Enantiomeric analysis of (+)-menthol and (−)-menthol by fluorogenic derivatization and liquid chromatography

A simple and sensitive liquid chromatography is described for the quantitative analysis of enantiomeric (+)-menthol and (−)-menthol that are lack of chromophore. The method is based on the derivatization of (+)-menthol and (−)-menthol with a fluorescent reagent, naproxen acyl chloride, in toluene. The resulting diastereomic derivatives were separated on a C8 column with methanol–water–tetrahydrofuran (80:18:2, v/v) as a mobile phase; they were sensitively monitored with a fluorimetric detector (excitation 235 nm and emission 350 nm). The linear range for the quantitation of the enantiomers was 5.0–50 μM with a detection limit (signal to noise ratio = 3, injected volume 10 μl) of about 1 μM. Application of the method to the enantiomeric analysis of menthol in mint plants proved simple and feasible. Toluene was used for the extraction of menthol from the leaves of mint, and the resulting toluene extract was directly used for subsequent derivatization without solvent replacement.