Chapter 19

the primary purpose of trace evidence analysis is to

Link people to places or items

Transfer

often microscopic, adding to the analysis challenge.

Persistance

How long evidence stays where it was deposited before it is lost or changed.

Prevalence

How common or rare a type of evidence is.

Recovery

The process of finding, collecting, and preserving evidence.

the

interaction [resulting in] depositing relevant materials is called

Action Of Interest (AOI)

Microanalysis

the application of a microscope and microscopical techniques to the observation, collection, and analysis of microevidence that cannot be observed or analyzed without such devices.

Refraction

The bending of light as it moves from one material to another (like air to glass).

Why it happens:

• Light changes speed when it enters a different material.

• This change in speed causes the light to bend

Refraction index

A number that tells how much a material bends light.

• A higher refractive index = light bends more.

• A lower refractive index = light bends less.

Focal Point

The point where a lens brings light rays together into focus.

In microscopes:

• This is where the image becomes sharp and clear.

Optic Axis

An imaginary straight line that passes through the center of a lens.

• Light is focused along this line.

Stereo Binocular Microscope

Often used in the preliminary evaluation of submissions and for locating and recovering microscopic particles and materials from their substrates, such as fibers from a carpet in a car trunk.

Total Magnification

computed by the power of the objective lens (OBJ), or first lens, multiplied by that of the eyepiece lens (EP), which is also called the ocular lens.

compound binocular microscope

they are found in schools and medical laboratories

micrometry

Dimensions and physical measurements are obtained using a specialized scale (a micrometer)

polarizer,

placed under the sample in the condenser mount

Analyzer

is positioned in the microscope’s body

plane-polarized light

propagating in a single plane

A polarized light microscope is a type of compound binocular microscope with two polarizing filters

the polarizer and analyzer

Isotropic

Light still interacts with the atoms, but the crystal’s effect is the same everywhere.

anisotropic

Light entering from different directions and angles will encounter different atomic environments.

retardation

difference in velocities

Birefringence

Birefringence is the property of some materials that split one beam of light into two separate beams as the light passes through them.

Interference colors

The bright colors seen when birefringent materials are viewed under a polarized light microscope (PLM).

privileged direction

Plane polarized light is obtained using polymer films in which the molecules are highly oriented and have been treated with a dye to absorb light vibrating in almost all but one direction.

Crossed polars

When two polarizers are placed so that light passes through one and then the second and the privileged directions of each are perpendicular, no light will emerge from the second.

Michel–Lévy chart

relates the birefringence, thickness, and retardation properties. Using the observed colors and measured sample thickness, the analyst can use the chart to obtain the birefringence, which is characteristic of the material.

Microspectrophotometry (MSP)

an area of microscopy that, over the past 25 years, has become widely used in trace evidence analysis.

What are the principle types of (MSP)?

visible and infrared microspectrophotometers

scanning electron microscope

permits viewing samples at much greater magnification and resolution than is possible with light microscopes. Magnification is possible in the range of 10–100,000 times. SEM is used with some trace evidence types like paint, but its’ primary application is for small particulates such as gunshot residue.

Backscattered electrons (BSEs) and secondary electrons

are emitted from the surface, converted to an electrical signal, and used to generate an image of the sample.

Radial Cracks

originating from the impact point and propagating away,

concentric cracks

which seem to circle the impact point.

When glass fractures, the edges often show characteristics referred to as

conchoidal lines.

Immersion Methods

The immersion method is a technique where a small piece of evidence (such as a glass fragment or fiber) is placed in a liquid with a known refractive index and viewed under a microscope.

The Becke line method

The microscope is focused on the sample, and then the focus is adjusted by raising the objective lens. This action creates a halo or brightness near the edge of the sample (the Becke line), which will appear to move into the material of greater RI, whether the sample or the mounting medium.

What is glass composed of?

composed of fused inorganic materials

What type of glass is resistant to thermal shock and used in laboratory glassware?

glass with a high boron content

glass refractive index measurement (GRIM)

• Determine the refractive index of a glass sample.

• Compare Questioned (Q) glass with Known (K) glass.

• Help determine whether two glass fragments could have come from the same source.

How it works

1. Place a small glass fragment in a special immersion liquid.

2. Heat the liquid slowly.

3. Watch the glass under a microscope.

4. At a certain temperature, the liquid and the glass have the same refractive index.

5. The instrument records the refractive index of the glass.

Elemental analysis

the most common type of chemical analysis used for glass.

Natural Fibers

(cotton, silk, wool)

Synthetic fibers

(nylon, polyester, etc.)

Delusterants (Physical methods)

used to reduce the shine of some fibers.

chemical methods link to the analysis of fiber color (dyes)

to determine if similarly

colored samples come from the same or different dye compositions, as an example

paint stratigraphy

Layering

Vehicle in paint

the binder that holds all the components together and is usually polymeric, consisting of natural or synthetic resins.

Lacquer

film forms by the simple evaporation of the solvent system of the liquid, the paint. dissolve in many organic solvents

Extenders

help paint flow smoothly when applied. As with many fibers, pigments supply paint with color, hue, and saturation. Pigments may be organic or inorganic.

Paint Data Query (PDQ)

international database maintained by the Royal Canadian Mounted Police. This database contains more than 21,000 entries with information on ~85,000 layers of automotive paint.