A mass spectrometer is disclosed comprising an ion guide. The ion guide comprises a hollow tubular conductor having a wall. One or more electrodes are provided in the wall of the tubular conductor. An exit aperture is provided in the wall of the tubular conductor downstream of the one or more electrodes. An AC or RF voltage is applied to the one or more electrodes and a DC potential difference is maintained between the wall of the tubular conductor and the one or more electrodes. The combination of a DC voltage gradient and applying an AC or RF voltage to the electrodes is that ions are confined radially to a region which is preferably close to the one or more electrodes. Ions are preferably extracted from the ion guide via the exit aperture.