Forensic Science: Evidence Collection and Analysis Techniques

Locard's Exchange Principle

Concept: "Every contact leaves a trace." When two objects come into contact, material is transferred between them.

Importance of Locard's Exchange Principle

Foundation of forensic science — supports the idea that physical evidence can link suspect, victim, and scene.

Cuticle

Outer layer (scaly) of human hair.

Cortex

Middle part of human hair, contains pigment.

Medulla

Central core of human hair, varies in humans.

Root

May indicate if hair was forcibly removed.

Growth stages of hair

Anagen, catagen, telogen.

Medullary Index

< 1/3 for human hair and > 1/2 for animal hair.

Human Hair Pigment Pattern

Consistent pigment distribution.

Animal Hair Pigment Pattern

Banded or varied pigment.

Cuticle pattern of Human Hair

Imbricate (flat).

Cuticle pattern of Animal Hair

Coronal or spinous (varies).

Natural fibers

Cotton, wool, silk.

Synthetic fibers

Nylon, polyester, acrylic.

Plain weave pattern

Simple over-under.

Twill weave pattern

Diagonal pattern.

Satin weave pattern

Smooth, few interlacings.

Collection of Fibers

Use tweezers, tape lifts, or vacuums. Package separately.

Microscopy in Fiber Testing

Shape, color, cross-section.

Burn test for Fibers

Odor, ash, melting.

Chemical tests for Fibers

Solubility, reactions.

FTIR/Spectrometry

Fiber composition analysis.

mtDNA (Mitochondrial DNA)

Found in mitochondria, inherited from mother.

Limitations of mtDNA

Less specific than nuclear DNA; can't distinguish between maternal relatives.

PCR (Polymerase Chain Reaction)

Amplifies small DNA samples to generate enough for analysis.

Steps of PCR

Denaturation → Annealing → Extension.

Benefit of PCR

Can analyze degraded or minute samples.

Electrophoresis

DNA fragments separated by size using an electric current.

STR (Short Tandem Repeats)

Examines repeating DNA sequences; highly specific to individuals.

Use of STR in forensics

DNA profiling and matching.

Types of Firearms

Handguns (pistols, revolvers), Rifles (long barrel, rifled), Shotguns (smooth barrel).

Ammunition components

Bullet: Projectile, Casing: Holds gunpowder and primer, Caliber: Bullet diameter.

Calibers

A measurement of the bore size of a rifle or handgun from land to land (how big the barrel is). Higher caliber= larger gun.

Gauge

The diameter of the shotgun barrel, indicating how many lead balls of that diameter it would take to weigh one pound. Higher gauge= more balls of shot, hence smaller.

Primer

An explosive chemical compound that ignites the gunpowder when struck by a firing pin.

Gunpowder

A chemical mixture that converts to an expanding gas when ignited.

Wad

In shotguns, a shot seal made of paper or plastic that separates the powder from the slug or shot.

Class characteristics

Model of firearm, ammo brand, caliber, gauge, or material.

Fiber Evidence Collection

Items are placed into individual bags with any fibers still attached. They are then hung up and scraped at the lab.

Flame Test for Fibers

Compare how the fibers react when on fire.

UV Fluorescence

Some fibers glow under UV light.

Chemical Composition of Fibers

Determine the exact chemical composition of a fiber and the colored dye in the fiber using gas chromatography-mass spectrometry.

Chemical Reaction of Fibers

Different fibers react differently to different chemicals.