Mass spectrometry (abbreviated as MS) has slowly emerged as a very powerful tool in analyzing the organic molecules including biomolecules. A mass spectrometer separates the molecules based on their mass and charge. The underlying principle is conceptually very simple: a moving charged particle can be deflected by applying electric and magnetic fields. The deflection caused by the electric and magnetic fields depends on the mass and the charge of the particle. Let us see what happens to a charged particle in an electric field (Figure 11.1).

Figure 11.1 Force experienced in an electric field by stationary charged particles (A) and charged particles with uniform motion (B). Panel B represents the situation when the charged particles are in uniform motion with their initial velocity vectors perpendicular to the electric field vector.

 

The force experienced, F by a particle with charge, q in an electric field, E is given by:

F = qE              ...............................................................................(11.1)

The force causes the particle to accelerate in the electric field which is given by

         ...........................................................................(11.2)
             ..........................................................................(11.3)

where, m is the mass of the particle and a is the acceleration under the electrostatic force.

Equation 11.3 shows that the acceleration of the particle depends on the mass to charge ratio, . A lighter particle is accelerated more than a heavier particle carrying the same charge. Similarly, a particle with higher charge is accelerated more as compared to the particle of same mass but having lesser charge.