Investigating the Future: Day 5 of International Forensic Science Week - PHYSICAL EVIDENCE

Principle Behind It

Trace evidence relies on Locard’s Exchange Principle, which states

"Every contact leaves a trace."

Types of Physical evidences

1. Biological Trace Evidence

   • Hair

   • Skin cells (DNA)

   • Blood (in tiny amounts)

   • Pollen or plant material

2. Physical Trace Evidence

   • Fibers (clothing, carpet)

   • Paint chips

   • Glass fragments

   • Soil or dust

   • Metal particles

   • Plastic fragments

3. Chemical Trace Evidence

   • Gunshot residue

   • Explosive particles

   • Drugs or toxins (in trace amounts)

Nature of Trace Evidence

1. Microscopic in Size

• Trace evidence is usually very small or even invisible to the naked eye.

• Requires special tools like microscopes or chemical analysis to detect and analyze.

2. Transferable

• Trace evidence transfers between people, objects, or environments during contact.

• Example: Fibers from a suspect’s shirt may transfer to a victim during a struggle.

3. Transfer Follows Locard’s Exchange Principle

• “Every contact leaves a trace.”

• This principle is the foundation of forensic trace analysis.

4. Persistent (but not permanent)

• It can remain on a surface for hours, days, or longer — until it's wiped, washed, or falls off naturally.

• Example: Gunshot residue may stay on hands for a few hours.

5. Associative

• Trace evidence helps link a suspect, victim, or object to a crime scene.

• It can prove or disprove contact between two surfaces.

6. Comparative in Nature

• Trace evidence is used to compare unknown samples (from the crime scene) to known samples (from a suspect, object, or place).

7. Fragile and Easily Contaminated

• Needs careful collection and storage to avoid loss or contamination.

• Investigators use gloves, tweezers, and sealed containers.

8. Subject to Scientific Analysis

• Can be analyzed using forensic techniques (e.g., DNA profiling, fingerprint comparison, ballistics).

9. Class or Individual Characteristics

• Class evidence: Shares traits with a group (e.g., shoe size, fiber type).

• Individual evidence: Unique to a single source (e.g., DNA, fingerprints).

10. Admissible in Court

• If collected and preserved properly, it can serve as legal proof.

• Must follow chain of custody and be uncontaminated.

Collection and Preservation of Physical Evidence

1. Hair

• Collection

  • Use clean tweezers, gloved hands, or tape lifts.

  • Collect both questioned hairs (from scene) and control samples (from known person).

• Preservation

  • Place in paper envelopes or pharmaceutical folds.

  • Avoid plastic (can cause moisture and mold).

2. Fibers

• Collection

  • Use tape lifts or vacuuming (in a clean area to avoid contamination).

  • Can also be picked with tweezers.

• Preservation

  • Place in small paper envelopes or glass vials.

  • Label carefully and seal.

3. Paint Chips

• Collection

  • Use clean tweezers or lift with tape.

  • Collect from damaged areas (like vehicles).

• Preservation

  • Wrap in paper folds and place in labeled containers.

  • Avoid crushing.

4. Glass Fragments

• Collection

  • Use forceps or tweezers.

  • Collect both large and small fragments (especially with edges).

• Preservation

  • Store in rigid containers (e.g., plastic boxes or pill bottles) to prevent breakage.

  • Wrap each piece in paper.

5. Soil

• Collection

  • Use a clean scoop or spoon.

  • Take from different depths or layers if needed.

• Preservation

  • Store in airtight containers (like plastic vials).

  • Air dry before sealing if moist (to avoid mold).

6. Gunshot Residue (GSR)

• Collection

  • Use GSR kits with sticky swabs or stubs, usually within a few hours of shooting.

• Preservation

  • Seal swabs in labeled containers.

  • Avoid contamination by wearing gloves and masks.

7. Documents

• Collection

  • Handle with gloves to avoid fingerprints.

• Preservation

  • Store flat in document protectors or envelopes.

  • Keep away from moisture, sunlight, and folding.

8. Tool Marks / Impressions

• Collection

  • Photograph first.

  • Make casts using dental stone or silicone (for impressions in soil, etc.).

• Preservation

  • Store casts in sturdy containers.

  • Tools themselves should be packaged separately to avoid altering marks.

9. Weapons (Knives, Firearms)

• Collection

  • Use gloves and secure the weapon.

  • For firearms: unload and document the condition.

• Preservation

  • Wrap in paper and place in rigid containers.

  • Tag with safety warnings if still functional.

10. Clothing

• Collection

  • Air dry if wet or bloody (to prevent mold).

  • Use gloves.

• Preservation

  • Store in paper bags, not plastic.

  • Label and seal carefully.

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