You Can't Touch This: Documents and Trace Evidence

Chain of Custody

  • Definition: Chronological documentation or paper trail showing seizure, custody, control, transfer, analysis, and disposition of physical electronic evidence.
  • Importance:
    • Maintains integrity of evidence.
    • If broken, the defendant can request evidence to be inadmissible.
  • Packaging:
    • Different evidence types require different packaging. For example:
      • Ignitable liquid residues: airtight containers.
      • Clothing: brown paper bags (allows breathability to prevent mold).
      • Glass controls: plastic pots.
      • Light globes: package in such a way that no further damage can occur.

Trace Evidence

  • Occurs when different objects contact.

  • Often invisible to the naked eye.

  • Examples: fingerprints, hairs, cosmetics, synthetic fibres, glass, paint chips.

  • Forensic Aspects:

    • Who was there? (Fingerprints, DNA).
    • How/why did something occur? (Paint, fibers, glass).

  • Locard's Exchange Principle:

    • Every contact leaves a trace.
    • Finding and examining the transfer of physical evidence.

  • Methods of Collection:

    • Hand picking.
    • Tape lifting.
    • Shaking to collect loose debris.

Paint Examination

  • Definition: Coating used to decorate, protect, add colour/luster, or smooth an item.

  • Types: Architectural and vehicle.

  • Common cases: Vehicle collisions, break-ins.

  • Paint Transfers:

    • Occur as a result of heat or friction.
    • Leave smears or chips of paint.
    • Can be one-way or two-way in vehicle collisions.

  • Steps in Comparison:

    • Optical microscopy: Examines layers.
    • Infrared microscopy: Identifies binder types, fillers, and pigments. Analyst requires a good understanding of paint technology to understand which polymers have been used to manufacture paints.
    • UV Vis: Objective color comparison (measures wavelength of light). Black, white and grey cannot be anlaysed because they either absorb or reflect back the light.
    • Scanning electron microscope with energy dispersive X-ray analyser: Determines elements present within paint layers. A SEM works by shooting electrons onto the surface to produce characteristic x-rays, which the equipment then analyses to provide information around what's actually present.

  • Practical Forensics:

    • Correct collection of controls (known samples) is critical. Collect samples from areas close to damage.
    • Significance depends on color, composition, and number of layers.
    • PDQ database to identify make/model of vehicle in hit and run cases.

Glass Examination

  • Backscatter Fragmentation: Broken glass transferred to nearby objects/persons.

  • Cases: break and enters, assaults, homicides, motor vehicle collisions.

  • Analytical techniques:

    • Physical characteristic: Refractive index.
    • Elemental/chemical characteristic.

  • Refractive Index:

    • Bending and slowing of light as it passes through a transparent object.
    • Measured using GRIM (glass refractive index measurement).
    • Indirect method: Oil immersion variable temperature method (measures to five decimal places).

  • Chemical Analysis:

    • Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).
    • Destructive.
    • Provides higher discrimination than refractive index.
    • Ablates glass with a laser, gaseous molecules are formed and pushed onto a spectrometer, where the percentages of each element can be measured.

  • Significance:

    • Glass is not generally found on random people (exceptions: smartphones).
    • Persistence of glass on clothing is limited.
    • Variability in glass composition assists in analysis.
    • Bayesian statistics (likelihood ratio) used to provide evidence to the court.

Fibre Examination

  • Cases: homicides, sexual assaults, assaults, break and enters.

  • White cotton fibers and blue cotton denim are of lower evidential value.

  • Fiber transfer occurs readily, but legitimate contact reduces evidential value.

  • Case Examples:

    • Louise Bell disappearance.
    • Wanaka homicide.
    • Daniel Hind homicide (fibers from victim's shows matched carpet from the residence).
    • Richard Kelvin homicide (foreign fibers led to suspect).

Document Examination

  • Comparison of unknown documents with known documents.

  • Connects crime to an individual, or excludes individual's involvement.

  • Types of examinations: Handwriting, signatures, paper, ink, alterations.

  • Wide range of cases: homicides, drug trafficking, etc.

  • Handling:

    • Protect documents in plastic bags/envelopes, ensure pockets are large enough so they wont bend.
    • Coordinate with fingerprint examiners to avoid damage from chemical processing.

  • Techniques:

    • ESDA (electrostatic detection apparatus): Reveals indented impressions.
    • Microscopy and different lighting: Detect alterations.
    • Signature comparison: Requested specimens vs. collected specimens.

  • Case Examples:

    • Snowtown case: forged signatures on Centrelink documents.
    • Mark Rust: letter directing police to body.

  • Other analyses:

    • Printing processes (counterfeit currency).
    • Additions (infrared fluorescence).
    • Obliterations (infrared or fluorescent lighting).
    • Chemical erasure (brake fluid).
    • Document restoration (reassembling shredded documents).