A method of isolating selected ion species in a quadrupole ion trap mass spectrometer is disclosed. One or more ranges of masses to be eliminated from the ion trap are ejected by applying a supplemental dipole excitation waveform, sparsely populated with frequency components, while the trapping field is modulated. The spacing of the frequency components in the supplemental excitation waveform varies across the range of frequencies in the waveform. Preferably, the frequency range is divided into a plurality of subranges, and the spacing of the frequency components in each of the subranges is constant. A method of creating a master set of frequencies used for generating a supplemental excitation waveform is also shown. Likewise, a method of calculating edge frequencies defining a gap in the mass spectrum that is excited by the supplemental waveform is also shown. Modulation of the trapping field may be varied while the supplemental excitation waveform is applied to change the width of the gap in the mass spectrum.