Stable isotope modelling of blackberry to trace the origin of cannabis

Abstract

The legality of marijuana and its products in different countries and cultures is subject of debate prompting scientific research about developing new techniques to enable verification of the region of origin of marijuana seizures. The overall aim of this PhD project was to develop methods using stable isotopes to assist investigators to determine the origin of marijuana. In this project, a link between the leaf and inflorescence of the same plants of marijuana was observed using bulk δ13C values, δ15N values and δ2H values. The bulk δ13C values and δ2H values in the leaf and inflorescence of marijuana implied fractionation processes taking place during the metabolic processes, and climatic and environmental conditions at the origin of their cultivation. Nonetheless, this had a limitation due to the number of authentic-origin marijuana samples obtained. Thus, to circumvent the lack of availability of spatially representative marijuana samples, blackberry was selected as a proxy after careful consideration of the commonalities between the two plants.

The relationship between the two plant types was validated using bulk δ2H values which indicated positive covariances at common regions of origin suggesting that blackberry is a suitable proxy for marijuana. Moreover, models developed in this study using blackberry data as input may serve for tracing the origin of marijuana in the future. In addition, the presence of terpenes, e.g. myrcene and linalool along with positive correlations of δ2H values and δ13C values in marijuana and blackberry indicated that similar isotopic fractionation occurs during common metabolic pathways under comparable climatic conditions. This means that the external climate factors were more important drivers of isotopic composition than interspecific differences. In future investigations should be carried out with focus on additional marijuana and blackberry samples from common regions of origin. Finally, a δ2H isoscape model of blackberry was developed using indirect and direct modelling approaches. These modelling approaches were examined by testing the source of four test blackberry samples of known origin. Both the approaches successfully assigned the region of origin of the test samples. However, the direct approach of modelling from authentic blackberry sample data was able to locate the origins with much better specificity than the indirect approach. The indirect approach does have the advantage of not requiring continual ground-truthing with authentic samples as its inputs are derived from the prediction of a meteorological model, hence the increased uncertainties. The isoscape approach can be readily applied to other plant species and is an effective means of providing information to verify claims of origin of natural products.