Trace Evidence(Part I)

Summary

Microscopic Analysis Types of Trace Evidence

Glass Hair (fur) Fibers

Paint Soil Gunshot Residue

What is Trace Evidence?

Trace analysis uses microscopes and other instrumentation.

Other sections (Drugs, Firearms) also use microscopes.

Loosely defined, trace evidence is small evidence used to link victims, suspects, objects, and crime scenes.

Microscopic Analysis

Who analyzes trace evidence?

Different evidence could be assigned to different sections (hair:biology, fiber:chemistry)

Some labs have trace or microscopy sections.

Labs sections are arranged differently in different labs.

Using the Microscope

Microscope magnifies sample.

Many different types of microscopes are used.

Also can look at more detailed traits of samples.

Trace > Microscopes

Microscope terminology

3-D object vs. Microscope slide 3-D object: put any object under the mic Slide: requires preparation

Surface analysis vs. Look through object Surface: Looking at surface of object (normal) Or can shine light through transparent sample

Trace > Microscopes

Microscope terminology

Reflected light vs. Transmitted light Refected: bounces off object (normal light) Transmitted: passes through object

Trace > Microscopes

Stereo Binocular Microscope

Look at 3-D objects Usually look at surface Usually use

reflected light

Trace > Microscopes

Stereo Binocular Microscope

Preliminary search of objects (clothes) for small fibers, hairs, etc.

Look at paint chip layers, measure fibers, bullet striations, etc.

Trace > Microscopes

Compound Microscope

Can be binocular or one ocular

Look at transparent object on prepared slide

Look through object Use transmitted light

Trace > Microscopes

Compound Microscope

Analyze fibers, hair, glass for optical traits.

Biological samples for sperm cells

Trace > Microscopes

Key terms:

Ocular lens Lens in the eyepiece

Objective lens Lens above the sample

Trace > Microscopes

Key terms:

Ocular lens Lens in the eyepiece

Objective lens Lens above the sample

Trace > Microscopes

Key terms:

Magnification - Amount the object is enlarged

Working Distance - Distance between the object and objective lens

Micrometer - “ruler” in the eyepiece, allows for measurement of sample.

Trace > Microscopes

Polarized Light Microscope (PLM)

Trace > Microscopes

Polarized Light Microscope (PLM)

Takes advantage of the optical properties of glass, crystals (chemicals), and fibers.

Sample absorbs light differently depending on its orientation in polarized light.

Trace > Microscopes

Comparison Microscope

Trace > Microscopes

Comparison Microscope

An optical bridge allows viewing of evidence side by side.

Useful for comparing bullets, fibers, hair…

Trace > Microscopes

Summary of Trace

Why is trace evidence useful?

Collecting trace evidence

Glass Hair (fur) Fibers

Paint Soil Gunshot Residue

Types trace evidence:

Why is Trace Evidence Useful?

Can link objects and people.

SuspectVictimLocationObject

Trace

Object

Trace evidence on hammer may include:

Blood/Tissue from Victim

Blood/Fingerprints from Suspect

Fibers from Rug in van

Trace

Location

Trace evidence on rug may include:

Blood/Tissue from Victim

Blood from Suspect

Trace

Victim

Trace evidence on Victim may include:

Blood/Semen from Suspect

Fibers from Rug in van

Trace

Suspect

Trace evidence on suspect may include:

Blood/Tissue from Victim

Fibers from Rug in van

Trace

How is Trace Evidence Transferred?

Locard Exchange Principle: Whenever there is contact between two objects, they will leave or pick up debris from the other object.

During a crime, there is always be a transfer of evidence.

The difficulty is finding & collecting this evidence.

Trace

Collecting Trace Evidence

Who collects the evidence?

Depends on the state/community

Police Officer Crime Scene Investigator Forensic Scientist

Often one person to ensure consistency of labeling

Collect trace or entire object?

Suppose a glove appears to have glass, fibers and blood on it.

Should the glass, fibers and blood be removed and packaged separately?

Should the entire glove be packaged?

Trace > Collecting

Considerations before packaging entire object:

Object may be too large or difficult to move Trace evidence may fall off item during transport. Trace Evidence may be transferred to different,

irrelevant area of object.

If packaging object, package objects separately.

Prevents trace being transferred to other objects.

Trace > Collection

NEVER package known material with evidence.

Example in book:

Suspect’s clothes had tar on the knees of pants.

His clothes were collected at the station.

Tar was collected at the crime scene.

The clothes and the tar were packaged in the same bag.

Whoops!

Trace > Collection

These 3 methods can be done at the crime scene or in the crime lab.

1. Visual Inspection

2. Tape Lift

3. Vacuum

Trace > Collection

Use naked eye or hand lens. Evidence removed and packaged for later

analysis Use bright light and forceps to collect.

Trace > Collection

Visible Inspection

Small paper envelopes are bad (Holes allow small objects to escape).

Use small plastic bags, glass vial or paper using a druggist fold.

Double package. Label each package.

Trace > Collection

Visible Inspection (Packaging)

Clear tape is used. Repeatedly apply tape to small area until

most of the stickiness is gone. Tape is folded back upon itself, taped to a

glass slide or taped to a piece of plastic. Put in separate labeled container.

Be sure to document specific area covered.

Trace > Collection

Tape Lift

Vacuuming

Nozzle should be short and transparent.

Debris is collected on a filter or membrane

Trace > Collection

Small area is vacuumed. (Filters changed frequently)

Filters packaged in separate labeled container. (Be sure to document specific area covered)

Most improperly used method because it often results in the collection of a lot of irrelevant material.

Vacuuming

Trace > Collection

What is the purpose of analysis?To identify the source of the collected evidence.

Fiber recovered from victim.Source: Matches fibers from rug in suspect’s van

Soil found on Suspect’s shoeSource: Matches soil at crime scene

Blood found on suspect’s couchSource: Matches blood of victim

Trace > Analysis

The Catch:

With trace evidence, an investigator usually cannot say that one piece of evidence definitely originated from a specific item.

The investigator can only tell the jury what similarities were found and give them an idea of how rare those similarities are.

Trace > Analysis

Classifying Evidence:

Most trace evidence is classified using class characteristics (color, shape, refractive index, etc.)

When examining class characteristics, absolute identification is not possible.

The Forensic Scientist’s main objective is to give the jury an idea about how rare the category is.

Trace > Analysis

Classifying Evidence:

If physical properties differ, they did not come from the same source.

Exclusion is possible.

Trace > Analysis