The Mechanism of Solute Retention

The separation of enantiomers on standard GC columns can be very difficult due to the close chemical similarity of the two solutes. The columns need to exploit the very small differences in structure to effect the separation (beitmay in the form of a resulting enthalpy or entropy change in the standard energy of distribution). Consequently, the stationary phases must be very carefully synthesized. It follows, that to understand the rational behind column selection in chiral gas chromatography (CGC) the function of the column and the factors that control resolution must be well understood.

In any chromatographic column, two physical chemical processes take place, more or less independent of one another. Firstly, the column moves the solutes apart so that the individual constituents are separated and, secondly, the peak dispersion (the spreading of the individual peaks) is constrained so that the components of the mixture remain isolated and can be eluted discretely. The two processes, component separation and peak dispersion, continue during the total period of development from the time of injection to the time of elution. The mechanism of retention, which arises largely from molecular interactions of the solute and the two phases and molecular restraint of the solute due to the structures of the solutes and the phases, is quite different from the mechanism that causes peak dispersion, which is mainly a result of molecular kinetics. The movement of the peaks apart is described by interaction theory and/or the thermodynamics of distribution whereas the dispersion of the solute peaks is described by the chromatographic dynamics of the system. It follows that the two processes should be considered separately and, in the first instance, the mechanism of retention (the movement of the peaks apart) will be considered. The mechanism of retention will be examined using the Plate Theory, then by Thermodynamics and finally by Interaction Theory. The three different approaches are necessary to exclusively describe three, quite different, aspects of the mechanism of solute retention.