Colour tests
Colour tests (sometimes referred to as chemical spot tests) provide toxicologists and drug analysts with one of the first tools for the presumptive identification of drugs and poisons. These colour tests are most usefully applied to pharmaceuticals and scene residues and, to a lesser extent, to biological fluids such as stomach contents, urine, etc. They are used to place the unknown into a specific class of compounds or to eliminate categories or classes of compounds. These colour tests remain popular for many reasons. They are simple to perform and no extensive training is required. As such, they appeal in situations where laboratory facilities may be very limited, for example in parts of the developing world. They can be performed in the field by police officers or technicians, require minimal reagents, are inexpensive, and give immediate results that can be viewed by the naked eye. In many instances, colour tests can also be used as TLC location reagents, applied by spraying or dipping (see p. 357). Some of the tests can also be carried out by the micro technique described by Clarke and Williams (1955). Colour tests are particularly important in clin ical toxicology (Chapter 8), especially where a patient is being treated in accident and emer gency, and clinical symptoms may indicate some form of poisoning. In these circumstances, the clinician needs to know as quickly as possible what substance or substances are involved in order to initiate treatment. Under these circum stances, colour tests can provide an indication of compound class far more rapidly than immunoassays and chromatographic techniques such as GC and HPLC. Examples of colour tests that are particularly important in this respect include Trinder’s test for salicylates; the Fujiwara test for halogenated hydrocarbons (e.g. dry cleaning reagents) (see Fig. 13.1); furfuraldehyde test for carbamate pesticides; test for cholin esterase inhibitors (organophosphate pesticides); sodium dithionite test for paraquat and diquat pesticides; and various tests for paracetamol including the ferric chloride test, Folin–Ciocalteu, Liebermann’s and Nessler’s tests. Colour tests are widely used by police authorities and customs to detect drugs of abuse. Import ant tests include the Marquis test (for opiates and amfetamines), Mandelin’s test for amfetamines, Scott’s test for cocaine and the Duqenois–Levine test for cannabis. Colour tests have also been developed forgamma-hydroxybutyricacid (GHB) and its precursor gamma-butyrolactone (GBL). Colour tests range from those that rely on reactions with certain functional groups (e.g. Folin–Ciocalteu for phenols), those that are almost specific for a given group (e.g. FPN reagent for phenothiazines), through to those that give diagnostic colours with a wide range of compounds (e.g. the Marquis test, see Fig. 13.1 where virtually the whole colour spectrum can be encountered, depending on the drug present). Some tests give similar colour response for markedly different classes of drugs. The Marquis test exemplifies this with drugs as chemically disparate as butriptyline, diamorphine, guiafenesin and procyclidine all giving a violet colour. It follows that colour tests are only an indication of the presence of a compound or class of compounds and that all tests must be confirmed by more specific methods. This is especially important where test results may ultimately result in custodial sentences.
