An ion guide is disclosed wherein an axial DC voltage barrier is created at the exit of the ion guide. A primary RF voltage is applied to the electrodes in order to confine ions radially within the ion guide. A supplemental RF voltage is also applied to the electrodes. The supplemental RF voltage has a greater axial repeat length than that of the primary RF voltage. The amplitude of the supplemental RF voltage is increased with time causing ions to become unstable and gain sufficient axial kinetic energy such that the ions overcome the axial DC voltage barrier. Ions emerge axially from the ion guide in mass to charge ratio order.