A beam correcting device which may be utilized with mass spectrometers for altering the configuration of the beam of ions passing through the mass spectrometer. The beam altering device may include at least four electrodes disposed at substantially equal positions around the beam of ions and positioned at substantially a point along the beam at which the beam is of a minimum thickness. An electrical control circuit is coupled to the electrodes for applying at least four electrical signals, each of a predetermined value, to the electrodes to establish an electrostatic field about the beam of ions. By varying the potentials applied to the various electrodes, a beam having an arcuate cross-sectional configuration may be altered to produce a beam having a generally rectangular cross-sectional configuration thereby compensating for the effect on the beam caused by undesirable fringing fields.