HPLC Enantioseparation of Menthol with Non-ultraviolet Detectors and Effect of Chromatographic Conditions

Menthol has been widely used in pharmaceuticals, food industry, oral health care products, cosmetic industry, and tobacco products, and its HPLC chiral separation has been less reported till now. Herein, a simple and direct method has been developed for separation of menthol by normal-phase HPLC coupled with refractive index detector and/or optical rotation detector, in which special sample pretreatment and derivatization were not required. The effects of chiral stationary phases, alcoholic modifiers, column temperature, and injection amount were studied in detail. The amylose tris(3,5-dimethylphenylcarbamate)-coated chiral stationary phase showed the best separation performance for menthol with the resolution of 2.84. Through the optical rotation detector, (+)-menthol was first eluted on the amylose-based chiral stationary phase in the hexane-2-propanol mixture, but later eluted on the cellulose-derived chiral stationary phases in the same mobile phase. In addition, retention and resolution of menthol displayed an upward trend along with the content of 2-propanol and column temperature reduction. Chiral separation of menthol could be attributed to an enthalpically driven process, since ΔΔH (− 3.04 kJ mol− 1) and ΔΔS* (− 8.57 J mol− 1 K− 1) for two enantiomers were negative in the experimental temperature range. Moreover, the limits of detection and quantitation for the menthol enantiomers with the refractive index detector were similar to the optical rotation detector. These results would provide useful information for direct resolution for chiral compounds without typical chromophoric groups.