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

The simultaneous enantiomeric separation of 30 racemic acids including 24 h ydroxy acids in a single analysis is described for the determination of the ir absolute configurations. It involves the conversion of each enantiomer i nto diastereomeric O-trifluoroacetylated (-)-menthyl ester for the direct s eparation by gas chromatography on achiral dual-capillary columns of differ ent polarities, with subsequent identification and chiral discrimination by retention index (I) library matching. Among the acids studied, the enantio mers 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 o f 0.7-7.7 and separation factors in the range of 1.002-1.021. Enantiomers o f 5-hydroxybutyric and alpha-methoxyphenylacetic acids were partially resol ved on DB-5 (resolution factor of 0.9), but not resolved on DB-17, while th e baseline resolution for 3-hydroxydecanoic acid and the minimal separation on the peak top (resolution factor less than 0.7) for 2-hydroxyglutaric ac id were achieved on DB-17 but not on DB-5. The temperature-programmed I val ues measured on both columns were characteristic of each enantiomer and thu s simple I matching with the reference values was useful in cross-checking for their chemical identification and the chiral discrimination as well. Wh en 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. (C) 2000 Elsevier Science B.V. All ri ghts reserved.