A number of immunological assay techniques are commercially available for detecting drugs through antigen-antibody reaction.
One such technique, the enzyme-multiplied immunoassay technique (EMIT), is used by toxicologists because of its speed and high sensitivity for detecting drugs in urine.
In a typical EMIT analysis, antibodies that will bind to a specific drug are added to the subject’s urine.
Other immunoassay procedures are also available, such as radioimmunoassay (RIA) which uses drugs labeled with radioactive tags.
In the EMIT assay, a drug that may be present in a urine specimen will compete with added labeled drug for a limited number of antibody binding sites. The labeled drugs are indicated by an asterisk. Once the competition for antibody sites is completed, the number of remaining unbound labeled drug is proportional to the drug’s concentration in urine.
When an animal, such as a rabbit or mouse, is injected with an antigen its body will produce a series of different antibodies, all of which are designed to attack some particular site on the antigen of interest.
This collection of antibodies is known as polyclonal antibodies.
Alternately, a more uniform and specific collection of antibodies designed to combine with a single antigen site can be manufactured.
Wet blood has more value than dried blood because more tests can be run.
For example, alcohol and drug content can be determined from wet blood only. Blood begins to dry after 3-5 minutes of exposure to air. As it dries, it changes color towards brown and black.
Blood at the crime scene can be in the form of pools, drops, smears, or crusts .
Pools of blood obviously have more evidentiary value in obtaining a wet sample.
Drops of blood tell the height and angle from which the blood fell. The forensic science of blood spatter analysis says that blood which fell perpendicular to the floor from a distance of 0-2 feet would make a circular drop with slightly frayed edges.
Drops from a higher distance would have more pronounced tendrils fraying off the edges (a sunburst pattern).
A blood smear on the wall or floor tells the direction of force of the blow.
The direction of force is always in the direction towards the tail, or smaller end, of the smear, or splatter. In other words, the largest area of the smear is the point of origin (a wave cast-off pattern).
Blood crusts need to be tested with crystalline methods to make sure it's blood.
Refrigerated red blood cells have a shelf life of about 42 days, and the serum containing white blood cells can be refrigerated much longer, almost up to a year. DNA can be extracted from blood (if white blood cells which always contain a nucleus are present), and also from sperm, bone marrow, tooth pulp, and hair roots. Blood, however, is commonly used in DNA testing, as per the following steps:
1. Blood samples are collected from the victim, defendant, and crime scene 2. White blood cells are separated from red blood cells 3. DNA is extracted from the nuclei of white blood cells 4. A restrictive enzyme is used to cut fragments of the DNA strand 5. DNA fragments are put into a bed of gel with electrodes at either end 6. Electric current sorts DNA fragments by length 7. An absorbent blotter soaks up the imprint; it is radioactively treated, and an X-ray photograph (called an autoradiograph) is produced
The crime scene investigator must remember that the location, distribution, and appearance of bloodstains and spatters may be useful for interpreting and reconstructing the events that produced the bleeding.
Surface texture and the stain’s shape, size, and location must be considered when determining the direction, dropping distance, and angle of impact of a bloodstain.
Surface texture is of paramount importance. In general, the harder and less porous the surface, the less spatter results.
The direction of travel of blood striking an object may be discerned because the pointed end of a bloodstain always faces its direction of travel.
The impact angle of blood on a flat surface can be determined by measuring the degree of circular distortion. At right angles the blood drop is circular, as the angle decreases, the stain becomes elongated.
The origin of a blood spatter in a two-dimensional configuration can be established by drawing straight lines through the long axis of several individual bloodstains. The intersection or point of convergence of the lines represents the origin point.
Illustration of stain convergence on a two-dimensional plane. Convergence represents the point from which the stains emanated. Courtesy Judith Bunker, J. L. Bunker & Assoc., Ocoee, FL
Categories of Blood Stains Passive Transfer Projected
Many of the cases sent to a forensic laboratory involve sexual offenses, making it necessary to examine exhibits for the presence of seminal stains.
The best way to locate and at the same time characterize a seminal stain is to perform the acid phosphatase (an enzyme secreted into seminal fluid) color test.
A purple color indicates acid phosphatase enzyme.
A screening test for semen by determining acid phosphatase content; because seminal fluid contains high concentrations of acid phosphatase, while other body fluids and extraneous foreign materials have very low concentrations, high values of acid phosphatase on vaginal aspirate or lavage, or on wash fluid from stains, render positive identification of semen, even if the male is aspermic.