Classification of poisons
Drugs and poisons can be classified alpha betically, pharmacologically (antidiabetic, anti-convulsant, etc.) or by chemical structure (barbiturates, phenothiazines, etc.). However, for analytical purposes it is more useful to classify poisons according to the method used for extraction. Five major groups are usually considered:
• gaseous and volatile substances isolated by distillation or, more usually, by sampling the headspace above the sample held in a closed container
• organic non-volatile substances isolated by solvent extraction (drugs and pesticides)
• metallic poisons isolated by ashing, by wet oxidation of the organic matter or by enzymatic hydrolysis of the tissue
• toxic anions isolated by dialysis
• miscellaneous poisons that require immunoassays or special extraction techniques, such as ion-exchange columns, formation of derivatives or ion-pairs, freeze drying and continuous extraction with a polar solvent. Some of these groups have been subdivided because they are too large or because alternative methods of extraction are available. For example, gases are considered separately from volatile substances. Pesticides are considered as a separate category from drugs, although both typically fall into the category ‘organic non-volatile substances isolated by solvent extraction’ and share similar methods of analysis. The seven groups so formed are illustrated in Figure 1 and were introduced earlier in the chapter. Most analyses require several unit operations, namely:
• separation of the poison and its metabolites from the biological material
• concentration
• identification
• confirmation of identity
• quantification.
Not all these steps will be required for all tests and particularly not where rapid screening methods are applied. The most useful methods are those that combine two or more of these unit operations. Thus, colour tests (Chapter 13), which can be applied to the sample directly without the need for any isolation or purification processes, are indispensable in the initial stages of an analysis. Immunoassay techniques (Chapter 14) also eliminate the need for many separate operations and, like colour tests, can provide a tentative identification and approximate quantification of the poison. However, a disadvantage of both these methods is that a negative result eliminates only a few of the possible toxic substances. Consequently, additional colour tests or immunoassays are required before that particular group of poisons can be excluded. This type of sequential testing can be time consuming and judgement must be made depending on the quantity of tissue available for analysis. A broad-spectrum screen, able to detect or eliminate most of the poisons in a group, usually requires a combination of three or more of the available techniques. For the drug and pesticide groups, the only combination potentially able to encompass all the required steps is mass spectrometry (MS) coupled with either gas chromatography (GC) or high-performance liquid chromatography (HPLC). However, a simple, direct solvent-extraction scheme is generally employed before MS analysis to eliminate endogenous substances that might otherwise reduce the efficiency of the system.
Figure 1 The seven major groups of poisons.
