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With temperature increases, selectivity of chiral separations decreases until enantiomers coelute at an isoelution temperature. Above this temperature, elution order should reverse and selectivity will increase with temperature. In this region, separation is termed "entropically driven". Entropically driven chiral separations hold the promise of being able to concurrently increase selectivity and column efficiency by means of increased temperature. The ability to achieve such separations is hindered by high isoelution temperatures. The isoelution temperature is determined by a balance of enthalpic and entropic contributions. A variety of mobile phase modifiers are evaluated for their ability to moderate these contributions. Results suggest that more use should be made of non-alcohol modifiers. The major barrier to entropically driven separations was found to be the nonspecific retention increase that is characteristic when the critical temperature is traversed. Use of hexane in place of CO(2) shifts the position of the retention increase away from the temperature range used in this study, and dramatically successful entropically driven chiral separations are obtained.

The effect of mobile phase additives is investigated for a variety of compounds under subcritical and supercritical conditions using packed columns. Retention of hydrogen bond donor/acceptor analytes was found to be more dependent on the presence of mobile phase additives than weak hydrogen bond acceptor analytes. The temperature and pressure of the mobile phase are major factors in the extent of this dependence. Consequently, selectivity between homologous compounds is dependent on both the additive used and the state of the mobile phase. Efficiency is nearly always improved by the presence of mobile phase additives, more so under supercritical conditions than under subcritical conditions. These observations suggest that surface molar excesses of mobile phase additives play a large part in the resulting character of the supercritical chromatographic system.

Chiral separations depend upon column efficiency and chiral selectivity. Supercritical fluid chromatography (SFC) has been shown to have pronounced advantages in chiral separations due to its enhanced column efficiency at normal flow rates. Examination of factors affecting selectivity in SFC is crucial to systematic chiral method development. Selectivity is a compromise between differences in enantiomeric binding enthalpy and disruptive entropic effects. Increased temperature will decrease the effect of differences in enantiomer binding enthalpy, eventually decreasing selectivity to a point where the enantiomers coelute. Extension of this thermodynamic theory predicts that further increases in temperature lead to selectivity values of less than 1.0, and elution order of the enantiomers reverses. In this region separations are said to be "entropically driven", and selectivity increases (decreases from 1.0) with temperature. Performing separations in this region is attractive because column efficiency is also expected to increase with temperature. Such entropically driven separations have been observed only in gas chromatography. Most data used to support the application of this theory in SFC have been generated at subcritical temperatures, while theory purports to predict behavior above the critical temperature (T(c)). This approach ignores the effects of traversing the critical temperature in SFC, which is known to have a variety of unpredictable consequences. Work presented here shows the effect of temperatures above T(c) on chromatographic behavior. Contrary to theory, capacity factors increase near T(c) and column efficiency declines. Use of pressures well above the critical pressure lessens these effects. In accordance with theory, selectivity does decrease with temperature through T(c) and isoelution temperatures and two instances of elution order reversal are observed here for the first time in SFC.