Inter-Laboratory Variation in Cannabis Analysis: Pesticides and Potency in Distillates

February 12, 2019
Volume: 
2
Issue: 
1
Table I
Table I (click to enlarge): Quantity of pesticide spiked into cannabis distillate samples and California state action limits
Table II: Boiling points of pesticides used in study
Table II (click to enlarge): Boiling points of pesticides used in study
Abstract / Synopsis: 

Cannabis distillate samples spiked with known amounts of pesticides were submitted to five cannabis testing laboratories. The false positive rate for pesticide detection was zero percent (%), but the false negative rate was 78%, indicating contaminated material may be reaching cannabis patients and users. Distillate samples were also analyzed for potency. The Δ9-tetrahydrocannabinol (THC) concentration values obtained varied from 77% to 94% with a 95% confidence limit of 8.08%, an unacceptably large error given the high concentration of the target analyte. Potency measurements on this material using mid-infrared (IR) spectroscopy gave a THC weight percent range from 89% to 93%, and a 95% confidence interval of 2.3%, 3.5x more precise than the cannabis laboratories. The consistent mid-IR results indicate the sample was homogeneous. The scatter observed in pesticide and potency results from the laboratories means inter-laboratory variation is a problem in the cannabis industry. Potential causes and solutions to the problem are discussed.

The problem of inter-laboratory variation in the cannabis testing industry, that is, different laboratories obtaining statistically different results on the same samples, is ongoing (1–3). One study (1) found most of the cannabidiol (CBD) oil samples analyzed were labeled with incorrect potencies. Reports in the popular media (2,3) have documented similar problems. We investigated inter-laboratory variation in our area (central California), and chose distillates because they are relatively high purity homogeneous liquids that should be easy to analyze.

We investigated pesticide contamination variability because one laboratory will pass and another fail the same sample. This conflicting data makes it difficult for cannabis businesses to ensure quality control and make decisions regarding material status. We submitted a set of cannabis distillate samples from the same batch known to be pesticide free (control), and a set of samples from the same batch spiked with known amounts of six pesticides (myclobutanil, paclobutrazol, pyrethrin, imidacloprid, spiromesifen, and abamectin). The samples were submitted to five different cannabis laboratories in the central California area to study the phenomenon of inter-laboratory variation.

We were also concerned about distillate potency inter-laboratory variation. Our experience has been that cannabis laboratories obtain markedly different Δ9-tetrahydrocannabinol (THC) values on the same sample. Traditionally, high performance liquid chromatography (HPLC) has been used to measure cannabinoid profiles in cannabis-containing materials (4–6). More recently, mid-infrared (IR) spectroscopy has been used to determine cannabinoid concentrations in cannabis oils and extracts (7,8). In this study, we extend the use of mid-IR to the analysis of potency in cannabis distillates. We found that mid-IR potency measurements gave better precision than HPLC. We believe this is because of the lack of sample preparation required for mid-IR. The results of all laboratory tests were collated, tabulated, and are presented below. Further explanations and possible solutions to the inter-laboratory variation problem are discussed.

Experimental

Pesticide and Potency Samples
To start, 60 g of a cannabis distillate batch made by the Herer Group were used. This distillate was found to be pesticide free by two different cannabis analysis laboratories. Next, 30 g of the distillate were set aside as a control. The second 30 g were spiked with known amounts of the pesticides listed in Table I, and compared to the state of California pesticide action limits (9). (See upper right for Table I, click to enlarge.)

Spiked samples were prepared by weighing commercial pesticides on an analytical balance, taking into account the concentration of pesticides in each product to determine the appropriate amount to measure. These quantities were dissolved in 100 mL ethanol to make a concentrated pesticide stock solution. Then 1 mL of concentrated solution was mixed with 1 L of ethanol to create a dilute pesticide solution. A blank sample of the ethanol was tested and found to be pesticide free.

Next, 300 mL of dilute pesticide solution were used to dissolve 30 g of distillate known to be pesticide free. Ethanol was removed using a rotovap whose water bath was heated to 40 °C. The solution was heated for 30 min at 50 Torr pressure. The boiling points of the spiked pesticides are listed in Table II. (See upper right for Table II, click to enlarge.)

Given the high boiling points of the six pesticides, and the gentle evaporation conditions used, little or no pesticide should have been lost. Regardless, the sample was small enough that any pesticide evaporation would have been uniform throughout the material, and all the laboratories should have obtained the same results. All samples were prepared at the same time by the same person, were always in his custody, and were hand delivered to the laboratories by him the day after preparation. This means there was no chance for the samples to change composition because of aging or tampering.

Two samples of spiked distillate and two samples of pesticide-free distillate were submitted to each laboratory. Five labs were used, each received four samples, so a total of 20 pesticide panels and cannabinoid profiles were measured. There were 10 pesticide free and 10 pesticide-spiked samples, the latter of which contained six different pesticides. A total of 60 potential pesticide detection events or hits were possible.

Our samples were analyzed for the presence of the 66 pesticides currently regulated by the state of California (9). The experimental methods for pesticide and potency measurements were laboratory dependent and their details were not made available to us. Typically, gas chromatography–tandem mass spectrometry (GC–MS/MS) or liquid chromatography–tandem mass spectrometry (LC–MS/MS) techniques are used to measure pesticides in cannabis-containing materials (10–13). HPLC is typically used for cannabis potency measurements (4–6).

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Brian C. Smith, PhD, is the Lab Director, Paul Lessard PhD, is the Chief Scientific Officer, and Rich Pearson is a Senior Scientist at the Herer Research Institute in Santa Cruz, California. Direct correspondence to: [email protected]

How to Cite This Article

B.C. Smith, P. Lessard, and R. Pearson, Cannabis Science and Technology 2(1), 48-53 (2019).