Trace Evidence

Trace evidence is used to…

Corroborate evidence, recreate the scene and witness statements

Does trace evidence conclusively link someone?

NO, it is supplemental

Types of Trace Evidence

Paint, fiber, impressions, and hair

Small and hard to detect

Paint evidence

• Most commonly in hit-and-runs or burglaries

  • Transfer between vehicles

  • Transfer from building to burglary tool

Paint components

Solvent, binder, pigment, and additives

Paint binder

Support medium, polymeric

Examples of binders

Acrylic, resin, enamels

Paint pigments can be…

Inorganic/organic, filler/extender, or metallic/pearlescent

White pigment is made of:

Titanium dioxide

The more matching layers of paint…

The more likely paint is from the same source

Paint ID Methods

Stereomicroscopy, visible microspectrophotometry, FTIR, Pyrolysis-GC-MS

Fiber comparisons

Used to link someone to a crime by determining donor and recipient fabric

Two methods of fiber comparison:

1. Scrape article over paper, into a holder

2. Roll with a piece of tape

Both examined microscopically

Natural fibers

Plant/animal: wool, mohair, cashmere, camel, cotton, bast, silk, flax, hemp

Cotton

Most common natural fiber with dog-bone cross-section and convoluted, longitudinal twists

Hemp

Produced by Cannabis sativa

Mohair

From angora goat

Cashmere

From Asiatic goat

As number of matching fibers increases….

Significance of giber comparison increases

Fibers are usually…

Class characteristics

Finer vs. Thicker fibers

Finer transfer more than thicker

Coarse vs. Smooth fabric

Coarse transfers more

Factors affecting fibers:

• Type of fiber (coarse vs. smooth vs. fine vs. thick)

• Area of contact (greater area = more fibers)

• Number of contact (more contact = more fiber)

• Force of contact (more force, more fiber)

Primary transfer

Direct transfer from donor to recipient (clothes on a person)

Secondary transfer

Fibers are left on an object, which comes into contact with someone else

Tertiary Transfer

Secondary transfer contact a third person

Fiber Persistence is affected by:

• Force of contact

• Wearing of garment

• Location/contact with other clothing

Does failure to find fibers mean lack of contact?

NO, unless suspect was immediately apprehended

(Remember: trace IS NOT conclusive)

Conclusions of fibers

1. Fibers are consistent with originated from known sample

2. Fibers are not consistent with originating from known sample

Strength of fiber evidence increases if….

1. Several fibers of the same type are consistent

2. More than one color/type match the known

3. Fibers match more articles of clothing

4. Two-way transfer and unexpected placement

Fiber evidence is weaker if:

1. Common type of fiber (ex. cotton)

2. Associated with large # of unassociated fibers

3. Contaminated

4. Not completely examined

Garments worn for a long time (old):

• Tangle/matted ends

• Foreign fibers enmeshed

• Matted yarn is soiled

New garments:

• Straight and untangled ends

• Few foreign fibers

• Brighter appearance of extended fibers

Cut fabric:

• Few protruding fibers

• Little distortion/puckering

• Direction not influenced by weave of fabric

Torn fabric:

• Direction follows weave of fabric

• Protruding fibers

• Puckered fabric

Detection of microtraces:

• Detect with magnification and bright light

• Isolate in the pure state

• Compare after identification

• Evaluate and draw conclusions

Collecting trace evidence:

• Hand-pick based on location, size, appearance, quantity, color

• Method of choice: adhesive tape

  • Secondary is vacuuming (indiscriminately deep particles)

Impressions

Footprints, fabrics (automotive finish), tire prints

Analysis of Impressions

1. Compare questioned to test impression

2. Test impression is made in lab

3. Compare to scene and other crime scenes

First step in impression analysis:

PHOTOGRAPH

How are photos taken?

• Axis of camera perpendicular to plane of substrate

• Scale included

• Oblique lighting (in case of 3-D impression)

Dental Stone

Used to cast impression in soil

Snow print wax

Used alongside dental stone to cast snow impressions

Common for tire and shoe impressions

Ink surface of shoe/tire and imprint on white paper, then make a transparency

Individual characteristics of impressions:

• Missing piece/cuts

• Little as one different marking

Absence of distinguishing mark DOES NOT exclude object

Footwear Impressions

Based on type, make, description, size of shoe

Tire Impressions

Used to determine tire size, manufacturer, sometimes the car

Hair main parts:

Cuticle, cortex, medulla

Hair grows from the….

Papillae (embedded in base of follicle of dermis)

Shaft of hair

Sticks out of follicle

Hair is composed of:

Keratin (salt, disulfide, and hydrogen bonds)

Cuticle

Outer scale layer of overlapping cells

Maintains structure/protects hair

Cortex

Major part of hair

Cortex components

Elongated cells, containing pigments: eumelanin and pheomelanin

Eumelanin

Brown/black pigment

Pheomelanin

Yellow-red pigment

Medulla

Central core of shrunken cells with or without pigment

Anagen phase

Active growth for 2-6 years

Catagen phase

Regression for 2 weeks

Telogen phase

Resting for 1 year

Forceful removal of hair can only be determined in which phase?

Anagen because there will be tissue present

If no tissue, CANNOT make conclusions

Factors in hair comparison

• Color

• Structure (diameter, medullation, cross-sectional)

• Treatment (cuts, dye, roots)

Fundamental concepts of hair:

• No two specimens from 1 person are identical

  • Do NOT need to match all characteristics

• Exemplars needed

• Unusual characteristics don’t mean association

• Significant differences = different sources

Hair recovery:

1. Picking off

2. Scrape

3. Tape

Head vs. Pubic Hairs

Pubic hairs are more probable to be from the same source due to less distinguishing characteristics