Figure 45. A Molecular Model of Cyclodextrin

The secondary hydroxyl groups can be reacted with appropriate reagents to introduce further interactive character to the cyclodextrin molecule. The strong chiral characteristics of the cyclodextrins structures arise from the multitude of chiral centers present in the molecule. For example, b-cyclodextrin has 35 stereogenic centers. They are probably the most effective stationary phases presently available for the GC separation of stereoisomers. Some examples of derivatized cyclodextrins are as follows.

Trifluoroacetyl (TA) (2,3-di-O-pentyl-3-trifluoroacetyl)

Dimethyl (DM) (2,3,di-O-methyl-6-t-butyl silyl)

Dipropionyl (DP) (2,3-di-O-propionyl-5- t-butyl silyl)

Dialkyl (DA) (2,3-di-O-pentyl-3-methoxy)

Propionyl (PN) (2,3-di-O-pentyl-3-3-propionyl)

Butyryl (BP) (2,3-di-O-pentyl-3-butyryl)

S-Hydroxypropyl (PH⁺) ((S)-2-hydroxy propyl methyl ether)

Permethyl (PM) (2,3,6-tri-O-methyl)

Their popularity is demonstrated by the results of a survey shown in figure 46.

Figure 46. The Number of Applications of Different Capillary GC Chiral Phases Appearing in Publication in July 2004

The a-, b- or g- cyclodextrins, chemically modified or unmodified, can be peralkylated, and the product dissolved directly in appropriate polysiloxanes. The polysiloxane solution can then be coated on the walls of glass or fused quartz open tubular columns using standard procedures that will be discussed later. The resulting columns are chirally selective and many alcohols, diols, carboxylic acids, alkanes and cycloalkanes can be separated directly on them without derivatization. By incorporating a phenyl polysiloxane into the stationary phase, the thermal stability of the resulting mixture can be significantly improved. The phenyl polysiloxane apparently increases the solubility of the cyclodextrin in the stationary phase mixture in addition to improving its resistance to oxidation at elevated temperatures.