Reference
Sampling for Dummies Part II - The Sequel
By Mark Hughes, P. Eng.
© 2000 Sintra Engineering Inc.
A few months ago, I wrote an article on some of the issues surrounding the black art of sampling for accelerants. This time I thought I would review how to select sample locations (including in vehicles) and what to do with the results from the laboratory that processed your sample.
Once you have selected a general area where you think there are accelerants present, it is important to try to narrow your sample location and pick an area where the sample will still have some accelerant present. This can be very tricky, particularly in areas that are severely burned.
When sampling, there are three factors that work in your favour (there are many more that work against you), these include the motility of the flammable liquids, fall down debris and the fact that only the vapour portions of liquids will burn.
The motility of the flammable liquids refers to the tendency of liquids to find and fill cracks, particularly in porous materials. An example of this is residential floor sheeting in a house. If you have a fire that was suppressed early enough, flammable liquid may have found some of the gaps between the sheeting and traveled downwards. This can often allow you to get a good sample a bit away from the burned area. Another example is the porous areas below an area of burn such as the underlay underneath burnt carpet.
Another possibility is to look for areas where fall down debris has covered the original surface. This can often lead to unburned areas that still have some flammable liquids present. Care should be taken to distinguish flammable liquids from pyrolysates that can sometimes mimic accelerants. Pyrolysates are the decomposed chemicals remains from a fire.
The third area to look for accelerants is in the center of the area where the fire was burning. As flammable liquids will tend to pool and burn at the edges before burning in the middle, sometimes this will leave a trace of the accelerant used.
You should not worry about the effect of water on the sample as water will often protect and suspend the flammable liquid. Water will also often enhance the success rate of finding a flammable liquid by slowing or preventing evaporation of the remaining trace liquids.
When looking for samples in vehicles, one must be very careful. Clues that an accelerant was used include burns on the exterior surface that do not communicate with the interior areas or damage that is away from the engine compartment.
Vehicles contain a great deal of plastics and sources of fuel for a fire. As the fire develops the plastics burn and decompose into pyrolysates. As plastics are based upon hydrocarbon chains, they decompose into other hydrocarbons such as Toluenes, Xylenes, Benzenes and Styrenes. All these chemical constituents will burn in and of themselves. The only reason they may not burn is because of a lack of oxygen or protection by falling debris.
Successful sampling from the engine compartment will prove quite difficult because the lack of porous surfaces prevent liquids from accumulating. In general, the only area of a vehicle that might get sampled is the occupant compartment and only after all potential accidental causes of the fire have been eliminated. In vehicles you will typically look for areas that were protected from the fire and may have absorbed accelerant. These include floor mats and upholstery.
Finally once the samples have been processed and completed, you need to review the lab results from the GC/MS/MS testing of your samples. It is often a good idea to talk to the analyst to make sure you understand what the results represent. Sometimes the test report indicates nothing to suspect that there was a flammable liquid, however, maybe there were other substances with unusually high traces that may prove useful in your investigation.
A laboratory analysis consists of a burning of a portion of the sample and monitoring the changes in a flame as the sample passes through. This process produces a trace that will show peaks that correspond to the molecular densities of various compounds. These peaks and traces can be compared to weathered exemplar samples to verify the found compounds correspond to a flammable liquid. This analysis is critical to the success of the sampling process. Sometimes the expected results are not what the process produces. Sometimes a search for an accelerant will produce lots of hits but all of them are pyrolysates. Interpreting the lab report is not a simple task and will often require the input of the lab technologist to clarify the results.
When dealing with a new lab, make sure you understand the technique for analysis that they are proposing to use. Some labs will offer to analyze your samples with little or no understanding of how to evaluate fire debris for accelerants. The sensitivity levels and particular features of fire weathered samples makes analysis a non-trivial exercise.
You are ready to go sample.
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