Double focussing analysers: 

In describing the magnetic sector analyser we mentioned that the ions entering the analyser have the same kinetic energy. In fact these fragments have a small range of kinetic energies and when passed through the magnetic field these get focussed at the same position. Thus, the spread of   kinetic energies cause a broadening of the beam reaching the detector leading to a loss in resolution.

For a better resolution and more accurate mass determinations double focussing instruments are used which employ an electrostatic field prior to the magnetic sector. This combination of electrostatic and magnetic fields causes the energy focusing as well as the directional focusing respectively. A schematic diagram of the double focussing mass analyser is given in Figure.

Figure: Schematic diagram of a double focussing mass analyser

The electrostatic analyser consists of two curved metallic plates as shown in Figure and a dc potential is applied across them. This permits the fragment ions along with only a limited range of kinetic energy to go over to the magnetic sector. As in case of the magnetic sector, here in addition the ions with greater and smaller kinetic energies are destroyed at the plates.