ABSTRACT
The findings in this paper are based on the results of our drug homogeneity studies and particle size investigations. Using that information, a general sampling plan (depicted in the form of a flow-chart) was devised that could be applied to the quantitative instrumental analysis of the most common illicit drugs: namely heroin, cocaine, amphetamine, cannabis resin, MDMA tablets and herbal cannabis in 'bud' form (type I). Other more heterogeneous forms of cannabis (type II) were found to require alternative, more traditional sampling methods. A table was constructed which shows the sampling uncertainty expected when a particular number of random increments are taken and combined to form a single primary sample. It also includes a recommended increment size; which is 1 g for powdered drugs and cannabis resin, 1 tablet for MDMA and 1 bud for herbal cannabis in bud form (type I). By referring to that table, individual laboratories can ensure that the sampling uncertainty for a particular drug seizure can be minimised, such that it lies in the same region as their analytical uncertainty for that drug. The table shows that assuming a laboratory wishes to quantitatively analyse a seizure of powdered drug or cannabis resin with a 'typical' heterogeneity, a primary sample of 15×1 g increments is generally appropriate. The appropriate primary sample for MDMA tablets is 20 tablets, while for herbal cannabis (in bud form) 50 buds were found to be appropriate. Our study also showed that, for a suitably homogenised primary sample of the most common powdered drugs, an analytical sample size of between 20 and 35 mg was appropriate and for herbal cannabis the appropriate amount was 200 mg. The need to ensure that the results from duplicate or multiple incremental sampling were compared, to demonstrate whether or not a particular seized material has a 'typical' heterogeneity and that the sampling procedure applied has resulted in a 'correct sample', was highlighted and the setting up of suitable control charts (R or S charts), for quality control purposes, was strongly recommended and examples given. Furthermore, although this particular study relates to the sampling of illicit drugs, it should be remembered that it is based on general sampling theory and therefore the same approach could be applied to other disciplines where 'correct sampling' of powders and solids is important.
ABSTRACT
The basic goal in sampling for the quantitative analysis of illicit drugs is to maintain the average concentration of the drug in the material from its original seized state (the primary sample) all the way through to the analytical sample, where the effect of particle size is most critical. The size of the largest particles of different authentic illicit drug materials, in their original state and after homogenisation, using manual or mechanical procedures, was measured using a microscope with a camera attachment. The comminution methods employed included pestle and mortar (manual) and various ball and knife mills (mechanical). The drugs investigated were amphetamine, heroin, cocaine and herbal cannabis. It was shown that comminution of illicit drug materials using these techniques reduces the nominal particle size from approximately 600 µm down to between 200 and 300 µm. It was demonstrated that the choice of 1 g increments for the primary samples of powdered drugs and cannabis resin, which were used in the heterogeneity part of our study (Part I) was correct for the routine quantitative analysis of illicit seized drugs. For herbal cannabis we found that the appropriate increment size was larger. Based on the results of this study we can generally state that: An analytical sample weight of between 20 and 35 mg of an illicit powdered drug, with an assumed purity of 5% or higher, would be considered appropriate and would generate an RSDsampling in the same region as the RSDanalysis for a typical quantitative method of analysis for the most common, powdered, illicit drugs. For herbal cannabis, with an assumed purity of 1% THC (tetrahydrocannabinol) or higher, an analytical sample weight of approximately 200 mg would be appropriate. In Part III we will pull together our homogeneity studies and particle size investigations and use them to devise sampling plans and sample preparations suitable for the quantitative instrumental analysis of the most common illicit drugs.
ABSTRACT
Sampling of illicit drugs for qualitative and quantitative analysis would normally be considered as routine and comparable tasks in forensic drugs laboratories and previously similar statistical sampling approaches have been applied. However, we believe that two different sampling approaches, based on two different theoretical and statistical backgrounds are more appropriate. Furthermore the application of the qualitative sampling approach can be impractical for quantitative sampling as it could generate many analytical samples from a single seizure. In some countries the purity of the illicit drug in a seizure may affect the criminal sentence and therefore, reliable results for quantitative analysis are crucial. It was decided to investigate a new approach, which although incorporating some statistics also took account of our background knowledge about the composition of the drugs we were analysing. The ultimate goal was to produce recommendations for a practical sampling plan for quantitative analysis. It was found that the two key factors which had a significant effect on obtaining a representative analytical sample from a bulk seizure were the heterogeneity of the drug powder and the particle sizes of its components. This article concentrates on drug heterogeneity. Particle size effects will be addressed in part II of this study. A sampling plan was devised for a range of drug seizure types and asked ENFSI member laboratories to use it when analysing real drug seizures to provide heterogeneity data for the most common illicit drugs (heroin, cocaine, amphetamine, MDMA and cannabis (herbal and resin)). It was found that for routine quantitative drugs analysis, the sampling problems caused by heterogeneity can be solved by using an incremental sampling protocol. Furthermore, the number of increments that need to be taken for a particular drug is dependent on the relative standard deviation (RSD) required by an individual laboratory and the analytical method that they employ. A 1g increment size was found to be suitable for powdered drugs and cannabis resin. However, 1g increments were not suitable for herbal cannabis, because of particle size issues. Sampling of herbal cannabis will be addressed in Part II of this study. Recommendations for a sampling plan, based on the heterogeneity and particle size of specific drugs seizures in casework will be discussed in Part III of this study.
