In states where cannabis is legalized, some analytical laboratories are tasked with identifying and quantifying pesticide content in plant material. This is a relatively new concept in the study of cannabis as most forensic laboratories that work with seized plant material are only concerned with positively identifying the sample as cannabis. Laboratories of this nature, often associated with police departments, the office of the chief medical examiner or the local department of public health are not required to identify the amount of THC and other cannabinoids in the plant. While data is abundant that compares the average THC content in today’s recreational cannabis to that commonly consumed in the 1960s and 1970s, limited scientific studies can be found that discuss the pesticide content in street-grade cannabis.
Using the QuEChERS approach, which is the industry gold-standard in food analysis for pesticides, a comparison study was carried out to analyze the pesticide and cannabinoid content in street-grade cannabis versus medicinal cannabis. For all samples, one gram of plant material was ground into a fine powder prior to hydration with methanol. The sample was then ready to be placed into an extraction tube, along with 10 mL of acetonitrile and one pouch of QuEChERS salts. After a quick vortex, all samples were then shaken for 1 minute using a SPEX Geno/Grinder prior to centrifugation.
For pesticide analysis, a one mL aliquot of the top organic layer was then subjected to additional dispersive solid phase extraction (dSPE) clean-up. The blend of dSPE salts was selected to optimize the removal of chlorophyll and other interfering compounds from the plant material without compromising the recovery of any planar pesticides. Shaken and centrifuged under the same conditions as described above, an aliquot of the organic layer was then transferred to an auto-sampler vial and diluted with deionized water. Cannabinoid analysis required serial dilutions between 200 to 2000 times, depending on the individual sample. Both pesticide and cannabinoid separation was carried out on a UCT Selectra® Aqueous C18 HPLC column and guard column coupled to a Thermo Scientific Dionex UltiMate 3000 LC System/ TSQ VantageTM tandem MS.
Due to inconsistent regulations among states that have legalized medicinal or recreational cannabis, a wide panel of commonly encountered pesticides was selected for this application. DEET, recognized by the EPA as not evoking health concerns to the general public when applied topically, was found on all medical cannabis samples tested. An average of 28 ng/g of DEET was found on medicinal samples analyzed. Limited research as to possible side effects, if any, of having this pesticide present within volatilized medical-grade product is available. Street-grade cannabis was found to have a variety of pesticides at concentrations higher than what was observed in the medical-grade product.
Tetrahydrocannabinolic acid A (THCA-A) is the non-psychoactive precursor to THC. Within fresh plant material, up to 90% of available THC is found in this form. Under intense heating such as when cannabis is smoked, THCA-A is progressively decarboxylated to the psychoactive THC form. Due to possible therapeutic qualities of this compound, medical cannabis samples specifically were tested for this analyte in addition to other cannabinoids. On average, 17% of the total weight in each medical cannabis sample came from the presence of THCA-A. In both medical and recreational samples, the percentage of THC contribution ranged from 0.9-1.7.
A fast and effective method was developed for the determination of pesticide residues and cannabis potency in recreational and medical cannabis samples. Pesticide residues and cannabinoids were extracted using the UCT QuEChERS approach, followed by either additional cleanup using a blend of dSPE sorbents for pesticide analysis, or serial dilutions for cannabinoid potency testing.
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