In mass spectrometer used for measuring the masses of ions, the are initially accelerated by an electric potential V and then made to describe semicircular paths of radius R using a magnetic field B. If V and B are kept constant, the ratio 
will be proportional to:
-
1/R
-
1/R2
-
R2
-
R
B.
1/R2
Centripetal force os provided by the magentic force qvB.
The radius of the orbit in which ions moving is determined by the relation as given below.
where m is the mass, v is velocity, q is charge of ion and B is the flux density of the magnetic field, so that qvB is the magnetic force acting on the ion, and mv2/R is the centripetal force on the ion moving in a curved path of radius R. The angular frequency of rotation of the ions about the vertical field B is given by
If ions are accelerated by electric potential V, then energy attained by ions
E =qV ..... (ii)
From Eqs. (i) and (ii), we get
If V and B are kept constant, then
