Evidence And Tools Used In Forensic Science
Long before DNA was recognized as the "ink" in the blueprints of life, blood samples were collected and analyzed in crime labs. Most tests used to tentatively identify a material as blood are based on the fact that peroxidase, an enzyme found in blood, acts as a catalyst for the reagent added to the blood and forms a characteristic color. For example, when benzidine is added to a solution made from dried blood and water, the solution turns blue. If phenolphthalein is the reagent, the solution turns pink. More specific tests are then applied to determine if the blood is human.
The evidence available through blood typing is not as convincing as genetic fingerprinting, but it can readily prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups–A, B, AB, or O. These blood groups are based on genetically determined antigens (A and/or B) that may be attached to the red blood cells. These antigens are either present or absent in blood. By adding specific antibodies (anti-A or anti-B) the presence or absence of the A and B antigens can be determined. If the blood cells carry the A antigen, they will clump together in the presence of the anti-A antibody. Similarly, red blood cells carrying the B antigen will clump when the anti-B antibody is added. Type A blood contains the A antigen; type B blood carries the B antigen; type AB blood carries both antigens; and type O blood, the most common, carries neither antigen. To determine the blood type of a blood sample, antibodies of each type are added to separate samples of the blood. The results,
|Antibody added to sample||Results of test indicates blood type to be|
which are summarized in the table, indicate the blood type of the sample.
If a person accused of a homicide has type AB blood and it matches the type found at the crime scene of a victim, the evidence for guilt is more convincing than if a match was found for type O blood. The reason is that only 4% of the population has type AB blood. The percentages vary somewhat with race. Among Caucasians, 45% have type O, 40% have type A, and 11% have type B. African Americans are more likely to be type O or B and less likely to have type A blood.
When blood dries, the red blood cells split open. The open cells make identification of blood type trickier because the clumping of cell fragments rather than whole red blood cells is more difficult to see. Since the antigens of many blood-group types are unstable when dried, the FBI routinely tests for only the ABO, Rhesus (Rh), and Lewis (Le) blood-group antigens. Were these blood groups the only ones that could be identified from blood evidence, the tests would not be very useful except for proving the innocence of a suspect whose blood type does not match the blood found at a crime scene. Fortunately, forensic scientists are able to identify many blood proteins and enzymes in dried blood samples. These substances are also genetic markers, and identifying a number of them, particularly if they are rare, can be statistically significant in establishing the probability of a suspect's guilt. For example, if a suspect's ABO blood type matches the type O blood found at the crime scene, the evidence is not very convincing because 45% of the population has type O blood. However, if there is a certain match of two blood proteins (and no mismatches) known to be inherited on different chromosomes that appear respectively in 10% and 6% of the population, then the evidence is more convincing. It suggests that only 0.45 X 0.10 X 0.06 = 0.0027 or 0.27% of the population could be guilty. If the accused person happens to have several rarely found blood factors, then the evidence can be even more convincing.
Since handguns are used in half the homicides committed in the United States and more than 60% of all homicides are caused by guns, it is not surprising that ballistic analysis has been an important part of the work performed in crime labs. Comparison microscopes, which make it possible to simultaneously view and compare two bullets, are an important tool for forensic scientists. When a bullet is fired, it moves along a spiral groove in the gun barrel. It is this groove that makes the bullet spin so that it will follow a straight path much like that of a spiraling football. The striations or markings on the bullet made by the groove and the marks left by the firing pin are unique and can be used to identify the gun used to fire any bullets found at the scene of a homicide. Similarly, tool marks, which are often left by burglars who pry open doors or windows, can serve as useful evidence if comparisons can be made with tools associated with a person accused of the crime. Particularly incriminating are jigsaw matches-pieces of a tool left behind that can be shown to match pieces missing from a tool in the possession of the accused.
In the event that bullets have been shattered making microscopic comparisons impossible, the fragments may be analyzed by using neutron activation analysis. Such analysis involves bombarding the sample with neutrons to make the atoms radioactive. The gamma rays emitted by the sample are then scanned and compared with known samples to determine the concentration of different metals in the bullet-lead. The technique can be used to compare the evidence or sample with bullet-lead associated with the accused.
Autopsies can often establish the cause and approximate time of death. Cuts, scrapes, punctures, and rope marks may help to establish the cause of death. A drowning victim will have soggy lungs, water in the stomach, and blood diluted with water in the left side of the heart. A person who was not breathing when he or she entered the water will have undiluted blood in the heart. Bodies examined shortly after the time of death may have stiff jaws and limbs. Such stiffness, or rigor mortis, is evident about ten hours after death, but disappears after about a day when the tissues begin to decay at normal temperatures. Each case is different, of course, and a skillful coroner can often discover evidence that the killer never suspected he or she had left behind.
Modern crime labs are equipped with various expensive analytical devices usually associated with research conducted by chemists and physicists. Scanning electron microscopes are used to magnify surfaces by as much as a factor of 200,000. Because the material being scanned emits x rays as well as secondary electrons in response to the electrons used in the scanning process, the microscope can be used together with an x ray micro analyzer to identify elements in the surface being scanned. The technique has been particularly successful in detecting the presence of residues left when a gun is fired.
The mass spectrometer and the gas chromatograph have been particularly effective in separating the components in illegal drugs, identifying them, and providing the data needed to track down their source and origin. Thin layer chromatography (TLC) has proved useful in identifying colored fibers. Although many fibers may appear identical under the microscope, they can often be distinguished by separating the component dyes used in coloring the fabric. Fusion microscopy—using changes in birefringence with temperature—has also proved useful in identifying and comparing synthetic fibers found at crime scenes. In addition to using such physical properties as density, dispersion, and refractive index to match and identify glass samples, the plasma emission spectroscope has proven helpful in analyzing the component elements in glass as well as distinguishing among various types of glass found in windows, bottles, and windshields.
The role and impact of forensic sciences came to the forefront of public attention during the highly publicized and televised O.J. Simpson murder trial. Lawyers for both sides offered a wide variety of forensic evidence—and disputed the validity of opposing forensic evidence—before the controversial acquittal of Simpson.
In 2002, forensic science specialists played an integral role in the tracking and eventual identification of evidence (e.g. similarities in ballistics, psychological profiles, etc.) that allowed investigators link a nationwide a string of crimes that culminated in several snipers attacks in the Washington-Virginia-Maryland area.
A new and emerging area of forensic science involves the reconstruction of computer data. High speed and large memory capacity computers also allow for what forensic investigators term as "virtual criminality," the ability of computer animation to recreate crime scenes.
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Sachs, Jessica S. Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death. An Exploration of the Haunting Science of Forensic Ecology. Perseus Publishing, 2001.
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