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.
Samples
Urine
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
substan(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.
Blood
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 sample
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. [1]
Oral
fluid
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
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.
Other
organisms
Bacteria,
maggots and other organisms that may have ingested some of the subject matter
may have also ingested any toxic substance within it.
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 the 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.
Gas chromatography
Gas-liquid
chromatography is of particular use in examining volatile organic compounds.
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.
References
External links
See also
