Forensic Sci 20- Unit 2 Trace Evidence

Locard’s Principle of Exchange and it’s importance

When a person comes into contact with an object or another person, a cross-transfer of physical evidence can occur.

Importance: guides investigators to thoroughly examine the scene for any potential evidence.

-can help link suspect to crime scene

-can help rule out suspects

-can help establish a timeline and build a case

can help identify victims

Because fibers are _____(1) they are considered ________(2) evidence.

1. mass produced

2. class

Why is early collection of fibers important?

Most fiber evidence (95%) falls off or is lost from a crime scene within 24 hours.

Types of fiber transfers

Direct: fibers are transferred directly from victim to suspect, or victim to scene (vice versa)

Secondary: fibers are transferred when an individual picks up fibers from the environment and transfers them to another person during contact

What information do forensic scientists want to determine from fiber evidence?

-type (composition/uniqueness)

-color (dye)

-number (more=more violent bc longer contact period)

-where

original of textile

-multiple types?

-type of crime

-time between crime and fiber discovery

Natural vs Synthetic fiber structure

Natural: derived from animal or plant sources.

• animal: made of proteins. EX: wool and cashmere (sheep), mohair (goats), silk (caterpillar cocoons)

• plant: made of cellulose- they absorb and cannot dissolve in water and are very resistant to damage from harsh chemicals. They become brittle quickly. EX: cotton, coir (coconuts), hemp

• mineral: EX: fibreglass and asbestos

Synthetic: manufactured from natural raw materials. Makes up 50% of fabrics. EX: rayon (common), celenese (carpets), nylon (easily broken by light and concentrated acid, similar to polyester), acrylic (inexpensive and balls easily, wool/fur substitute), polyesters (wrinkle resistant and added for strength as it’s not easily broken down)

Common fibers and structural differences:

cotton: flattened hose appearance, up to 2 inches long tapering to blunt end, may have frayed root, hollow core

flax: “bamboo stick”, straight with angles, “nodes” every inch or so, often in bundles

silk: do not taper, small variations in diameter, no internal structure

wool: surface scales may be visible, hollow or partial hollow core, fibers up to 3 inches long tapering to a fine point

synthetic: vary widely in cross-sectional shape and diameter, generally straight to gentle curves, uniform in diameter, may have surface treatments that appear as spots

Basic structure of hair

All hair consists of a follicle and shaft. It is produced from the follicle during fetal development. The end of the follicle is the papilla, a network of blood vessels that supply nutrients to feed the hair and help it grow. The bulb also contains a sebaceous gland to secrete oil. Arrector pili muscles cause hair to stand upright and nerve cells to respond to the environment. The hair shaft is composed of the protein keratin that makes it strong and flexible.

Shaft:

• cuticle- transparent outer layer that protects hair. made of scales that overlap

• cortex- middle layer and largest. it contains pigment granules. (eumelanin gives color to brown/black, pomeranian gives color to blonde or red). It provides hair with strength, elasticity, and determines the texture and quality

• medulla- central core that can be hollowed or filled with cells.

Medulla: hair is classified into five different groups depending on the appearance of the medulla. Human hairs generally have no medulla or one that is fragmented. Continuous medulla is often found in indigenous peoples and asians.

• continuous- one unbroken line

• interrupted (intermittent)- dotted line at regular intervals

• fragmented/segmented- unevenly spaced dotted line

• solid- pigmented filing both medulla and cortex

• non- no separate pigmentation in the medulla

Differences in hair

Shape, length, diameter, texture, and color

What type of evidence is hair

class- no follicles attached

accidental- follicle with DNA present

Why are follicles important?

It contains DNA and can lead to individual identification

What is blood made of

Red blood cells: erythrocytes, 45% of blood cells, contain hemoglobin and carry oxygen and carbon dioxide, no nucleus and no nuclear DNA

White blood cells: leukocytes, immune system cell that produces antibodies, contain DNA

Platelets: thrombocytes, small cell fragments that assist in blood clotting

Plasma: 90% water, 10% dissolved proteins, nutrients, and waste, they transport nutrients, hormones, and proteins to parts of the body that need it, they hold RBCS and WBCS in suspension

Where is blood made?

Bone marrow (spongey material in middle of bones)

What type of evidence is blood?

RBC: class- blood type, but no nucleus or mitochondria (no DNA)

White: individual/accidental evidence

Blood drops/spatter: circumstantial (location of wound, movement of a victim, type of injury, how a crime might’ve happened)

Blood types and their role in crime scene investigation

A and B proteins called antigens are found in the surface of some RBCs that determine the type. It can help narrow down and link a suspect to a crime.

What can analysis of blood spatter determine?

Direction, Angle, Point of origin, Velocity, Manner of death

How does blood fall?

Cohesion: blood sticks together as it falls, it resists flattening out

Satellite (small secondary droplets around the main drop): from height or high velocity, blood overcomes its natural cohesiveness

Spikes/extensions: when blood falls on a less than smooth surface

Circular vs Tear drop blood stain

circular: blood drop fell straight down without force (typically drip wound)

elongated/tear drop: blood travelled from a different direction when it landed. Point of impact may appear darker and wider than the rest of the drop, with a tail pointing in the direction of travel. Smaller secondary droplets may break off and will land in front of the moving droplet.

How to determine area of convergence

Tracing blood drops back to a point of intersected axis.

How to calculate angle of impact

sin-1 (width/length)

Blood smear/ spatter patterns

Smear patterns form a large volume of blood and are often distorted so much that further classification is not possible. Transfer patterns occur when a wet bloody surface contacts a section installed surface creating a recognizable portion.

swipe pattern: transfer of blood into a a surface not already contamined with blood. one side is usually feathered and indicates direction of travel.

wipe pattern: created when an object moves through blood that has not dried and alters it.

passive drops: created from force of gravity

arterial gush: blood exiting out of pressure from an artery and striking a surface

expiratory blood: blood blown out of a nose or mouth as a result of air pressure, often with air bubbles. injury is often caused by internal bleeding

shadowing/void: area or object devoid of blood spatter, leaving void spot

Blood velocity and spatter size

high velocity (less than 1 mm): fine mist, gun shot

medium velocity (1-4mm): beating or stabbing

low velocity (4-6mm): beating with bat or pipe

Documentation and preservation of blood evidence

• Luminol can be used to detect hemoglobin left behind. the area with be luminescent for about 30 seconds. it destroys blood evidence

• deliver blood or stained objects to lab immediately

• if unable to deliver, object must be mailed and air dried completely before packaging

• blood in pools should be absorbed by a gauze pad and allowed to air dry. then it should be refrigerated or frozen asap. delays beyond 48 hours to the lab may make the samples useless.

• lab needs to confirm if it is human blood