Contemporary Chiral Stationary Phases

Contemporary chiral stationary phases suitable for GC are either based on polysiloxane polymers or, probably the more popular, based on bonded cyclodextrin derivatives. Very few GC chiral columns that are used for analytical purposes are packed, the vast majority being open tubular columns having a range of different lengths and, in general, one of two internal diameters, i.e. I.D's. of about 125 m or 250 m.

Chiral Polysiloxane Stationary Phases

The poor stability and relatively high volatility of those phases so far discussed severely limits their versatility and does not allow a mixture of substances of biological origin to be separated in a single analysis. Bayer's answer to high temperature operation was to attach a chiral agent by an imide linkage to a carboxyl group of a polymer matrix of dimethylsiloxane(2-carboxypropyl) methylsiloxane.

Figure 40. The Separation of the Protein Amino Acids as N-(O,S)-Pentafluoro-Propanoyl-Isopropyl Esters on Chirasil-Val.

His stationary phase combined the chiral selectivity of L-valine-t-butylamide with the high thermal stability and low vapor pressure of the polysiloxanes. This historic stationary phase was eventually made commercially available as Chirasil-Val and performed satisfactorily over the temperature range 30 C to 230 C. Chirasil-Val separated all the racemic protein amino acids in 30 min. An example of the separation of the N-(O,S)-pentafluoro-propanoyl-isopropyl esters of the amino acids is shown in figure 40.

Although significant, it is seen that the column bleed is quite manageable at 185 C. and an excellent separation of the enantiomers is obtained. Chirasil-Val has been widely used for chiral separations including aryl glycols (7), drug metabolites 2- and 3- hydroxy carboxylic acids (23) 2-halo carboxylic acids (24) and many underivatized substances such as alcohols, diketones and hydroxy lactones.