Migration of ions in an electric field at a definite pH is called elecrophoresis. This method was developed by Arne W.K. Tiselius in 1937 and is based on the principle that the proteins migrate in an electric field except at the pH of their isoelectric point. And in a mixture of proteins/chemicals, each sample with its characteristic electrical charge will respond differently to an applied electric potential.
The rate of this electrophoretic migration (or mobility) depends on the pH of medium, strength of the electric field, magnitude of the net charge on the molecule and the size of the molecule.