Moving Boundary Electrophoresis

The moving boundary method of electrophoresis separations was the first type to be used to demonstrate the efficacy of the electrophortic process. The apparatus was simple and consisted of a U tube the horizontal lower portion of the U tube being filled with a mixture of the substances under examination dispersed in a suitable buffer. Initially the two vertical limbs contained only buffer and the electrodes (cathode and anode) dipped into the buffer at the top of each limb respectively. The system is depicted in figure 2. As an example, assume that the sample contains cations A⁺, B⁺, C⁺ and anions A^-, B^- and C^- As with normal electrolytical processes the cations and anions will be attracted to the cathode and anode respectively and, as a result, bands of ions will be formed in both limbs.

Figure 2. Moving Boundary Electrophoresis

In the limb containing the cathode, the highest band will consist exclusively of the fastest moving cation (A⁺) (i.e. the cation with the greatest mobility). The adjoining band will contain both (A⁺) the cation with the greatest mobility and (B⁺) the cation with the next greatest mobility. Finally, the third, and lowest band will contain a mixture of all three cations including the cation with the lowest mobility. Clearly, in the limb containing the anode, the same process will occur, the highest band will contain the fastest moving anion (A^-) (the anion with the greatest mobility). The next band will contain both (A^-) the anion with the greatest mobility and (B^-) the anion with the next greatest mobility. While the third, and lowest band, will contain a mixture of all three anions including the anion with the slowest mobility. This type of separating process can be compared with frontal analysis in chromatography.