Migration rate, differential

Separation mainly of charged materials by differential migration across a surface or through a column in an applied potential gradient migration rates dependent upon size, shape and charge of species. 

As mentioned earlier as an axiom, separation between two components is only possible if they have different migration rates through the column. Selectivity or separation factor (a) is a measure of differential retention of two analytes. It is defined as the ratio of the capacity factors k ) of two peaks as shown in Figure 4. Selectivity must be >1.0 for peak separation. 

The aim of any chromatographic separation may be defined as the achievement of an optimal combination of speed of elution, sample size, and resolution of the solutes. Good resolution can only be obtained if there is adequate control over the differential migration rates of a group of solutes as they move down a column (column selectivity) and over the extent of zone dispersion for each of the solutes (column efiiciency). Historically, the various modes of liquid chromatography have been considered as separate and independent phenomena. It is now clear that they all have a common theoretical basis. Column selectivity in HPLC, irrespective of the mode, arises due to differences in the distribution equilibria... 

As In all differential migration methods, the criterion for separation In isotachophoresls depends simply on the fact that two lonegenlc constituents will separate whenever their migration rates In the mixed state are different. For two constituents 1 and j, this means that according to equation (1) their effective mobilities In the mixed state must be different. 

Capillary zone electrophoresis (CZE) is a relatively recent separation technique based on the differential migration rates of ionic species in an electrical... 

Two types of results can be obtained in the case of separation of LiCl from LiOH. In a mixture diluted to about 0.5 N, all constituent ions can be expected to exist in a fixUy solvated state. The largest hydrated ion of lithium that is common for the two electrolytes determines the migration rates of both LiCl and LiOH fronts. In accordance with the size exclusion principle, both fronts arrive at the colunm outlet before the hold-up volume with a minimal separation of only Ai=0.1 bed volumes. This selectivity of salt/base differentiation rises by a factor of ca 20 when LiCl is taken at the high concentration of 3.5 N (Fig. 12.9). Now, the selectivity amounts to more than 2 bed volumes (Aj > 2) and LiOH behaves as a... 

This constant is a true thermodynamic value which is temperature dependent it expresses the relative tendency of a solute to distribute itself between the two phases. Differences in distribution constants result in differential migration rates of solutes through a column. 

The possibility of differential migration rates of individual constituents has presumably prevented wider application of the method to alloys and compounds, although recent work at Birmingham on a Pr-Nd alloy has demonstrated the... 

DUAL CYCLODEXTRIN SYSTEMS Applying Only One CD in CE occasionally is insufficient to completely resolve chiral compounds as can be seen above. Therefore, either another CD or a combination of two CDs in dual systems can be tested to enhance both enantioselectivity and resolution. The second CD then provides enantioselectivity itself, competes with the other CD, or provides a differential migration rate. A dual system usually combines a neutral and a charged CD, or more rarely either two charged or two neutral CDs. CDs might also be combined with carbohydrates, ion-pairing reagents, surfactants, crown ethers,... 

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