A method is described that produces product ions for mass analysis, the method comprising the steps of: introducing precursor ions into an RF electric field ion containment device, introducing reagent ions into the RF electric field ion containment device and performing an ion-ion interaction in the RF electric field ion containment device by co-trapping the precursor ions with the reagent ions. Precursor ions and product ions may be retained and/or isolated in the RF electric field ion containment device. The steps above may be repeated until a predetermined amount of reaction completeness is attained. Mass analysis of at least some of the ions in the RF electric field ion containment device may be performed where the ions are mass analyzed either directly from the RF electric field ion containment device.