A method of operating a mass spectrometer having a rod set is provided. The rod set has a first end, a second end opposite to the first end, and a longitudinal axis extending between the first end and the second end. The method comprises a) admitting ions into the rod set; b) trapping at least some of the ions in the rod set by i) producing a first barrier field at a first end member adjacent to the first end of the rod set, ii) producing a second barrier field at a second end member adjacent to the second end of the rod set, and iii) providing an aggregate field comprising an RF field between the rods of the rod set; c) selecting a first selected mass to charge ratio of a first group of ions in the ions; d) determining a first excitement level of a selected characteristic of the aggregate field for the first group of ions; e) adjusting the selected characteristic of the aggregate field to the first excitement level to resonantly excite the first group of ions to mass selectively eject the first group of ions from the rod set past the barrier field; and, f) maintaining the selected characteristic of the aggregate field at the first excitement level during an excitement time interval wherein the excitation time interval is at least 1 millisecond.