During an accumulation time period of each time cycle of an ion guide and before a ramped AC voltage is applied to at least one set of axial rods to eject ions according to m/z value, a number of steps are performed. Ions are received from outside of the ion guide through an entrance aperture and into a first cell. A low DC voltage is applied to a barrier electrode to receive ions from the first cell into a second cell. And, a high DC voltage is applied to an exit electrode to prevent ions from exiting the ion guide. During a cooling time period before the AC time period, a high DC voltage is applied to the barrier electrode to trap and cool ions in the second cell and to continue to receive ions into the first cell without being affected by the ramped AC voltage.