Electrophoresis-convection principle

Countercurrent electrophoresis can be nsed to split a mixtnre of mobile species into two fractions by the electrical analog of elntria-tion. In such countercurrent electrophoresis, sometimes termed an ion still, a flow of the suspending flnid is maintained parallel to the direction of the voltage gradient. Species which do not migrate fast enough in the applied electric field will be physically swept out of the apparatus. An apparatus based mainly on this principle bnt nsing also natural convection currents has been developed (Bier, Electrophoresis, vol. II, Academic, New York, 1967). 

Capillary electrophoresis is based on the same principle as gel electrophoresis. Charged analytes can be separated in an applied electric field according to their mobility. In contrast to gel electrophoresis, however, separations are carried out in a small diameter capillary containing a free solution of electrolyte rather than on a slab gel. Moreover, convective flows due to Joule heating occur more easily in a free solution than in the gel. In contrast to GE, electroosmotic flow is often part of the separation process. 

In capillary gel electrophoresis (CGE), the capillary is fllled with a gel rather than with a free solution. The separation in CGE works on the same principle as slab gel electrophoresis The analytes migrate depending on their electrophoretic mobility but they are retarded by the gel pores depending on their size. The gel acts as an anti-convective medium, reducing band broadening and as a molecular sieving material. 

The principle of compensation can, unfortunately, be employed to its whole extent only in very dilute solutions. In more concentrated solutions there is always a false boundary of low mobility (Tiselius and Rabat, 1939 Longsworth and Macinnes, 1940 Svensson, 1946). Thus, even in the presence of only two components, there are always three boundaries. If the false 6 boundary is compensated down to the bottom of the U-tuhe, it will cause convection in the other limb. It is true that this may he permitted if the chief interest is to isolate the faster component in the ascending limb, but the electrophoresis is no longer con-... 

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