Sector instrument

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A sector instrument is a general term for a class of mass spectrometer that utilizes 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) also referred to as a double focusing sector instrument.

[edit] Theory

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

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) will be:

F = qE, 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) will be:

F = qvB, 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.

The force in the magnetic sector is complicated by the velocity dependence but with the right conditions (uniform velocity for example) ions of different masses will separate physically in space into different beams as with the electric sector.