Forensic Science

V

Techniques of Forensic Science

Forensic science uses sophisticated laboratory techniques to detect the presence of substances in the victim, in the suspected criminal, or at the crime scene. For example, in determining whether alcohol was involved in a crime, the amount of alcohol in the blood can be measured in two ways. One is to measure the amount of alcohol exhaled in the breath of an individual, which reveals the concentration of alcohol in the person's blood. Recent advances in technology have produced alcohol breath-testing instruments so accurate that their results are evidential (capable of providing evidence in court). Blood-alcohol level can also be determined by actual blood tests, usually through gas chromatography. In this method, the blood sample is vaporized by high temperature, and the gas is then sent through a column that separates the various chemical compounds present in the blood. Gas chromatography permits the detection not only of alcohol but also of other drugs, such as barbiturates, cocaine, amphetamines, and heroin.

When a body is discovered in a lake, stream, river, or ocean and the lungs are found to be filled with water, the medical examiner must determine if the drowning occurred where the body was found or elsewhere. A standard microscope that can magnify objects to 1500 times their actual size is used to look for the presence or absence of diatoms, single-celled algae that are found in all natural bodies of water. The absence of diatoms raises the possibility that the drowning took place in a sink or bathtub, not where the body was found, since diatoms are filtered from household water during treatment.

A scanning electron microscope that can magnify objects 100,000 times is used to detect the minute gunpowder particles present on the hand of a person who has recently fired a gun. These particles can also be chemically analyzed to identify their origin from a particular type of bullet.

Forensic examination of substances found at a crime scene can often establish the presence of the suspect at the scene. One of the oldest techniques of forensic science is dusting the scene of a crime for fingerprints, impressions of the fingertips left on surfaces touched bare-handed. In one method of obtaining a fingerprint, a technician spreads fine powder over a surface with a brush or magnetic wand. The powder sticks to proteins secreted by the sweat glands on the skin ridges of the fingertips. When the excess powder is removed, an outline of the contours of the ridges remains. In other methods, the print may be chemically treated to reveal the contours. Because no two fingerprints are the same, fingerprinting provides a positive means of identification. Computer technology now allows law-enforcement officers to record fingerprints digitally and to transmit and receive fingerprint information electronically (see Electronics) for rapid identification.

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Other evidence present at a crime scene may include blood, hair, skin, or semen. Recent developments in technology now allow scientists to examine the deoxyribonucleic acid (DNA), or genetic material, of these substances to establish whether they belong to the victim or to a suspected assailant. By means of a high-technology method known as the polymerase chain reaction (PCR), a laboratory can rapidly clone, or multiply, the DNA from a tiny sample of any of these substances. This process produces enough DNA to compare with a sample of DNA taken from a suspected criminal. The use of DNA for such identification purposes is commonly referred to as DNA fingerprinting.

Human bite marks can also serve as circumstantial evidence. Such bites may be found upon the body of a homicide victim or within pieces of food or other objects, such as chewing gum, found at the crime scene. A forensic scientist can fill the impressions caused by these bites with liquid plastic. Upon hardening, the cast formed is an extremely accurate replica of the assailant’s teeth, which can be compared with a cast made from the teeth of the suspect.

VI

Forensic Science and the Legal System

District attorneys routinely call upon medical examiners to give expert testimony in a trial, especially concerning the findings of an autopsy and the results of laboratory tests. Expert testimony is that given by a specialist who has been recognized by the court as having expert knowledge about evidence in the case. Such testimony is governed by different rules than the testimony of ordinary witnesses in a trial. Ordinary testimony is restricted to statements concerning what the witness actually saw or heard. An ordinary witness is prohibited from stating opinions about the case and from quoting statements made by other people. In contrast, an expert witness is allowed to express an opinion about the validity of the evidence in a case and may quote the statements of other experts in support of an opinion.

VII

History

The combination of a medical and legal approach to dealing with crimes used in the United States today had its origin in England in the 12th century, when King Richard I established the Office of the Coroner. Although the coroner's main duty was to keep a record of all criminal matters in the county, he was also responsible for investigating all deaths thought to be the result of suicide or homicide.

With time, the need for a more scientific investigation of unnatural deaths became apparent, and coroners began calling on physicians for help. Over the centuries, it became clear that medical schools needed to prepare doctors for this responsibility. As a result, in 1807 the University of Edinburgh in Scotland established a Department of Legal Medicine.

The early American colonists brought the coroner system with them, including the appointment of coroners by a representative of the crown. After the formation of the United States, elected officials appointed coroners. As medical involvement in the investigations of violent and unexplained deaths increased, communities began to require that the individual in charge of such investigations have a specific academic background. In 1877 Massachusetts adopted a statewide system requiring that the coroner's office be replaced by an Office of the Medical Examiner, to be headed by a physician. A number of other states also adopted this requirement. In 1915 New York City established a comprehensive program in which the medical examiner was specifically authorized to investigate all deaths resulting from criminal violence, accidents, or suicides, and those that occurred suddenly to people who appeared to be in good health.

Forensic science as practiced today is a high-technology field using electron microscopes, lasers, ultraviolet and infrared light, advanced analytical chemical techniques, and computerized databanks to analyze and research evidence.

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