A mass spectrometer is disclosed having a mass analyzer with a mass-to-charge dispersive element for separating ions according to their mass-to-charge ratios along a dispersive plane and an ion deflector to deflect ions leaving the mass analyzer in the dispersive plane. A shielding arrangement, located between the dispersive element and the ion deflector is arranged to define the portion of the beam to be deflected by the ion deflector. The deflected beam is steered onto a beam defining aperture, located at the focal plane of the mass analyzer is detected by at least one ion detector, located downstream from the beam defining aperture.