Quantitative gas chromatographic analysis of mandelic acid enantiomers

Effect of the mobile phase on the retention behavior of optical isomers of the mandelic acid derivative methyl 2-phenyl-2-(tetrahydropyranyloxy) acetate by chiral HPLC

Conditions for separation of enantiomers of a mandelic acid derivative, methyl 2-phenyl-2-(tetrahydropyranyloxy) acetate (the analyte) were studied. Because of the presence of two chiral carbons, the analyte consists of four stereoisomers stable at ambient temperature. Chiral HPLC of the analyte resulted in four peaks, using an (S,S)-Whelk-O1 column with the mobile phase consisting of hexane and the t-butyl methyl ether (TBME). It was found that TBME dramatically changed the retention of the isomers, though it produced the best enantioseparation on (S,S)-Whelk-O1. The amount of TBME in the mobile phase influenced the degree of retention shift; 5% (v/v) TBME gave a bigger shift than 8% (v/v) and 10% (v/v). 2-Propanol did not produce the same results. The chiral separation was also tried on cellulose tris (3, 5-dimethyl phenylcarbamate) (CDMPC), but only three peaks were seen, indicating some but not full enantiomer resolution.

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.

A note on individual differences in the urinary excretion of optical enantiomers of styrene metabolites and of styrene-derived mercapturic acids in humans

Urine samples from 20 male workers in the polyester industry exposed by inhalation to styrene concentrations ranging from 29 to 41 ppm were investigated. Excretion products of styrene metabolism, mandelic acid and mercapturic acids, were purified from the urine over an extraction column packed with Porapak Q, with subsequent ether elution. The optical enantiomers R- and S-mandelic acid were then determined by thin layer chromatography (TLC) using chiral plate material and selective staining with vanadium pentoxide. Quantitative analysis of these compounds was performed using commercial reference substances. Styrene-specific mercapturic acids were analyzed by a modified TLC method, using synthesized reference substances. The concentration of racemic mandelic acid in the individual urine samples ranged from 80 to 1610 mg/l, and the ratio of the R- and S-enantiomers ranged from 0.7 to 2.2. These individual variations are not explained by differences in individual styrene exposure levels, or by differences in the concentration of the urine samples (in relation to creatinine excretion). Styrene-specific mercapturic acids were detected in the urine of only 1 of the 20 workers, at a concentration much lower than expected from previous investigations by others in humans and laboratory animals, in which less specific analytical methods had been used. The results point to marked interindividual differences in metabolism of styrene, probably related to enzyme polymorphisms.