Forensic Science Overview: Fingerprints, Bloodstains, Bodily Fluids, Arson, and Firearms

Formation of Fingerprints:

• Form during embryonic development from the dermal papillae.

• Created by ridges in the epidermis over the dermis.

• Friction ridges are permanent and unique, even in identical twins.

• Cannot be changed (unless deep dermal damage occurs).

Fingerprint Classification & Types:

• Three Main Patterns:

  • Loops (most common): Radial (toward thumb), Ulnar (toward pinky), have one delta and a core.

  • Whorls: Plain, Central Pocket Loop, Double Loop, Accidental; two deltas.

  • Arches (least common): Plain Arch (gentle wave), Tented Arch (sharp spike or angle); no deltas or cores.

Collection, Examination, and Enhancement:

• Types of Prints:

  • Patent: visible (e.g., in blood, ink).

  • Plastic: 3D (e.g., clay, wax).

  • Latent: invisible; need development.

• Collection Methods:

  • Physical: powder dusting, hinge lifters, Magna brush, SPR (wet surfaces).

  • Chemical:

    • Ninhydrin: reacts with amino acids (paper).

    • Iodine fuming: sublimates into vapor (non-porous).

    • Cyanoacrylate fuming: (SuperGlue for non-porous).

    • Silver nitrate: reacts with salts.

  • Special Lighting: alternate light source, lasers, luminescence.

• Documentation:

  • Photograph first!

  • Use measuring scales.

Three Basic Principles:

1. Individuality: No two fingerprints are the same.

2. Permanence: Remain unchanged throughout life.

3. Classifiable: Based on ridge patterns (loops, whorls, arches).

BLOODSTAIN PATTERNS:

Properties of Blood:

• Composed of plasma, RBCs, WBCs, and platelets.

• Surface tension gives droplets spherical shape.

• Blood is 6x more viscous than water.

• Passive, projected, and transfer stains differ based on force, surface, and direction.

Directionality, Area of Convergence, Area of Origin:

• Direction is indicated by the tail of the stain.

• Area of convergence: 2D location where bloodstains' paths meet.

• Area of origin: 3D location determined using angle of impact.

• Calculate angle: sin⁻¹(width ÷ length).

• Stringing method, lasers, and software help visualize it.

Spattered Blood:

• Caused by force applied to a blood source.

• Size/quantity/distribution depends on:

  • Force.

  • Volume of blood.

  • Surface texture.

• Impact mechanisms:

  • Gunshots: small mist-like droplets.

  • Stabbings/Beatings: 1–3 mm spatters.

  • Back spatter: entrance wound; can land on weapon/shooter.

  • Forward spatter: exit wound.

Other Bloodstain Types:

• Cast-offs: blood flung from moving object (linear pattern).

• Arterial spray: pulsing pattern due to breached artery.

• Expirated blood: expelled from lungs/mouth; may contain bubbles.

• Satellite spatters: secondary stains around a parent stain.

• Splashing/Projected: blood hits surface with force; ricochet possible.

• Transfer patterns: object moves through existing bloodstain (e.g., handprint).

Collection & Enhancement:

• Document direction, size, and location.

• Use Luminol or chemical reagents to enhance invisible stains.

• Photograph before collection.

• Collect entire item or cut out the stained area if possible.

ARSON, FIRE, & EXPLOSIVES:

Fire-Related Terms:

• Flashpoint: temp at which liquid gives off enough vapors to ignite (but may not sustain).

• Firepoint: temp at which vapors sustain combustion.

• Flammable liquids: flashpoint below 100°F.

• Combustible liquids: flashpoint above 100°F.

• Fire tetrahedron: fuel, heat, oxygen, chain reaction.

5 Ways Heat Is Produced:

• Chemical: combustion/oxidation.

• Mechanical: friction (e.g., machinery overheating).

• Electrical: shorts, arcing.

• Compressed Gas: diesel engines.

• Nuclear: atomic energy (e.g., power plants).

Fire Scene Examination:

• Exterior and interior examined for damage pattern.

• Identify point of origin (least damage → most damage).

• Determine cause: accidental, natural, incendiary, undetermined.

• Look for multiple points of origin = likely arson.

• Overhaul by firefighters can damage evidence — document carefully.

• Void patterns suggest objects were moved.

Accelerants:

• Substances used to start/spread fire: gasoline, alcohol, paper.

• Two types:

  • Petroleum-based (gasoline, kerosene).

  • Non-petroleum (ethanol).

• Detection: K9s, chemical sensors, lab analysis.

• Residue collected from porous items (carpet, wood).

Sample Collection & Analysis:

• Use unlined metal paint cans for debris samples.

• Collect 3 control samples from unaffected areas.

• Analyze using Gas Chromatography:

  • Headspace sampling (cold or heated).

  • Purge & trap, SPME, extraction.

Explosives & Alfred Nobel:

• Alfred Nobel invented dynamite by stabilizing nitroglycerin.

• Types:

  • Low explosives: slow burn (e.g., gunpowder, fireworks, pipe bombs).

  • High explosives:

    • Primary: sensitive, used as initiators.

    • Secondary: stable, powerful (e.g., TNT, PETN, RDX/C4).

    • Plastique (e.g., C4): moldable RDX-based explosive.

• Detection:

  • Dogs, ion mobility spectrometry, X-ray, GC-MS, LC-MS.

• Residue: low explosives leave more; high explosives less.

• Shockwave & fragmentation pattern can indicate explosive type.