Abstract:
Forensic laboratories examining Cannabis may be asked to determine if a sample is from a particular geographical location. They may also be asked to determine if two separate seizures of cannabis were once part of one larger sample. A variety of methods have been published for investigating these two related origin determination questions. This thesis presents an examination of the elemental profiles of cannabis plant material and the soil the plants were grown on, as a method for origin determination.
Cannabis plants were cultivated on three different soil profiles under controlled conditions. Plant samples, harvested at various stages of growth, were subjected to a nitric acid digestion procedure developed specifically for this investigation. The soil samples were extracted with dilute acetic acid to determine the plant available concentration of soil elements. Both the plant and soil samples were analysed on a HP 4500 Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) operated in the semiquant mode. Fifty-eight different elements were quantified in every sample. Plant and soil samples were also subjected to total element analysis methods.
A high level of precision was achieved for both the plant and soil analysis procedures. The accuracy of the plant digestion process was determined with the analysis of a standard reference material and showed good agreement with certified and recommended values. The accuracy of the soil extraction method was not determined, as a reference soil extraction sample was not available
From the comparison of plant results it was possible to conclude that plants grown on the same soil display the same element profile. This was true for plants of different sex and plants material harvested at different stages of development all from the same soil. In addition to this, plants from different soils had different elemental profiles. Twenty-two elements were identified as being significantly different between plants from different soils with the ANOVA univariate statistical test. These differences lead to clear group separation when samples were analysed with the canonical discriminant analysis multivariate statistical test.
The comparison of plant concentrations and extracted soil elements identified a limited number of positive correlations between plants and the soil they were grown on. These results indicate that a relationship does exist between Cannabis plants and the soil it was grown, however the acetic acid soil extracted process does not precisely represent the elements absorbed by Cannabis.
The elemental method for the origin determination of cannabis presented in this thesis answered one of the two questions posed. It was possible to identify cannabis plants that had been grown on the same soils and to separate plants from different soils. However, using the correlation between plant elements and extracted soil elements it would not be possible to offer strong conclusions about the relationship between a cannabis sample and possible soil origin.