Forensic Arson Analysis

When detectives arrive at the scene of a fire, the first thing that they do is interview any witnesses to the fire, for example, the person who called the fire brigade and those who arrived before the fire department, whom may have seen how the fire began. When the fire has been controlled, the temperature falls, allowing the firefighters to make the building safe for investigation.  Similar to burglary and theft, arson is also a crime of stealth. The perpetrator of arson does not want to be seen committing his cowardly act. Most arson fires, therefore, occur at night and normally when no one is in the structure. Arson-for-profit is usually planned well ahead and the insured usually has a solid alibi far from the scene. 

 The investigation into how the fire started involves beginning at the lower levels, as fire travels upwards. The signs investigators search for when looking for the place the fire may have started include lingering heat, how deep the charring is, the flaking of building materials like cement and plaster, distorted plastic, metal and glass resulting from prolonged burning, damaged ceiling and structural damage.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.

References:

1. How Stuff Works
2. Fire Cops: On the Case with America's Arson Investigators
3. Fire Investigator Field Guide
4. Thinkquest team