Electrophoretic mobility inclusion

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

When cyclodextrins are added to the run buffer in MEKC, they can improve the separation of enantiomeric mixtures. The choice of cyclodextrin is important because the size of the cavity can affect the separation. Since the run buffer is primarily aqueous, it is assumed that the primary separation mechanism is inclusion into the cyclodextrin cavity. If the molecule is too small for the cyclodextrin cavity, then it will be included in a random order, and if the molecule is too large, then inclusion will not occur. Some of the native cyclodextrins have low solubilities in the aqueous run buffers and have to be added at low concentrations. These low concentrations may not be enough to affect the MEKC separation. Modified cyclodextrins have a higher solubility and are often used in place of the native cyclodextrins. In MEKC, cyclodextrins are considered electrically neutral and have no electrophoretic mobility. They are not assumed to be incorporated into the micelle because of the hydrophilic nature of the outside surface of the molecule. Therefore, when cyclodextrin is added to the run buffer, the separation is based on the equilibrium distribution among the analyte in the aqueous phase, the cyclodextrin cavity, and the... 

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