Forensic Chemistry of Alkaloids: Presumptive Color Test

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Evidence collected from crime scenes often includes unknown powders, pills, and tablets, many of which are illicit drugs difficult to identify visually. Presumptive color tests help with the on-scene recognition of drug materials via rapid color changes. Most of these tests are based on qualitative chemical reactions and have since been standardized. Although simple and rapid, qualitative tests provide only preliminary analytical data. Nevertheless, these tests are still important components of crime scene investigations, and government authorities deploy them to detect illicit drugs in the field. Understanding the chemistry behind presumptive color tests makes it possible to predict reactions to known drug standards. However, in the presence of cutting agents and other chemicals, the results of presumptive color tests may not be predictable due, in part, to interference by contaminating chemicals. Most described presumptive tests are developed for ‘classic’ drugs, such as opiates, amphetamines, and cocaine, but new psycho-active substances and cutting agents emerge every day on the market. Among them, alkaloids (e.g., lobeline, caffeine, piperine) can be purchased easily and legally via the Internet or in local shops and are often utilized to lace or cut drugs. This study is the first to predict and document the results on 7 of the most common presumptive color tests with various alkaloid standards.

Weassessed these tests with mixtures of alkaloids to ascertain interference, if any, in the color results. We performed presumptive color tests on various popular cutting agents and, finally, tested several mixtures of drugs/alkaloids/cutting agents potentially similar to samples seized in the field. The results showed that color prediction worked well with pure standards, but color tests could not be predicted for mixtures in most cases. Also, alkaloid cutting agents often interfere with presumptive color test results, affecting outcomes. Better understanding of presumptive color tests, coupled with better populated databases of color results involving cutting agents, will help in reducing false positives and false negatives, thereby improving initial testing of seized evidence.