Forensic Science - Trace Evidence

What is trace evidence?

Tiny fragments of physical evidence transferred during contact or movements, often at crime scenes.

How can trace evidence be transferred?

• Direct contact (person to object).

• Secondary transfer (object to object).

• Contactless transfer.

Give 4 examples of trace evidence.

• Glass.

• Paint.

• Fibres.

• Soil.

Name some methods for collecting trace evidence.

• Shaking.

• Vacuuming.

• Scraping.

• Combing.

• Swabbing.

• Tape lifts.

• Extracting.

• Hand picking with tweezers.

How is GSR analysed?

Using SEM-EDX (Scanning Electron Microscope - Energy Dispersive X-ray).

What forensic techniques are used for documents?

• ESDA.

• Handwriting analysis.

• Ink analysis.

What are the main components of paint?

• Pigment.

• Solvent.

• Resin.

• Additives.

Where might paint evidence be found?

On tools, victims, clothing or crime scenes.

How is paint collected?

• Tape lifts.

• Tweezers.

• Lifting flakes.

• Cutting.

• Dislodging.

What analysis techniques are used for paint?

• Microscopy/macroscopy.

• FTIR.

• GC-MS.

Why is paint valuable evidence?

It has a rare composition and multiple layers can strongly link a suspect to a crime scene.

Who was Malcolm Fairley and what was significant about the paint?

Malcolm Fairley was a British serial rapist in the 1980’s. Yellow paint found on a tree matched a rare car which linked to Malcolm.

Name 5 types of glass.

• Float.

• Borosilicate.

• Toughened.

• Fibre.

• Specialist.

What does GRIM stand for and how does it work?

Glass Refractive Index Measurement:

• Put the glass in a special liquid that bends light.

• Look at this under a microscope - a bright halo (Becke line) appears around the glass.

• Change the liquid until the halo disappears. When it disappears, the liquid bends light exactly like the glass does.

• Compare the measurement to glass from a suspect source. If they match, the fragments could come from the same place.

What factors affect glass refractive index?

• Wavelength.

• Temperature.

• Cooling rate.

• Pressure.

• High-energy radiation.

Name 4 natural fibres.

• Animal (silk, wool).

• Human hair.

• Mineral (asbestos).

• Vegetable (flax, hemp).

What are man-made fibres?

• Regenerated (viscose).

• Synthetic (nylon).

• Other (glass fibres).

How is fibre evidence collected?

• Tape lifting.

• Serial taping.

• Vacuum.

• Forceps.

How are fibres analysed in the lab?

• Visually.

• Microscopy (stereomicroscope, SEM).

• Fluorescence.

• Birefringence.

• Microspectrophotometry.

• TLC.

• IR spectroscopy.

What does birefringence mean?

The difference between 2 refractive indices in textile fibres (parallel vs perpendicular).

What can hair/fibre evidence indicate?

• Human/animal origin.

• Body location.

• Pulled/fallen.

• Part of crime reconstruction.

What are the 3 regions of the hair?

• Cuticle.

• Cortex.

• Medulla.

What is the cuticle?

Overlapping external cells, scale pattern used for species ID.

What is the cortex?

Inner spindle-shaped cells, gives strength, elasticity and contains pigments.

What is the medulla?

Central canal, patterns differ between species, may be absent, continuous, interrupted or fragmentary.

How can hair reveal human ethnicity?

Differences in medulla structure, thickness, pigmentation and growth patterns.

What types of animal hair are forensic significant?

• Guard hairs.

• Under-hairs.

• Whiskers.

• Wool.

• Dog/cat hair.

How was fibre evidence used in the Sarah Payne case?

Matched fibres from victim’s clothing to fibres in Roy Whiting’s van and clothing; hair evidence supported identification.

What was the outcome for Roy Whiting?

Life imprisonment, “Sarah’s Law” launched in 2011.

What are the main components of soil?

Mineral matter, soil water, soil air, organic material.

Why is soil valuable as evidence?

Highly individualistic, transferable between locations, can narrow crime scenes.

How is soil analysed?

• Density, texture, characterisation (carbonates, colour, structure).

• Nutrients.

• Microscopy.

• Particle size.

• SEM.

• Munsell Colour Chart.

What can changes in soil layers indicate?

Disturbance at the crime scene?

What must be considered when interpreting trace evidence?

• Circumstances.

• Transfer type (one-way/two-way).

• Persistence.

• Fibre/soil background.

• Effective recovery.

• Number & type of samples.

Why is DNA often prioritized over general fibre characteristics?

Provides unique identification, fibres are less definitive without DNA confirmation.