A mass spectrometer is disclosed having a gas collision cell. An AC or RF ion guide comprising a plurality of ring electrodes which preferably have the same internal diameter is provided within the gas collision cell. The ion guide extends upstream and/or downstream of the gas collision cell so that ions may be continuously radially confined as they pass from a vacuum chamber maintained at a relatively low pressure, through an inlet differential pumping aperture to the gas collision cell, through the gas collision cell and then out of the gas collision cell through an outlet differential pumping aperture.