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The complexity of the cannabis plant and the wide range of potential products add up to a complex matrix of problems for analytical chemists and their labs. For example, a recent paper by Professor Luca Zamengo of the Department of Environmental Hygiene and Forensic Toxicology in Venice, Italy, examines the role of time and storage conditions on the composition of hashish and marijuana samples.1 The authors focus on D9-THC and its degradation products, principally cannabidiol (CBN). Tetrahydrocannabinol (THC) degrades with time, producing CBN and other degradants. The stoichiometry is interesting: Only about one-third of the THC winds up as CBN. The remaining material in mass balance is not identified. The main purpose of the paper appears to be modeling of the kinetics of degradation to trace the history of marijuana and hashish for law enforcement purposes.
Experimental
Six representative samples of marijuana (#3, #4, and #6) and hashish (#1, #2, and #5) from law enforcement seizures were stored for four years. The samples were stored under the conditions shown in Table 1.
Assays were run periodically about every 100 days. Condition D gave almost no change, while conditions A and B showed significant conversion of THC to CBN. Conditions A and B were open to ambient air with no humidity control. The results show a 50% reduction in THC in 700 days when stored under condition A. Interestingly, only about one-third of the loss shows up in increasing CBN content. In contrast, material stored under condition D showed a loss of about 10% in THC upon termination of the study after four years. The CBN content line shows no significant change over four years for condition D.
Storage under condition B (20 °C, no light), but open to the atmosphere at 20 °C, showed a similar decay curve for THC with a T½ of about 900 days. For condition C, the T½ for THC was extrapolated to about 1600 days. The authors noted a periodic dependence that could correlate with laboratory humidity extremes in May and November. It is important to note that conditions C and D were isolated from the laboratory environment after removing a portion of the sample for the periodic analysis.
Humidity (water activity, aW) appears to be an uncontrolled potential variable in this study. Also, the effect of oxygen on stability was confounded by the lack of inclusion in the experimental design. For example, storage under condition B was open to oxygen in the air, while conditions C and D were closed-storage contains between the sampling events about every 100 days.
The paper goes on to introduce models to estimate the harvest date backwards from the seizure date based on the current profiles and stoichiometry. At first glance, kinetics seems to be first order in time when aged isothermally the region of 4 °C to about 20 °C.
This study is an interesting initial step in elucidating environmental effects on storage of THC and cannabidiol (CBD) to CBN. But commercial products often contain other important analytes that may be involved in degradation during storage. Much more research is needed before one can say we have confidence in how to store complex cannabis products such as hashish and natural product plant material, including marijuana.
The mathematical models of degradation are interesting but complex. However, the focus on THC misses the importance of volatile and semivolatile components that are critical quality attributes of different plant strains used in smoking. There are reports that freezing the plant material causes the trichomes to rupture and release the semivolatile plant components responsible for the flavor of the smoke. Smokable cannabis (hashish and marijuana) is the largest market segment quantity and monetary value.
I want to thank the authors for organizing and executing a very time-consuming experimental plan. Real-time data on natural products is important to develop. Plus, the huge number of variables compounds the problem.
Reference
- Zamengo, L.; Bettin, C. et al. The role of time and storage conditions on the composition of hashish and marijuana samples: a four-year study. Forens. Sci. Int. May 2019, 298, 131–7; doi: 10.1016/j.forsciint.2019.02.058. Epub 2019 Mar 8.
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]