TRACE EVIDENCE

Locard’s Exchange Principle

Edmond Locard was the director of the very first crime laboratory in existence, located in Lyon, France.

Locard’s Exchange states that "with contact between two items, there will be an exchange."

What is Trace Evidence?

Material found at a crime scene or accident scene in small (maybe almost invisible) but measurable amounts

It can definitively link an individual or object to the crime or accident scene

Microscopy is the single most important method of examination & comparison

Vocabulary

Compound microscopeMonocular BinocularStageBody TubeArmObjectiveNosepieceBaseLensFocus

MicrobeTrace evidenceWet mount slidePollenSporePalynologyPhysical propertyChemical propertyDecantCheiloscopy

Examples of Trace Evidence

Paint

Physical and chemical analysis of paint evidence (chips or residue) can indicate it’s class, such as automobile paint, house paint, nail polish, etc.

Most paint evidence submitted to a lab will come from hit-and-run cases involving automobiles.

Paint Transfer on a Car

Glass

Analysis of tint, thickness, UV fluorescence, density, and refractive index can help determine the original source of the glass.

The pattern of cracks in a windshield fracture can reveal information about speed, occupant position, and angle of impact.

Explosives

Explosive substances can be examined to determine its chemical composition to identify the type of explosive used and its origin.

Traces of explosives found on a suspect’s clothing, skin, hair, or other objects may be matched to explosives from the crime scene.

Materials used to make an explosive device will be compared to evidence found in the suspect’s possession to confirm a match.

Ballistics

Characteristics of ammunition, firearms, and residue are examined to find matches between suspects and the evidence found at a crime scene.

Chemical tests can reveal gunshot residue (GSR) on the hands, face, or clothing of a victim or suspect to indicate how close a person was to a fired gun.

Rifling (grooves) in a gun barrel causes distinctive grooves, indentations and scratches upon fired bullets, which can be matched to the weapon that fired them.

Police are able to search the Integrated Ballistics Identification System (IBIS) database to compare markings from bullets, cartridge cases, and shotgun shells to ballistic evidence.

Dust, Dirt, Pollen

Dust, dirt, or sand evidence can reveal where a person has traveled and may be picked up at a crime scene or left behind.

Pollen can provide information on the time of year or how long an evidence item was exposed to the elements.

Impression Evidence

Shoe prints and Tire tracks Shoes and tires show wear patterns after being used for a

period of time as well as other features (scratches, nicks, and cuts) that can be used to match evidence to specific items.

Bite marks Can be matched to dental records

Tool marks Tiny nicks and chips form on the edges of a tool as it is used Tools may also pick up traces of blood or other substances

that can be tested

Physical Matches

Comparison between two cut, broken, or torn objects to determine if they were at one time part of the same object

DNA and Body Fluids

Investigators can extract DNA from almost any tissue, including hair, fingernails, bones, teeth and body fluids. The DNA is used to create a profile that can be compared to profiles from suspects or victims.

Blood, semen, saliva, sweat, and urine can be analyzed to give investigators information about the crime as well as its victim or the suspect.

Insects

Flies, beetles, and other insects can provide useful clues about a corpse.

Forensic entomologists use factors such as weather conditions, the location and condition of the body, and their knowledge of the life cycles of insects to help them estimate the postmortem interval or PMI (the time between death and the discovery of the body).

Hairs and Fibers

Hairs and fibers may be transferred from the suspect or the suspect’s clothes to the victims’ and vice versa. For example, a suspect may pick up carpet fibers on his shoes or leave hairs behind at a crime scene.

Hairs can be examined to identify their origin, such as human or animal. Hairs with roots intact can be tested for DNA.

Fibers are used to make clothing, carpeting, furniture, beds, and blankets. They may be natural fibers from plants or animals or synthetic fibers that are man-made.

T. Trimpe 2006 http://sciencespot.net/

Reference

MICROSCOPES!

A. Types of microscopes 1. Compound microscope.

A. The eyepiece system may be monocular, binocular, or trinocular.

B. Monocular observation uses one eyepiece, binocular observation uses two. Trinocular microscopes contain an additional upright ocular which is utilized when the microscope is used with a photo or video system.

C. Compound microscopes are used to observe small animals in water, sections of plants, and/or animal and plant cells, hair & fibers

MICROSCOPES!.

B. Parts of a microscope1. Eyepiece (Ocular)—contains lenses to increase magnification. It may be replaced with lower or higher magnification.

2. Body Tube—holds lenses of ocular and objectives at the proper working distance from each other.

3. Nosepiece—permits interchange of objectives.

4. Objectives—Contains lenses of different magnifications: usually low, medium and high power objective magnifiers.

5. Stage—supports slide over opening that admits light from mirror or lamp.

MICROSCOPES!

6.Stage Clips—holds slide firmly in place.

7. Course Adjustment—moves body tube or stage up and down.

8. Fine Adjustment—permits exact focusing by moving stage or body tube up or down very slightly.

9. Diaphragm—regulates the amount of light passing through the specimen.

10.Light Source—directs light upward through the diaphragm and hole in stage.

11. Arm—supports the body tube and course adjustment.

12. Base—firm support that bears the weight of the microscope.

MICROSCOPES!

How to use a microscope Carry the microscope with one hand on the arm and

one hand on the base. Carry it close to the body. Remove the cover and plug in the microscope. If the

cord is too long, place the excess cord on the table (do not let the cord dangle over the edge of the table).

Always start with the lowest power. Place the slide on the microscope stage with the specimen directly over the center of the glass circle on the stage (directly over the light).

DEMONSTRATE PROPER USE, CLEANING, AND STORAGE OF A COMPOUND MICROSCOPE AND STEROSCOPE.

On the low power, lower the objective lens using the course (large) adjustment until it is at the lowest point. Then slowly raise the lens using the course (large) adjustment until you see the specimen within the field of view. Then use the fine (small) adjustment to see the specimen clearly.

At this point adjust the diaphragm while still looking through the eyepiece. This will allow more or less light. More detail will be seen when less light is allowed in through the diaphragm. Too much light will give the specimen a washed-out appearance.

DEMONSTRATE PROPER USE, CLEANING, AND STORAGE OF A COMPOUND MICROSCOPE AND STEROSCOPE.

Once the specimen is clear on low power, center the specimen in the field of view, then, without changing the focus knobs, switch to the next higher power.

Once on this higher power only the fine adjustment can be used. Use of the course adjustment at this point may scratch or crack the slide.

The highest power is an oil immersion lens. If this lens is used without oil, it may ruin the lens.

MICROSCOPES!

a. Trouble shooting. If you wear glasses take them off. If you see only your eyelashes, move closer. If using a monocular microscope, cover or close your

other eye. If you see a dark line that goes part way across the

field of view, turn the eyepiece. The dark line is a pointer that is a very valuable tool when you want to point out something to your lab partner or your teacher.

DEMONSTRATE PROPER USE, CLEANING, AND STORAGE OF A COMPOUND MICROSCOPE AND STEROSCOPE.

How to clean and store a microscope Only use lens paper to clean the lenses. A paper towel

may be used to clean the stage. Lower the stage completely. Return the lens to the lowest power. Wrap cords loosely around the microscope and replace

the cover. Return the microscope to the storage area carrying the

microscope with one hand on the arm and one hand on the base and close to the body.

3.10 PREPARE A WET MOUNT SLIDE

A. Wet mount slide preparation. Use a flat glass slide. The slide should be clean and free

of dust or other particles. Draw a few drops of the liquid specimen into a medicine

dropper. Pick up the glass slide with one hand grasping it by the

outer edges. Place one drop of the liquid specimen from the medicine

dropper on top of the slide. The drop should be placed in the center of the slide.

Do not tilt the slide in your hand.

PREPARE A WET MOUNT SLIDE

Use your free hand to carefully pick up a cover slip (they are extremely fragile) grasping it by the outer edges.

On an angle, place the cover slip on top of the specimen on the slide. The edges of the cover slip should match up with the edges of the slide. DO NOT PRESS DOWN ON THE COVER SLIP.

Holding the slide by the outer edges, keep the slide as horizontal and steady as possible. Place the slide on the stage of the microscope. The specimen should be placed over the opening of the diaphragm.