Trace evidence analysis

What is forensic evidence?

Any and all objects that can establish:

1. A crime has been committed.

2. Links between crime scenes, victim, offender.

What is trace evidence?

Microscopic quantities of material that are of probative value in a forensic investigation.

Types of trace evidence

•Biological

•Physical

•Particles, substances, marks/impressions.

•Natural vs manufactured materials.

Forensic value

•Mute witnesses

•Microscopic

•Determine circumstances of what happened.

•Associate a person with a crime

•Who? What? When? Why? How?

•Establish spatial and temporal links between persons and scenes of interest.

Limitation of trace evidence

-Can often be imprecise.

-Its interpretation can cause error.

-Only human failure to find, study, and understand it can diminish its value.

Forensic process of trace evidence

Transfer - Persistence - Collection - Analysis - Interpretation - Presentation

Evidence dynamics

Any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical evidence, regardless of intent.

Transfer - during crime

Persistance - Post crime.

Transfer

•Every contact leaves a trace.

•Evidence deposited/removed.

Often undetected.

Modes of transfer

1. One way

2. Two way - back and forth transfer of material

3. Direct vs Indirect

4. Secondary transfers.

Variation in transfer

•Amount and distribution of transferred evidence varies.

•Characteristics include - trace material, properties of source, environmental conditions, force, duration and type of contact.

Examples of variation in transfer

•Clothing

•Weather.

•Trace characteristics

Clothing

•Varies in composition and texture.

•Influences fibre shedding.

•Influences adhesion of other traces.

•E.g. nylon vs wool.

Weather

•Local conditions affect transfer of environmental trace evidence.

•Influences deposition of other evidence types at the scene.

•Rain/wind can affect how much evidence transfers.

•More mud if it's been raining.

•Wet evidence degrades quicker.

Trace characteristics

•Particle size/ shape.

•Glass - fragments (mm) or particles (um).

•Pollen - different sizes and surface textures.

Examples of frequency

•Pollen

•Glass

Frequency

•Is the trace easily transferred?

•How commonly is the trace naturally encountered?

•Transfer is not a discrete event - pre and post crime transfers.

Pollen (frequency)

•Abundant in the environment.

•Transport mechanisms = direct/indirect transfers.

•Present on most peoples hair, shoes and clothing.

•Individual pollen profile.

Glass (frequency)

•Population studies - frequency in general populations.

•Rarity amongst general public - forensic significance.

•Classroom example.

Examples of trace persistence

•Wear

•Offender activity

Trace persistence

•Temporal dynamics of transferred evidence.

•How long is evidence preserved for after crime event?

•Impacted by - source, environment, evidence characteristics, initial transfers, activity...

Wear

•Loss of evidence over time.

•Fibre persistence on different clothing items.

•Similar with most traces (glass, paint etc.).

•Other types of trace evidence more complex.

Offender activity

•Post-crime activity may also contribute to loss of trace evidence; offender, bystanders and police officers.

•Washing, cleaning, burning and vacuuming.

•Packaging exhibits - loss of redistribution of evidence?

Evidence dynamics

•Important to consider dynamics during collection, analysis, interpretation

•Need to consider limits to prevent misinterpretation/ misuse 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).

•Aim to establish point of similarity between samples.

•If properties differ = not from same source.

•Absolute identification often not possible - exclusion.

Classifying evidence

•Morphology, optical, physical, chemical properties.

•General physical characteristics include: Melting point, boiling point, refractive index, absorption and emission spectra, density, molecular mass, colour and species characteristics.

Techniques

•Non-destructive (priority) - physical and morphological e.g. microscopy.

•Destructive (further info) - chemical e.g. chromatography and mass spectrometry.

•Depends on amount/ value of evidence, type of crime ,aims of enquiry and resources etc.

Microscopy

•Microscope is the most fundamentally important tool to the trace evidence examiner.

•Analyse many different evidence types: 3D object and microscope slide.

•Surface analysis and 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 and specimen.

•Light passes through objective lens and ocular lens.

Scanning electron microscope

•Higher magnification (10x to 500,000x mag).

•Sample gold coated.

•Scans surface with focused high-enegery electron beam.

•Secondary electrons detected - create image.

•Combine with chemical analysis (XRD, EDX, EDS).

Interpretation

•Comparison and exclusion of samples.

•Physical evidence is circumstantial - no certainty.

•Refer to available info - databases, scientific reports, scientific surveys and examiner experience.

•Flawed interpretation has serious consequences.

Examples of trace evidence analysis in practice

•Paint

•Glass

•Fibres

•Environmental evidence.

Paint

•Various crime scenes - burglary, vandalism, assault etc.

•1,00 paint types - different chemical compounds.

•Often applied in sequences 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 and pyrolysis gas chromatography.

Glass

•Various crime scenes.

•Sand + sodium carbonates + calcium oxides + impurities.

•Different glass types = different characteristics.

•Whole shards - microscopic particles of glass.

•Classification: pattern matching, fracture shape, density and refractive index.

•IR - how light bends as it passes through - oil immersion method, temperature varied until line disappears.

Fibres

•Smallest unit of textile material.

•Natural and synthetic fibres.

•Indicate direct contact between persons and or scenes.

•Prevalent within the environment.

•Classification - nature of the fibre, colour and pigment distribution, geometry , surface characteristics, additives.

•Techniques - microscopy, microspectrophotometry, Fourier transform infrared spectroscopy and chemical composition.

Environmental evidence

•Environmental transfer from crime scenes to persons.

•Soil most frequently transferred - physical,chemical and biological analysis.

•Plant traces also transferred in abundance.

Summary of trace evidence

•Trace evidence valuable when recognised, detected and collected.

•Important to use non-destructive analytical techniques first.

•Understanding trace evidence dynamics assists with crime reconstruction.

•Anything can be trace evidence - diverse analysis approach.

•Importance to exclude rather than match.