Electrophoretic mobility calculation methods

Thus, as shown earlier, CE represents a suitable technique for the determination of enantioselective binding constants between chiral drugs and cyclodextrins. The results obtained under appropriate conditions are reasonable and can be applied for optimization purposes as well as for a better understanding of the fine nuances of chiral CE separations. On the other hand, some care must be taken for the proper application of CE methods for the determination of the binding constants as well as when applying these data. A critical review of the calculation of stability constants for the chiral selector-enantiomer interactions from electrophoretic mobilities has been published by Vespalec and Bocek (40). 

The electrophoretic mobility of a neutral base B in a buffer at a given pH is correlated to its thermodynamic dissociation constant. Such correlations were used for determination of the pKd of HAA belonging to classes B and C of Table 2, by CZE-UVD (at 214 nm) of these compounds in buffers of pH from 3 to 9. The pKd values obtained by the CZE method are the same as those obtained from UW spectrophotometric measurements256. The pKd values derived from these experimental measurements are different from those appearing in Table 2, because the latter values are purely calculated ones (see note a in the table). 

Some types of electrophoretic cells are stationary layer problem free , but in the other cells the electroosmotic flow can lead to erroneous results. The observed velocity of particles is a sum of the electroosmotic flow of the fluid and the velocity of particles with respect to the fluid. The latter is a function of the potential of the particles and the former is a function of the position in the cell cross section. Hydrodynamic calculations make it possible to find the stationary levels, i.e. the positions in the cell cross section where the electroosmotic flow equals zero. Certainly the position of stationary levels in commercial electrophoretic cells can be found in the user s manual, and there is no need to perform any calculations. The fastest method to determine the electrophoretic mobility is from the velocity at one stationary level, but such a procedure can lead to substantial errors. For example, when the cell position is adjusted at room temperature and then measurements taken... 

To calculate the retention factor or electrophoretic mobility either marker compounds [172] or a homologous series iterative method is generally used [173,174]. Typical marker compounds for teo include neutral compounds with strong UV absorption and high solubility in the electrolyte solution (e.g. formamide, thiourea, dimethylsul-foxide). These are added in small amounts to each sample and must not be retained by interactions with the column wall or pseudostationary phase. Alternatively, teo can be estimated from the first deviation of the baseline of the electropherogram when an... 

Differences in the potentials calculated using Equations 10.76 and 10.77 can reach up to 25% (Miller et al. 1992, Miller and Berg 1993). One can estimate the k values according to Xu (1998) and the average size of primary particles from the specific surface area (or from direct observation of primary particles using TEM, AEM, or SEM methods) or the primary particle size distribution fla) with minimal J and maximal a ) radii. Equation 10.77 for the electrophoretic mobility can be written as follows ... 

Notably, dh calculated using the above simple equation shows a dependence on electrolyte concentration and hence the method cannot be used in a straightforward manner. Cohen-Stuart et al. [43] showed that the measured electrophoretic thickness approaches <5h only at low electrolyte concentrations. Thus, to obtain dh from electrophoretic mobility measurements, results should be obtained at various electrolyte concentrations and 4 should be plotted versus the Debye length (1/k) to obtain the limiting value at high (l//c) (i.e. low electrolyte concentration), which now corresponds to <5h-... 

The instrument displays the zeta potential as calculated from the Smoluchowski equation. At the point where the particles image is stationary, the displayed potential is the zeta-potential of the sample. The zeta potential can then be converted to electrophoretic mobility by deviding [sic] by an appropriate constant. The concentration of the sample was 0.15% for the cotton fibers. This method allows the measurement of the entire particle cloud simultaneously yielding average mobility values. [2]... 

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