The Mechanism of Chromatographic Retention - Chiral Chromatography > Page 67
It is seen that, a silica blend can be designed to provide an exclusion media that has a definitive molecular range appropriate for a specific separation problem. However, despite this simple blending procedure being available, there appears to be relatively few 'tailor made' exclusion media that are targeted for particular analytical applications.
Chiral selectivity, although always augmented by interactive mechanisms, is primary due to a form of exclusion, but on a molecular scale (molecular shape) as opposed to a microscale (particle pores). The most successful method for separating enantiomers is to exploit the differential interactions that can take place as a result of their unique spatial orientation on the molecular scale. This is brought about by introducing another, different enantiomer, into the distribution system to induce particular selectivity. The introduction of a specific extra enantiomer (which may be in the mobile or stationary phase, the latter being most common) to cause one solute enantiomer to be selectively retained relative to another is presently the most direct and technically viable way to resolve enantiomeric mixtures. The extremely high efficiencies available from modern chromatographic apparatus makes this approach the most effective.
The use of GC for the separation of stereoisomers is not nearly so common as liquid chromatography, but nevertheless there are a number of very effective optically active stationary phases that can be used in GC for the separation of volatile enantiomers. The first effective GC chiral stationary phase adopted derivatized amino acids to provide chiral selectivity (18) in 1966. These types of stationary phase had very limited temperature stability and the optimum temperature for separation can often be greater than that at which the stationary phase was stable. The first relatively stable chiral stationary phase was introduced by Bayer (19) who combined the derivatized optically active component in a polysiloxane gum.