The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 78

The cyclodextrins can tolerate a wide range of buffer concentrations and are stable from a pH 3 to a pH 14. Unfortunately, as silica is significantly soluble at a pH of 8.0 and higher, the stability of the silica matrix restricts the practical pH range from 3.0 to about 7.0. Cyclodextrin type stationary phases may be operated as a polar phase with a dispersive mobile phase or as a reversed phase mode with a polar mobile phase. As with many the other water resistant LC stationary phases, mobile phases with high water contents that have little dispersive properties can be used and thus the dispersive interactive properties of the stationary phase can be exploited. Conversely, if dispersive solvents are used, the dispersive character of the stationary phase is swamped, and the polar and or ionic interactive character of the stationary phase can be utilized. These polar interactions will take place with the many hydroxyl groups that are intrinsically present or with other polar or ionic centers that may have been introduced by derivatization. The sites of derivatization are shown in figure 41 together with an example of a naphthyl carbamate derivative attached to the 2 or 3 positions.

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Figure 41. The Bonding and Derivative Positions on the Cyclodextrin Structure