There are a number of presumptive tests for cocaine available from the literature. These are described below.
Although there are several presumptive tests available for cocaine and related compounds, none alone is specific to cocaine itself. A very simple test for cocaine is the addition of the material under investigation to a 2% (wt/vol) solution of cobalt isothiocyanate in water. The presence of cocaine and related alkaloids results in a blue colour being formed. Interestingly, diamorphine and temazepam also result in a blue reaction, but none of the other opiates or benzodiazepines result in this colour reaction. In addition, a number of local anaesthetics also produce a positive colour reaction. It should be remembered that positive and negative controls should always be undertaken at the same time as the sample is being examined.
What is the purpose of the positive and negative controls? Answer
The negative control is undertaken to demonstrate that the equipment is clean and that any colour change observed is due to the actual presence of the drug. The positive control demonstrates that the test is working and provides a reference colour against which the reaction due to the sample may be compared.
This is a modification of the cobalt isothiocyanate test, and involves a 2% (wt/vol) solution of cobalt isothiocyanate in water, diluted with an equal volume of glycerine (Reagent 1), concentrated hydrochloric acid (Reagent 2) and chloroform (Reagent 3). In order to test the sample for the presence of cocaine, a small amount of material is placed in a test-tube and 5 drops of the first reagent are added. A blue colour develops if cocaine is present. One drop of concentrated hydrochloric acid is then added and the blue colour, if it results from the presence of cocaine, should disappear, leaving a pink solution. Several drops of chloroform should then be added. An intense blue colour forms in the chloroform (lower) layer if cocaine is present. Again, positive and negative controls should be performed concurrently with analysis of the test sample.
What advantage does this modified test have over the cobalt isothio-cyanate approach?
This test allows greater discrimination. It is known that a number of products form blue complexes with the cobalt isothiocyanate, although these do not extract into the chloroform as readily, or if at all.
This test relies on the hydrolysis of the benzoate ester of cocaine and benzoyl ecgonine by potassium or sodium hydroxide . A 5% (wt/vol) solution of the hydroxide is prepared in methanol. The sample under test is mixed with a small volume of this reagent in a suitable container and warmed gently, for example, on the palm of the hand. The excess alcohol is allowed to evaporate and the smell arising from the mixture noted (very great care should be exercised when assessing this). A positive and a negative control should also be undertaken. The resulting smell is that of methyl benzoate (familiar in the smell of oil of wintergreen). Piperocaine and benzoylecgonine (both benzoate esters) also result in a positive test.
What precautions should be taken with this test?
If positive tests are obtained in the presumptive tests, the next stage in the analysis is to undertake TLC or to proceed directly to a spectroscopic evaluation. The use of TLC will allow a rapid, inexpensive means of discriminating samples containing cocaine from the false positives and a determination of what should be used in the standard mixtures for GC-MS analyses.
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