Chapter 10: Trace Evidence

Intro to Forensic Chemistry

Trace Evidence

refers to materials transferred between people, objects, or the environment during a crime

Edmond Locard

forensic geologist who was most interested in dust that was transferred from the crimes scene to the perpetrator

Locard Exchange Principle

It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence

Types of trace evidence

Glass

Polymers

Plastic

Paints

Paper

Hair

Fiber

Soil

Tape

Physical Properties

Size

Diameter

Color

Morphology

Fracture edge matching

Refractive index

Chemical Properties

Composition (GC, HPLC, GC-MS)

pH

Spectroscopic properties (IR, NMR, UV-Vis, Raman)

Elemental composition (AA, Emission Spectroscopy)

Glass can be differentiated from

Color

Thickness

Texture

Uniformity

Curvature

Fracture edge matching

Types of Glass

Soda silicate (Water glass)

Soda lime silicate

Borosilicate

Aluminium silicate

Lead silicate

Density of Silica glass

2.20

Density of Soda lime silicate glass

2.49

Density of Sodium borosilicate glass

2.23

Density of Alkali silicate

3.02

Density of Aluminosilicate glass

2.64

Quartz Index of Refraction

1.41

Acrylic glass Index of Refraction

1.49

Polycarbonate Index of Refraction

1.58

Dense crown glass Index of Refraction

1.67

Diamond Index of Refraction

2.24

Forensic Geology

the application of earth sciences- rock, minerals, soils, fossils, and water- to criminal investigations and legal matters

Characteristics of soil

Percentage of components

Color

Density

Particle size distribution

Mineralogical content

Composition of Soil- Minerals

45%

Composition of Soil- Organic matter

5%

Composition of Soil- Water

25%

Composition of Soil- Air

25%

Types of Minerals

Quartz

Feldspars

Micas

Clays

Paint smears

Paint top layer of loose oxidized layer is transferred, or smeared, to another surface

Paint chips

Small piece of the paint layer of layers that have detached from the surface

Binder

film former/ adhesion

typically polymers such as epoxides or urethanes

colorant

dyes or pigments

additives

thickeners, fungicides/ biocides, dispersants

solvent

organic or aqueous

automobile paint

Four layers (electrocoat primer, primer surface, base coat, clear coat)

electrocoat primer

epoxide

primer surface

epoxide modified polyester or urethane

Basecoat

acrylic containing pigments and/or other additives

clearcoat

acrylic or urethane

Polymers

Polyethylene

Polypropylene

Polystyrene

Polyvinylchloride

Polyethylene terephthalate

natural fibers

25% of textiles

cotton, wool, silk, jute, hemp, mohair, cashmere

Synthetic fibers

75% of textiles

nylon, acetate, rayon, polyester, orlon

Acid dyes

used in basic conditions, functional group of fiber is protonated and bonds to functional group of dye

basic dyes

used in acidic conditions, functional group of fiber is deprotonated and bonds to functional group of dye

Azoic dyes

use diazo salt and a coupling agent to dye fibers

direct dyes

dye is applied using heat and electrolytes

disperse dyes

Van der Waal forces and hydrogen bonding dye the fibers

metallized dyes

metal complexes forms between the dye and fiber

sulfer dyes

dye is reduced and then oxidizes with the fiber to bond, the dye becomes insoluble

vat dyes

similar process to sulfur dyes, dye becomes insoluble with fiber once oxidation occurs