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Database searches

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p360-361

2026-11-03

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Database searches

Database searches are used in systematic toxicological analysis to identify suspect substances in biological fluids and post-mortem tissue samples. Extracted samples are separated in one or more standard TLC systems. The corrected RF values, often combined with the results of sequential post-chromatographic colour reactions, are then entered into the search program. The input data are automatically compared against a database of reference drugs, common metabolites, natural contaminants, etc., for identification. A number of chemometric procedures can be used for data analysis, but the most common approach is based on the mean list method.

It is assumed that the errors in individual measurements are random and can be described by a standard deviation. The precision of the separation system can then be described as the mean of the standard deviation of all substances separated in the system, called the system mean standard deviation. This allows a confidence interval or window to be assigned to the system as some multiple (typically three) of the system mean standard deviation. Each RF value in the system database that appears in the window could be confused with the original substance. The number of substances identified as above is called the list length. Repeating the process for all RF values in that system and averaging the individual list lengths provides the mean list length for that system. The mean list length indicates, on average, the number of substances in the database that qualify as candidates for the identification of a single drug. The shorter the mean list length, the greater the information potential of the system. Combining the results from additional retention parameters in complementary standard separation systems, colour   reactions, spectroscopic data, etc., minimises the mean list length to the point that only a small number of candidate compounds for the unknown are indicated. More specific tests can then be used to identify the unknown from among the small number of indicated possibilities. For systematic drug identification in forensic toxicology, commonly two or more complementary TLC systems combined with the results from several in-situ sequential colour reactions are used. For drugs of toxicological interest, a mean list length from 2 to 10 is possible.