Mass spectrometer

Ions are formed at D and pass through the cathode C and then through a slit S₁. They then travel between two plates A and B, between which a potential (V) is applied. A magnetic field (strength B) is applied at right angles to the electrostatic field and so the electrostatic and electromagnetic forces act in opposite directions to each other.

A particle with a charge q and velocity v will only pass through the next slit S₂ if the resultant force on it is zero – that is it is travelling in a straight line. That is if:

Electromagnetic force (Bqv) = Electrostatic force (qE) Velocity of particle (v) = E/B

Only particles with a certain velocity enter the deflection chamber F. For this reason the combination of slits and deflecting plates is called a velocity selector.

Mass of ion (M) = rB2q/E

The radius of the path in the deflection chamber is directly proportional to the mass of the ion.