The spectrometer includes a single magnet having two opposing pole faces each spanning at least a 270.degree. sector. A 270.degree. sector nonmagnetic chamber is juxtaposed in the air gap of the magnet and is provided with partitions for forming 90.degree. and 180.degree. sector portions. An ion source generates a relatively wide, parallel beam of ions which is accelerated into the 90.degree. sector portion. A common partition between the 90.degree. and 180.degree. sector portions and the partition at the exit end of the 180.degree. sector portion are each provided with a narrow slit through which particular ions are focussed to obtain the mass separation function of the spectrometer.
A quadrupole mass spectrometer includes means for measuring total ion current having an electron repeller cage disposed about an electron source filament. The electron repeller cage repels electrons emitted by the electron source filament, urging them away from both the repeller and the filament, while also attracting positive ions. When the positive ions contact the electron repeller cage, a current is induced that is measured at the electron repeller cage. The measured current represents total ion current transmitted through the spectrometer. Thus, the need to include a total pressure measurement plate to measure total ion current is eliminated, and since the ion-exposed surface area of the electron repeller cage is greater than the ion-exposed surface area of the total pressure measurement plate, the invention provides improved total pressure measurement sensitivity.
