Published on January 9, 2008
Physical evidence: Physical evidence Types of physical evidence: Types of physical evidence Biological fluids Blood, semen, saliva Documents Drugs Explosives Fibers Fingerprints Firearms and ammunition Glass Hair Impressions Paint Accelerants Man-made polymeric Soil, vegetative matter Tool marks Powder Residues Comparison : Comparison A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examinations for the ultimate purpose of determining whether or not they have a common origin. Only tells probability of a common source Individual vs. Class characteristics: Individual vs. Class characteristics Evidence which is unique and has a high probability of coming from the same source as a comparison standard has individual characteristics. Example DNA, fingerprints Evidence which has some characteristics which make it somewhat likely that it came from the same source as a comparison sample possess class characteristics. Examples: paint color, fiber type, hair, blood type Product rule: Product rule Probability of finding a number of different pieces of evidence with class characteristics is determined by multiplying individual probabilities Williams trial: Home carpet : 1 in 7,792 Car carpet: 1 in 3,828 Odds of finding two fibers at random: 1 in 29 million Individualizing evidence: Individualizing evidence Even evidence which has class characteristics can be individualized by additional factors. Piece of tape was torn from roll found in suspects vehicle Properties of samples: Properties of samples Physical properties Describe a substance without reference to any other substance Weight, volume, color, bp, mp Intensive properties don’t depend on sample size: density, bp, mp Chemical properties Describes the behavior of a substance when it reacts or combines with another substance In the presence of Heroin, the Marquis reagent turns purple Properties used to analyze glass samples: Properties used to analyze glass samples Density = weight per unit volume Refractive index = velocity of light in vacuum/ velocity of light in medium Edge thickness Lamination, tempering, etc. Chemical composition is not used Determining density of glass sample: Determining density of glass sample Archimedes’s principle An object immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. Water has a density of 1 g/cm3 or 1 g/mL Therefore a 2.5 g piece of glass with a volume of 1mL will weigh only 1.5 g in water Density = mass of sample/volume of sample Comparing density by flotation column: Comparing density by flotation column Place some bromoform (CHBr3, d=2.89) in a test tube. Add glass fragment and bromobenzene (C6H5Br, d=1.52) until fragment is suspended. Add known glass fragment and observe whether it floats, sinks or is suspended with other glass fragment. Both fragments suspended would indicated that both had same density. Refractive index: Refractive index A measure of the bending of a ray of light as it passes from air into a solid or liquid. Tempered glass will change its refractive index as it is annealled (slowly heated and cooled) Immersion method Silicone oil with fragment is heated until Becke line disappears. GRIM 2: GRIM 2 Instrument (Glass Refractive Index Measurement) Automated determination of RI Uses immersion method and analyzes video of glass fragment as temperature is altered to determine when Becke line disappears. Can determine RI to +0.0002 Glass fractures: Glass fractures Fracture patterns can be used to determine: Whether glass was broken from outside or inside Size of object which pierced glass Velocity of object which pierced glass Radial vs. concentric fractures: Radial vs. concentric fractures 3R rule Radial cracks form a Right angle on the Reverse side of the force. Soil evidence: Soil evidence Soil composition differs in different areas and also at different depths. Soil samples can contain Minerals organic matter botanical matter Specific seed and spores man-made substances Dust from inside can also be identifiable Kitchen: flour, spices Bathroom: powder, make-up Soil analysis: Soil analysis Soil properties can be changed with amount of moisture present so must first be dried Soil color- observed by smear test Soil pH measured Microscopy Determine soil structure and presence of unusual material Minerals Particle size distribution Established by passing through series of nesting sieves with decreasing nest size Density Gradient Tubes: Courtyard College Gardens Sports Pitch Crime Scene Density Gradient Tubes Formed by layering liquids with decreasing density Most dense at bottom, least dense at top Produces distribution pattern which can be compared to known sample Soil sedimentation rate: Soil sedimentation rate Suspend soil sample in water and observe Visible light absorbance over time Graph absorbance values Two sample which have same sedimentation rate graphs likely came from a common source Soil is nearly impossible to individualize.