Determination of mandelic acid enantiomers in urine by gas chromatography and electron-capture or flame ionisation detection

A sensitive and stereospecific GC method was developed for the analysis of R- and S-enantiomers of mandelic acid (MA) in urine, using a chiral CP Chirasil-Dex-CB column. The enantiomers of MA were derivatised with isopropanol into their corresponding isopropyl esters and determined either directly with flame ionisation detection (FID) or after subsequent derivatisation of a hydroxy group with pentafluoropropionic anhydride with electron-capture detection (ECD). Both derivatisation steps proceeded with negligible inversion of enantiomers (<1%). The limit of detection of the FID determination was 8 and 5 mg/l for R-MA and S-MA, respectively and of the ECD determination 1 mg/l for both enantiomers. Repeatability (within-day precision) and reproducibility (day-to-day precision) was for both enantiomers below 7.5% for the FID and below 5.8% for the ECD analysis. The method was applied to urine of volunteers exposed to 105 and 420 mg styrene/m3 air. In the urine of the exposed volunteers, the S-enantiomer showed higher excretion compared to that of the R-enantiomer, with marked interindividual differences in excretion of both enantiomers.