4.2 Physical evidence - presentation

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Unformatted text preview: YSCN0017 From Sherlock Holmes to modern forensic science “It is a capital mistake to theorize before you have all the evidence. It biases the judgment.” ▬▬▬ Sherlock Holmes ▬▬▬ PHYSICAL PHYSICAL EVIDENCE Hit-and-Run Accident in Tsuen Wan on Jan 9, 2008 A 23 years old woman was hit by an unknown vehicle in Tsuen Wan,EVERYmarks running LEAVES ITS TRACES ” “ with tire CONTACT from her head to her right leg. Edmond Locard’s Exchange Principle Suspect vehicle found on January 10, 2008 Suspect vehicle found on January 15, 2008 3 Types of Physical Evidence Blood, semen, and saliva Blood, Documents Documents Drugs Drugs Explosives Explosives Fibers Fibers Fingerprints Fingerprints Firearms and ammunition Firearms Glass Glass Hair Hair Impressions Impressions Organs and physiological fluids Organs Paint Paint Petroleum products Petroleum Plastic bags Plastic Plastic, rubber, and other polymers Plastic, Powder residues Powder Soil and minerals Soil Tool marks Tool Vehicle lights Vehicle Wood and other vegetative matter Wood 4 Almost anything can be physical evidence. The investigator must be able to recognize the The physical evidence before it can be collected and analyzed – crime scene investigation. The weight of a given piece of evidence in a case The is ultimately decided by a jury. 5 Examination of physical evidence by a forensic scientist involves identification or comparison. Identification is to determine the physical or chemical Identification identity of a substance. The certainty of identification can be near absolute using modern analytical techniques. In comparison analysis, a suspect specimen and a In comparison standard/reference specimen (the control) are subjected to the same tests. The goal is to determine whether or not the suspect specimen and the control have a common origin. 6 Common Types of Evidence that require identification include: Illicit drug – is it cocaine or sugar? Illicit Gasoline in the debris of a fire. Gasoline Explosive residues – dynamite or TNT? Explosive Blood, semen, hair, or wood. Blood, 7 Comparison is to determine whether or not a suspect specimen and a standard/reference specimen (the control) have a common origin. Comparison of tool marks using a comparison microscope. 8 In comparison analysis, the probability that two specimens have the same origin that is estimated, if possible. Evidence that can be associated to a common Evidence source with an extremely high degree of probability is said to possess individual individual characteristics. Evidence associated only with a group is said to Evidence have class characteristics. class 9 Fingerprints possess individual characteristics. The chance that two persons has the same fingerprints is estimated to be one out of 1 × 1060. 10 Irregular and random wear patterns in tire or footwear impressions have individual characteristics 11 DNA profile, sort of a biological barcode of an individual, is unique to each individual. DNA are inherited partly from the mother and partly the father. Parents’ DNA profiles will not be the same as their child’s DNA profile. 12 The forensic laboratory often cannot link physical evidence to a common origin with a high degree of certainty. Evidence is said to possess class Evidence class characteristics when it can be associated only associated with a group and never with a single source. 13 Blood type is a class characteristics. Bloodstains can become individual characteristics if more blood factors are determined. The probability of specifying the origins of the specimens is increased. 14 In the O.J. Simpson case, a bloodstain located at the crime scene has the same factors as the blood of O.J. Blood Factor A EsD 1 PGM 2+2■ ■ Frequency 26% 85% 2% Consider only the blood type: 26% of population is type A. Consider all 3 factors: 0.26×0.85×0.02 = 0.44% of the population would have this particular combination of blood factors. 15 Paint chips of one layer of paint possess class characteristics (the color). ■ Paint chips can have individual characteristics if more layers of paint are compared. will have better chance of individualizing the evidence. ■ Combining the properties Combining cross sections of red paint from different automobiles and household. 16 Collective presence of more than one type of Collective class evidence may lead to an extremely high certainty that they originated from the same source. As the number of different objects linking an As individual to a crime scene increases, so does the likelihood of that individual’s involvement with the crime. 17 The value of class physical evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias. Just as important, a person may be exonerated or Just excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject. 18 Firearms Firearms Firearms Death Rate (per 100,000) of selected countries in one year between 1990 and 1995 http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNSTAT.html 21 Rifling of Gun Barrel ■ Rifling is a process in manufacturing of gun barrel that the inner surface of the gun barrel is impressed with spiral grooves. No two rifled barrels have identical striation markings. ■ 22 Firearms Identification The inner surface of the barrel of a gun leaves characteristic characteristic markings on a bullet that passes through it. There is no practical way of making a direct comparison of the markings on the fired bullet and those found within a barrel. The examiner must obtain test test bullets fired through the suspect barrel for comparison. 23 Laser pointers can be used to set a search path for cartridges and other evidences of the shooter but cannot determine the distance between the firearm and the target. 25 Distance between the firearm and a target can be determined by studying the distribution of powder patterns or the spread of a shot pattern. ■ Gunpowder is not completely consumed during a discharge. ■ Unburned as well as partially burned residue would be distributed around the bullet hole with a characteristic pattern. 26 Fire and Explosion Fire Most arsons are started with petroleumbased accelerants such as gasoline or kerosene. Petroleum residues in a fire scene may evaporate Petroleum within a few days or even hours. The arson investigator needs to begin examining a The fire scene for signs of arson as soon as the fire has been extinguished. The search of the fire scene must focus on finding The the fire’s origin, which may be most productive in any search for an accelerant or ignition device. 28 Signs of arson: Evidence of separate and unconnected fires Evidence Evidence of the use of “streamers” Evidence Evidence of severe burning found on the floor as opposed to Evidence the ceiling of a structure, due to a flammable liquid. Fortunately, combustible liquids are rarely entirely consumed Fortunately, during a fire. Traces of flammable liquid residues may be located with a vapor detector (sniffer). 29 Collection of Evidence At the suspect point of origin of a fire, ash and soot, At along with porous materials which may contain excess accelerant, should be collected and stored in airtight containers, leaving an airspace for vapor sampling in the lab. It is important that a sampling of similar but It uncontaminated control specimens be collected. A search for ignitors such as matches, an electrical search sparking device, or parts of a “Molotov cocktail” must also be conducted. 30 In the laboratory, the gas chromatograph (GC) is the most sensitive and reliable instrument for detecting and characterizing flammable residues. The vast majority of arsons are initiated by petroleum The distillates such as gasoline and kerosene. The gas chromatograph separates the hydrocarbon The components of a sample (i.e., the fire debris) and produces a chromatogram (peak pattern) characteristic of a particular petroleum product. By comparing selected gas chromatographic peaks of By the vapor of the fire-scene debris to that of known flammable liquids, a forensic analyst may be able to identify the accelerant used to initiate the fire. 31 Gas Chromatography Figure 11–8 (top) Gas chromatogram of vapor from a genuine gasoline sample. (bottom) Gas chromatogram of vapor from debris recovered at a fire site. Note the similarity of the known gasoline to vapor removed from the debris. Courtesy New Jersey State Police. 32 Explosive: ion mobility spectrometer (IMS) Figure 11–14 Schematic diagram of an ion mobility spectrometer. A sample is introduced into an ionization chamber, where bombardment with radioactive particles emitted by an isotope of nickel converts the sample to ions. The ions move into a drift region where ion separation occurs based on the speed of the ions as they move through an electric field. 33 Fibers Fibers Case 5: Pat Sin Leng (八仙嶺 ) Corpse Burning Back of body Recovered fibers 35 Types of Fibers Natural fibers are derived in whole from animal or Natural plant sources. Examples: wool, mohair, cashmere, furs, and cotton. Examples: Man-made fibers Man Regenerated fibers are manufactured from natural raw Regenerated materials and include rayon, acetate, and triacetate. Synthetic fibers are produced solely from synthetic chemicals Synthetic and include nylons, polyesters, and acrylics. Polymers, or macromolecules, are synthetic fibers Polymers, composed of a large number of atoms arranged in repeating units known as monomers. 36 The quality of the fiber evidence depends on the ability of the criminalist to identify the origin of the fiber or at least be able to narrow the possibilities to a limited number of sources. If the examiner is presented If with fabrics that can be exactly fitted together at their torn edges, it is a virtual certainty that the fabrics were of common origin. 37 The color and diameter of the questioned and standard/reference fibers can be compared under a microscope. Other morphological features: Other Lengthwise striations on the Lengthwise surface of the fiber Presence of delustering Presence particles (TiO2) that reduce shine Cross-sectional shape of the Cross fiber 38 Hairs human (450x) dog (450x) deer (100x) Medulla pattern of hair rabbit (450x) cat (450x) mouse (450x) 39 Synthetic fibers, such as acetate, nylon and vivrelle, display discernible differences in structure and color under the polarizing microscope. 40 Birefringence (double refraction) of crystalline solids A beam of light is refracted into two different beam light-ray components of different refractive indices. 41 Methods of Fiber Comparison Comparison and polarizing microscope. Comparison Visible light microspectrophotometer for comparison Visible of colors of fibers as spectra. Infrared spectrophotometry for identification of the Infrared generic class of fibers. Chromatographic separation for detailed analysis of the Chromatographic fiber’s dye composition. 42 Case 6: Fiber Evidence and the Wayne Williams Trial Paint Paint Paint Paint spread onto a surface will dry into a hard film Paint consisting of pigments and additives suspended in the binder. A chip of automobile dried paint can be transferred to chip the clothing of a hit-and- run victim during the impact. Automobile manufacturers normally apply a variety of Automobile coatings to the body of an automobile. The wide diversity of automotive paint contributes to The the forensic significance of an automobile paint comparison. 45 Methods of Paint Comparison Questioned and known specimens are best Questioned compared side by side under a stereoscopic microscope for color, surface texture, and color layer sequence. Crime laboratories are often asked to identify the Crime make and model of a car from a small amount of paint and will make use of color charts for automobile finishes. Pyrolysis gas chromatography and infrared Pyrolysis spectrophotometry are invaluable techniques for distinguishing most paint binder formulations, adding further significance to a forensic paint comparison. 46 Methods of Paint Comparison: Pyrolysis gas chromatography Figure 8–21 Paint pyrograms of acrylic enamel paints. (a) Paint from a Ford model and (b) paint from a Chrysler model. Courtesy Varian Inc., Palo Alto, Calif. 47 YSCN0017 From Sherlock Holmes to modern forensic science “It is a capital mistake to theorize before you have all the evidence. It biases the judgment.” ▬▬▬ Sherlock Holmes ▬▬▬ ...
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