double focussing mass spectrometer

A mass spectrometer consisting of a magnetic sector and an electric sector in series which  focusses ions that have differing initial momentum and kinetic energies. When the electric sector  comes first, in the EB configuration, the arrangement is known as forward geometry.

Conversely, the BE configuration is known as  reverse geometry. Two EB configurations are Nier-Johnson geometry and Mattauch Herzog geometry.  In the former, a deflection of π/2 rad in E is  followed by a deflection of π/3 rad in B to focus the ions at a focal point. In the latter, a deflection  of π/4v2 rad in E is followed by a deflection of  π/2 rad in B to focus the ions on a focal plane.  Also called a sector mass spectrometer.