at the centre, separating into individual bands or spots. Detection techniques such as spraying with a
chromogenic reagent or staining with a dyestuff are then used to visualize the developed
electropherogram. A schematic diagram of a paper electrophoresis apparatus is shown in Figure 4.51
and a typical electropherogram in Figure 4.52. The technique differs from chromatography in that only
a single phase is involved, i.e. the electrolyte solution which essentially remains stationary on the
supporting medium. High-performance capillary electrophoresis (HPCE) is a comparatively recent
development that is proving to be a very powerful separation technique of growing importance (vide
infra).
Figure 4.51
Apparatus for paper electrophoresis.Factors Affecting Ionic Migration
If charged species, dissolved or suspended in an electrolyte solution, are subjected to a uniform
potential gradient, they rapidly assume a constant rate of migration. As in conventional electrolysis
cations migrate towards the cathode (negative electrode) and anions to the anode (positive electrode).
The rate of migration reaches a constant value when the attractive force exerted by the electrode is
balanced by the frictional force due to movement of the species through the electrolyte solution. Each
species is characterized by its electrophoretic mobility, μ, which is determined by its total charge, its
overall size and shape and the viscosity of the electrolyte solution. The distance d travelled by a given
species is related to its mobility μ and to the time of application and size of the potential gradient by the
equation
where t denotes time, E is the applied potential and S is the distance between the electrodes. Typically,
50 – 150 V is applied across a distance of 10–20 cm. For two species of differing mobility, μ 1 and μ 2 , the
separation after time t is given by:
Separation is hindered by diffusion which increases in proportion to the square root of the time taken.
Resolution is therefore maximized by applying a large potential gradient E/S for a short time.