A pulse‐counting mass spectrometer is described which is comprised of a new ion source of cylindrical geometry, with exceptional optical properties (the Baur source), a dual focal plane externally adjustable collector slits, and a 17‐stage Allen‐type electron multiplier, all housed in a metal 21 cm radius, 90° magnetic sector flight tube. Mass discrimination of the instrument is less than 1 per mil per mass unit; the optical transmission is more than 90%; the source sensitivity (Faraday collection) is 4 ma/torr at 250&mgr;m emission; and the abundance sensitivity is 3×104. A low blank extrction system is also described which routinely provides precedural blanks at 1600°C of approximately 10,000 atoms of132Xe. The corresponding blanks for84Kr,36Ar, and20Ne are 70,000, 4×106, and 5×107atoms respectively.