Systems and methods for filtering a continuous beam of ions are provided. An acceleration electric field is applied to a continuous beam of ions using an accelerator to produce an accelerated beam of ions. A field is applied to the accelerated beam to separate ions in time and space using a deflector producing a separated beam of ions. The field applied by the deflector is a rotating field or a circulant rastering field. The rotating field can be a rotating magnetic or electric field. Only accept those ions from the separated beam whose m/z values lie within a range centered around a target m/z value using an aperture. The aperture can include a pinhole aperture in a rotating disk or an annular aperture in a first stationary disk, a second deflector, and a pinhole aperture in the center of a second stationary disk.