To analyse a sample you need to use its physical, chemical and biochemical properties to distinguish it not just from the other possible analytes, but also from other substances in the matrix of the sample. This sounds intimidating, but by breaking it down into separate steps, it becomes a sort of logic puzzle.
Readily made observations is where this analysis starts. It is the observation of easily ascertainable physical properties, such as the colour, shape and phase of the substance. These can all easily be seen with the naked eye or the aid of a light microscope. These aspects can be surprisingly informative, however care must be taken as powders and pills can easily be mislabelled or tampered with. Some drugs are easy to identify by just their morphology, for example magic mushrooms and marijuana, however others require more attention to detail.
For example, say you find a brown powder in an unlabelled plastic bag. Based on it's appearance you could identify it as being heroin. However to decide it is heroin would be unwise as there are many other substances it could be. Furthermore, inspecting the substance with a light microscope, rather than jumping to conclusions may reveal the powder to be made of multiple morphologically distinct constituents. Some common drug appearances are as follows:
Cocaine White Crystalline Powder
Ecstasy Capsules or Tablets of various sizes, shapes and colours
Heroin White or Brown Powder often mixed with cutting agents
LSD Very small brightly coloured tablets or pieces of blotting paper
Once you have noted the physical characteristics, you can proceed to presumptive tests. Most presumptive tests are colour tests, where a small amount of the substance is treated with reagents which will produce specific colours on reacting with the suspected analyte. Typically these tests will produce a positive test in the presence of 1mg of the analyte. A blank test and positive control should always be carried out in order to increase your certainty. A blank test is where you test using the reagents only, while the positive test involves using a pure sample of the analyte along with the reagents.
Thin-layer Chromatography is one of the most useful presumptive tests. It is a separative technique that provides numerical data about the chemical species in both bulk and trace samples. The process involves dissolving a solid in methanol, spotting it onto a silica gel TLC plate with both positive and negative control samples. The plate is then developed with a 25/6/0.4 by volume mix of methanol, propanone and ammonia. A number of techniques can be employed to visualise the spots, for example spraying the plate with dilute (0.5 M ) NaOH, leaving t dry and then spraying with aqueous Fast Black K. If this is done, amphetamine spots will appear purple, will methamphetamine will appear orange-red. This is especially useful as amphetamine and methamphetamine appear the same colour in most other presumptive tests.
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| Thin-layer Chromatography |
