directions rather than toward a detector mounted along the z-axis. Also preferably, the trapping field has a DC component selected so that the trapping field has both a high frequency and low frequency cutoff, and is incapable of trapping ions with resonant frequency below the low frequency cutoff or above the high frequency cutoff. Application of the filtered noise signal to such a trapping field is functionally equivalent to filtration of the trapped ions through a notched bandpass filter having such high and low frequency cutoffs. Application of filtered noise in accordance with the invention avoids accumulation of contaminating ions during the process of storing desired reagent ions, and permits ejection of unwanted ions in directions away from an ion detector to enhance the detector's operating life and rapid ejection of unwanted ions having mass-to-charge ratio below a minimum value, above a maximum value, and outside a window (between the minimum and maximum values) determined by the filtered noise signal.