Sector instrument

A sector instrument is a general term for a class of mass spectrometer that uses a static electric or magnetic sector or some combination of the two (separately in space) as a mass analyzer. A popular combination of these sectors has been the BEB (magnetic-electric-magnetic). Most modern sector instruments are double-focusing instruments (first developed by A. Dempster, K. Bainbridge and J. Mattauch in 1936) in that they focus the ion beams both in direction and velocity.

The behavior of ions in a homogeneous, linear, static electric or magnetic field (separately) as is found in a sector instrument is simple. The physics are described by a single equation called the Lorentz force law. This equation is the fundamental equation of all mass spectrometric techniques and applies in non-linear, non-homogeneous cases too and is an important equation in the field of electrodynamics in general.

where E is the electric field strength, B is the magnetic field induction, q is the charge of the particle, v is its current velocity (expressed as a vector), and × is the cross product.

So the force on an ion in a linear homogeous electric field (an electric sector) is:

in the direction of the electric field, with positive ions and opposite that with negative ions.

The force is only dependent on the charge and electric field strength. The lighter ions will be deflected more and heavier ions less due to the difference in inertia and the ions will physically separate from each other in space into distinct beams of ions as they exit the electric sector.

And the force on an ion in a linear homogeous magnetic field (a magnetic sector) is:

perpendicular to both the magnetic field and the velocity vector of the ion itself, in the direction determined by the right-hand rule of cross products and the sign of the charge.

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