Profiling and comparison
A more detailed analysis of drug samples can be used to provide ‘collective’ information. This is generally called profiling when it involves the chemical analysis of powders, but is known as characterization when the physical properties of tablets and other dosage forms are measured. Chemical profiling has been the technique used most widely and is often based on the chromatographic separation of impurities and precursors (as in the case of amfetamine and methamfetamine) or other naturally occurring components and adulterants (e.g. diamorphine, cocaine, cannabis resin). Detection may range from GC with flame-ionisation to isotope-ratio MS. Non separation methods, for example using IR, Raman, X-ray fluorescence (XRF) spectroscopy or X-ray diffraction (XRD) spectroscopy, have only limited scope for identifying individual components in a drug sample because other substances present will also be detected by these techniques, giving rise to spectra which are difficult to interpret. Drug profiling may be used for two quite separate purposes. In the first case, it can establish connections between a number of exhibits suspected to be linked as part of a local distribution chain. This is known as comparison or tactical profiling, and may be carried out as a routine requirement in forensic casework. There is a second stage (intelligence or strategic profiling) in which answers to wider questions may be sought. These depend on the drug concerned, but include:
• estimating the number of different profile types in circulation and relating them to the number of active laboratories and the period for which they have been in operation
• determining the extent of importation by comparing the profiles of police and customs seizures
• identifying the route of synthesis and types of precursors used
• creating large-scale maps of drug distribution and identifying the country or region of origin.
Figure 3.30 illustrates how analysis can be used to identify region of origin. Ehle ringer et al. (2000) used isotope ratio mass spectrometry (IRMS) (Fig.2) to determine the carbon (d13C) and nitrogen (d15N) isotope-ratio signatures of 200 samples of coca leaves from the five primary coca growing regions of Bolivia, Columbia and Peru. They then combined these data with information on the trace alkaloids truxilline and trimethoxy cocaine and d13C and d15N ratios in cocaine samples extracted from the coca leaves. This enabled them to predict correctly the country of origin of 96% of the coca samples (Fig. 1). Tablet comparisons using general physical features, gross drug content and microscopic examination of defects and punch marks (so called ballistic analysis) can be of some value, but they suffer because at any one time a large fraction of illicit tablets in circulation may be almost identical. Such small differences as may exist could simply reflect inherent differences in the punches and dies of a multiple-stage tableting machine. This is illustrated by the Mitsubishi logo , which was found on over half of all tablets seized in Europe in the late 1990s. A similar pattern was also found in the UK for amfetamine in the early 1990s, when nearly half of all samples belonged to one profile type. In these circumstances, any connection between two separate seizures of otherwise identical tablets or powders may be purely fortuitous. This, in turn, raises other problems with profiling. It is necessary to maintain a database of profiles such that the significance of any ‘match’ or ‘non-match’ can be assessed critically. However, for a situation in which profiles may change with time, what constitutes a ‘current’ database is not always clear. In the case of determination of country of origin, authentic samples are required to provide a statistical ‘training set’, yet such samples may be difficult to obtain and their true provenance uncertain.
Figure 1 Isotope ratio mass spectrometry has been used to identify the geographic origin of illicit cocaine. (A) Regions producing illicit cocaine. (B) Identification of cocaine-growing regions based on a combined model derived from carbon- and nitrogen-isotope ratios as well as abundance of minor alkaloid components: squares, Bolivia; triangles, Colombia; circles, Peru. Regions within a country are distinguished by solid and open symbols. Trux, truxilline; TMC, trimethoxy cocaine. Isotope ratios are expressed as (Rsample /Rstandard -1) X 1000 ppt, where R is the molar ratio of heavy to-light stable isotope; standards for carbon and nitrogen are PDB and air, respectively. (Reprinted by permission from Macmillan Publishers Ltd (Nature Publishing Group) from Ehle ringer J, Casale J, Lott M and Ford V, Tracing the geographical origin of cocaine. Nature 408 311–312; copyright (16 Nov. 2000).
Figure 3.31 Schematic of isotope ratio mass spectrometer (IRMS).
