Grayce P. Storey
Physical evidence includes any and all objects that can establish that a crime has been committed or that can provide a link between a crime and its victim.
Listed are common types of physical evidence: blood, semen, and saliva (these fluids are subjected to serological biochemical analysis for determination of identity and possible origin), documents, drugs, explosives, fibers, fingerprints, paint, glass, hair, impressions, powder, residue, took marks, soil, and minerals, firearms, ammunition, serial numbers, petroleum products, organs and physiological fluids, wood, and other vegetative matter.
The search for physical evidence at a crime scene must be thorough and systematic. When and what to search for will be determined by the particular circumstances of the crime.
Collecting, Preserving Physical Evidence
The forensic scientist seldom personally supervises the collection of evidence at the crime scene. It is left to the field investigator who must be well aware of the requirements that will arise out of the future utilization of such evidence in a legal proceeding.
Physical evidence should be handled in a manner as to prevent any change from taking place between the time it is received in the crime laboratory.
Some instruments may be used at the crime scene to retrieve smallitems and place them in appropriate containers, such as unbreakable plastic pill bottles with pressure lids for collecting hair fibers and glass. Also, envelopes of various shapes and sizes make versatile evidence containers. Bloodstain materials are recommended to be wrapped in paper or paper bags.
The discovery of each item must be accompanied by a detailed description and location of each item, also by whom and when it was discovered. The description must also include how the evidence was packaged, marked and transported to the laboratory.
Methods Used in the Laboratory
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1.) Quick test in celluloid films
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The hair is placed on a thick transparent celluloid film and a drop of acetone is added to flow along the hair, it dries, leaving a clear impression on the surface of the celluloid. After the hair is removed, it can be studied in transmitted or incident light.
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2.) Test in the Gelatin Layer of Photographic Plates of Films
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Unexposed negative material is fixed and washed. As the gelatin dries a hair is embedded by pressing gently with a slide. After a few minutes the slide and the hair can be removed.
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3.) Semi-embedding of Hairs
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The lower half of the fiber placed on a slide is embedded in a mounting medium with identical refractive index to the fiber. The scale can be studied directly in the freeze zone. This procedure requires more skill and yields excellent results.
The goal of forensic serology is to individualize blood stains by identifying genetic markers whose population frequencies have been established. This includes a series of analyses utilizing several components of the blood from which a profile of genetic markers can be established. The markers in the blood are inherited independent of one another and their frequencies within a given population are known. The profile obtained will permit a mathematical probability or uniqueness to be calculated.
Blood is a multi-component system made up of red and white blood cells, platelets and plasma. They each contain a vast array of biochemical constituents. The forensic serologist has chosen three classes of the blood constituents for their genetic information and use in individualization endeavor. These constituent classes are: 1) the blood grouping and typing antigens, 2) the polymorphic enzymes and 3) the polymorphic protein.
Since Landsteiner discovered the ABO system of typing antigens around the 1900’s, over 246 published antigens have been found. However only three of these antigens, the ABO, MN, and Rh, have received crime laboratory acceptance.
The ABO, MN, and Rh system have useful population frequencies. Four groups belonging to the ABO system occur approximately the following percentage of frequency: O 44%, A 44%, B 8%, and AB 3%. The MN has three groupings with the following frequencies: M 30%, MN 50%, and N 20%. The Rh system has basically a five component antigen system giving rise to eight gene complexes or agglutinogenes. Phenotyping use Rh’s is quite useful in obtaining individualizing information.
A procedure for obtaining blood group antigens information for ABO involves about 45 minutes. This includes a ten minute preparation period and the collection of threads, a ten minute antibody incubation, a three minute wait, a ten minute solution and a ten minute rotation and examination period. An advantage to this technique is that it permits more samples to be processed in a given time and only three bloodstained threads are used from the questioned source.
The second class of blood constituents used as genetic markers is the polymorhic enzymes. These enzymes of interest are within the red blood cells are commonly referred to as isoenzymes. These are described as those enzymatically active proteins which catalyze the same biochemical reactions and occur in the same species but differ in certain of their physiochemical properties. Many of the forensic serologists are not privileged to use electrophorensic and isoenzyme determination, is the technical capabilities shared by only a few laboratories.
Many isoenzymes have been identified from various human tissue sources but six erythrocytic systems have received routine crime laboratory status. These are phosphoglucomutase (PGM), adenylate kinase (AK), adenosine deaminase (ADA), glucose-6-phosphate dehydrogenase (G-6-PD), 6-phosphatase (EAP).
The third class of constituents used as genetic markers in the blood are polymorphic proteins. Hemoglobin and the haptoglobins are the most important members of this classification. The haptoglobins are Alpha 2 globinlins which are responsible for binding free homoglobin released into the plasma after the destruction of red blood cells. Genetically they exist in three forms: Hp1, Hp2, and Hp2-1, with the following population distribution: hp1 14%, Hp2 32%, and Hp2-1 53%. These frequencies are useful in screening blood differences.
Blood Typing
Serology is use to describe a broad scope of laboratory tests that utilize specific antigen and serum antibody reactions. The most widespread application of serology is the typing of whole blood for its A, B, O identification. In determining the A, B, O blood type, only two antiserums are needed: anti-A and anti-B.
The identification of natural antibodies present in blood offer another route to the determination of blood type.
Techniques for Grouping Dried Blood
Blood of recent origin, whether in the form of a powder or flake, should be dissolved in a small quantity of distilled water and grouped.
Blood stains associated with non-absorbed substrata such as knives or bottles should be scraped off or dissolved off with distilled water and tested.
Blood associated with absorbent substrata includes many different types of substratum. Blood stained material should be placed in a small quantity of distilled water and left to soak for up to an hour to allow proper extraction. A little water may be applied to the stain and in a few seconds it is possible to scrape it off with a scalpel. These methods can be applied to cloth and paper. Fragments of bloodstained materials is very effective and can easily be made to stick in the wells of cavity slides by the standard method. Fingernail fragments have been grouped successfully utilizing this method.
Tube Technique
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1.) Place small fragments of blood stain in the bottom of three 5 cm x 0.5 cm test tubes.
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2.) Dispense small amounts of 0.5% v/v A1, B, and O suspensions into the test tubes. The amount of cell suspension used is the smallest volume commensurate with being able to recover a sufficient amount for recordings to be made.
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3.) Leave cells and blood stain in contact for two hours then remove the cell suspension gently with a pipette and spread carefully ontoa microscope slide.
Cavity Slide Technique
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The cavity slide technique has not been too effective in dealing with small quantities of test material in the well of a three cavity 3 in. x 1 in. slide.
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1.) Place fragments of bloodstain material in the well of a three cavity 3 in. x 1 in. slide.
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2.) Dispense one drop of a 0.01% v/v indicator cell suspension of the appropriate type to each well (A, B, and O) using a very fine borepipette. The test material should be covered by the cell suspension.
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3.) Place the slide in a moist chamber to prevent evaporation.
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4.) The results are read with a low-power microscope. Pipetting is not required. A reaction may be evident within a few minutes or up to two to three hours.