Patent ID: 7709786

Claim:
A method of processing ions in a quadrupole rod set, the method comprising a) establishing and maintaining a two-dimensional substantially quadrupole field for processing the ions, the field having a quadrupole harmonic with amplitude A 2 and a selected higher order harmonic with amplitude A m wherein m is an integer greater than 2, and the magnitude of A m is greater than 0.1% of the magnitude of A 2 ; b) introducing the ions to the two-dimensional substantially quadrupole field and subjecting the ions to both the quadrupole harmonic and the higher order harmonic of the field to radially confine 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 including the auxiliary excitation field to place ions within a selected range of mass-to-charge ratios within a selected stability island in the plurality of stability islands to impart stable trajectories to the selected ions within the selected range of mass-to-charge ratios for transmission through the rod set, and to impart unstable trajectories to unselected ions outside of the selected range of mass-to-charge ratios to filter out such ions.