Determination of menthol and menthol glucuronide in human urine by gas chromatography using an enzyme-sensitive internal standard and flame ionization detection.

The rate of peppermint oil absorption and excretion, following peroral administration, was determined by measuring urinary levels of menthol glucuronide. Menthol, a major component of peppermint oil, was liberated from its glucuronide metabolite by treating raw urine with beta-D-glucuronidase (Patella vulgata). Phenyl glucuronide was employed as an enzyme-sensitive internal standard. Menthol and phenol were recovered by ethyl acetate extraction and quantitated by capillary gas chromatography using flame ionization detection. Standard curves were linear between 25 and 250 micrograms/ml with a detection limit (signal-to-noise ratio = 2) of 0.25 microgram/ml. Assay precision was shown to be +/- 1.2% relative standard deviation.