A colorimetric 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 the air under ambient conditions. For quality assurance purpose, there is a need in the industry for rapid testing methods 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, monoterpenes 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. Upon the palladium-facilitated oxidation of monoterpenes via the Wacker process, the complex was destabilized, and free pararosaniline molecules were released into the solution, exhibiting an intense magenta color. 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 elicited greater extents of color changes as compared to other monoterpenes (limonene, α-pinene, β-pinene and terpinolene). The colorimetric indicator is potentially useful for rapid assessment of monoterpene oxidation in hemp essential oil and other cannabis-based products.