Figure 38. The Separation of the Enantiomers of N-TFA-( )-a Amino Acid Isopropyl Esters

Thermodynamically, this implies an increase in standard entropy that would also be accompanied by an even greater change in standard enthalpy and, consequently, result in greater retention. It is also seen that the separation takes a very long time that results largely from the low operating temperature. Unfortunately, the low operating temperatures were essential due to the inherent poor thermal stability of these types of stationary phase.

It follows, that despite this relatively early successful application of the technique to a chiral separation, due to the thermal instability of the system, the use of GC for the separation of enantiomers was still rather slow to develop. The stability problem arose due to the racemisation that occurred at the elevated temperatures that resulted in a continual loss of selectivity. Although the thermal lability of the enantiomers of the solute cannot be completely avoided, the key to the use of GC techniques to separate enantiomers proved to be the development of thermally stable chiral stationary phases.

However, although the need for thermally stable stationary phases was evident suitable phases were still not developed for over a decade. It was not until 1977 that Frank, Nicholson and Bayer (17) produced a chiral stationary phase by the co-polymerization of dimethylsiloxane with (2-carboxypropyl) methoxysilane and L-valine-t-butylamide that the separation time of most amino acids could be drastically reduced by operating at higher temperatures. The polysiloxane copolymers had a much lower volatility and higher thermal stability than previous chiral stationary phases and, in fact, could be used up to temperatures of 175 C.