Principle of the Magnetic Sector

The Lorentz Force Law can be used to describe the effects exerted onto a charged particle entering a constant magnetic field. The Lorentz Force Fl depends on the velocity v, the magnetic field B, and the charge of an ion. In the simplest form the force is given by the scalar equation [3,4,70,71] 

Equation 4.11 is valid if v and B (both are vectors) are perpendicular to each other. Otherwise, the relationship becomes 

Upon rearrangement we obtain the radius r of this circular motion 

This shows the working principle of a magnetic sector the radius r depends on the momentum mv of an ion, and therefore the momentum depends on m/z- 

Note The magnetic sector is a momentum analyzer rather than a direct mass analyzer as commonly assumed. 

Finally, dispersion in momentum causes a dependence of on the square root of mass becoming obvious by substitution of v from Eq. 4.3 and q = ez 

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