The analytical toxicologist may be required to detect, identify, and in many cases measure a wide variety of compounds in samples from almost any component of the body or in related materials such as residues in syringes or in soil. Many difficulties may be encountered. The analytes may include gases such as carbon monoxide, drugs, solvents, pesticides, metal salts, and naturally-occurring toxins. Some poisons may be individual chemicals and others complex mixtures. New drugs, pesticides, and other substances continually present novel challenges in analysis and in the interpretation of the results of the analysis. The analyte might be an endogenous compound such as acetone, or an exogenous compound such as a drug and/or metabolite(s) of the drug, whilst the sample matrix may range from urine to bone. Many biological samples contain muscle, connective tissue, and so forth, which may have to be separated or degraded prior to an analysis, as well as a multitude of small and large molecular weight compounds. The concentration of the analyte to be measured can range from g L–1 (parts per thousand) in the case of blood ethanol to μg L–1 (parts per thousand million) in the case of plasma digoxin, and even ng L–1 (parts per million million) in the case of the potent opioid carfentanil. The stability of the analytes in biological samples also varies considerably, ranging from a few minutes for protease sensitive peptides and esters such as aspirin and diamorphine, to several years for some other drugs and pesticides.