Configurational analysis of chiral acids as O-trifluoroacetylated (−)-menthyl esters by achiral dual-capillary column gas chromatography

Abstract

The simultaneous enantiomeric separation of 30 racemic acids including 24 hydroxy acids in a single analysis is described for the determination of their absolute configurations. It involves the conversion of each enantiomer into diastereomeric O-trifluoroacetylated (−)-menthyl ester for the direct separation by gas chromatography on achiral dual-capillary columns of different polarities, with subsequent identification and chiral discrimination by retention index ( I) library matching. Among the acids studied, the enantiomers of 27 acids were discriminatively resolved on both non-polar DB-5 and the intermediate-polar DB-17 columns with resolution factors in the range of 0.7–7.7 and separation factors in the range of 1.002–1.021. Enantiomers of 3-hydroxybutyric and α-methoxyphenylacetic acids were partially resolved on DB-5 (resolution factor of 0.9), but not resolved on DB-17, while the baseline resolution for 3-hydroxydecanoic acid and the minimal separation on the peak top (resolution factor less than 0.7) for 2-hydroxyglutaric acid were achieved on DB-17 but not on DB-5. The temperature-programmed I values measured on both columns were characteristic of each enantiomer and thus simple I matching with the reference values was useful in cross-checking for their chemical identification and the chiral discrimination as well. When applied to a clinical urine sample, the present method allowed positive identification of endogenous ( S)-lactic acid and ( S)-2-hydroxybutyric acid along with ( R)-3-hydroxybutyric acid.