Being the most abundant constituents of hemp essential oil, monoterpenes exhibit various important bioactive functions. However, monoterpenes are prone to oxidation at ambient environmental conditions. To assure the bioactivity and functionality, there is a need to determine monoterpene contents in hemp products. For quality assurance purposes, the developing of a rapid testing approach will be beneficial for the industry. In this study, a colorimetric indicator assay was developed to evaluate terpene oxidation, using hemp (Cannabis sativa) essential oil as a model system, which was compared with the essential oils of lavender (Lavandula angustifolia), eucalyptus (Eucalyptus globulus) and sweet orange (Citrus sinensis). The indicator solution contained palladium chloride that complexed with an indicator dye, pararosaniline hydrochloride. Upon the exposure to monoterpenes, which was oxidized with palladium under Wacker process, the indicator dye’s color reappear and the solution changed to intense magenta. The indicator solution was characterized by ultraviolet–visible (UV–VIS) and nuclear magnetic resonance (NMR) spectroscopies. The chemical compositions of essential oils were characterized by Fourier transform–infrared (FT–IR) spectroscopy and gas chromatography–mass spectrometry (GC–MS). The color response rate of the indicator solution was dependent on terpene structure, type of essential oil, terpene loading and the reaction temperature. The main components in hemp essential oil, myrcene and ocimene, elicited greater extents of color changes as compared to other monoterpenes within the same timeframe up to 6 h exposure at room temperature. This study shows that the colorimetric indicator system is potentially useful for rapid assessment of monoterpene oxidation in hemp essential oil and other cannabis-based products.

Colorimetric detection of monoterpenes in hemp (Cannabis sativa) essential oil

ACS Fall 2021

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