A sector instrument is a general term for a class of mass spectrometer that uses a static electric (E) or magnetic (B) sector or some combination of the two as a mass analyzer. Popular combinations of these sectors have been the EB, BE, three-sector BEB and four-sector EBEB (electric-magnetic-electric-magnetic) instruments. Most modern sector instruments are double-focusing instruments in that they focus the ion beams both in direction and velocity.

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.