In a mass spectrometer used for measuring the masses

In a mass spectrometer used for measuring the masses of ions, the ions are initially accelerated by an electric potential V and then made to describe semicircular path of radius R using a magnetic field B. If V and B are kept constant, the ratio
change on the ion will be proportional to
mass of the ion

In mass spectrometer, when ions are accelerated through potential V

	1	mv² = qV...........(i)
	2

As the magnetic field curves the path of the ions in a semicircular orbit

Bqv =	mv²	⇒ v =	BqR	...........(ii)
	R		m

Substituting (ii) in (i)

	1	m			BqR		²	= qV
	2				m

or	q	=	2V
	m		B²R²

Since V and B are constants,

∴	q	∝	1
	m		R²

		change on the ion		will be proportional to
		mass of the ion