Tracing the arsonist
More than 23,000 cases of arson were recorded in Germany in 2011 according to the official police statistics of the Federal Ministry of the Interior. However, only about half of them have been cleared up. In the case of deliberate acts, the detection rate drops to 35 percent. In retrospect, it is often very difficult to decide whether it was arson. Timely investigations are necessary in order to identify possible fire accelerants such as petrol. For this, samples must be taken at the site of the fire. The methods of sampling and analysis have been studied as part of a research project at the BAM Federal Institute for Materials Research and Testing. Here, the Berlin State Criminal Investigation Department (LKA) provided advice and support. For fire trace interpretation, solid, liquid and gaseous combustion products were tested using chemical analysis in order to draw conclusions about the fire scenario.
Accelerants could all be detected in the burnt remains of the fire and in swab samples taken from the walls in both laboratory fire tests and five room fire tests. "Taking swab samples from the walls is an easy-to-perform application in the field for investigating the cause of a fire on site", says BAM fire expert Simone Krüger. To detect the accelerant used, a combined methodology of enrichment (headspace solid-phase microextraction) and detection (gas chromatography combined with mass spectrometry) has been used.
In order to enhance detectability, BAM analysts took samples using a particular type of fabric and covered them with water. It is important to take the sample as soon as possible: "Laboratory studies have shown that it is difficult to prove the accelerant after just six hours", says the chemist Christian Piechotta.
In the five room fire tests which were performed at the "Leipzig Society for Materials Research and Testing for Construction Industry mbH" (MFPA) in Leipzig, scientists have worked through various scenarios. In the rooms, each equipped with the same furniture, a mixture of petrol and diesel was used as a fire accelerant. In addition to chemical analysis detection of fire accelerants, how an accelerant influences fire development was also investigated in these fires.
The identified parameters included heat release, room and combustion gas temperatures, mass loss and combustion gas composition. The data obtained can provide further information about the temperature profile and combustion gas toxicity in connection with a room fire.
The experiments also have shown that the fire accelerant greatly reduced the time to "flashover". Due to the heat of the fire, the furniture disintegrates and as a consequence, the gases produced ignite suddenly. This is known as flashover, which is the transition from the incipient phase to the full fire phase.
The results and data will be fed into a computer model. Computer models can make a significant contribution in the assessment of possible development patterns of a fire. In retrospect, it is often difficult to reconstruct how a fire proceeded. Currently, a cross-departmental research project is being carried out at BAM with the aim of combining analytical studies and numerical methods of fire reconstruction. Numerical simulations can help decide which fire development patterns are probable and which ones can be excluded. In addition, data recently acquired in room fire tests are being added to the computer models which can then be compared to the calculations. "The work goes well beyond the currently published approaches and models and will be presented next spring at a workshop jointly organised with the Berlin LKA", says Simone Krüger.