Fundamentals of Analytical Toxicology. Robin Whelpton

others, especially in the US, continue to use mass concentration [so-called ‘traditional’ units (mg L^–1, etc. or even mg dL^–1)]. Most published analytical toxicology and pharmacokinetic data are presented in SI mass units per millilitre or per litre of the appropriate fluid [the preferred unit of volume is the litre (L)], or units that are numerically equivalent in the case of aqueous solutions:

      When preparing written statements for a court of law or other purpose outside the normal reporting channels it is advisable to write out the whole unit of measurement in full (milligrams per litre, for example).

      An exception to the above is carboxyhaemoglobin saturation (COHb), which is usually reported as a percentage of the total haemoglobin present in the sample (% COHb) – the SI convention is that fractions of one should be used rather than percentages, but this is generally ignored.

      We have followed the recommendations of the UK NPIS/ACB (National Poisons Information Service/Association for Clinical Biochemistry and Laboratory Medicine) for reporting analytical toxicology results and have used the litre as the unit of volume and SI mass units except for lithium (and sometimes toxic metals/trace elements), methotrexate, and thyroxine (NPIS/ACB. Laboratory analyses for poisoned patients: Joint position paper. Ann Clin Biochem 2002; 39: 328–39). More information on SI is available (Flanagan RJ. SI units – Common sense not dogma is needed. Br J Clin Pharmacol 1995; 39: 589–94).

      Conversion from mass concentration (ρ) to amount concentration (c) (‘molar units’) and vice versa is simple if the molar mass (M) of the compound of interest is known. Thus, if a solution contains 302.4 g L^–1 of a compound of M_r 151.2 g mol^–1

      However, such conversions always carry a risk of error. Especial care is needed in choosing the correct M_r if the drug is supplied as a salt, hydrate, etc. This can cause great discrepancies especially if the contribution of the accompanying anion or cation is high. Most analytical measurements are reported in terms of the free acid or base, and not the salt.

Acknowledgements

      We thank Drs Andrew Taylor and Ian Watson for contributions incorporated from Edition 1 of this book, Dr Lewis Couchman, Dr Natalia Kroupina, Prof John Langley, Ms Karen Morgan, Dr Simon Nelms, Prof Fritz Pragst, Ms Annie Sanger, Mr Peter Streete, and Dr Nicholas Tiscione for their help, and Prof Olaf Drummer and Prof Ilkka Ojanperä for commenting on drafts of the manuscript. We also thank Ms J Cossham (Wiley) for her help and encouragement.

Section A The Basics

1 Analytical Toxicology: Overview

      1.1 Introduction

      Analytical toxicology is concerned with the detection, identification, and measurement of drugs and other foreign compounds (xenobiotics) and their metabolites, and in some cases endogenous compounds, in biological and related specimens. The analytical toxicologist can play a useful role in the diagnosis, management, and indeed the prevention of poisoning, but to do so a basic knowledge of clinical and forensic toxicology is essential. Moreover, the analyst must be able to communicate effectively with clinicians, pathologists, coroners, police, members of the legal profession, and a range of other people. In addition, a good understanding of analytical chemistry, clinical chemistry, pathology, clinical pharmacology, pharmacokinetics, and occupational and environmental health is essential.

      The use of physicochemical techniques in the detection, identification and measurement of drugs and other poisons in body fluids and tissues has its origins in the development of forensic toxicology. Important contributions came later from work to improve food safety and from occupational toxicology. Major advances in analytical methodology followed the introduction and application of refined physicochemical techniques such as spectrophotometry and chromatography in the late 1940s. In particular, ultraviolet (UV) and infra-red (IR) spectrophotometry, together with visible spectrophotometry (colorimetry), and paper and ion-exchange column chromatography were widely used. In the 1960s paper chromatography was largely superseded by thin-layer chromatography (TLC) as this latter technique offered advantages of speed of analysis and lower detection limits.

Improved instrumentation for UV/visible spectrophotometry (UV/Vis), spectrophotofluorimetry, atomic absorption spectrophotometry (AAS), electrochemistry, X-ray diffraction, nuclear magnetic resonance (NMR), and neutron activation analysis led to these techniques being applied to particular problems. However, whilst some more traditional methods still have their uses, gas and liquid chromatography (GC and LC, respectively), often linked to mass spectrometry (MS) in its various modes, on the one hand and immuno- and enzyme-based assays on the other, are the techniques that are used most widely today (Table 1.1).

Table 1.1 Methods for the analysis of drugs and other organic poisons in biological samples

Principle	Technique
Chemical	Colour test
Electrochemical	Biosensors Differential pulse polarography (DPP)
Spectrometric	High resolution mass spectrometry (HRMS)Mass spectrometry (MS)Nuclear magnetic resonance (NMR)Spectrophotofluorimetry (SPFM)Ultraviolet/visible absorption spectrophotometry (UV/Vis)
Kinetic	Flow-injection analysis (FIA)
Chromatographic	Gas chromatography (GC), includes gas–solid chromatography (GSC) and gas–liquid chromatography (GLC)(High performance) liquid chromatography [(HP)LC](High performance) thin-layer chromatography [(HP)TLC]Ion-exchange chromatography (I-EC)Supercritical fluid chromatography [SFC]
Electrophoretic	Capillary (zone) electrophoresis [C(Z)E]Capillary electro-chromatography (CEC)Ion mobility spectrometry (IMS)Micellar electrokinetic (capillary) chromatography (MEKC)
Ligand immunoassay	Cloned enzyme donor immunoassay (CEDIA)Enzyme-linked immunosorbent assay (ELISA)Enzyme-multiplied immunoassay technique (EMIT)Fluorescence polarization immunoassay (FPIA)Latex agglutination test (LAT)Microparticle enzyme immunoassay (MEIA)Radioimmunoassay (RIA)
Enzyme-based assay	Alcohol dehydrogenase – ethanolAryl acylamide amidohydrolase – paracetamol

1.2 Modern analytical toxicology

Recent years have seen many advances in methods for detecting, identifying, and measuring drugs and other poisons in biological fluids with consequent improvement in the scope and reliability of analytical results. The value of certain emergency assays and their contribution to therapeutic intervention has been clarified. Some such assays are performed for clinical purposes, but have overt medico-legal implications and require a high degree of analytical reliability. Examples include ‘brain death’ and child abuse screening (Flanagan, 2019), and instances of suspected iatrogenic poisoning. In addition, demand for the measurement of plasma drug and sometimes metabolite concentrations to aid treatment (therapeutic