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Practical points , Performing colour tests

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  340-341

2026-11-01

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Practical points

Performing colour tests

Tests are usually carried out either in clear glass test-tubes or on white glazed porcelain tiles (spotting tiles), which give a uniform back ground against which colours can be assessed. For drugs, most colour tests are designed to work on about 1 mg, either as the solid form or as a dried extract of this amount. A sample known not to contain the compound of interest should be tested at the same time as the test sample. This enables a comparison of the colours produced by the sample and by the reagent blank. Ideally, the blank sample should have the same matrix as the test sample (e.g. for urine tests use analyte-free urine), since this takes account of the effects of extraneous materials. Otherwise, water is usually adequate.

Before making a final decision on the result of a test, the reaction of the unknown should be compared with that of a reference substance tested under exactly the same conditions. The reference substance should, ideally, be spiked a blank sample of the same type as the sample; for example, if the sample to be tested is a urine sample then the reference sample should be added to a sample of urine known not to contain the compound of interest.