Analysis of complex molecular systems using two-dimensional gas chromatography (GCxGC-TOFMS) and chemometrical data analysis - forensic application of chemical signatures of drugs of abuse
M. Schäffer (HMGU), T. Gröger (HMGU), M. Pütz (BKA), B. Ahrens (BKA), R. Zimmermann (UR/HMGU)
The chemical characterisation of drugs gained importance as analytical tool in supporting the work of investigating and law enforcing authorities often successfully providing additional information about the clandestine manufacturing process, the used precursors, distribution networks, linkage of different seizures and sometimes hints of source or even geographical origin. Chemical profiling of illicit drugs is a complex process. The results are affected by many different factors such as sample size, the sample processing conditions, the used analytical technique as well as the statistics that are applied. In order to get an efficient separation of main- minor- and trace compounds a high separation power of the employed (chromatographic) system is required. Since mostly minor- and trace compounds are of interest a high sensitivity of the analytical system is preferred in addition. Comprehensive two-dimensional (2D) gas chromatography (GC×GC) meets these requirements. The obtained higher-dimensional data are then used to identify possible marker compounds for the discrimination of the samples according to their chemical profile. The non-targeted and pixel-based chemometric process includes preprocessing steps (background correction, alignment of chromatograms and normalization) followed by an adaptation of hierarchical clustering to identify chemically similar samples and analysis of variances / discriminant analysis techniques for the identification of promising discriminative compounds.
- Fig. 1 (A): Calculated mean chromatogram of all cannabis resin samples. (B) Calculated mean chromatogram of all marijuana samples.
In a proof-of-concept study we demonstrated the adaptability of two-dimensional gas chromatography (GCxGC) combined with a picture-based chemometric data processing method for batch-to-batch comparison of real seizures of heroin and cannabis based on the obtained two-dimensional impurity profiles. Samples of heroin and cannabis were extracted and analyzed with GC×GC–TOF-MS (time-of-flight mass spectrometry) and GC×GC-FID (flame ionization detection). The used pixel-based hierarchical clustering was able to group the samples according to their chemical profiles. The grouping matched the results found by well-established comparative studies and/or the forensic background of the samples.
- Fig. 2 (A): Hypothetical dendrogram illustrating the samples linkage according to the forensic background. Hierarchical clustering according to the 2D chemical signature based on GCxGC-TOFMS analysis is shown in (B) Line I cuts the dendrogram according to the main groups. Line II according to the subgroups.
3,4 methylenedioxymethamphetamine (“ecstasy”) is a structural derivate of betaphenylethylamine and belongs to the group of amphetamines. The popularity of MDMA as a drug of abuse increased in the 1990`s and as a matter of course the illicit production raised. Nowadays MDMA is still one of the most popular drugs of abuse. Samples were extracted according to an established and harmonised extraction method. For evaluation of the discrimination potential a comparable analysis of several real samples based on the obtained two-dimensional impurity profiles was part of this work. The application of chemometric data analysis allowed the identification of potential marker compounds useful for discrimination of the set of samples. Besides known target analytes various additional compounds could be determined based on the comprehensive two-dimensional analysis.
- Fig. 3: The result of the data analysis routine provides statistical significant peaks for discrimination of the given sample set. Many of the markers found (numbered) are target compounds of the one-dimensional standard profiling method. The known forensic links among the samples are illustrated on the left side of the figure.
“Spice” is a generic term for a blend of different herbal mixtures with synthetic cannbimimetic compounds. In 2008/2009 a couple of these compounds could be identified and are now banned in Germany and large parts of Europe. Beside these main compounds very little is known about the chemical composition of “Spice”. To gain more information about the chemical composition samples from different Spice products were analysed in cooperation with the University Hospital Freiburg (Forensic Department). Ethanolic extracts were analysed by one- and comprehensive two-dimensional gas chromatography for additional cannabimimetic compounds as well as for the presence of impurities. First results indicate the existence of possible characteristic impurities.
- Fig. 4: Part of a GCxGC-TOFMS chromatogram of an ethanolic Spice extract. Synthetic cannabinoids and possible impurities which show some correlation to the cannbinoid concentration are indicated.
