Resolution of enantiomeric amides on a Pirkle-type chiral stationary phase. A comparison of subcritical fluid and liquid chromatographic approaches.

Subcritical and supercritical fluid chromatography (SubFC and SFC) have been evaluated for the resolution of an homologous series of enantiomeric amides. The solutes were the 2-naphthoyl amides of an homologous series of amines, ranging from 2-aminobutane to 2-aminoctane, and the p-methyl-, p-methoxy- and p-chlorophenylamides of 2-aminoheptane. The chiral stationary phase (CSP) used was the covalent form of (R)-N-(3,5-dinitrobenzoyl)phenylglycine. In liquid chromatography (LC) the mobile phase comprised hexane-2-propanol--acetonitrile (97:3:0.5) at a flow-rate of 2 ml/min and temperatures of 20-35 degrees C. In SFC, the mobile were various mixtures of carbon dioxide and polar modifiers, such as alcohols, chloroform and water. For the best conditions in LC, the chiral resolution, alpha, increased through the homologous series from alpha = 1.03 for the amide derived from 2-aminobutane to alpha = 1.11 for the 2-aminooctane amide. The values of alpha observed for the pi-basic amides of 2-aminoheptane (p-methyl and p-methoxy) were greater than that observed for the pi-acidic amide (p-chloro), i.e., alpha = 1.08 versus 1.04. The selectivities, resolutions and efficiencies obtained by LC and SubFC were similar. These results indicate that the mechanism of chiral recognition is the same in LC and SubFC and that the methods should be interchangeable. The actual analysis time for SubFC was significantly shorter than that required for LC: as short as 2 min for the 2-aminooctane amide, whereas LC takes over 10 min under the best conditions.