analyte ions , particularly biopolymer ions , stored in an rf ion trap are ergodically fragmented by bombarding the analyte ions with collision ions , for example medium - mass , mono - atomic ions having a charge of opposite polarity to the charge of the analyte ions . since the analyte ions are not fragmented by accelerating and / or exciting them to oscillations , as is the case with conventional collision - induced dissociation , the rf voltage of the ion trap can be set low enough that daughter ions with light charge - related masses that are produced by the fragmentation can also remain trapped in the ion trap .