Lecture 3: Trace evidence analysis Flashcards
Forensic evidence
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
Trace evidence
-Microscopic quantities of material that are of probative value in a forensic investigation
-Almost anything can be encountered as trace evidence
Types of trace evidence
- Biological (human)
- Physical
> Incl. biological (non-human) - Particles, substances, marks/impressions…
-Natural v. manufactured materials
Forensic value
-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
The Forensic Process
Transfer –>
Persistence –>
Collection –>
Analysis –>
interpretation –>
Presentation
Evidence dynamics
“any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent”
>Transfer- During crime
>Persistence- post-crime
Transfer
- Locard’s Exchange Principle:
-Every contact leaves a trace - Evidence deposited/removed
- Often undetected
Modes of transfer
-Direct v. indirect
-Secondary transfers
Variation in transfer
- 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
Variation in transfer: clothing
-Varies in composition and texture
-Influences fibre shedding (creation of evidence)
-Influences adhesion of other traces
-E.g. nylon v. wool
Variation in transfer: weather
- Local conditions affect transfer of environmental trace evidence (source)
-Influences deposition of other evidence types at the scene
Variation in transfer: trace characteristics
-Particle size/shape
-Glass : fragments (mm) or particles (μm)
-Pollen : different sizes and surface textures
Frequency
- 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
Frequency: Pollen
-Abundant in the environment
-Transport mechanisms = direct/indirect transfer
-Present on most people’s hair, shoes, & clothing
-Individual pollen profile
Frequency: glass
-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
Trace persistence
- 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…
Trace persistence: wear
-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
Trace persistence: offender activity
-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?
Trace collection
- 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!
Trace analysis
> 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
Classifying evidence
- 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
Techniques
> 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
Microscopy
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…
Stereo microscope
> 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
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
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)
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…
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
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
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
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
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)
Environmental evidence
> Environmental transfer from crime scenes to persons
Soil most frequently transferred -Physical - Chemical - Biological analyses
Plant traces also transferred in abundance
What is forensic evidence?
Any and all objects that can establish:
* A crime has been committed
* Links between crime scene(s), victim, offender
Helps investigators reconstruct crime event: Who? What? Why? When? How? Must be recognised, recovered, & preserved.
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.
List the types of trace evidence.
- Biological (human)
- Physical
- Biological (non-human)
- Particles, substances, marks/impressions
- Natural vs. manufactured materials
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
True or False: Physical (trace) evidence can be wrong.
False
Physical (trace) evidence cannot be wrong; only in its interpretation can there be error.
What does evidence dynamics refer to?
Any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent.
What is Locard’s Exchange Principle?
Every contact leaves a trace.
What are the modes of transfer?
- Direct
- Indirect
- Secondary transfers
What factors influence variation in transfer?
- Trace material
- Properties of source/recipient
- Environmental conditions
- Force, duration, type of contact
How does clothing affect trace evidence?
- Varies in composition and texture
- Influences fibre shedding
- Influences adhesion of other traces
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.
What influences the frequency of trace evidence transfer?
- Is the trace easily transferred?
- How commonly is the trace ‘naturally’ encountered?
What is trace persistence?
Temporal dynamics of transferred evidence, including how long evidence is preserved for after a crime event.
What types of activities contribute to the loss of trace evidence post-crime?
- Offender activity
- Bystanders
- Police/CSI’s
What are the techniques for trace collection?
- Picking
- Scraping
- Brushing
- Combing
- Tape lifts
- Vacuum
- Swabs
What is the aim of trace analysis?
To establish points of similarity between samples and determine how rare they are.
What are class characteristics used for in evidence analysis?
Identifying evidence via properties such as colour, shape, refractive index, etc.
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
What is the priority in techniques for evidence analysis?
Non-destructive techniques are the priority.
What is a stereo microscope used for?
Preliminary examination to segregate evidence from other material.
How does a compound microscope function?
High power examinations using transmitted light through the sample.
What is the magnification range of a scanning electron microscope?
10x to 500,000x magnification.
What are the analytical tools used to classify paint evidence?
- Stereo microscopy
- Solvent tests
- IR spectrophotometry
- SEM-EDX
- XRD
- Pyrolysis gas chromatography
What are the components of glass?
- Sand
- Sodium carbonates
- Calcium oxides
- Impurities
What techniques are used for fibre evidence classification?
- Microscopy
- Microspectrophotometry (MSP)
- Fourier Transform Infrared Spectroscopy (FTIR)
- Chemical composition (e.g. GC/MS)
What is the most frequently transferred environmental evidence?
Soil.
What types of analyses are used for environmental evidence?
- Physical
- Chemical
- Biological analyses
