An ion-ion reaction cell is provided comprising a plurality of electrodes (1) forming an ion guide (2). One or more transient DC voltage waves (8, 9) are applied to the electrodes (2). Reagent anions and analyte cations are arranged to undergo ion-ion reactions within the reaction cell and the resulting fragment ions which are formed within the reaction cell are then subsequently translated out of the reaction cell by means of one or more transient DC voltage waves (8, 9).