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An Evaluation of Sample Preparation Techniques for Cannabis Potency Analysis

By Kelsey Cagle, Frank L. Dorman, Jessica Westland
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Sample preparation is an essential part of method development and is critical to successful analytical determinations. With cannabis and cannabis products, the analyst is faced with a very challenging matrix and targets that may range from trace level through percent level thus placing considerable demands on the sample preparation techniques.1 The optimal sample preparation, or “extraction”, method for potency analysis of cannabis flower was determined using a methanol extraction coupled with filtration using regenerated cellulose filters. 

In the United States (US), Canada, and other countries where medicinal and/or adult recreational cannabis has been legalized, regulatory entities require a panel of chemical tests to ensure quality and safety of the products prior to retail sales2. Cannabis testing can be divided into two different categories: Quality and Safety. Quality testing, which includes potency analysis (also known as cannabinoid testing or cannabinoid content), is performed to analyze the product in accordance with the producer/grower expectations and government regulations. Safety testing is conducted under regulatory guidelines to ensure that consumers are not exposed to toxicants such as pesticides, mycotoxins, heavy metals, residual solvents and microbial contaminates.

Potency testing evaluates the total amount of cannabinoid content, specifically focusing on tetrahydrocannabinol (THC) and cannabidiol (CBD). In the US, the biggest push for accurate total THC is to differentiate between hemp (legally grown for industrial or medicinal use), which is defined as cannabis sativa with a THC limit ≤ 0.3 %, and cannabis (Cannabis spp.), which is any cannabis plant with THC measured above 0.3 %3. Potency testing is typically performed by liquid chromatography (LC) with UV detection to determine the quantity of major cannabinoids.

In addition to reporting THC and CBD, their respective precursors are also important for reporting total potency. Tetrahydrocannabinolic acid (THCA) is the inactive precursor to THC while cannabidiolic acid (CBDA) is the precursor to CBD.4,5

Methods and Materials

Sample Preparation

All samples were homogenized using an immersion blender with a dry material grinder. The nominal sample amounts were 200 mg of flower, 500 mg of edibles, and 250 mg of candy samples.

Potency Extraction Method (1)

Twenty milliliters (mL) of methanol (MeOH) was added to each sample. The samples were mechanically shaken for 10 minutes and centrifuged for 5 minutes.

Potency Extraction Method (2)

Ten mL of water was added to each sample. The samples were mechanically shaken for 10 minutes. 20 mL of acetonitrile (ACN) was then added to each sample and vortexed. An EN QuEChERS extraction salt packet was added to the sample. The samples were placed on a mechanical shaker for 2 minutes and then centrifuged for 5 minutes.

Each extract was split and evaluated with two filtration/cleanup steps: (1) a regenerated cellulose (RC) syringe filter (Agilent Technologies, 4 mm, 0.45 µm); (2) a PFTE syringe filter (Agilent Technologies, 4 mm, 0.45 µm). The final filtered extracts were injected into the ultra-performance liquid chromatograph coupled with a photodiode array detector (UPLC-PDA) for analysis.

Figure 1: Calibration curve for THC potency

Calibration

Standards were obtained for the following cannabinoids at a concentration of 1 mg/mL: cannabidivarin (CBDV), tetrahydrocannabivarin (THCV), cannabidiol (CBD), cannabigerol (CBG), cannabidiolic acid (CBDA), cannabigerolic acid (CBGA), cannabinol (CBN), tetrahydrocannabinol (9-THC), cannabichromene (CBC), tetrahydrocannabinol acid (THCA). Equal volumes of each standard were mixed with MeOH to make a standard stock solution of 10 ug/mL. Serial dilutions were made from the stock to make concentrations of 5, 1, and 0.5 ug/mL for the calibration curve (Figure 1).

Instrumental Method

All instrument parameters were followed from Agilent Application Note 5991-9285EN.8 A UPLC with a PDA (Waters Corp, Milford, MA) detector was employed for potency analysis. An InfinityLab Poroshell 120 EC-C18, 3.0 x 50 mm, 2.7 um column (Agilent Technologies, Wilmington, DE) was utilized for compound separation. The organic mobile phase composition was 0.05 % (v/v) formic acid in HPLC grade MeOH and the aqueous mobile phase composition was 0.1 % (v/v) formic acid in HPLC grade water. The mobile phase gradient is shown in Table 1. The flow rate was 1 mL/min (9.5 minute total program), injection volume was 5 uL, and column temperature was 50 °C.

Table 1: LC mobile phase gradient for potency samples6

Discussion and Results

Table 2 summarizes the relative standard deviations (% RSD) were found for the THC calibrator (at 1 ug/mL) and one extract of a homogeneous sample (utilizing 7 replicates).

Table 2- %RSD values for the instrument response precision for THC in both the calibrations and the homogeneous extract.

The cannabinoid potency of various cannabis plant and cannabis product samples were determined for the various extraction techniques In the chromatograms THC was observed ~8.08 minutes and CBD was observed ~4.61 minutes (Figure 2).

Figure 2: Chromatogram of the 10ug/mL calibrator for potency/cannabinoid analysis

Total potency for THC & CBD were calculated for each sample using the equations below. Equation 1 was used because it accounts for the presence of THCA as well as the specific weight difference between THC and THCA (since THCA will eventually convert to THC, this needs to be accounted for in the calculations).

Table 3 shows the % THC and the total THC potency values calculated for the same flower samples that went through all four various potency sample preparation techniques as described earlier. Figure 3 also provides LC chromatograms for flower sample 03281913A-2 and edible sample 03281912-1.

Table 3-THC and Total THC potency values for the same cannabis flower sample processed through the combination of extractions and cleanups.
Figure 3: Potency/Cannabinoid analysis chromatogram for flower sample 03281913A-2 (red trace) and edible sample 03281912-1 (green trace).

The results indicated that with the “Potency Extraction Method 2” (ACN/QuEChERS extraction) coupled with the RC filter provided a bias of 7.29 % greater for total THC % over the other extraction techniques. Since the other 3 techniques provided total THC values within 2% of each other, the total THC of the sample is more likely ~14%.

Since the sample dilution for the above data set reduced the CBD content, an undiluted sample was run and analyzed. This data is reported in Table 4.

Table 4- CBD and Total CBD potency values for the same cannabis flower sample processed through different sample preparation techniques.

The CBD results indicated that with the “Potency Extraction Method 1” (methanol extraction) coupled with RC filter, allowed for a greater CBD recovery. This may indicate the loss of CBD with an ACN/QuEChERS extraction.

With an average ~14% total THC and 0.06% total CBD for a homogenous cannabis flower sample, the optimal sample preparation extraction was determined to be a methanol extraction coupled with filtration using a regenerated cellulose filter. Since potency continues to remain at the forefront of cannabis regulatory testing it is important to utilize the right sample prep for your cannabis samples.


References

  1. Wang M, Wang YH, Avula B, Radwan MM, Wanas AS, Mehmedic Z, et al. Quantitative Determination of Cannabinoids in Cannabis and Cannabis Products Using Ultra-High-Performance Supercritical Fluid Chromatography and Diode Array/Mass Spectrometric Detection. Journal of Forensic Sciences 2016;62(3):602-11.
  2. Matthew Curtis, Eric Fausett, Wendi A. Hale, Ron Honnold, Jessica Westland, Peter J. Stone, Jeffery S. Hollis, Anthony Macherone. Cannabis Science and Technology, September/October 2019, Volume 2, Issue 5.
  3. Sian Ferguson. https://www.healthline.com/health/hemp-vs-marijuana. August 27, 2020.
  4. Taschwer M, Schmid MG. Determination of the relative percentage distribution of THCA and 9-THC in herbal cannabis seized in Austria- Impact of different storage temperatures on stability. Forensic Science International 2015; 254:167-71.
  5. Beadle A. CBDA Vs CBD: What are the differences? [Internet]. Analytical Cannabis. 2019 [cited 2020 Apr 22]; https://www.analyticalcannabis.com/articles/cbda-vs-cbd-what-are-the-differences-312019.
  6. Storm C, Zumwalt M, Macherone A. Dedicated Cannabinoid Potency Testing Using the Agilent 1220 Infinity II LC System. Agilent Technologies, Inc. Application Note 5991-9285EN
amandarigdon
The Practical Chemist

Internal Standards– Turning Good Data Into Great Data

By Amanda Rigdon
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amandarigdon

Everyone likes to have a safety net, and scientists are no different. This month I will be discussing internal standards and how we can use them not only to improve the quality of our data, but also give us some ‘wiggle room’ when it comes to variation in sample preparation. Internal standards are widely used in every type of chromatographic analysis, so it is not surprising that their use also applies to common cannabis analyses. In my last article, I wrapped up our discussion of calibration and why it is absolutely necessary for generating valid data. If our calibration is not valid, then the label information that the cannabis consumer sees will not be valid either. These consumers are making decisions based on that data, and for the medical cannabis patient, valid data is absolutely critical. Internal standards work with calibration curves to further improve data quality, and luckily it is very easy to use them.

So what are internal standards? In a nutshell, they are non-analyte compounds used to compensate for method variations. An internal standard can be added either at the very beginning of our process to compensate for variations in sample prep and instrument variation, or at the very end to compensate only for instrument variation. Internal standards are also called ‘surrogates’, in some cases, however, for the purposes of this article, I will simply use the term ‘internal standard.’

Now that we know what internal standards are, lets look at how to use them. We use an internal standard by adding it to all samples, blanks, and calibrators at the same known concentration. By doing this, we now have a single reference concentration for all response values produced by our instrument. We can use this reference concentration to normalize variations in sample preparation and instrument response. This becomes very important for cannabis pesticide analyses that involve lots of sample prep and MS detectors. Figure 1 shows a calibration curve plotted as we saw in the last article (blue diamonds), as well as the response for an internal standard added to each calibrator at a level of 200ppm (green circles). Additionally, we have three sample results (red triangles) plotted against the calibration curve with their own internal standard responses (green Xs).

Figure 1: Calibration Curve with Internal Standard Responses and Three Sample Results
Figure 1: Calibration Curve with Internal Standard Responses and Three Sample Results

In this case, our calibration curve is beautiful and passes all of the criteria we discussed in the previous article. Lets assume that the results we calculate for our samples are valid – 41ppm, 303ppm, and 14ppm. Additionally, we can see that the responses for our internal standards make a flat line across the calibration range because they are present at the same concentration in each sample and calibrator. This illustrates what to expect when all of our calibrators and samples were prepared correctly and the instrument performed as expected. But lets assume we’re having one of those days where everything goes wrong, such as:

  • We unknowingly added only half the volume required for cleanup for one of the samples
  • The autosampler on the instrument was having problems and injected the incorrect amount for the other two samples

Figure 2 shows what our data would look like on our bad day.

Figure 2: Calibration Curve with Internal Standard Responses and Three Sample Results after Method Errors
Figure 2: Calibration Curve with Internal Standard Responses and Three Sample Results after Method Errors

We experienced no problems with our calibration curve (which is common when using solvent standard curves), therefore based on what we’ve learned so far, we would simply move on and calculate our sample results. The sample results this time are quite different: 26ppm, 120ppm, and 19ppm. What if these results are for a pesticide with a regulatory cutoff of 200ppm? When measured accurately, the concentration of sample 2 is 303ppm. In this example, we may have unknowingly passed a contaminated product on to consumers.

In the first two examples, we haven’t been using our internal standard – we’ve only been plotting its response. In order to use the internal standard, we need to change our calibration method. Instead of plotting the response of our analyte of interest versus its concentration, we plot our response ratio (analyte response/internal standard response) versus our concentration ratio (analyte concentration/internal standard concentration). Table 1 shows the analyte and internal standard response values for our calibrators and samples from Figure 2.

 

Table 1: Values for Calibration Curve and Samples Using Internal Standard
Table 1: Values for Calibration Curve and Samples Using Internal Standard

The values highlighted in green are what we will use to build our calibration curve, and the values in blue are what we will use to calculate our sample concentration. Figure 3 shows what the resulting calibration curve and sample points will look like using an internal standard.

Figure 3: Calibration Curve and Sample Results Calculated Using Internal Standard Correction
Figure 3: Calibration Curve and Sample Results Calculated Using Internal Standard Correction

We can see that our axes have changed for our calibration curve, so the results that we calculate from the curve will be in terms of concentration ratio. We calculate these results the same way we did in the previous article, but instead of concentrations, we end up with concentration ratios. To calculate the sample concentration, simply multiply by the internal standard amount (200ppm). Figure 4 shows an example calculation for our lowest concentration sample.

Figure 4: Example Calculation for Sample Results for Internal-Standard Corrected Curve
Figure 4: Example Calculation for Sample Results for Internal-Standard Corrected Curve

Using the calculation shown in Figure 4, our sample results come out to be 41ppm, 302ppm, and 14ppm, which are accurate based on the example in Figure 1. Our internal standards have corrected the variation in our method because they are subjected to that same variation.

As always, there’s a lot more I can talk about on this topic, but I hope this was a good introduction to the use of internal standards. I’ve listed couple of resources below with some good information on the use of internal standards. If you have any questions on this topic, please feel free to contact me at amanda.rigdon@restek.com.


Resources:

When to use an internal standard: http://www.chromatographyonline.com/when-should-internal-standard-be-used-0

Choosing an internal standard: http://blog.restek.com/?p=17050