A mass spectrometer operating according to the iso-tach principle in which a mass filter accelerates ions to nominally equal velocities irrespective of their mass-to-charge ratios. The mass spectrometer is provided with an improved detector based on an electrostatic lens arrangement made of a concave lens followed in the beam path by a convex lens. These lenses deflect ions away from the beam axis by a distance from the beam axis that is inversely proportional to their mass-to-charge ratios. The mass-to-charge ratio of the ions can then be determined by a suitable detector array, such as a multi-channel plate placed in the beam path. This provides a compact and sensitive instrument.