In an RF ion trap, analyte ions are fragmented by applying a moderately high RF storage voltage to the trap. The ions are then excited via dipolar excitation, and after a short time, the ions are forced into a resting state, again using dipolar excitation. The RF storage voltage is then rapidly reduced to a low value thereby making it possible to store small fragment ions produced by ergodic decompositions that occur subsequent to the reduction of the RF storage voltage.