Capillary Chromatography - Applications of Capillary Columns in GC Analysis 5

A number of examples of some specifically derivatized cyclodextrin stationary phases have been reported. In one example, the positions 2 and 6 are alkylated (pentylated) thus providing very dispersive (hydrophobic) centers that will interact strongly with any alkyl chains contained by the solutes. After the pentylation of the 2 and 6 positions, the hydroxyl group in the 3 position can then be trifluoroacetylated which produces a very different type of stationary phase. which has a wide field of application. It has been reported that the derivatized g-cyclodextrin is distinctly more selective than the b material. It has been employed in the separation of a very wide range of compound classes, and from very small to very large molecules. Yet another stationary phase has been synthesized by substituting the cyclodextrin hydroxyl groups with pure the 'S' hydroxypropyl groups followed by permethylation. As a result, the size selectivity of the material is reduced but more polar (hydrophilic) groups are introduced. The b material has a greater chiral selectivity than the a or g phases. This material provides a good general purpose column. It is clear that there are many possibilities for derivatizing the cyclodextrins to provide unique interactive character; there are a large number commercially available and many more are likely to be synthesized in the future.

 

A number of examples of some specifically derivatized cyclodextrin stationary phases have been reported. In one example, the positions 2 and 6 are alkylated (pentylated) thus providing very dispersive (hydrophobic) centers that will interact strongly with any alkyl chains contained by the solutes. After the pentylation of the 2 and 6 positions, the hydroxyl group in the 3 position can then be trifluoroacetylated which produces a very different type of stationary phase. which has a wide field of application. It has been reported that the derivatized g-cyclodextrin is distinctly more selective than the b material. It has been employed in the separation of a very wide range of compound classes, and from very small to very large molecules. Yet another stationary phase has been synthesized by substituting the cyclodextrin hydroxyl groups with pure the 'S' hydroxypropyl groups followed by permethylation. As a result, the size selectivity of the material is reduced but more polar (hydrophilic) groups are introduced. The b material has a greater chiral selectivity than the a or g phases. This material provides a good general purpose column. It is clear that there are many possibilities for derivatizing the cyclodextrins to provide unique interactive character; there are a large number commercially available and many more are likely to be synthesized in the future.

 

It is obvious, that the properties of the different chiral stationary phases available will differ considerably. Moreover, as the nature of many of the synthetic procedures involved can be very complex, the properties of the products have the potential for varying significantly from batch-to-batch. It follows, that test mixtures are needed to quality control the columns and also to demonstrate the nature of the stationary phase. The results from such test mixtures should not only reveal the general chromatographic properties of the stationary phase but also confirm its capacity for separating enantiomeric pairs. The latter point is important, as most application samples contain many compounds other than those of a chiral nature and all, or most, will require to be resolved. An example of a chromatogram of a test mixture used by Supelco to demonstrate the chromatographic characteristics of their a-DEX column is shown in figure 28. The stationary phase is claimed to have a strong shape selectivity for positional isomers (e.g. xylenes, menthols, cresols etc.) and the small internal cavity of the permethylated a-cyclodextrin gives it a rigid character and unique chiral selectivities.