Abstract The mass filter is a dynamic mass spectrometer and its performance is very dependent on fringing fields at the entrance and exit. The influence of these fields was investigated by a computer calculation of ion trajectories. Entrance ramps, where the static and dynamic components of the field increase gradually to their full values, create a near equivalence of ion behaviour for all initial phases of the r.f. field. This leads to good resolution and good peak shape. In this sense, entrance ramps are preferable to perfect, abruptly beginning, fields. For coincident (d.c. and a.c.) ramps of less than about three r.f. cycles there is also a larger effective aperture than for the “perfect” field, but long ramps lead to small effective apertures. This may cause discrimination against ions of low velocity (high mass or low energy). Ramp effects have undoubtedly been present in many existing quadrupole mass filter designs. The “delayed” d.c. ramp proposed by Brubaker retains the near-equivalence of ion behaviour for all phases of the r.f. field and solves the aperture problem in the case of the slow ions. At the exit from the quadrupole, it is suggested that an advanced d.c. ramp may sometimes be advantageous to ensure total ion collection.