Principles of forensic toxicology

The objective of the forensic toxicologist is to attempt to provide answers to questions that may arise during criminal investigations or in subsequent court proceedings. The traditional question that must be answered is ‘Has this person been poisoned?', together with the supplementary queries that follow if the result is affirmative, such as ‘What is the identity of the poison?', ‘How was it administered?', ‘What are its effects?' and ‘Was it a dangerous or lethal amount?'. Note that it is not the role of the forensic toxicologist to determine who administered the poison. That is typically the role of the police or the courts. Nor is it the role of the forensic toxicologist to confirm the cause of death. That is the role of the pathologist.

Chemical analyses are used to detect the presence of the poison, measure its concentration and relate this to its known toxicity or effects on the organism.

Generally, the forensic toxicologist is involved in the following situations:

Toxicological investigations

•  to establish poisoning as the cause of death

•  to investigate unlawful poisoning by a third party (e. g. in suspicious deaths; in cases of non-accidental child poisoning; in cases of drug-facilitated sexual assault (DFSA))

•  to establish the presence of substances that may affect a person’s behaviour or ability to make rational/reasoned judgement, e. g. DFSA and driving under the influence of drugs or alcohol.

Human and animal performance testing

•  to investigate incidents of driving under the influence of drugs and alcohol

•  to detect the use of performance-enhancing drugs in human and animal sports.

Forensic drug testing

•  to detect non-compliance with policies governing the use of drugs in the workplace

•  to provide evidence in cases where parents may be denied access to young children on the basis of a history of drug abuse and where continued abuse may endanger the child.

If the poison for which the forensic toxicologist has to test is not specified by name, the request to ‘‘test for poisons’’ is a major problem. Given that all substances can be poisons, depending on the dose, in theory this means that an exceedingly broad range of substances with very different chemical natures may need to be tested for. Thankfully for the forensic toxicologist, in practice the number of substances encountered as poisons is considerably less than the total number of compounds that exist in the world. However, this still leaves a relatively large number of substances as poisons likely to be encountered, and there is always the possibility of an unexpected substance being involved in any particular case. By their very nature, most chemical methods of analysis employ some form of detection that relies on a specific interaction with some aspect of the physicochemical properties of the compound under test. There is no universal method of analysis for all substances, particularly where the requirement is to be able to detect, identify and quantify the substance, typically at low concentrations and in complex matrices such as organ tissues and blood. At least seven different analytical schemes are required to exclude even the most commonly encountered poisons (Fig. 1.1).

Forensic toxicology demands an overall analytical system designed to exclude or indicate the presence of any poison in each of the chemical groups shown in Fig. 1.1. Most of the numerous screening procedures reported in the literature are too limited to permit a confident negative report.

Apart from these analytical problems, the legal aspect of the work demands a scrupulous attention to detail. Failure to make full descriptive notes on the items received, a simple error in the date the analysis was performed or neglecting to check reagent purity can be presented as evidence of careless work by an astute lawyer. The lawyer may, with justification, explore the extent of the toxicologist’s experience and knowledge, demand a detailed account of the analytical methods and challenge the integrity of any opinion. The crucial evidence of identification and quantification of the poison may be faultless and the conclusions correct, but if the court’s confidence in the forensic toxicologist as an unbiased scientific expert is destroyed, the case may be lost. A secure chain of custody of all the exhibits submitted also has to be proved. Aspects of sampling, avoidance

Figure 1.1 The seven major groups of poisons.


Of contamination, appropriate packaging of samples, chain of custody and recording sample information at all stages of the process are emphasised throughout this book. As will be seen in the various chapters, the types of body fluids, tissues and other samples of importance in forensic toxicology can vary depending on the analytes under consideration, the aim of the analysis and individual case circumstances. As will also be seen in the chapters that follow, the nature of the sample can also impact on the methods used for analysis and the interpretation of results.

Orfila was well acquainted with the aspects of forensic toxicology outlined above, and the guiding principles he established nearly 200 years ago are still applicable. These may be summarised as follows:

•  all chemists who undertake this work must have toxicological experience

•  the analyst must be given a complete case history that contains all the information available

•  all the evidential material, suitably labelled and sealed in clean containers, must be submitted and examined

•  all the known identification tests should be applied and adequate notes made at the time

•  all the necessary reagents used for these tests should be pure, and blank tests should be performed to establish this fact

•  all tests should be repeated, and compared with control samples to which the indicated poison has been added.

Strict adherence to these principles makes forensic toxicology one of the slowest and most expensive forms of analysis. However, this must be accepted not only to ensure justice for the poisoned victim and for the accused, but also to protect the integrity and reputation of the analyst and the laboratory he or she represents.

A comparison of the principles listed above with the modern requirements of quality control and assurance may be made by reference to Chapter 22.

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