A Cannabis QC System Between Two Thorns

Cannabinoid testing is the highest volume and (arguably) the most important test performed in cannabis labs. Clients are highly concerned about cannabinoid concentrations (potency) when it comes to meeting dosage regulatory requirements for formulated products – even more so when it comes to flowers. Flower valuation is primarily based on THC concentration – the higher the concentration, the higher the flower’s value. And this desire places pressure on all parties to obtain “high enough potency.” Requests for retests after flowers fail to meet client expectations are a regular occurrence.

Most labs will test a variety of products, including those with high and low THC concentrations. A system of different sample preparation methods is often developed to cover the in-commodity cannabinoids concentration range on a high performance liquid chromatography-ultraviolet (HPLC-UV) calibration curve. However, the result is a complex workflow that often includes analysis of one sample at multiple dilutions and frequent retests – and a negative impact on profitability.

There are a few different ways for labs to combat this. To improve profitability, I advise labs not to “leave meat on the bone” when it comes to the cannabinoids calibration curve. Customization is key for high-volume commodities, as well as best instrument performance. I regularly see methods that still have calibration curve ranges from 1 to 200 or 250 µg/mL (ppm, injection concentration). In the past, 200 or 250 ppm was about the highest concentration of mixed cannabinoid standards. Individual cannabinoids could be purchased at 1000 ppm but they had to be combined by the laboratory. If you wanted to have higher calibration levels – for example, 500 ppm – you could only combine two compounds. The result is multiple injections of various cannabinoids at different levels to create a calibration curve.

Today, products of mixed cannabinoids at high concentrations of 1000 ppm are readily available and DEA exempt. This opens opportunities to easily extend the calibration range. You can also customize curve ranges per cannabinoids, taking the typical range seen in various commodities into consideration. For example, you can have an extended upper calibration range (1–600 ppm) for “the big four” (THC, THCA, CBD, and CBDA) and a lower upper limit (1–200 ppm) for all other cannabinoids. This approach saves costs as expensive cannabinoids aren’t required, and the range of THC, THCA, CBD, and CBDA is tripled. 

Some adjustment is usually needed when the calibration curve range becomes wider. Typically, I eliminate low calibration levels to produce a calibration curve model with improved accuracy that increases the s/n of the lower calibration levels. Calibration levels of 5–600 ppm for the “big 4” and 5–250 ppm for other cannabinoids seem to be the sweet spot to capture a wide in-commodity range – keeping standards reasonable and choosing levels based on accuracy criteria. In some cases, I’ve found it useful to increase the lowest calibration to 10 ppm to improve accuracy. However, for flowers, the big four in-commodity calibration range is 0.3-36 percent wt, whereas 0.3-15 percent wt caters to all other cannabinoid results.

I typically see nonlinear signals starting between 600–750 ppm injection concentration. At these concentrations, the UV signal is quite high and limits the upper calibration level. However, you can still extend the range if you’re feeling adventurous, such as by calibrating on an alternative wavelength and assuming a multiple wavelength or DAD detector. Looking at the full UV spectra of cannabinoids, acids have a couple of apexes and neutral cannabinoids have what can be described as an apex region. 

Wavelengths near these apexes are at relatively low intensity. A low intensity quant wavelength (such as 308 nm) for high concentrations of acidic cannabinoids (THCA), or 274 nm for neutral cannabinoids (THC), where signals at 228 nm could be saturated. This would result in fewer dilution schemes or repeated sample preparations at different dilutions – providing an alternative signal if matrix interference occurs at the common wavelength. 

In many labs, profit is literally being injected away with multiple injections per sample, constant retests and costly matrix interference investigations. Squeezing the most out of testing methods is critical in the economically difficult world where most cannabis testing labs must exist.