Effective migration velocity

Theoretical calculations will always overestimate precipitator efficiencies, probably because of reentrainment. This overestimation could be as large as a factor of 2 or more (Rose and Wood, 1966). Even so, drift velocity or effective migration velocity is the basis for all precipitator calculations and does provide a good base for the comparison of various designs. 

To achieve a separation, the operating parameters are selected on the basis of the effective migration velocity of the components in a counter-current bed (discussed in Chapter 7). 

It is important to note that the value derived from Deutsch, is not equal to the theoretical value uith. derived from Eq. (14). 

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... 

The selectivity of a separation is determined by the effective mobility because the effect of the electroosmotic mobility is equal for all the sample constituents. In order to obtain /(err from gapp, knowledge of the magnitude of /(,.<>( is required. Therefore, it is necessary to measure the velocity of the EOF. This can be done by several methods [2] however, the procedure of applying a neutral marker is commonly used. The neutral marker is a neutral compound and thus migrates only because of the EOF. Its migration velocity represents the velocity of the... 

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.

The nonequilibrium effect is due to the different velocities at which the components of the analyte are carried down the channel. The different velocities, in turn, originate from the laminar nature of the flow since the constituents of the analyte are dispersed in these laminae, they undergo differential migration velocities in the axial direction. The expression of the nonequilibrium system dispersion takes the following form ... 

Figure 8.5—Effect of the nature of the capillary inner wall on migration velocities If the inner wall has not been treated (glass or silica naturally have a negative polyanionic layer) the liquid is pumped from the anodic towards the cathodic reservoir. This is called the electro-osmotic flow. Thus an anion can move towards the cathode. Between pH 7 and 8. vE0S can increase by 35%. However, if the wall is coated with a nonpolar film (e.g. octadecyl) this flow does not exist.

Each ion has an apparent migration velocity napp easily obtained from the electro-pherogram. If / designates the effective length of the capillary between the injector and the detector, and migration time, then app can be obtained using the equation app == l/tm. 

Method validation includes determination of performance characteristics such as selectivity (which determines accuracy), linearity, precision, and sensitivity (limit of detection). This work evaluated linearity, precision, and sensitivity for specific CZE separation conditions selectivity was reported previously (15). Factors that contribute to assay imprecision by affecting peak shape (such as the pH of the mobile phase) or migration velocity (pH effects on the electrophoretic velocity) were evaluated also. 

Increased voltage (and the associated gain in electrical field strength E) increases ion migration velocity U and thus separation speed (providing 0 does not increase too severely as a side effect). For many purposes, it is useful to have a convenient parameter to characterize migration velocities independent of E. The electrophoretic mobility p fulfills this role it is defined by... 

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