Improved Spectrophotometric Method for Fast and Accurate Quantitative Determination of Menthol in Essential Oils

Mint is an industrial crop cultivated worldwide at millions of hectares. A considerable variation has been reported in the yield and quality of oil obtained from the crops of menthol mint varieties depending on its menthol contents. The menthol percentage of mint is usually determined by gas chromatographic (GC) analysis, which needs high capital investments and is expensive, requiring special expertise and considerable sample preparation and analysis time. It is also difficult to have onsite GC analysis in many rural areas of mint cultivation. The present study was aimed to develop a simple, rapid, reproducible, and cost-effective spectrophotometric method for the direct estimation of menthol in essential oil samples. The analytical process was optimized for preparation of coloring reagent, salicylaldehyde. The results of the modified spectrophotometric assays (menthol: 74.72 ± 0.13%) attained comparable accuracy and reproducibility to gas chromatographic analysis (menthol: 74.78 ± 0.09%). The modified method can be used as a reference method for quick analysis at lab, pilot, and commercial scale.