FORS-2107EL Study Guide - Quiz Guide: Heat Sink, The Strokes, Cuticle
27 Jun 2018
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FORS-2107 - Quiz 3 Review
Paint
Definitions
-Dye: a colouring agent that is soluble in the medium in which it is dispersed
-Latex: A suspension of a pigment in a water based emulsion of a resin
-Paint: A suspension of a pigment in a liquid vehicle, or any surface coating
designed for protection and/or decoration of a surface
-Pigment: A fine powder that is insoluble in the medium in which it is dispersed
Collection and Preservation of Paint Evidence
-Questioned samples (Q) are those that will be compared against samples from a
known (K) source to determine whether or not they share a common origin
-Q samples should inclose all lose and transferred garments
-K samples are derived from a source under investigation
-When paint evidence is found, it is very important to manually recover all fragments,
prior to tape lifting of other trace evidence, since tape can complicate handling since
it may contaminate chemical features of the fragments by altering the apparent
chemical make up of the fragment by ripping layers off of the paint chip
-K samples should be collected from multiple sites close to the areas of damage, or
even those suspected as the site of transfer, because the process of damage may
have led to contamination or altering the layer structure and/or chemical makeup of
the fragments
Analysis of Paint
-It is critical that the known fragments (K) contain all layers of the original film for
analysis
-Vehicle paint chips have a layered structure that is usually characteristic of the
manufacturer, model and year
-1) Microscopy and spectrophotometry (UV-VIS or IR) can be combined to make
microspectrophotometry, allowing for spectroscopic measurements (absorbance) of
each layer to be done, which will give us chemical information and spectral colour
interpretations (what a colour actually is)
-This uses an FTIR micrscosope
Other examinations
-Other examinations can look at other paint features, like polymeric backbone, or
elemental consistuents
-2) Pyrolysis GC-MS pyrolyzes (decomposes with heat) the paint chip will be
analyzed to characterize the polymeric backbone constituents and dye in paint,
based on the pattern of products on the GC/MS chromatogram
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-This involves:
-Placing the paint sample in a heated vessel, with a very high temperature (about
800 degrees), under inert atmosphere (an unreactive atmosphere like N2)
-Then the pyrolysis products are transferred to GC column and into MS, then
products are identified by retention time and mass spectrum
Glass Analysis
-Glass is an amorphous material comprised of a variety of minerals and metal oxides,
SiO2 (silicon dioxide) and Al2O3 (aluminum oxide)
-When a crack occurs in a surface, it happens by the impact finding the most
thermodynamic weak spot of the surface, and then it will propagate out from here
Fracture Patterns
-Cracks around holes may be radial (straight lines that originate at the hole) or
concentric (circular lines that circle around the hole)
-Sequence of fractures can be found from the line patterns, since a crack will stop one
it meets another crack
-The energetics of fractures result in observable stress lines that can indicate the
directionality of the impact (conchoidal lines)
-In Radial lines: curve will originate on the side of impact, and curve away
-In Concentric lines: curve will originate on the opposite side of impact, and curve
towards the side of impact
Analysis of glass evidence
-Small fragments are usually recovered by shaking or brushing a garment over sheets
of paper, and then using a stereo microscope to examine the debris
-Microscopic techniques are used to:
-Examine the physical shape and colour of the glass fragment, to provide info on
the glass type, colour can be compared through Munsell charts or spectroscopically
-The issue with this is that colour is subjective, therefore it is not an accurate
measurement of analysis
-Also the cards used will fade in colour which will change the results
-We compare glass fragments with a sample of glass taken from scene, through
comparing the refractive index values (nD)
Refractive Index (GRIM)
-Refractive index refers to the modification of the speed of light in a medium, relative
to that in a vacuum
-nD = c/v, where c is the speed of light in a vacuum, and v is the speed of light in the
medium in question
-This can be measured by placing the fragment into an oil medium of a known nD and
known temperature dependency of nD , then the oil is changed, or the temperature of
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the oil is changed to view for becke lines (lines that show the outline of the piece of
glass)
-When beck lines are no longer visible, then the nD of the oil matched that of the glass
fragment
Trace Element Analysis
-Trace elements of glass are present at levels of nanograms/gram or micrograms/
gram, so the concentrations are typically too low to influence the nD measurements
-Trace elements can be analyzes through Atomic Emission Spectroscopy (AES) or
Atomic Mass Spectrometry (AMS)
-Here, the glass fragments are dissolved in strong acid (HF/HClO4) to make a
solution, the solution is then places into an ICP torch (flame) which is about 10 000
Kelvins, so hot enough to atomize/ionize any trace elements
-This excites the electrons in the solution, and when they come down to ground
state they will give off photons, which have wavelengths that can be characteristic
of elements present in the glass
-Then the vapours produced will be funnelled into a MS, and measures the m/z
(charge to size ratio) of the ion, allowing for determination of elements
Hair and Fibre Analysis
-Fibres, hairs, and other trace evidence can be transferred between people, and
between people and objects, with each contact event (Locard’s Principle and Trace
Evidence)
-Process of hair and fibre analysis:
-Collection of trace evidence
-Screening: hair and/or fibre identification
-Comparison: known hair/fibre vs questioned hair/fibre
-Textile damage assessment
Collection
-The collection process must prevent/minimize the potential for contamination of items
between each other or from the external environment
-Collections are normally done with tape, and sometimes can be picked up with
tweezers (in microscopic examination)
Evidence Handling
-Items are separated in to group that originate from common source, therefore 2
things being analyzed will not be touching each other so no accidental transfer occurs
-Items from different sources are taped in different rooms
-The examiner’s clothes are taped for control purposes as well, and a different lab
coat is worn for taping items from different sources
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Document Summary
Dye: a colouring agent that is soluble in the medium in which it is dispersed. Latex: a suspension of a pigment in a water based emulsion of a resin. Paint: a suspension of a pigment in a liquid vehicle, or any surface coating designed for protection and/or decoration of a surface. Pigment: a ne powder that is insoluble in the medium in which it is dispersed. Questioned samples (q) are those that will be compared against samples from a known (k) source to determine whether or not they share a common origin. Q samples should inclose all lose and transferred garments. K samples are derived from a source under investigation. It is critical that the known fragments (k) contain all layers of the original lm for analysis. Vehicle paint chips have a layered structure that is usually characteristic of the manufacturer, model and year. Other examinations can look at other paint features, like polymeric backbone, or elemental consistuents.
