Cannabis Monitoring using Near IR spectroscopy technote

With the growth of cannabis industry and legalization of it in many countries such as Canada, Netherlands, Spain, etc. and some states of USA such as Oregon, California and Colorado, the need for quantification of cannabinoids such as THC and CBD in Cannabis has become more in demand. Analytical techniques such as Gas Chromatography (GC) and High-Performance Liquid Chromatography are all very expensive, require sample preparation, take time and in the process destroy the sample. For this reason, Near IR spectroscopy has emerged as a power method to monitor Cannabis quality and quantify THC, CBD and other ingredients of cannabis by measuring their absorption spectra. Chemometric models are developed from these sets of spectra which are called calibration models. The calibration models are applied to a set of validation spectra to determine the accuracy of prediction. If this accuracy is suitable, the model can be applied to unknown samples and make a prediction of their THC and CBD.

Figure 1 shows the trichomes on the cannabis plant which contain a liquid full of cannabinoids.

Figure 1: Trichomes in a cannabis plant

This technote focuses on four aspects of the Near IR technique of measuring Cannabis which are as follows

• Differences between grinding and whole flower methods

• Water activity and moisture content

• Spectral features of Cannabis in near IR 

• Batching

Differences between grinding and whole flower methods

When it comes to measuring the spectral characteristics of the cannabis sample, one could either grind it or measure it as the full flower. There are different types of instruments for these two different styles of measurement. For example, the beacon analyzer by Sage Analytics (Distributed by Allied Scientific Pro) requires the samples to be grinded to a fine powder and be placed in the sample chamber. Figure 2 shows the beacon analyzer.

Figure 2: Beacon analyzer requires sample to be grinded

On the other hand, there are other analyzers that measure THC and CBD in the flower form. For these analyzers, the flower is mounted at the central part of the instrument and is rotated. Several imaging cameras take pictures of the trichomes and also a spectrometer measures the reflectance spectra from different angles. Instrument’s software will correlate the spectra and the images to come up with an average THC/CBD measurement. Figure 3 shows the Gemmacert spectrometer that measures the flower as a whole1,2  .

Figure 3: Gemmacert measures the flower as a wholer

Water activity and moisture content

The amount of moisture in a Cannabis plant affects the storage conditions of the plant. A cannabis plant with too much water could run the risk of mold infection and if there is too little water in it, there will be a loss of potency. Trichomes become fragile and breakdown causing loss of cannabinoids and terpenes. The moisture is measured in terms of water activity in certain instruments which analyze the sample as a plant sample without grinding. Although moisture content defines the amount of water present in the plant, the water activity defines how the water in the plant reacts with the micro-organisms such as mold and bacteria. The higher the water activity, the faster micro-organisms such as mold, yeast and bacteria are able to grow, and this results in higher and more strict standards for storage. At this point it is necessary to define the water activity (Aw). The water activity is defined as the ratio of vapour pressure in food to the vapour pressure of pure water3

Water activity = P / Po

Where P is the vapour pressure in food and P0 is the vapour pressure of pure water.

 The water pressure is closely related to the moisture content which is defined as

Where w is the wet weight, d is the fry weight and MC is the moisture content.

Here are the moisture contents of a few food items

When you measure water activity, you are in fact measuring the excess of water in a food item that can be used by micro-organism. Table 2 classifies different water activities leading to emergence of different microorganisms.

Table 2: Water activity and the corresponding vulnerability to microorganisms3

For cannabis, the best water activity is in the range 0.6 to 0.85. Lower than this range, the flower will lose potency and the leaves break. Higher than this range, the flower is susceptible to mold infection.

Spectral features of cannabis in near IR

Several bands of importance can be named in the NIR spectra of cannabis4

• OH groups, mostly present in the water, manifests themselves in the form of 1450 and 1930 nm overtones and combination bands.
  

• As as lipids are concerned, their associated bands can be found around 1210 nm (second overtone of stretching vibrations), 1730 and 1760 nm (first overtone of stretching vibrations), and around 2310 and 2350 nm (combination of stretching and bending vibrations) of -CH2 and -CH3 functional groups. 

• Bands observed at 2058 and 2166 nm have been associated with absorption of protein, and the ones observed at around 2078–2110 nm and 2268 nm are related to fiber content. The band observed at  1666 nm corresponds to aromatic hydrocarbons of the Cannabis terpenes (first overtone of CH stretching). This band is more visible in samples with more than 4% CBD content. 

Figures 4 and 5 show the near-IR absorption spectrum of Cannabis in the ranges 900-1700 nm and 1350-2150 nm measured with the NIRvascan spectrometer (Offered by Allied Scientific Pro, refer to the link at the end of the article) in the standard and extended ranges respectively