where, θ is the angle between the velocity vector, v and the magnetic field vector, B.
In a mass spectrometer, v and B are generally orthogonal to each other; in that case:

         ...................................................................................(11.8)

Equation 11.8 shows that the deflection caused by a magnetic field in a moving charged particle is proportional to the mass to charge ratio. For the two particles having same charge but different masses, the one with lesser momentum deflects more (r ∝ mv and smaller r means larger deflection). In mass spectroscopy, the charge is usually represented as z and we shall be sticking to the same convention. A mass spectrum is a two dimensional plot between ion abundance and ratio (Figure 11.3)

Figure 11.3 A typical mass spectrum

Let us see the design of a typical mass spectrometer (Figure 11.4). The basic requirement for an analyte molecule to be studied using mass spectrometry is that it has to be charged. A large number of molecules, however, may not be charged. The first step in an MS experiment is therefore to ionize the molecules. The spectrometer therefore has an ionization source. The ions generated are then separated by one or more mass analyzers which are then detected by a detector.

Figure 11.4 The components of a mass spectrometer