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Screening tests

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p60-61

2026-09-19

14

Screening tests

Colour and/or spot tests give a valuable indication of the content of any particular item tested, but it is stressed that positive results to colour tests are only presumptive indications of the possible presence of the drug. They must be followed up by other tests which offer confirmation of the presence of a drug. Colour tests have the advantage that they are rapid and inexpensive and unskilled operators can use them as field tests, with the obvious need for follow-up analyses in the laboratory. For example, they could be used at customs to give an indication as to whether a suspicious substance is likely to be a controlled drug or not. One of the most important and widely used colour tests is the Marquis reagent test. Opiates, including heroin, give a purple colour in response to the test. Amfetamine and methamfetamine give an orange-red colour. Other useful tests are the Van Urk test for LSD and the cobalt thiocyanate test for cocaine. Another important screen is thin-layer chromatography. It has many advantages as an analytical and/or screening tool. It is quick, easy to use, has a low cost, is relatively sensitive and can give a good degree of discrimination. A wide variety of solvent systems are available (Moffat et al. 2004, and Table 3.4 below) but the following solvent systems are suitable for many drugs:

• methanol–strong ammonia solution (100:1.5 v/v)

• cyclohexane–toluene–diethylamine (75:15:10 v/v). Visualisation of many of the drugs may be achieved by a variety of methods ; however, spraying with acidified potassium iodoplatinate reagent is suitable for many drugs.

A variety of immunoassay-based test kits are available for screening for drugs. These have the advantage of avoiding the use of hazardous chemicals, are simple to use and offer a quick test result with good drug specificity, but can be expensive on a per test basis compared with some of the simpler colour tests.