We know that moving electric charges create around them a field of electric property which we call an electric field. This field will interact with another electrical charge, producing a force of attraction and repulsion. Thus, we define the electric field as the force acting on a positive test charge (q0) divided by the value of that charge.
By applying a potential difference between the electrodes, an electric field is established in the region between them. The ions of the molecules to be studied undergo an electrical force given by F = qE. This force displaces the ions in the direction of the field. Larger ions move at a slower speed than smaller ions.
After some time, the field is turned off and the ions stop at different positions. By measuring the final positions of the ions, two samples can be compared.