From Sample Collection to Analysis and Interpretation
By Marc-André Morel, Chemist
It is 4:00 a.m. at the Pepperoni-Cheese Pizzeria. Upon arrival, it seems obvious to the firefighters that the main entrance has been forced open, the front window is broken, and the burn pattern on the tiled floor is particularly unusual. In short, the fire appears to be suspicious. Immediately, police assistance is requested to determine the cause and origin of the fire.
Later, a fire and explosion investigator from CEP Forensic is appointed by the building’s insurer to scientifically determine the origin, circumstances and probable cause of the fire. How does the investigator proceed in such a situation?
In this hypothetical scenario, preliminary information suggests that a flammable or combustible liquid may have been used to intensify the fire.
The Importance of Sampling
In such scenarios, an investigator must pay particular attention to sampling and collection to confirm the presence of a flammable or combustible liquid on the site. To do this, the investigator will apply rigorous decontamination techniques for sampling tools and uses inert containers that are vapour-tight to contain the flammable liquids.
Porous materials around the area where a spill is suspected will be prioritized (carpet, wood, insulation wool, grout between tiles, etc.). When possible, the expert will collect a comparison sample from an area where no flammable liquid is evident. This sample will allow laboratory experts to distinguish between the chemical signature of the substrate and the sample with the suspected flammable liquid.
It should be noted that suspicious odours can guide where the expert will choose to collect a sample. If necessary, using a dog handler with a dog trained to detect hydrocarbon vapours can greatly increase the relevance of the sampling and the likelihood of a positive result once tested in the lab. Another option is to use a handheld combustible gas detector, which is a device used to detect areas where vapours from an ignitable fluid may still be present.
Residual Gasoline After a Fire?
While we might think that gasoline would be completely consumed by the fire, experience and theory suggest otherwise. Traces can be found afterward. The chemical composition of gasoline and its physicochemical properties make it easy to be absorbed when in contact with porous substrates; traces of it, therefore, can remain after the fire has been extinguished.
Moreover, through research and experience, we now know that the gasoline vapours above the liquid are consumed by the fire, not the liquid itself. Thus, traces of the liquid can be protected, for example, by falling debris.
External Laboratory Analyses and Their Basic Principles
Once a sample has been collected, it will then be sent to an external laboratory. With the aid of advanced techniques in analytical chemistry, a specialist will examine the sample and search for the presence of a flammable (e.g., gasoline, lighter fluid, alcohol) or combustible liquid (e.g., diesel, fuel oil). Additionally, the specialist will have to account for several typical phenomena of flammable liquids that could alter the results displayed by the device used (e.g., evaporation, degradation).
During the laboratory analysis, the specialist will use the volatile property of flammable liquids to extract them from the sample. The sample is gently heated, and a porous material (e.g., activated charcoal) suspended above the sample will absorb the hydrocarbon vapours present. The extracted portion is then analyzed with specialized instruments, such as a gas chromatograph coupled with a mass spectrometer (GC-MS).
The GC-MS instrument executes two functions. First, it separates the molecules in a mixture by passing them through a 30-meter-long tube with an internal diameter of about 150 micrometers (a hair has an external diameter of about 75 micrometers). This part of the sequence is like a race, resulting with the smaller molecules finishing first, and the heavier ones finishing last. The molecules then pass, one by one, in front of a mass spectrometry detector. This detector allows the molecules to be fragmented and the mass of the fragments to be analyzed.
The results are then analyzed and interpreted based on the presence and proportions of certain major families of components from petroleum refining.
Sample Traceability and Analysis of Overall Results
Throughout the process, sample traceability is ensured to maintain the integrity of the sample. Indeed, the flammable liquids commonly used in suspected fires are ubiquitous in everyday environments.
Finally, the fire/explosion investigator from CEP Forensic will analyze the specialist’s report, produced by the external laboratory, to evaluate the results based on the sample and the observations made on site. Indeed, the results can be positive for a multitude of flammable liquids.
Depending on any remaining containers that are still-identifiable, at the fire site, the presence of a flammable liquid could be attributable to a leak from one of them or by recent work that had been done by the insured (such as engine part cleaning, etc.). For example, finding gasoline in a garage would be perfectly normal. But finding gasoline in the master bedroom of a home is not typical and could indicate that the fire had been intentionally set.
It is important to follow precise and rigorous rules when investigating a fire or explosion scene that appears suspicious. Don’t hesitate to call upon the investigators from CEP Forensic, who, thanks to their expertise, can guide you in managing your claim file.