Forensic toxicology

A urine sample is urine that has come from the bladder and can be provided or taken post-mortem. Urine is less likely to be infected with viruses such as HIV or Hepatitis B than blood samples.[1] Many drugs have a higher concentration and can remain for much longer in urine than blood. Collection of urine samples can be taken in a noninvasive way which does not require professionals for collection. Urine is used for qualitative analysis as it cannot give any indication of impairment due to the fact that drug presence in urine only indicates prior exposure.[2]

A blood sample of approximately 10 ml (0.35 imp fl oz; 0.34 US fl oz) 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.[3]

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, and spleen.

The inspection of the contents of the stomach must be part of every postmortem examination if possible 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.

Bacteria, maggots and other organisms that may have ingested some subject matter may have also ingested any toxic substance within it.

Gas-liquid chromatography is of particular use in examining volatile organic compounds.

The compounds suspected of containing a metal are traditionally analyzed 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.

Drugs, both prescribed and illicit, pesticides, natural products, pollutants and industrial compounds are some of the most common nonvolatile 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, preceded with sample workup as liquid-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 spectrometry, or spectrophotometry.