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Lecture 4

FSCI 1010U Lecture Notes - Lecture 4: Right Angle, Generic Point, Tiffany Glass


School
UOIT
Department
Forensic-Science
Course Code
FSCI 1010U
Professor
Nelson Lafreniere
Lecture
4

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Forensic Science Lecture 6 Trace Evidence
Trace Evidence
What is Trace Evidence?
Can literally be anything…
Locard’s Exchange Principle tells us that: Every contact leaves a trace
oPerson to person
oPerson to place
Many different types of transfers
oPrimary: Direct contact. (Ex. Soil from gravesite)
oSecondary: Indirect contact – An intermediate object is the site of transfer (ex.
Dog – Car – Dog hair on human)
Quite often the evidence that is transferred is small or even microscopic in size, making
analysis quite difficult
Should be of no surprise that our microscope is one of the most important tools of the
forensic trace analyst
oAnalysis is typically called “microanalysis”
Microanalysis
The application of a microscope and microscopical techniques to the observation,
collection, and analysis of micro-evidence that cannot be clearly observed or analyzed
without such devices.
Generally deals with samples in the microgram (Mg) or milligram (mg)
Often operate using different forms of energy: Visible, Ultraviolet, Infrared, Electron
Can get useful information like:
oMorphology: Size, Shape, etc.
oAnalytical data: What chemicals are inside?
How does it behave when exposed to light?
Instruments of Microanalysis
Many types of microscopes available for use in a forensic laboratory for a wide variety of
materials, we will only discuss a few:
1. Stereo microscope
2. Compound binocular microscope
3. Comparison microscope
4. Basic scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy
(EDS)
Light and Matter
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Forensic Science Lecture 6 Trace Evidence
If you probe matter with energy, you can obtain useful information used to tell you the
chemical composition of the sample.
Spectroscopy: the science of how radiation/energy interacts with matter
The higher the wave, the lover the energy, therefore the lower the frequency
The higher the frequency, the shorter the wavelength
Speed of light = Wave length x Frequency
Speed of light (c) in a vacuum = 3.0 x 108m/s
Stereo Binocular Microscope
The most likely to be employed first in the examination of evidence
Used in preliminary evaluation of submissions, and for the location of microscopic
particles and materials
Recovery of fragments of red wool fibers from a victim’s sweater found on the denim
jacket of a suspect in an assault case
Recovery of glass particles from the jeans of a burglary suspect
Stereo Binocular Microscope
Often employed first
This microscope is of a compound type
Total magnification is computed by multiplier the power of the
objective lens (OBJ) by that of the eyepiece lenses (EP)
Objective power x EP power = Total Magnification
Microscope is constructed from two similar, but separate
microscopes
The views are offset by 15° so that each eye sees a slightly different
perspective
Results in 3D image
Many significant preliminary and analytically important
observations are made with this microscope
This microscope works with reflected light, just like our eyes.
Compound Binocular Microscope
Second most common microscope
oCommonly used in science laboratories
Has two eyepieces and only one objective
o25-1200x is possible
o40-400x commonly employed in forensics
Mainly used to collect morphological information:
information relating to its structure (size, shape)
Micrometry: A calibrated micrometer placed in the EP of a
microscope enables accurate measurements to be taken
This microscope works with transmitted light
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Forensic Science Lecture 6 Trace Evidence
Comparison Microscopy
At least one type will be present in every major crime
laboratory
Can simultaneously view two independent images in one
field
oAlso allows superposition of the two images
Classical, transmission illumination, and even PLMs can be
oriented similarly
oCritical for a side-by-side comparison of evidence
Many experts suggest this is: the only valid way to compare
two pieces of evidence
Two microscopes joined by an optical bridge.
Scanning Electron Microscope (SEM) *Do not need to know
Enables far greater magnification (10 – 100,000x)
Principle of operation:
oBeam of electrons is swept over a small spot
oBeam of electrons interacts with surface, and
area slightly below the surface of the sample
oInitial beam of electrons removes additional
electrons from the sample, which are converted
to an image
oX-rays are also created, which can be measured
with an energy-dispersive x-ray spectrometer
(EDS) to obtain chemical information
Microscopic Evidence and Its Analysis
Glass Fragments
Common type of microscopic evidence, frequently encountered as transfer evidence
Broken and shattered glass fragments can place a suspect at a crime scene
Chips of broken glass from a window may lodge in a suspect’s shoes or garments
Particles of headlight glass found at the scene of a hit-and-run can lead to clues about the
identity of a suspect vehicle
What is Glass?
Glass is an amorphous solid
oIt does not have a regular/repeating shape
oA hard, brittle material that is usually transparent and lacks ordered arrangement
of atoms found in most solids
Borosilicate – Glass used for cooking
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