Committee on Systematic Toxicological Analysis & Guidelines

Supplement to TIAFT Laboratory Guidelines for Toxicological Analysis (1993)

3.3.1. QUALITATIVE ANALYSIS
In qualitative analysis, the first aim is to detect all substances of toxicological relevance. Then, the next aim is to unambiguously identify the substances found in the detection stage.
Since the outcomes of these analyses can have substantial legal and/or social consequences, all approaches and procedures should be scientifically undisputable and legally defensible.
In all cases, the relevant properties of the analytical procedures used (e.g. selectivity, sensitivity, robustness, reproducibility, etc.) have to be adequately ensured and considered and documented in the analytical report.

3.3.1.1. Detection
Depending on the reason for analysis, different analytical strategies may be pursued. If the toxicological analysis is intended to detect a single substance or a group of substances, e.g. in workplace testing, specifically designed analytical procedures can be applied (directed analysis).
If the analysis is required to detect or exclude a wide range of (potentially toxic) substances without specific direction (undirected analysis or 'general unknown'), the comprehensive strategy of Systematic Toxicological Analysis (STA) is required. Its aim is to detect all substances of (actual) toxicological relevance. To this end, a number of analytical procedures should be run in parallel or in sequence, representing a multitude of analytical principles.
Prior to the systematic analytical approach, thorough consideration should enable to reasonably confine the scope of the detection stage to compounds relevant to the actual problem. Experience with similar tasks may also be considered when possible.
Therefore criteria to define the group of relevant compounds for a given area of interest are desirable. The various areas of interest (such as forensic and clinical toxicology, workplace testing, drugs of abuse testing, drugs and driving, doping analysis, environmental analysis, residue analysis) represent analytical challenges of their own, that should be taken into consideration when embarking on the systematic analytical approach.

3.3.1.2. Identification
When the detection procedures indicate the possible presence of one or more toxicologically relevant compounds, the latter have to be unambiguously identified.
This can be done by comparing the signals (results) of the various tests applied in the detection and/or identification process from the unknown sample with data from authentic reference standards analysed under the same actual conditions and/or (Iess reliably due to additional variables) with data on reference compounds stored in appropriate data bases on relevant substances.
The ultimate aim of the identification process is that for a given unknown substance only one suitable candidate is found (because all measured signals of the unknown and the reference candidate match adequately) and that all other relevant substances can be excluded (because one or more signals do not match). Experience has learned that a single analytical method, even when it is based on a highly informative principle, is not always sufficient to reach unambiguous identification. The large number of substances, sometimes widely different, sometimes with very close structural resemblance, make it hardly possible to really fulfil the exclusion criterion.
Therefore, proper identification requires as a rule two, if not more analytical methods (their number depending on their information gain), to exclude all possible candidates except one.

Ideally, the analytical signals for the unknown(s) should be compared with those of authentic reference standards run in parallel with the case sample. This is more secure than comparisons with literature data or such stored in data bases, because the data may be influenced by the actual analytical conditions. However, keeping up an adequate collection of reference substances is easiest for certain areas in which the number of relevant substances is small (e.g. workplace testing). When the number of compounds of interest is very large (e.g. in forensic and clinical toxicology, control of drugs and driving), it can become very difficult for single laboratories to set up and maintain adequate supplies of all reference substances (and of their metabolites). In these instances, the use of reliable, interlaboratory data bases might be the only feasible solution. The data collection must then contain not only the toxicologically relevant substances, but also metabolites, related substances (including isomers, sometimes enantiomers), endogenous substances, and the like. In addition to the data themselves, the interlaboratory reproducibilities of the analytical techniques must be available and have to be included in the evaluation of the compared results and in the conclusions.
In recent years, many analytical toxicologists have come to use the term 'confirmation' of a first analytical 'screening' step as a substitute for 'identification'. When this relates to cases in which the results from the detection or screening phase lead to the presumption that a certain substance is present, and in the confirmatory stage one or more signals from the unknown are matching those of the presumed candidate, the presumption is considered 'confirmed'. However it should be realized, that such an approach does not necessarily provide unambiguous identification: it will always depend on the existence of similar analytical signal patterns of other compounds and on the actually provided information capacity, whether another substance cannot be distinguished from the presumed one. Thus, it has always carefully considered whether the exclusion criterion mentioned above is fulfilled.
In order to enable others to estimate the degree of certainty of the result of a qualitative analysis, the methods applied to draw conclusions should be stated in the report (see also chapters 4. and 5.), eventually together with their appropriate properties. Special circumstances, such as limited specimen supply, unavailable or improperly functioning detection and/or identification techniques, unexpected interferences, etc., must be mentioned if occurring in the report as well.

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