Thin Layer Chromatography

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Prior to the use of any further method for the analysis of the material and identification of cocaine, the latter must be extracted. For powdered samples, which have been thoroughly homogenized, a 1 mgml-1 solution in methanol will suffice. For herbal material, the situation is rather more complex if the problems associated with extraction of chlorophyll, etc. are to be avoided. One study, where methanol and ethanol alone and in the presence of either diethylamine or ammonia were used, demonstrated that refluxing in ethanol for 15 min resulted in a 93% recovery of the cocaine from the leaf material. This investigation further demonstrated that using an acid back-extraction, followed by a chloroform extraction, could allow the extract to be 'cleaned up' sufficiently enough for good, quantitative chromatography by packed-column GC to be achieved [2]. However, care should be used when undertaking a cocaine extraction. In aqueous solutions, if the pH is above 5.5, decomposition of the cocaine to benzoyl ecgonine and methanol will occur. At pH 5.8, a 10% loss is observed after 13 d, while at pH 8, 35% of the cocaine is lost after 4.5 h [5].

The presence or absence of cocaine in a sample may qualitatively be determined by using TLC, carried out on silica gel. A number of different solvent systems are available for this (Table 6.1). The compounds can be examined visually

Table 6.1 TLC Rf data for cocaine and local anaesthetics obtained under the operating conditions described in the text

Compound Solvent system

Table 6.1 TLC Rf data for cocaine and local anaesthetics obtained under the operating conditions described in the text

Compound Solvent system

MeOH/NH3 (100:1.5)"

Cyclohexane/toluene/Et2NH (75:15:10)"

Cocaine

0.59

0.56

Ecgonine

0.84b

0.00

Ecgonine methyl ester

0.65b

0.44

Benzoyl ecgonine

0.25b

0.00

Lidocaine

0.69

0.40-0.55b

Procaine

0.55

bStreaking observed, i.e. not a rounded spot.

aBy volume.

bStreaking observed, i.e. not a rounded spot.

under (i) white light, (ii) short-wavelength UV light (254 nm), and (iii) long-wavelength UV light (360 nm), and after using visualization reagents. These include general reagents such as 1% KMnO4 in water, those used for other drugs of abuse, for example, acidified potassium iodoplatinate, or those used for alkaloid detection, for example, Dragendorff's reagent (see Section 5.5.2 above). As before, it should be remembered that positive and negative controls should be included in every chromatographic investigation.

What are the difficulties associated with interpreting data from TLC? Answer

There are two principle problems. The first is lack of resolution, for example, in the methanol/ammonia system, cocaine/procaine and ecgo-nine methyl ester/lidocaine are not well resolved and yet may occur together in the mixture. A further difficulty is added by the lack of specificity of the visualization reagents. None of the reagents used here are specific to this group of compounds.

By comparison of the Rf data and the colours observed under different light conditions and with different visualization reagents, the compounds thought to be present in the mixture can be determined and standard mixes prepared for GC-MS analysis. In addition, it may be possible to make comparisons between samples.

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