Читать книгу Chemical Analysis - Francis Rouessac - Страница 85

Unlike the direct process, this process consists in creating a chemical reaction between the analyte and a chiral reagent, prior to the injection, to form diastereomers. As these have different physical properties, they should be separable with conventional phases. This longer process is not used as often as the direct one. It may cause a partial racemization of the analyte.

Chiral stationary phases for GC are mainly obtained from cyclodextrins (Figure 2.10), ring‐shaped macromolecules composed of six, seven or eight units of D(+)‐glucose. They look like cones, with a hydrophobic central cavity, which enables the selective and reversible inclusion of a large variety of compounds, such as diastereomeric complexes.

After possible chemical transformation, cyclodextrins are either deposited on the internal surface of the capillary column or incorporated into a poly(dimethylsiloxane)‐type polymer or grafted onto the silanol functions of silica via a short carbonated chain. These can be used up to 200°C. Above this temperature, analytes can racemize.

As a reminder, the other chiral vectors for GC, much less in use, are crown ethers and some asymmetrical diamides, which are progressively being abandoned (Figure 2.10).