Sampling of illicit drugs for quantitative analysis – Part III: Sampling plans and sample preparations
Résumé
The findings in this paper are based on the results of our drug homogeneity studies [1] and particle size investigations [2]. 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.