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Forensic science module > Lecture 3: Trace evidence analysis > Flashcards

Lecture 3: Trace evidence analysis Flashcards

1
Q

Forensic evidence

A

Any and all objects that can establish:
1. A crime has been committed
2. Links between crime scene(s), victim, offender
* Varies based on circumstances of crime
* Helps investigators reconstruct crime event
-Who? What? Why? When? How?
* Must be recognised, recovered, & preserved

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2
Q

Trace evidence

A

-Microscopic quantities of material that are of probative value in a forensic investigation
-Almost anything can be encountered as trace evidence

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3
Q

Types of trace evidence

A
  • Biological (human)
  • Physical
    > Incl. biological (non-human)
  • Particles, substances, marks/impressions…
    -Natural v. manufactured materials
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4
Q

Forensic value

A

-Mute witnesses
-Microscopic
-Determine circumstances of what happened
-Associate a person with a crime
-Establish spatial & temporal links between persons and scenes of interest
-BUT… trace evidence analysis can often be imprecise
-Important to consider different stages of the ‘forensic process

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5
Q

The Forensic Process

A

Transfer –>
Persistence –>
Collection –>
Analysis –>
interpretation –>
Presentation

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6
Q

Evidence dynamics

A

“any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent”
>Transfer- During crime
>Persistence- post-crime

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7
Q

Transfer

A
  • Locard’s Exchange Principle:
    -Every contact leaves a trace
  • Evidence deposited/removed
  • Often undetected
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8
Q

Modes of transfer

A

-Direct v. indirect
-Secondary transfers

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9
Q

Variation in transfer

A
  • Amount and distribution of transferred evidence varies
    • Where is the evidence?
  • Characteristics including:
    -Trace material
    -Properties of source/recipient
    -Environmental conditions
    -Force, duration, type of contact
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10
Q

Variation in transfer: clothing

A

-Varies in composition and texture
-Influences fibre shedding (creation of evidence)
-Influences adhesion of other traces
-E.g. nylon v. wool

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11
Q

Variation in transfer: weather

A
  • Local conditions affect transfer of environmental trace evidence (source)
    -Influences deposition of other evidence types at the scene
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12
Q

Variation in transfer: trace characteristics

A

-Particle size/shape
-Glass : fragments (mm) or particles (μm)
-Pollen : different sizes and surface textures

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13
Q

Frequency

A
  • Is the trace easily transferred?
    • E.g. pollen, hair, soil…
  • How commonly is the trace ‘naturally’ encountered?
    -Significance in a case?
  • Transfer is not a discrete event:
    -Pre- and post-crime transfers
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14
Q

Frequency: Pollen

A

-Abundant in the environment
-Transport mechanisms = direct/indirect transfer
-Present on most people’s hair, shoes, & clothing
-Individual pollen profile

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15
Q

Frequency: glass

A

-Population studies -> frequency in general populations
-E.g. Jackson et al (2013) -> glass on hair/headwear
–> 6 fragments found on 232 members of public
–> 138 fragments found on 15 people in industry
-Rarity amongst general public = forensic significance

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16
Q

Trace persistence

A
  • Temporal dynamics of transferred evidence
  • How long is evidence preserved for after crime event?
    -Quantity / Quality
  • Impacted by:
    • Source/recipient, environment, evidence characteristics, initial transfer, activity…
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17
Q

Trace persistence: wear

A

-Loss of evidence over time –> the decay ‘J’ curve
-Fibre persistence on different clothing items
-Similar trends with most traces (e.g. GSR, paint, glass)
-Other types of trace evidence more complex

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18
Q

Trace persistence: offender activity

A

-Post-crime activity may also contribute to loss of trace evidence:
–>Offender
–>Bystanders
–> Police/CSI’s
-E.g. washing, burning, cleaning, vacuuming (cars, shoes, clothing)
-Packaging exhibits -> loss or redistribution of evidence?

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19
Q

Trace collection

A
  • Scene, lab, person (or their possessions…)
    • Range of techniques :
    • Picking, scraping, brushing, combing
    • Tape lifts
    • Vacuum
    • Swabs
  • Uncertainty re. type of evidence
    Control samples important!
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20
Q

Trace analysis

A

> Identified via class characteristics (colour, shape, refractive index etc.)
Aim to establish points of similarity between samples - how rare?
If properties differ = not from same source
Absolute identification often not possible → EXCLUSION

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21
Q

Classifying evidence

A
  • Morphology, optical, physical, chemical properties…
  • General physical characteristics:
    • Melting point
    • Boiling point
    • Refractive index (RI)
    • Absorption and emission spectra (IR, vis, UV)
    • Density
    • Molecular mass
    • Colour
  • Species characteristics
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22
Q

Techniques

A

> Non-destructive techniques the priority
-Physical & morphological
-e.g. microscopy
Destructive analyses = further information
-Chemical
-e.g. chromatography, mass spectrometry
Depends on: amount/value of evidence, type of crime, aims of enquiry, resources etc

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23
Q

Microscopy

A

Microscope is the most fundamentally important tool to the trace evidence examiner
>Analyses many different evidence types:
- 3D object - Microscope slide
>Surface analysis & look through object
>Many different types of microscopy…

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24
Q

Stereo microscope

A

> Preliminary examination (30x magnification)
Segregate evidence from other material:
- Substance type, size, form, colour, texture, appearance
Guides more in-depth analysis
Isolation of individual particles

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25
Compound microscope
>High power examinations (100-1000x mag.) >Transmitted light = sample preparation >Light from the base, through condenser & specimen >Light passes through objective lens & ocular lens
26
Scanning electron microscope
>Higher magnification (10x to 500,000x mag.) often needed >Samples gold coated >Scans surface with focused high-energy electron beam >Secondary electrons detected = creates image >Combine with chemical analysis (XRD, EDX, EDS)
27
Interpretation
>Comparison & exclusion of samples >Physical evidence is circumstantial – no certainty >Refer to available information: -Databases - Scientific reports - Scientific surveys - Examiner experience >Flawed interpretation has serious consequences…
28
Paint
>Various crime scenes: burglary, vandalism, assault, vehicular etc. >1,000s paint types = different chemical components >Often applied in sequence of layers >Fragments, chips, particles… >Classification: -Number of layers, colour, surface texture, chemical composition >Analytical tools: -Stereo microscopy - Solvent tests - IR spectrophotometry - SEM-EDX, XRD -Pyrolysis gas chromatography
29
Glass
>Various crime scenes: burglary, vandalism, assault, vehicular etc. >Sand + sodium carbonates + and calcium oxides + impurities >Different glass types= different characteristics >Whole shards → microscopic particles of glass
30
Glass evidence
>Classification: - Pattern matching (physical fit) - Fracture shape -Density (flotation method) - Refractive index (RI) >RI : how light bends as it passes through -Oil immersion method (Becke-line) - Temperature varied until line disappears
31
Fibres
>Smallest unit of textile material − e.g. clothing, carpets, furniture >Natural and synthetic fibres >Indicate direct contact between persons and/or scenes BUT… prevalent within the environment
32
Fibre evidence
>Classification: - Nature of the fibre, colour and pigment distribution, geometry, surface characteristics, additives >Techniques: - Microscopy - Microspectrophotometry (MSP) - Fourier Transform Infrared Spectroscopy (FTIR) - Chemical composition (e.g. GC/MS)
33
Environmental evidence
>Environmental transfer from crime scenes to persons >Soil most frequently transferred -Physical - Chemical - Biological analyses Plant traces also transferred in abundance
34
What is forensic evidence?
Any and all objects that can establish: * A crime has been committed * Links between crime scene(s), victim, offender ## Footnote Helps investigators reconstruct crime event: Who? What? Why? When? How? Must be recognised, recovered, & preserved.
35
What is trace evidence?
Microscopic quantities of material that are of probative value in a forensic investigation. Almost anything can be encountered as trace evidence.
36
List the types of trace evidence.
* Biological (human) * Physical * Biological (non-human) * Particles, substances, marks/impressions * Natural vs. manufactured materials
37
What are the forensic values of evidence?
* Mute witnesses * Microscopic * Determine circumstances of what happened * Associate a person with a crime * Establish spatial & temporal links between persons and scenes of interest
38
True or False: Physical (trace) evidence can be wrong.
False ## Footnote Physical (trace) evidence cannot be wrong; only in its interpretation can there be error.
39
What does evidence dynamics refer to?
Any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent.
40
What is Locard’s Exchange Principle?
Every contact leaves a trace.
41
What are the modes of transfer?
* Direct * Indirect * Secondary transfers
42
What factors influence variation in transfer?
* Trace material * Properties of source/recipient * Environmental conditions * Force, duration, type of contact
43
How does clothing affect trace evidence?
* Varies in composition and texture * Influences fibre shedding * Influences adhesion of other traces
44
What environmental conditions affect trace evidence transfer?
Local conditions affect transfer of environmental trace evidence and influence deposition of other evidence types at the scene.
45
What influences the frequency of trace evidence transfer?
* Is the trace easily transferred? * How commonly is the trace ‘naturally’ encountered?
46
What is trace persistence?
Temporal dynamics of transferred evidence, including how long evidence is preserved for after a crime event.
47
What types of activities contribute to the loss of trace evidence post-crime?
* Offender activity * Bystanders * Police/CSI’s
48
What are the techniques for trace collection?
* Picking * Scraping * Brushing * Combing * Tape lifts * Vacuum * Swabs
49
What is the aim of trace analysis?
To establish points of similarity between samples and determine how rare they are.
50
What are class characteristics used for in evidence analysis?
Identifying evidence via properties such as colour, shape, refractive index, etc.
51
What are the general physical characteristics used in classifying evidence?
* Melting point * Boiling point * Refractive index * Absorption and emission spectra * Density * Molecular mass * Colour * Species characteristics
52
What is the priority in techniques for evidence analysis?
Non-destructive techniques are the priority.
53
What is a stereo microscope used for?
Preliminary examination to segregate evidence from other material.
54
How does a compound microscope function?
High power examinations using transmitted light through the sample.
55
What is the magnification range of a scanning electron microscope?
10x to 500,000x magnification.
56
What are the analytical tools used to classify paint evidence?
* Stereo microscopy * Solvent tests * IR spectrophotometry * SEM-EDX * XRD * Pyrolysis gas chromatography
57
What are the components of glass?
* Sand * Sodium carbonates * Calcium oxides * Impurities
58
What techniques are used for fibre evidence classification?
* Microscopy * Microspectrophotometry (MSP) * Fourier Transform Infrared Spectroscopy (FTIR) * Chemical composition (e.g. GC/MS)
59
What is the most frequently transferred environmental evidence?
Soil.
60
What types of analyses are used for environmental evidence?
* Physical * Chemical * Biological analyses

Forensic science module (14 decks)

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