Sourcing the provenance of cannabis crops using inter-element association patterns ‘fingerprinting’ and laser ablation inductively coupled plasma mass spectrometry

Abstract

This paper describes the development and application of laser ablation ICP-MS methodologies for determining the trace element association patterns ‘fingerprint’ of cannabis crops and the potential tracing of these crops to specific geological environments. It also details the specific identification of single- and multi-sourced material collected during police drug raids. Cannabis samples are ground to a fine powder under liquid nitrogen and the resulting material pressed, under 20 t pressure, into cardboard mounts. An Nd:YAG laser is used to ablate this pressed powder and high sensitivity ICP-MS is used to determine the elemental association patterns ‘fingerprint’ derived from the analysis of the material. These associations of elements, rather than the traditional element concentrations, form the basis of the approach for the determination of provenance of cannabis crops and samples recovered in individual police drug raids. Variations in analyte signal between different ablation events, because of variations in coupling and transport efficiency, number of shots and morphology and colour of the substrate, are obviated by the use of ternary ratio per cent. discriminant diagrams. Ternary ratio per cent. plots represent the direct comparison of the relationship between three components in a system. The use of these plots effectively eliminates any variability in absolute values by concentrating on inter-comparison of components rather than absolute values. Positioning a ratio per cent. data point anywhere within the ternary plot indicates the relative percentile inter-association of the three components defined by the diagram. Using this method all ternary association plots of elements from the same sample will plot in the same position on the diagram, whereas those from different samples will plot elsewhere. Reproducibility on a day to day basis is confirmed by analysis of in-house standard cannabis materials with each sample batch.