Physical evidence

1
Physical evidence
2
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

3
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

4
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

5
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

6
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

7
Properties of samples

• Physical properties
• Describe a substance without reference to any
  other substance
• Weight, volume, color, bp, mp
• Intensive properties dont 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

8
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

9
Determining density of glass sample

• Archimedess 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

10
Comparing density by flotation column

• Place some bromoform (CHBr3, d2.89) in a test
  tube. Add glass fragment and bromobenzene
  (C6H5Br, d1.52) 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.

11
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.

12
(No Transcript)
13
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

14
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

15
Radial vs. concentric fractures

• 3R rule
• Radial cracks form a Right angle on the Reverse
  side of the force.

16
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

17
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

18
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

Courtyard College Gardens Sports Pitch Crime
Scene
19
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.