Quality control of dried cannabis

Cannabis has a long history as a recreational drug and medically as an analgesic and antispasmodic agent. Discovery of the major cannabinoids tetrahydrocannabinol, cannabidiol, and cannabigerol (THC, CBD, and CBG) has resulted in increased interest in their medical effects. While THC is psychoactive and classified as an illicit drug of abuse in most countries, CBD has a legally accepted status in many parts of the world for medicinal purposes. CBD is believed to relieve anxiety and stress and to promote better sleep. Because of the different effects and possible legal consequences, determining the cannabinoid profile of cannabis is of high interest.

Typically, cannabis potency testing is performed by HPLC analysis. While HPLC can detect low concentrations of less abundant cannabinoids, it requires chemical reagents and it is quite time-consuming. This Application Note explains why near-infrared spectroscopy (NIRS) is a superior method for the quantification of THC, CBD, and CBG in dried cannabis because it provides results in less than a minute and does not require any chemicals.

DS2500 Solid Analyzer, Open Lid, Small Cup, DS2500 Sample Holder, Sample Glass, Transflection Vessel, Optically Flat, Gold Diffuse Reflector

Figure 1. Metrohm DS2500 Solid Analyzer with a NIRS transflection vessel shown instead of a mini sample cup.

A total of 702 dried cannabis samples with varying THC, CBD, and CBG concentrations were used for this study. The Vis-NIR spectra were acquired on a Metrohm DS2500 Solid Analyzer (Figure 1) equipped with the DS2500 Holder. A 400 mg portion of each sample was ground by hand using a grinder, then placed in a NIRS mini sample cup. Afterward, a 4 mm total pathlength diffuse gold reflector was positioned on top of the prepared cannabis sample. Data collection and model development were carried out with the Vision Air Complete software package.

**Table 1.** Hardware and software equipment overview
Equipment	Metrohm number
DS2500 Solid Analyzer	2.922.0010
DS2500 Holder	6.7430.040
Mini sample cups, 10 pcs incl. 100 disposable backs	6.7402.030
NIRS gold diffuse reflector, 4 mm total pathlength	6.7420.020
Vision Air 2.0 Complete	6.6072.208

The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for THC, CBD, and CBG content in dried cannabis. To verify the quality of the prediction models, correlation diagrams were created which display a correlation value (R²) between the Vis-NIR prediction and primary method (HPLC) values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figures 3–5).

Figure 2. Selection of Vis-NIR spectra of different cannabis samples measured with a Metrohm DS2500 Solid Analyzer.

Result THC

Correlation diagram and the respective figures of merit for the prediction of the THC content in dried cannabis using a DS2500 Solid Analyzer.

Figure 3. Correlation diagram and the respective figures of merit for the prediction of the THC content in dried cannabis using a DS2500 Solid Analyzer.

Figures of Merit	Value
R²	0.979
Standard Error of Calibration	0.71%
Standard Error of Cross-Validation	0.73%

Result CBD

Correlation diagram and the respective figures of merit for the prediction of the CBD content in dried cannabis using a DS2500 Solid Analyzer.

Figure 4. Correlation diagram and the respective figures of merit for the prediction of the CBD content in dried cannabis using a DS2500 Solid Analyzer.

Figures of Merit	Value
R²	0.978
Standard Error of Calibration	0.74%
Standard Error of Cross-Validation	0.75%

Result CBG

Correlation diagram and the respective figures of merit for the prediction of the CBG content in dried cannabis using a DS2500 Solid Analyzer.

Figure 5. Correlation diagram and the respective figures of merit for the prediction of the CBG content in dried cannabis using a DS2500 Solid Analyzer.

Figures of Merit	Value
R²	0.849
Standard Error of Calibration	0.16%
Standard Error of Cross-Validation	0.17%

This Application Note presents a Vis-NIR method that is excellently suited for the fast quantification of the three main cannabinoids in cannabis (i.e., THC, CBD, and CBG). Compared to the standard HPLC method (Table 2), cannabis analysis with near-infrared spectroscopy saves up to 30 minutes of time per analysis. Additionally, NIRS requires no chemical reagents and is a non-destructive analytical technique.

**Table 2.** Time to result with conventional HPLC method.
Parameter	Method	Time to result
THC, CBD, CBG	HPLC	∼10 min (preparation) + ∼20 min (HPLC)

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