The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 68

Chiral Polysiloxane Stationary Phases

Bayer and his coworkers (19), synthesized a stationary phase that consisted of a chiral agent attached by an amide linkage to a carboxyl group of a polymer matrix of dimethylsiloxane or (2-carboxypropyl)-methylsiloxane. This combined the chiral selectivity of L-valine-t-butylamide with the high thermal stability and low volatility of the polysiloxanes. This stationary phase could be used over the temperature range of 30˚C to 230˚C. and would allow the separation of all the racemic protein amino acids to be separated in one chromatogram taking only about 30 min. An example of the separation of the N-(O,S)-pentafluoro-propanoyl-isopropylesters of the amino acids, employing this phase system, is shown in figure 32. It is seen that the column bleed is quite manageable at 185˚C and that a clean separation of all the enantiomers is achieved. Chiral polysiloxanes can also be prepared in a number of ways. For example, the cyano groups of OV-225 can be hydrolyzed and the carboxy groups coupled with L-valine-t-butylamide though the amino group. However, some of the more popular chirally selective phases are synthesized from cyclodextrin.

Figure 32 The Separation of the Protein Amino Acids as N-(O,S)-Pentafluoro-Propanoyl-Isopropyl Esters on Chirasil-Val