Paint Chip Analysis by Stereo Microscope and IR Spectroscopy
As part of Arson Case # 1140509, a fire at a Grand Rapids area college, you are receiving 6 paint chip
samples for testing. One of the samples was found at a parking structure nearest the building where the fire
took place. The paint was found on a pole in the structure and it appeared to have been left there by a car
that was leaving in a hurry. The parking structure was just recently painted as part of college renovations, so
this scrape mark was very noticeable. A chip of the paint was removed from the pole. The paint appears to be
a dark blue/grey color, but analysis needs to be conducted. Samples from the five suspects cars were also
collected. Three of the cars had damage that could have coincided with scraping of a parking structure pole.
You will do two separate analysis of the paint chips. The first analysis will be microscopic. Paint is applied
to cars in layers similar to the pattern below:
If a piece of paint is removed from the body of the car and observed on its edge, these layers can be
observed. The chip of paint from the scene and chips from each of the five suspects have been placed in
putty on a microscope slide. By observing these under a Stereo Microscope, you will be able to compare the
layering pattern of the chips to see if any of them are possible matches. Complete your observations below:
Magnification of samples:______________
Sample:
Apparent surface color
Pattern of layering - outside color first
Scene (S)
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1
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2
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3
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4
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5
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Does the scene sample appear to match any of the suspects’ car chip samples? Explain:
What are some problems with using only visual microscopic comparison of color?
Analysis using IR Spectroscopy:
Infrared spectroscopy involves shining infrared light, light waves that have less energy than visible light,
through a liquid or gaseous sample or reflecting them from a solid sample. As the infrared light hits a
compound, certain types of bonds found in the compounds absorb the IR energy causing the bonds to rotate
or vibrate. The beam that is shined on the sample is split and the light that interacts with the sample is
compared to the light that was originally emitted. A pattern of absorbance/transmittance is graphed and
appears as a fingerprint for the compound. This is especially useful with larger organic compounds because
of the multiple bonds present in the compound.
Infrared Light has lower energy and longer wavelengths
than visible light.
As IR hits a compound, certain wavelengths
are absorbed by the bonds of the compoundthe transmitted waves can be measured.
The graphs you will obtain will be similar to the one below: You will do a comparison of the graphs to see if
your scene sample (the unknown) is a match with one of the five suspects’ car samples (knowns).
PROCEDURE FOR IR SPEC:
Your group will go into the IR Spec lab and work with a teacher to get transmission graphs for all six samples.
The samples will be analyzed one at a time. Be sure to place the samples color side down.
Instructions for Jasco IR
Set up: Turn on the IR and wait until you hear 3 short beeps. Then turn on the computer and monitor. Open
Spectra Manager by clicking the icon. Then open Spectra Measurement, so that data can be collected, and
Spectra Analysis, so that data can be worked up, by clicking on these items.
Data Collection:
Bring the Spectra Measurement window forward by clicking on it. Make sure the sample compartment is
empty and clean. If not, use methanol and soft tissue to clean the crystal and the metal upper part of the
sample holder. Next collect a background spectrum by clicking on B-> (Monitor Background).
To collect a sample spectrum, cover the crystal with sample. Be careful not to touch the crystal with any sharp
object like a spatula as it is easily scratched and is very expensive. Close the sample compartment by
pushing the lever on the left side of the dial toward the dial and then move the dial forward. Rotate the dial
clockwise until it comes to a natural stopping point. Do not force.
After the sample has been added, click S-> (Monitor Sample). If the spectrum is not what you expected, it is
likely that your sample compartment was not clean. Go to FILE and select Analysis Send. Wait until the
spectrum appears in the Spectra Measurement Window. Print.
-Do your results from the IR spec match up with what you found from the Microscopic examination of the paint
chips?
-What could this mean for the crime scene/ suspects overall? Is this information important to this case?
What other types of crime might paint chip samples be important for?