Mass spectrometers operate by ionizing neutral molecules or atoms and separating these ions according to their mass to charge ratio. In most commercial gas analysis mass spectrometers the ionization is carried out by bombarding the gas sample with an electron beam from a hot filament. The separation of the ions is typically carried out by either a quadrupole or magnetic sector analyzer.

The trajectory of an ion through a magnetic field is described by the following equation:

M/e = (B²R²)/2V

where:
M = Mass of particle
e = Charge on particle
R = Radius of curvature of path
B = Magnetic field "flux density"
V = Accelerating voltage used for ions

In the magnetic sector analyzer, the trajectory of the ions is controlled by a variable magnetic field allowing ions of interest to be sequentially collected onto a single detector. Thus, the user is able to scan the whole gas sample to identify both known and unknown species. The mass range is tuned by varying the current through the electromagnet, the heavier masses being tuned with a higher current. The mass of a tuned particle is therefore related to the current through the electromagnet.

The diagram shows the characteristic flat top peak of the magnetic sector analyzer. Magnetic sector is preferred for industrial process applications because it offers greater precision, long-term stability and resistance to contamination.