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How Forensic Lab Techniques Work

Forensic Paint Analysis and Arson Investigation

Forensic scientists are sometimes called to help analyze evidence left from a hit-and-run or possible case of arson. They have special techniques to study what's often small or extremely damaged evidence.

Paint Analysis

Sometimes forensic scientists need to analyze a paint sample -- for example, if a paint chip is found on the body of a hit-and-run victim and investigators are trying to match it to a make and model of car.

First, the scientists look at the appearance of the sample -- its color, thickness and texture. They examine the sample under a polarized light microscope to view its different layers. Then they can use one of several tests to analyze the sample:

  • Fourier transform infrared (FTIR) spectrometry determines the type of paint (chemicals, pigments, etc.) by analyzing the way in which its various components absorb infrared light.
  • Solvent tests expose the paint sample to various chemicals to look for reactions such as swelling, softening, curling and color changes.
  • Pyrolysis gas chromatography/mass spectrometry helps distinguish paints that have the same color, but a different chemical composition. The paint sample is heated until it breaks into fragments, and then is separated into its various components.

Arson Investigations

To light a fire, arsonists need a flammable material and an accelerant (such as kerosene or gas). Arson investigators look for these items when they're investigating the crime scene. Because all that's usually left of the evidence is charred remains, the investigators will collect fire debris and take it back to the forensics lab for analysis.

Arson investigation.
Gary Tramontina/Getty Images
Investigators look through the remains of the Morning Star Missionary Baptist Church on Feb. 8, 2006, near Boligee, Ala. Forensic technicians will examine the fire debris.

Samples are sealed in airtight containers and then tested for residues of accelerant liquid that might have been used to start the fire. These are the most common tests performed by forensics labs during an arson investigation:

  • Static headspace heats the sample, causing the residue to separate out and vaporize into the top, or "headspace" of the container. That residue is then injected into a gas chromatograph, where it's broken apart to analyze its chemical structure.
  • Passive headspace heats the sample and the residue collects onto a carbon strip in the container. Then the residue collected is injected into a gas chomatograph/mass spectrometer for analysis.
  • Dynamic headspace bubbles liquid nitrogen gas through the sample and captures the residue onto an absorbent trap. The trapped compounds are then analyzed using gas chromatography.

How do technicians analyze biological evidence like blood, semen or the oils left behind by fingerprints? In the next section, we'll find out.