Analysis of cocaine
Colour test Cobalt thiocyanate test or modified cobalt thiocyanate test (Scott test) that gives a blue colour indicates the presence of cocaine. Odour test A 5% methanolic solution of sodium hydroxide added to the test sample and warmed gives a characteristic odour for the presence of cocaine (although other compounds may give a similar odour).
In addition to GC and HPLC, IR spectroscopy is routinely used in cocaine cases if a distinction is to be made between cocaine as a salt (e.g. cocaine hydrochloride) and cocaine in base form. The cocaine base is known as crack and, unlike cocaine hydrochloride, can be consumed by smoking. The differences in the IR spectra are shown in Fig. 1. Differences at 1736 cm–1 and 1709 cm–1 for the base and 1729 cm–1 and 1711 cm–1 for the hydrochloride are explained by the effect of the hydrochloride ion on the C=O stretching bands (Elsherbini 1998). A simple laboratory test also exists for the determination of the chemical form of cocaine (Logan et al. 1989). This utilises a series of liquid/liquid extractions with testing of the organic or aqueous phases using the cobalt thiocyanate test. The base form of the drug partitions into the hexane used in the test but not into the aqueous phase, while the reverse is true for the salt form of cocaine.
Quantitative analysis and profiling of cocaine samples Quantitative analysis of cocaine samples may be carried out by GC or HPLC. The unsophisticated nature of the cocaine manufacturing process means that a multitude of trace-level alkaloid impurities are present in illicit cocaine. Many of these impurities are naturally-occurring alkaloids that originate from the coca leaf and are carried through the manufacturing process; they include cis- and trans-cinnamoyl cocaine, tropa cocaine, truxillines and hydroxycocaines. In addition, cocaine is also contaminated with a variety of manufacturing by-products, which include hydrolysis products, such as benzoyl ecgonine, ecgonine methyl ester, ecgonine and benzoic acids. The relative amount of these compounds can be used to compare cocaine samples (Moore and Casale 1998). Oxidation by products also arise and include N-norcocaine and N-norecgonine methyl ester. In addition to the above, solvent residues may be detected by NMR spectroscopy or headspace GC. The solvents detected include acetone, methyl ethyl ketone, benzene, toluene and diethyl ether (Cole 1998).
Figure 1 Infrared spectra of (A) cocaine base and (B) cocaine hydrochloride.
A comparison of cocaine samples can be achieved by a combination of qualitative analysis for the presence or absence of certain trace impurities and by quantitative analysis of the cocaine and other ingredients.
