Presentation Transcript
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.