A mass spectrometer is disclosed comprising an ion mobility spectrometer or separator and an ion guide arranged downstream of the ion mobility spectrometer or separator. A plurality of axial potential wells are created in the ion guide so that ions received from the ion mobility spectrometer or separator become confined in separate axial potential wells. The potential wells maintain the fidelity and/or composition of ions received from the ion mobility spectrometer or separator. The potential wells are translated along the length of the ion guide.