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Forensic toxicology is the use of toxicology and other disciplines such as analytical chemistry, pharmacology and clinical chemistry to aid medicolegal investigation of death, poisoning, and drug use. The primary concern for forensic toxicology is not the legal outcome of the toxicological investigation, but rather the technology and techniques for obtaining and interpreting the results. A toxicological analysis can be done to various kinds of samples.
A forensic toxicologist must consider the context of an investigation, in particular any physical symptoms recorded, and any evidence collected at a crime scene that may narrow the search, such as pill bottles, powders, trace residue, and any available chemicals. Provided with this information and samples with which to work, the forensic toxicologist must determine which toxic substances are present, in what concentrations, and the probable effect of those chemicals on the person.
Determining the substance ingested is often complicated by the body's natural processes (see ADME), as it is rare for a chemical to remain in its original form once in the body. For example: heroin is almost immediately metabolised into another substance and further to morphine, making detailed investigation into factors such as injection marks and chemical purity necessary to confirm diagnosis. The substance may also have been diluted by its dispersal through the body; while a pill or other regulated dose of a drug may have grams or milligrams of the active constituent, an individual sample under investigation may only contain micrograms or nanograms.
A urine sample is quick and easy for a live subject, and is common among drug testing for employees and athletes. Urine samples do not necessarily reflect the toxic substance(s) the subject was influenced by at the time of the sample collection. An example of this is THC from cannabinoid (for example, marijuana) use, which in heavy users can be detected in urine for up to 14 days following use. Note also that it can take as long as 8 hours until a given substance can be detected. Specific to workplace drug testing, urine collection MUST be directly observed due to the prevalence of substance abusers "beating the test" via sample substitution or adulteration.
A blood sample of approximately 10 cm³ is usually sufficient to screen and confirm most common toxic substances. A blood sample provides the toxicologist with a profile of the substance that the subject was influenced by at the time of collection; for this reason, it is the sample of choice for measuring blood alcohol content in drunk driving cases.
Hair is capable of recording medium to long-term or high dosage substance abuse. Chemicals in the bloodstream may be transferred to the growing hair and stored in the follicle, providing a rough timeline of drug intake events. Head hair grows at rate of approximately 1 to 1.5 cm a month, and so cross sections from different sections of the follicle can give estimates as to when a substance was ingested. Testing for drugs in hair is not standard throughout the population. The darker and coarser the hair the more drug that will be found in the hair. If two people consumed the same amount of drugs, the person with the darker and coarser hair will have more drug in their hair than the lighter haired person when tested. This raises issues of possible racial bias in substance tests with hair samples. 
Oral fluid is the proper term, however Saliva is used commonly. Saliva is a component of oral fluid. Oral fluid is composed of many components and concentrations of drugs typically parallel to those found in blood. Sometimes referred to as ultra filtrate of blood, it is thought that drugs pass into oral fluid predominantly through a process known as passive diffusion. Drugs and pharmaceuticals that are highly protein bound in blood will have a lower concentration in oral fluid. The use of oral fluid is gaining importance in forensic toxicology for showing recent drug use, e.g in clinical settings or investigation of driving under influence of substances.
Other bodily fluids and organs may provide samples, particularly samples collected during an autopsy. A common autopsy sample is the gastric contents of the deceased, which can be useful for detecting undigested pills or liquids that were ingested prior to death. In highly decomposed bodies, traditional samples may no longer be available. The vitreous humour from the eye may be used, as the fibrous layer of the eyeball and the eye socket of the skull protects the sample from trauma and adulteration. Other common organs used for toxicology are the brain, liver, spleen and stomach contents
The inspection of the contents of the stomach must be part of every postmortem examination because it may provide qualitative information concerning the nature of the last meal and the presence of abnormal constituents. Using it as a guide to the time of death, however, is theoretically unsound and presents many practical difficulties, although it may have limited applicability in some exceptional instances. Generally, using stomach contents as a guide to time of death involves an unacceptable degree of imprecision and is thus liable to mislead the investigator and the court. Characteristic cell types from food plants can be used to identify a victim's last meal; knowledge about which can be useful in determining the victim's whereabouts or actions prior to death (Bock and Norris, 1997). Some of these cell types include (Dickison, 2000):
- sclereids (pears)
- starch grains (potatoes and other tubers)
- raphide crystals (pineapple)
- druse crystals (citrus, beets, spinach)
- silica bodies (cereal grasses and bamboos)
In a case where a young woman had been stabbed to death, witnesses reported that she had eaten her last meal at a particular fast food restaurant. However, her stomach contents did not match the limited menu of the restaurant, leading investigators to conclude that she had eaten at some point after being seen in the restaurant. The investigation led to the apprehension of a man whom the victim knew, and with whom she had shared her actual final meal (Dickison, 2000). Time since death can be approximated by the state of digestion of the stomach contents. It normally takes at least a couple of hours for food to pass from the stomach to the small intestine; a meal still largely in the stomach implies death shortly after eating, while an empty or nearly-empty stomach suggests a longer time period between eating and death (Batten, 1995). However, there are numerous mitigating factors to take into account: the extent to which the food had been chewed, the amount of fat and protein present, physical activity undertaken by the victim prior to death, mood of the victim, physiological variation from person to person. All these factors affect the rate at which food passes through the digestive tract. Pathologists are generally hesitant to base a precise time of death on the evidence of stomach contents alone.
Detection and Classification
Detection of drugs and pharmaceuticals as in biological samples are usually done by an initial screening and then a further confirmation of right compound and quantification of that compound. The screening and confirmation is usually done with different analytical methods. Every analytical method used in forensic toxicology should be carefully tested with preforming a validation of the method to ensure correct and indisputable results at all time. A testing laboratory involved in forensic toxicology should adhere to some quality programme to ensure the best possible results and safety of any individual.
The choice of method for testing is highly dependent on what kind of substance one can suspect to find and what the material is that the testing is performed on. Biological samples have are more complex as factors as matrix effect, metabolism and conjugation of compounds has to be considered. Substances as such (powders, pills and liquids) have a much higher concentration and toxicology is aimed at finding what compounds constitutes the sample and at what concentrations.
Detection of Metals
The compounds suspected of containing a metal is traditionally separated by the destruction of the organic matrix by chemical or thermal oxidation. This leaves the metal to be identified and quantified in the inorganic residue, and it can be detected using such methods as the Reinsch test, emission spectroscopy or X-ray diffraction. Unfortunately, while this identifies the metals present it removes the original compound, and so hinders efforts to determine what may have been ingested. The toxic effects of various metallic compounds can vary considerably.
Nonvolatile organic substances
Drugs, both prescribed and illegal, pesticides, natural products, pollutants and industrial compounds are some of the most common compounds encountered. Screening methods include thin-layer chromatography, gas-liquid chromatography and immunoassay. For complete legal identification, a second confirmatory test is usually also required. The trend today is to use liquid chromatography tandem mass spectrometry, predeced with sample workup as liqiud-liquid extraction or solid phase extraction. Older methods include: spot test (see Pill testing), typically the Marquis Reagent, Mecke Reagent, and Froehde's Reagent for opiates, Marquis Reagent and Simon's reagent for amphetamine, methamphetamine and other analogs, like MDMA, the Scott's test for cocaine, and the modified Duquenois reagent for marijuana and other cannabinoids. For compounds that don't have a common spot test, like benzodiazepines, another test may be used, typically mass spectroscopy, or spectrophotometry.
- The Further Mismeasure: The Curious Use of Racial Categorizations in the Interpretation of Hair Analyses Tom Mieczkowski, Ph.D., The University of South Florida