Working Principle of a Magnetic Analyzer

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In a magnetic field of strength B, an ion of charge q and velocity v experiences a force given by 

The force is perpendicular to the direction of the magnetic field as well as to the direction of the motion of the ion. As a result, the ion travels in a curved path that has a radius r in a plane perpendicular to the direction of the magnetic field. The force Fb is counterbalanced by the centrifugal force such that 

To acquire a mass spectrum, a common practice is to scan the magnetic field while keeping V constant. Alternatively, all ions can be collected simultaneously in the same plane by keeping B fixed, as is the usual practice in Mattauch-Herzog geometry double-focusing mass spectrometers. The third, not so common option is to keep B constant and vary the value of V. 

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