The magnetic sector mass spectrometer

These mass spectrometers essentially consists of the ion source, the magnetic separation system and the ion trap.

The ion source ionizes neutral gas particles and produces an ion beam. The positively charged ions are accelerated out from the ion source and end-up into a homogeneous magnetic field. Subsequently they are steered into a circular path, whose radius depends on the ratio mass-load of the ions.

The Ion trap is designed thatway, so only the He-ions can end-up in the trap on their circular path. Other ions miss the Ion trap due to their larger or smaller circular path radius. At the Ion traps the incoming He-ions are caught and the generated signal is proportionally to the number of those ions. – therefore the signal represent the current partial pressure of the helium.