A Colorimetric Indicator Assay for the Detection of Monoterpenes in Hemp (Cannabis Sativa) Essential Oil

Abstract

Monoterpenes are the most abundant constituents in hemp essential oil that contribute to its sensory properties and bioactivities. However, monoterpenes are prone to oxidation when exposed to air under ambient conditions. For quality assurance purpose, there is a need in the industry for a rapid testing method to determine monoterpene contents in hemp products. In this study, a colorimetric indicator assay was developed for rapid determination of monoterpenes in hemp (Cannabis sativa) essential oil. For comparisons, monoterpene contents in lavender (Lavandula angustifolia), eucalyptus (Eucalyptus globulus) and sweet orange (Citrus sinensis) essential oils were also determined. The developed assay was based on an indicator solution containing palladium chloride that complexed with pararosaniline hydrochloride dye. The palladium-mediated oxidation of monoterpenes in the essential oil samples through Wacker process, the released pararosaniline dye from the complex revealed an intense magenta coloration. From ultraviolet-visible and nuclear magnetic resonance spectra, the color response rate of the indicator solution was dependent on terpene structure, type of essential oil, terpene loading and the reaction temperature. Fourier transform-infrared (FT-IR) spectroscopy and gas chromatography-mass spectrometry (GC-MS) results showed that the main components in hemp essential oil are myrcene and ocimene, which elecited greater extents of color changes as compared to other monoterpenes (limonene, alpha-pinene, beta-pinene, and terpinolene). The colorimetric indicator is potentially useful for rapid assessment of monoterpene oxidation in hemp essential oil and other cannabis-based products.

Keywords: terpene, hemp essential oil, colorimetric indicator, industrial hemp