A mass filter apparatus for filtering a beam of ions is described. The apparatus comprises an ion beam source and first and second mass filter stages in series to receive the ion beam. A vacuum system maintains the first and second filter stages at substantially the same operating pressure, below 10−3 torr. The first mass filter stage transmits only ions having a sub-range of mass-to-charge ratios including a selected mass-to-charge ratio. The second filter transmits only ions of the selected mass-to-charge ratio.The second mass filter can achieve high accuracy detection without being subjected to problems such as build-up of material on quadrupole rods, resulting in a distorted electric field close to the rods. The first mass filter acts as a coarse filter, typically transmitting 1% of ions received from the ion source. Thus, the detection accuracy and lifetime of mass spectrometers embodying this invention are greatly improved.