A mass selector which efficiently produces mass filtered beams of both primary and secondary ions in the medium current ranges is described. Currents up to 10−6 A for the mass filtered beam can be obtained with beam energies of 3–5 keV. Ions are produced in a hot cathode, gas discharge ion source of relatively simple construction, and are extracted as a parallel beam. Mass analysis is accomplished over a short pathlength with a compact, high current, sector field electromagnet which is entirely contained within the analyzer chamber. An electrostatic quadrupole lens is used for optimum shaping of the incident ion beam for high transmission through the magnetic sector. A second quadrupole lens is used to restore parallelism to the mass filtered ion beam and is mounted beyond the image slit, which was made wide enough to pass the entire image focused beam. The instrument has no object slit, as the entire incident beam is brought to a crossover at the object position. Beam transmission through all elements in the analyzer chamber, including the two quadrupole lenses, the sector magnet, and the image slit, is approximately 50%. Oscillating beam scanners are used to measure the beam density profile and to monitor the mass spectrum. The resolving power for spatial separation is about 60. Secondary ions such as N4+ and CH5+ can be generated with intensities comparable to those of primary ions.