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Best practises in analysis of fire debris


Following a contingency, it may be important to understand the causes, obtain experience feedback and determine accountability.

Insurers generally use an expert in Fire Origin and Cause Investigations (OCI), specifically trained to read fires. In cases of suspected arson, the expert will carry out targeted sampling in the area identified as those in which the fire(s) started, to search for the presence of accelerators.
The successful highlighting of products of this type requires:
  • Correct sampling methods.
  • Suitable sample conservation.
  • Analyzes carried out with rigor. In this document we focus on the latter issue.
   

How to analyze fire débris

The reference method used is gas chromatography.
As with all chromatography techniques, gas chromatography is a technique that makes it possible to separate molecules from a mixture that may be highly complex of very different types.
  
  
  
  

Sample preparation: a key step

This involves extracting the debris, usually in solid form, to be injected into the column of the chromatograph for analysis.
The methods used to obtain this extract will be decisive for the precision of the analysis. Conventionally, three methods are used to cover the spectrum of products used as fire accelerators (hydrocarbons and alcohols)
  • A "solvent" extraction, which enables the extraction of heavy and light hydrocarbons.
  • A "volatile" extraction, which discriminates heavy hydrocarbons and thus enables a more concentrated analysis on the light hydrocarbons
  • An "alcohol" extraction, which discriminates heavy and light hydrocarbons and enables only the selection of polar liquids (alcohols).
As soon as the extract to be analyzed is injected into the injector, it is vaporized and mixes with the carrier gas* which permanently scans a capillary column coated on its internal walls with an active substance called the stationary phase, specific to the compounds being sought.   
Progressing at the same time as the carrier gas, the different molecules of the extract will separate from each other depending on:
  • their boiling point
  • their molecular weight
  • their affinity for the stationary phase
Each passage of a molecule in the detector, positioned at the end of the capillary column, results in a peak, the set of peaks representing a chromatogram, which then needs to be interpreted.
  

Three gases are used in chromatography

  • Hydrogen, the most sensitive gas which therefore enables more accurate results but may present a risk for the user (related to the explosive limits of the gas).
  • Nitrogen, less efficient, but which does not involve any safety concerns.
  • And finally helium.
For the sake of sensitivity, we use hydrogen.
The chain of analysis is regularly calibrated.
  

All that remains is to  interpret the chromatograms.

There are databases with options for reading peaks and each laboratory has its own reference library.
Chromatic profile study of domestic fuel oïl or diesel fuel  according to the % of combustion Chromatic profile study of white spirit according to the % of combustion
The detection and identification criteria were defined after numerous comparative studies on calcined materials carried out in our laboratory, after being soaked or not with flammable liquids. So we have a wide-ranging, accurate library.
 
In case of a peak unknown to the library, the extract is injected into a mass spectrometer coupled to the gas chromatograph, in order to determine the molecule with a very high degree of precision 
In addition to 30 years of experience, we are taking part in a (blind) quality testing program organized by the CTS (Collaborative Testing Service - USA).
CNPP is registered in the program for the analysis of flammable liquids in fire debris.
 
   
  

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