Electrical migration velocity

The drift velocity is the mean velocity experienced by the particle population due to the presence of the external force Fe . For example, in the case where the external force is simply gravity, Fe = mpg, and the drift velocity (or settling velocity) will simply be Vdriit = gt (see also (9.41)]. When the external force is electrical, the drift velocity is the electrical migration velocity [see also (9.49)]. Therefore our analysis presented in the previous sections is still valid even after the introduction of Brownian motion. 

Particle diam., i Number of elementary electrical charges, Nq Particle migration velocity, u, ft/sec... 

Migration velocity The electrophoretic velocity of a charged particle in an electric field. 

Particle migration velocity The velocity at which a charged particle moves in a given direction in an electric field. 

The separation of charged compounds is based on the differences in migration velocity (v) when the electric field is applied. Migration velocity is derived by dividing the length of the capillary from injection to detection (1) by the measured migration time (t) ... 

In order to influence a migration it is obvious that one can alter the charge of the compounds, the viscosity of the medium and the dynamic radius of the compounds. According to Eq. 17.5, the electrophoretic mobility is the proportionality factor in the linear relationship of the migration velocity and the electric field strength... 

The migration in CE is obviously influenced by both the effective and the electroosmotic mobility. Therefore, the proportionality factor in the relationship of the migration velocity and the electric field strength in such a case is called the apparent electrophoretic mobility (/iapp) and the migration velocity the apparent migration velocity (vapp). The /iapp is equal to the sum of /migration velocity is expressed as... 

An important pre-concentration method is the sample-stacking procedure [68,69]. When the sample is dissolved in a solvent with an electrical conductivity lower than that of the buffer electrolyte, sample stacking occurs because of the difference in electric field strength in the sample and the electrolyte medium. As the field strength in the sample zone is higher, the migration velocity in the sample zone is also higher. At the interface of the sample and the buffer electrolyte zone... 

The separation mechanism is based on stereoselective ion-pair formation of oppositely charged cationic selector and anionic solutes, which leads to a difference of net migration velocities of the both enantiomers in the electric field. Thus, the basic cinchona alkaloid derivative is added as chiral counterion to the BGE. Under the chosen acidic conditions of the BGE, the positively charged counterion associates with the acidic chiral analytes usually with 1 1 stoichiometry to form electrically neutral ion-pairs, which do not show self-electrophoretic mobility but... 

As can be observed in Equation (11) the migration velocity is directly proportional to the ionic charge of a compound and the applied electric field strength. It is inversely proportional to the viscosity of the medium and the hydrodynamic radius of the compound. The electric field strength is determined by the applied voltage difference (V, in V) and the... 

During electrophoresis, a steady state establishes where the vector sum of electrical force and frictional force is zero therefore, a constant migration velocity v is attained, given by ... 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

---