A method of processing ions in a quadrupole rod set is provided, comprising a) establishing and maintaining a two-dimensional substantially quadrupole field having a quadrupole harmonic with amplitude A2 and a selected higher order harmonic with amplitude Am radially confining ions having Mathieu parameters a and q within a stability region defined in terms of the Mathieu parameters a and q; c) adding an auxiliary excitation field to transform the stability region into a plurality of smaller stability islands defined in terms of the Mathieu parameters a and q; and, d) adjusting the two-dimensional substantially quadrupole field to place ions within a selected range of mass-to-charge ratios within a selected stability island in the plurality of stability islands.