Abstract: An apparatus for the in-situ detection of ions in a beam of an ion implanter device includes a mass spectrometer device having inner and outer walls and, a system for generating and directing an ion implant beam through the mass spectrometer device. The mass spectrometer device generates a magnetic field for directing ions of the ion implant beam of a desirable type through an aperture for implanting into a semiconductor wafer, and causing ions of undesirable type to collide with the inner or outer wall. For in-situ detection, a detector device is disposed on the inner and outer walls of the mass spectrometer for detecting the undesirable type of ions deflected. In one embodiment, the detector device comprises electronic sensor devices for detecting a concentration of the undesirable type ions which comprise undesirable elements and compounds.