4.2 Electrophoresis and Capillary Electrophoresis

The separation of compounds in capillary electrophoresis depends on the velocity of individual compounds. The velocity is given by the electrophoretic mobility (µe) of the compound and the applied electric field, (E)

Eqn 4.1

The electrophoretic mobility is a measure of the particle's tendency to move through the medium given the applied electrical field and thus changes with the medium and the particle; tabulated values of electrophoretic mobility often differ from the experimentally observed electrophoretic mobility. Consequently, experimentally determined mobilities are called "effective mobilities," and can change radically with different solvents and with different solution pH. The electrophoretic mobility depends on the frictional drag (Ff) the medium exerts on the particle and the electrical force (Ef) exerted to move the particle such that

Eqn 4.2

where the electrical force is dependent on the charge of the ion (q) and the strength of the electrical field (E)

Eqn 4.3

The frictional force can be described as

Eqn 4.4

where r is the ion radius, v is the ion velocity, and η is the solution viscosity. In slab electrophoresis, the sample is placed on the gel, and then the electrical field is applied. The force of the electrical field causes the sample to progress down the slab, accelerating until the drag--or friction--experienced by intermolecular interactions with the medium equals the force caused by the electrical field, and then the sample proceeds at a constant rate. Once the sample has reached a steady velocity, it can be shown that

Eqn 4.5

This equation shows that highly charged, small particles move quickly—especially with a low viscosity fluid—whereas less-charged, larger particles move more slowly, especially through a more viscous medium.

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