Cannabis and near-infrared spectroscopy
Near-infrared spectroscopy (NIRS) is a fast analytical technique with low running costs used to perform quality control (QC) and product screening on various cannabis products, including hemp. It can determine the cannabinoid profile or even the potency of dried cannabis flowers. NIRS can also be used for QC of cannabis oils, waxes, tinctures, and other formulations.
«Cannabis» is the general term covering a group of three plants with varying psychoactive properties: Cannabis sativa, Cannabis indica, and Cannabis ruderalis . The cannabis plant has been used as medicine, food, and for its fiber for thousands of years. The flowers and some leaves from the cannabis plants are coated with tiny crystal-like structures called «trichomes», and it is these structures which are responsible for producing the cannabinoids used in many other products.
When the flowers of these plants are harvested and dried, this results in one of the most common drugs in the world.
What are the most important and well-known cannabis compounds?
Cannabis contains more than 120 compounds known as «cannabinoids» . Each cannabinoid’s effects are not yet completely known, but experts have a decent understanding of three of them: cannabidiol (CBD), cannabigerol (CBG), and tetrahydrocannabinol (THC, or Delta-9-THC). Each of them has its own effects and uses :
- CBD. This is a psychoactive cannabinoid, yet it’s non-intoxicating and non-euphoric, meaning it won’t get you «high». It’s often used to help reduce inflammation and pain. Researchers are still trying to fully understand the effectiveness of CBD’s medical use.
- CBG. This cannabinoid was first isolated in 1964. Research into CBG is still at preclinical stages, but available studies suggest that it holds considerable therapeutic promise. CBG's properties may surpass those of THC without the intoxicating effect. There is also evidence to suggest that CBG may offer anticancer, antidepressant, and antibacterial qualities.
- THC. This is the main psychoactive compound in cannabis. THC is responsible for the «high» that most people associate with cannabis.
Legality of cannabis and hemp
Cannabis is illegal in many countries. However, more and more places are starting to legalize it for medical as well as recreational uses. Hemp, which typically has a low THC content, is still used in many places for its fiber to create products like textiles, rope, and even biodegradable plastics.
The laws regarding cannabis possession and use also vary for each country. Some consider any kind of cannabis possession a serious crime, others permit products containing only CBD, and very few allow people to use products with THC (whether for medical or recreational purposes).
What cannabis products are available on the market?
Legal cannabis can appear in a wide assortment of products other than the typical dried flower or wax that is smoked or otherwise ingested. Several options are available, depending on the country you live in and the legality of different cannabidiol compounds. The list below (not comprehensive)  shows how the market for cannabis products has evolved over the past decade as it becomes more accepted in various places.
- Cannabis oils
- Cannabis beauty and skin care products
- Cannabis edibles (e.g., beverages, chocolates, gummies, and other snacks)
- Cannabis capsules
- Cannabis dog treats
Drying, curing, and processing cannabis
Drying and curing are important for successfully processing cannabis plants after harvest to end products like cannabis oils, cannabis capsules, etc. The plant material must be dried immediately after harvest to avoid spoilage. Most of the moisture should be removed within the first three days, then slowed to avoid drying out over the next several days . The curing process is an extension of drying, but in an altered environment to facilitate plant processes over a period of weeks. The dried cannabis is placed in airtight containers only opened periodically. This draws out moisture from within the flower buds while enhancing quality and potency . After curing, the cannabis is ready to be processed into its end product.
One such product, CBD oil, is made by extraction from dried cannabis flower and then diluting it with a carrier oil (e.g., coconut or hemp seed oil, Figure 1). Two suitable options are available for the extraction process: ethanol extraction or supercritical CO2 extraction.
To produce cannabis capsules or tablets, the dried cannabis needs to be heated for 10 minutes at 99–104°C. This will decarboxylate the raw plant material and render it more potent.
To screen these processes, Metrohm offers applicative solutions for ground plant materials as raw material testing and for oils. NIR spectroscopy is an analytical technique that is particularly suited for making quality control of these products more efficient and cost-effective.
In the remainder of this article, a short overview of NIRS is presented followed by an application example for the medicinal cannabis industry, concluding with how medicinal cannabis producers can benefit from NIRS in their QC process.
NIRS technology – a short overview
The interaction between light and matter is a well-known process. Light used in spectroscopic methods is typically not described by the applied energy, but in many cases by the wavelength or wavenumbers.
A NIR spectrometer (such as the Metrohm DS2500 Liquid Analyzer or the Metrohm DS2500 Solid Analyzer) measures this interaction to generate spectra such as those displayed in Figure 2. NIRS is especially sensitive to the presence of certain functional groups (e.g., -CH, -NH, -OH, and -SH). Therefore, NIR spectroscopy is an ideal method to quantify chemical parameters including the amount of THC, CBD, CBG, and water content (moisture) in cannabis and its end products.
All of this information is contained in a single NIR spectrum, making this method suitable for quick multiparameter analysis.
NIRS measuring modes
The measuring mode is dependent on the sample type. The transmission mode is generally an appropriate procedure for analyzing liquids like cannabis oils. For transmission (Figure 3b), the NIR light will travel through the sample while being absorbed. Any unabsorbed NIR light reflects to the detector.
The transflection mode is generally an appropriate procedure for analyzing creams. This mode is also used when only very small sample quantities of sample are available, as in the case of cannabis. For transflection (Figure 4b), the NIR light passes through the sample, then reflected by the diffuse reflector travels again through the sample while being absorbed. Unabsorbed NIR light is then detected at the end of the analysis.
The diffuse reflection mode is generally an appropriate procedure for analyzing solid samples like dried cannabis flower. For diffuse reflection (Figure 5b), the NIR light illuminates the sample and any unabsorbed NIR light reflects back to the detector.
The procedure to obtain NIR spectra as shown in Figure 2 already highlights two major advantages of NIR spectroscopy: simplicity regarding sample measurement and speed:
- Fast technique with results in less than a minute.
- Little to no sample preparation required.
- Low cost per sample – no chemicals or solvents needed.
- Environmentally friendly technique – no waste generated.
- Non-destructive – precious samples can be reused after analysis.
- Easy to operate – inexperienced users are immediately successful.
For more detailed information about NIRS as a secondary technique, please read our previous blog posts.
NIRS as an ASTM compliant tool for QC
ASTM E1655 (Standard Practices for Infrared Multivariate Quantitative Analysis) is a guide for the multivariate calibration of infrared spectrometers used for determining physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near-infrared (NIR) spectral region (roughly 780 to 2500 nm) through the mid-infrared (MIR) spectral region (roughly 4000 to 400 cm−1).
Applications and parameters for QC of medicinal cannabis with NIRS
Medicinal cannabis products are subjected to standardized test methods to determine their chemical properties. Laboratory testing is an indispensable part of R&D and quality control. Table 1 outlines some of the most relevant test parameters for QC of medicinal cannabis.
A turnkey solution for dried cannabis analysis
The Metrohm NIRS solution for medicinal cannabis analysis comes with ready to use pre-calibration models for the determination of THC, CBD, CBG, and even moisture content based on more than 700 real product spectra (Table 2). These pre-calibrations allow the Metrohm solution to be used as a validated calibration model without any method development.
Table 2. Available NIRS pre-calibrations for medicinal cannabis analysis.
Figure 6 shows the results of an example of the determination of THC, CBD, and CBG content in medicinal cannabis. The corresponding correlation plots show that the models for these three parameters are robust. Furthermore, the Standard Error of Cross Validation (SECV) values are close to the Standard Error of Calibration (SEC) for each parameter measured.
NIR spectroscopy is excellently suited for the analysis of several key quality parameters in dried cannabis. Compared to the primary method (HPLC), the time to result is a major advantage of NIRS since a single measurement is performed within one minute versus approximately 30 minutes with HPLC. In addition, samples measured by NIRS are not destroyed, and chemical reagents are unnecessary – saving even more costs. Another positive aspect of using near-infrared spectroscopy as an alternative technology for QC of medicinal cannabis and its products is the ease of handling so that even shift workers can perform these analyses without issue. Furthermore, the Vision Air software from Metrohm is fully compliant with 21 CFR Part 11 and USP <856> guidelines.
- What Is Cannabis? Facts About Its Components, Effects, and Hazards. Healthline. https://www.healthline.com/health/what-is-cannabis (accessed 2022-07-14).
- Fiorillo, S. 7 Cannabis Products On the Rise. TheStreet. https://www.thestreet.com/lifestyle/5-cannabis-products-on-rise-14578907 (accessed 2022-07-14).
- dhydratech. Cannabis Drying & Curing 101. Dhydra Technologies. https://dhydra.com/cannabis-drying/ (accessed 2022-07-14).