Determination of mandelic acid enantiomers in urine by derivatization in supercritical carbon dioxide prior to their determination by gas chromatography

The use of a supercritical carbon dioxide reaction medium for the determination of the (R)- and (S)-enantiomers of mandelic acid (MA) is proposed. The process involves a previous derivatization step under supercritical conditions by which the carboxyl group is esterified with methanol, then followed by acylation of the hydroxyl group in methyl MA with pentafluoropropionic anhydride in the absence of a catalyst. These derivatization steps cause no enantiomeric inversion. The derivatized enantiomers are extracted and quantified by gas chromatography. A BETA DEX 225 capillary column allows the separation of (R)-MA and (S)-MA as pentafluoropropionyl methyl esters with good resolution and precision. The overall method was used to determine both enantiomers in urine samples.