UNI 2nd:
Biological and Trace analysis-
• Trace evidence is material found at a crime scene or accident scene in small but
measurable amounts.
• Small, often microscopic material
• Huge variety of material
Edmond Locard: (1977-1966)
• Developed the first crime lab in Lyons, France (1910, when he was 33)
• Locard’s Principle of Exchange:
Every contact leaves a trace.
• Locards principle of exchange:
Traces of the scene and the victim will be carried by the perpetrator.
Traces of the perpetrator will be left at the scene and on the victim.
12 print finger identification.
Qs for evidence:
• What is it?
• Is it man-made or natural?
• What is its source?
• How common is it?
• Can it be identified to a single source?
• Increased sensitivity brings increasing potential for:
• CONTAMINATION
• MIS-INTERPRETATION
• Physical evidence cannot be wrong.
• Only interpretation can err.
Contamination
(example from Schewndener et al, 2016)
• Contamination can lead to incorrect conclusion and miscarriage of justice.
• Fifteen unused body bags from various Australian Jurisdictions were examined.
• On average, 3603 fibers and 1429 particles were detected in each body bag.
• Some strategies are proposed to mitigate problems caused by these impurities.
• Forensic material should never be assumed to be exempt of impurities.
Biological and trace evidence:
• Hair
, • Fibres
• Glass
• Paint
• Soil
• Pollen
• Blood
• Explosive residues
• Drug traces
• Gunshot residue
Benefits of trace evidence:
• Cannot be hidden / erased / forged.
• Preliminary analysis often quite easy – to identify type for example.
• Often links suspect to victim to crime scene (multiple transfer)
• Multiple evidential types at a crime scene – makes each scene unique.
• Evidence can link crime scene, suspect/perpetrator, and victim.
• Trace evidence may help to place a suspect at a crime scene, but it does not
necessarily mean that they were involved in the commission of the crime.
• Associative evidence - evidence that links two separate entities, people, or objects.
• Think of examples of crimes where this transfer could occur.
• Can you think of examples of two-way transfer between two entities? One-way
transfer?
Drawbacks of trace evidence:
• Can often be contaminated.
• Analysis time often outweighs evidential value.
• Quantities can be small – hard to see, collect and analyse.
• Very rarely allows individual identification.
• Mass production can limit value – common fibres, pens etc.
Targeting potential traces:
Time needs to be prioritised: cost/benefit analysis of evidence.
E.G. paint:
• Peeling paintwork at a point of entry is more likely to yield tiny fragments of paint
which can be transferred to and remain on the clothes of an intruder rather than a
smooth, undamaged surface.
• Traces are more likely to lodge on a rough surface (chunky knit jumpers) than on
smooth ones (silk jumpers or shell-suits).
• The more forceful the contact, and the longer it endures, the greater the amount of
material transferred.
Biological and Trace analysis-
• Trace evidence is material found at a crime scene or accident scene in small but
measurable amounts.
• Small, often microscopic material
• Huge variety of material
Edmond Locard: (1977-1966)
• Developed the first crime lab in Lyons, France (1910, when he was 33)
• Locard’s Principle of Exchange:
Every contact leaves a trace.
• Locards principle of exchange:
Traces of the scene and the victim will be carried by the perpetrator.
Traces of the perpetrator will be left at the scene and on the victim.
12 print finger identification.
Qs for evidence:
• What is it?
• Is it man-made or natural?
• What is its source?
• How common is it?
• Can it be identified to a single source?
• Increased sensitivity brings increasing potential for:
• CONTAMINATION
• MIS-INTERPRETATION
• Physical evidence cannot be wrong.
• Only interpretation can err.
Contamination
(example from Schewndener et al, 2016)
• Contamination can lead to incorrect conclusion and miscarriage of justice.
• Fifteen unused body bags from various Australian Jurisdictions were examined.
• On average, 3603 fibers and 1429 particles were detected in each body bag.
• Some strategies are proposed to mitigate problems caused by these impurities.
• Forensic material should never be assumed to be exempt of impurities.
Biological and trace evidence:
• Hair
, • Fibres
• Glass
• Paint
• Soil
• Pollen
• Blood
• Explosive residues
• Drug traces
• Gunshot residue
Benefits of trace evidence:
• Cannot be hidden / erased / forged.
• Preliminary analysis often quite easy – to identify type for example.
• Often links suspect to victim to crime scene (multiple transfer)
• Multiple evidential types at a crime scene – makes each scene unique.
• Evidence can link crime scene, suspect/perpetrator, and victim.
• Trace evidence may help to place a suspect at a crime scene, but it does not
necessarily mean that they were involved in the commission of the crime.
• Associative evidence - evidence that links two separate entities, people, or objects.
• Think of examples of crimes where this transfer could occur.
• Can you think of examples of two-way transfer between two entities? One-way
transfer?
Drawbacks of trace evidence:
• Can often be contaminated.
• Analysis time often outweighs evidential value.
• Quantities can be small – hard to see, collect and analyse.
• Very rarely allows individual identification.
• Mass production can limit value – common fibres, pens etc.
Targeting potential traces:
Time needs to be prioritised: cost/benefit analysis of evidence.
E.G. paint:
• Peeling paintwork at a point of entry is more likely to yield tiny fragments of paint
which can be transferred to and remain on the clothes of an intruder rather than a
smooth, undamaged surface.
• Traces are more likely to lodge on a rough surface (chunky knit jumpers) than on
smooth ones (silk jumpers or shell-suits).
• The more forceful the contact, and the longer it endures, the greater the amount of
material transferred.
