Module 2: Trace Evidence

Evidence Overview

  • Focus on trace evidence

General Nature of Trace Evidence

  • Trace evidence consists of a large variety of materials.

  • Characteristics of trace evidence:

    • Typically provides class characteristics.

    • In rare cases, may help establish individualization.

Types of Trace Evidence

  • Common materials found as trace evidence include:

    • Hair

    • Glass

    • Fibers

    • Paints

    • Soil

  • Instruments used in analysis include:

    • Microscopes (including comparison microscopes)

    • Infrared spectrometry

    • Automated refractive index systems

    • Scanning Electron Microscopes (SEM)

Hair Evidence

Distinction and Analysis

  • Distinguishing Characteristics:

    • Can differentiate by species and race.

    • General location of the body can sometimes be identified.

  • Components of hair structure:

    • Diameter

    • Scales

    • Medulla

    • Cortex

    • Pigment

  • Hair analysis tends to focus on class characteristics.

  • In certain cases, individualization may occur if:

    • Hair has been treated (e.g., dyed).

    • Follicle is present for DNA extraction.

  • Mitochondrial DNA analysis is often preferred over traditional hair analysis for objective data.

Fiber Evidence

Classification and Analysis

  • Fibers can be classified based on:

    • Originating source: Natural, manufactured, or synthetic.

  • Analysis focuses on:

    • Dye and composition characteristics, often using SEM.

    • Optical differences assessed through microscopy and infrared spectrometry.

  • Transfer Characteristics:

    • Hair and fiber transfers result from contact between:

    • People

    • Objects

    • Clothing

    • Noted that 90% of transferred hairs or fibers may be lost within the first eight hours following contact.

  • Collection Protocols:

    • Hair and fibers are fragile and must be documented and collected immediately.

    • Use forceps or tweezers to collect and place fibers in envelopes.

    • Seal envelopes to prevent loss.

Collection Methods for Hair and Fibers

Alternative Light Sources

  • Can aid in locating fibers as some fluoresce under various wavelengths of light.

Search Techniques

  • Common collection methods:

    • Taping:

    • Use clear tape to create lengths of 8-10 inches.

    • Apply adhesive side to the surface, transfer to a document protector for examination.

    • Use different backings for different areas to prevent cross-contamination.

    • Vacuuming:

    • Crime labs may have specific protocols regarding the use of vacuums.

Glass Evidence

Analysis and Determination

  • Glass examination focuses on:

    • Type determination: Compare fragments to known samples for exclusion/inclusion.

    • Direction of force: Evaluate radial fractures to determine force direction on glass.

    • Sequence of force: Analyze multiple bullet defects to establish shot sequence.

  • Variability in glass results from:

    • Different manufacturing recipes and temperature variations.

  • Analysis involves measuring the refractive index and other optical properties.

Mechanical Fit

  • Glass fragments can potentially be matched to their source through mechanical fit.

Direction of Force in Glass

The 4R Rule

  • The 4R rule states that ridge lines on radial fractures are at right angles to the rear of glass.

  • Process of establishing direction involves:

    • Identifying primary radial fractures within the first concentric fracture.

    • Examining the conchoidal fractures:

    • Parallel lines indicate the side where force was applied.

    • Right angles indicate the opposite side.

Analysis of Paint Evidence

Characteristics

  • Paint types vary based on function and manufacturing ingredients.

  • Paint analysis may involve evaluating:

    • The sequence/layers of components in paint chips.

  • Instruments commonly used:

    • Fluorescent microscopy

    • Infrared spectrometry

    • Gas chromatography

    • Scanning Electron Microscopy (SEM)

Collection Techniques

  • Paint chips should be collected from their source, preferably by cutting beneath the transfer.

  • Known samples should be obtained from areas near where the questioned sample originated.

Soil Evidence

Composition and Collection

  • Soil is complex, comprising inorganic and organic materials, with variability even within small areas.

  • Analysis includes the examination of:

    • Mineral content and pollens.

    • Environmental variations affecting soil.

  • Soil collection guidelines:

    • Obtain samples from probable contact locations (walkways, fight sites).

    • Collect at appropriate depth corresponding to tire marks or other disturbances.

Gunshot Residue (GSR)

Composition and Analysis

  • Gunshot residue consists of elements:

    • Antimony

    • Barium

    • Lead

  • Previous methods of analysis included neutron activation, but current practices focus on:

    • Sampling through swabbing surfaces with adhesive materials.

  • Challenges in GSR analysis:

    • Presence may indicate proximity to discharge rather than direct handling of the weapon.

    • Current evaluation methods look for unique fused particles containing all three elements for higher accuracy.