A mass spectrometer is disclosed comprising an Electron Transfer Dissociation cell (1). Positive analyte ions are fragmented into fragment ions upon colliding with singly charged negative reagent ions with the cell (1). The cell comprises a plurality of ring electrodes (1) which form a spherical trapping volume. Ions experience negligible RF heating over the majority, of the trapping volume which enables the kinetic energy of the analyte and reagent ions to be reduced to just above thermal temperatures. An Electron Transfer Dissociation cell (1) having an enhanced sensitivity is thereby provided. Fragment ions created within the cell (1) may be cooled and may be transmitted onwardly to an orthogonal acceleration Time of Flight mass analyser enabling a significant improvement in the resolution of the mass analyser to be obtained.