Capillary Chromatography - Applications of Capillary Columns in GC Analysis 3
The arrangement of the glucose units in the three different forms of cyclodextrin are shown in figure 26. The mouth of the torus-shaped cyclodextrin molecule has a larger circumference than at the base and is linked to secondary hydroxyl groups of the C2 and C3 atoms of each glucose unit (see figure 27). The primary hydroxyl groups are located at the base of the torus on the C6 atoms. As these hydroxyl groups are free to rotate, they partially block the base aperture. The size of the cavity increases with increasing number of glucose units (figure 26).The secondary hydroxyl groups can be reacted with appropriate reagents to introduce additional and different interactive character to the cyclodextrin molecule. The strong chiral selectivity of the cyclodextrins structures is thought to arise from the many chiral centers they contain (e.g., b-cyclodextrin has 35 stereogenic centers). The cyclodextrins are probably the most effective chirally selective stationary phases presently available for the GC separations. When the a, b, or g cyclodextrins are derivatized, the hydroxyl group on the 2-position reacts first. Nevertheless, the derivative is still size selective and interaction will be determined by the size and neighboring functional groups on the interacting molecule.
To give an example, very small molecules might be separated on the derivatized a-cyclodextrin whereas, in contrast, larger molecules might be better separated on the derivatized g-cyclodextrin. It follows, that the derivatized b-cyclodextrin might be selected for separating the isomers of solute molecules of intermediate size. In contrast, if the 3-position hydroxyl group is derivatized all the cyclodextrins completely loose their size selectivity. Derivatizing the 6-hydroxyl position has little or no effect on chiral selectivity but does appear to enhance the loading capacity of the stationary phase. This position is also used to anchor the cyclodextrins to the surface of silica gel in the preparation of LC stationary phases. After permethylation the a-, b- or g- cyclodextrins must be dissolved directly in appropriate polysiloxanes mixtures, and the coated on the walls of the capillary column (glass or fused quartz) using the techniques already described. The underivatized cyclodextrins, and those that have been derivatized but not permethylated, can be coated directly onto the walls of the column in a suitable solvent. Many isomers of alcohols, diols, carboxylic acids, alkanes and cycloalkanes can be separated directly on such columns without derivatization.