A mass spectrometer is disclosed comprising an ion mobility spectrometer or separator (6) arranged upstream of a collision or fragmentation cell (8). Ions are separated according to their ion mobility within the ion mobility spectrometer or separator (6). The kinetic energy of the ions exiting the ion mobility spectrometer or separator (6) is increased substantially linearly with time in order to optimise the fragmentation energy of ions as they enter the collision or fragmentation cell (8). During the time that the potential of the ion mobility spectrometer or separator (6) is being varied, the potential of ion-optical components upstream of the ion mobility spectrometer or separator (6) such as an ion source (1), ion guide (2), quadrupole mass filter (3), optional second collision or fragmentation cell (4) and an ion trapping device (5) are kept constant.