Stability of amphetamine impurity profiles during 12 months of storage.

Impurity profiling is a well described forensic tool that may be applied to gain information about the illegal drug market. However, it requires experience to assess the correlation between chemical profiles thereby separating linked from unlinked samples. One of the challenges in this context is that the chemical profiles may change over time, thus complicating an assessment if samples are stored under different conditions. In this study, the impact of different storage conditions on the stability of amphetamine impurity profiles was investigated. We examined the influence of storage time, temperature, sample purity, sample quantity and the presence of methanol on the amphetamine profile stability when stored for up to 12 months. We find that the target compounds in amphetamine impurity profiles are susceptible to all the examined storage conditions. Consequently, this unstable nature of amphetamine profiles may complicate the assessment when comparing amphetamine seizures that has been separated for longer time periods or stored under different conditions prior to seizure. Knowledge about the seizure history is rarely available to the forensic analyst. Therefore, sample stability issues should be taken into account when comparisons of chemical profiles are made.

Variation in chemical profiles within large seizures of cocaine bricks.

Cocaine is usually trafficked from South America throughout the world in packages of approximately one kilogram shaped as bricks and imprinted with a logo. Seizures consisting of multiple cocaine bricks gives the opportunity to examine the variation in the chemical profile within cocaine bricks assumed to originate from the same manufacturer and maybe even the same production batch. This knowledge may be important to the forensic investigator when chemical profiles from cocaine samples of unknown origin are compared. In the present study, the alkaloid and residual solvent profiles from three large cocaine seizures each containing identical cocaine bricks was examined. The three cases consisted of 36, 84 and 100 cocaine bricks, respectively. Each cocaine brick was profiled according to its cocaine alkaloid and residual solvent content using gas chromatography-mass spectrometry (GC-MS) and headspace GC-MS. The study showed that each of the three identical looking seizures consisted of up to four groups of cocaine bricks displaying the same cocaine alkaloid and residual solvent profile. The size of the groups varied from 2.4 to 63.3kg cocaine. The study also showed that the residual solvent profile within each of the three large seizures exhibited very little variation whereas the alkaloid profile varied considerably more. This finding suggest that the same organic solvent is used for the production of several batches of cocaine HCl. Therefore, the residual solvent profile may be a tool to link different production batches from the same manufacturer even though the alkaloid profile are different.

Profiling of cocaine using ratios of GC-MS peaks.

Illicit cocaine seizures are often compared to each other by using gas chromatography-mass spectrometry (GC-MS) data from cocaine alkaloid compounds to determine whether two specimens originate from the same production batch or not. This can provide intelligence or investigative information at the early stages of an investigation or evidence in court. Traditional classification methods assume high stability of all alkaloids, use all of them to calculate the correlation between two profiles and use a threshold to classify samples. Unstable alkaloids will have a strong influence on the performance. We show that comparing each alkaloid target compound individually improves the classification. Unfortunately, it requires normalization and is also sensitive to the stability. Instead we suggest to use ratios of all possible pairwise combinations of the GC-MS peaks. These ratios are scale free and directly comparable between samples. The peaks can be given different weights in the comparison of profiles using appropriate classification methods and we show that randomForest classification using these ratios have a high and reproducible performance in comparison with other methods. The performance of this method is not affected by noise, transformation or normalization and should be considered for future comparison of chromatographic profiles in general.

Cocaine classification using alkaloid and residual solvent profiling.

Many different groups of chemical compounds can be used in statistical impurity-profile comparison in order to establish links between different seizures of illicit drugs. For cocaine, some of these compounds descent directly from the coca leaf while others are remnants from the manufacturing process; and each of the compound groups exhibit different degrees of stability and discrimination power. Information obtained from the different groups can be handled in numerous ways and selecting the right method using a balanced combination of the compound groups is highly important in order to provide investigators and courtrooms with accurate conclusions. By using logistic regression or discriminant analysis (linear and quadratic), cocaine alkaloid and residual solvent distances can be combined in order to obtain probabilities for the two possibilities: linked or unlinked. We examined different data transformations and distance methods and ranked the different models using cross validation. Validation in an unrelated data set proved the consistency of the results. Data consisted of five large groups of linked samples exposed to different storage conditions during 12 months, 124 different cocaine sample seizures and 15 smaller groups of linked samples stored at room temperature for up to 15 months. The alkaloid and residual solvent impurity profiles of the samples were analysed using gas chromatography-mass spectrometry (GC-MS) and headspace GC-MS, respectively. Residual solvent profiles exhibited considerable higher discrimination power than cocaine alkaloid profiles. Thus, a residual-solvent-weighted model (log10 transformation and cosine distance) was found superior at distinguishing correctly between linked and unlinked seizures compared to models using alkaloid distance alone. The model only gives weight to the residual solvents when the alkaloid profiles are very similar. This finding demonstrates the possibility to combine information from the highly stable, non-coca leaf-descent residual solvent profiles and the less stable cocaine alkaloid profiles for statistical comparative analysis of cocaine seizures in a simple and easy-to-implement way.