Investigation of fires by unconventional IID (improvised incendiary device) and combustion residues on different materials by SPME-GC-MS

Incendiary events are phenomena that have a different evolution depending on the place in which they occur, the type of fuel, combustive and ignition, the type of material involved and the time that elapses between ignition and extinction. In the past few months numerous riots or attacks in different part of the world were carried out by using petrol bombs or incendiary devices. To establish the dynamics of the incendiary event it is necessary to analyze the debris but to make the results reliable it is necessary that the sampling is carried out by expert personnel to ensure the chain of custody of the evidence collected. Once sampled, the samples must be properly stored and sent to the competent laboratory for subsequent analyzes, which must be conducted in line with the reference standards. For the analysis of volatile substances, headspace extraction (HS) with the microextraction in solid phase (SPME) is used with subsequent analysis in gas chromatography coupled to the mass spectrometer (GC-MS). Given the variability of the operating conditions, the materials involved and the ignition methods, it can be said that there are no identical incendiary events, so through the multivariate analysis of the data it is possible to highlight correlations and differences between the various events. The creation of a model that allows the classification of a sample since the comparison with the typical compounds of a primer allows to facilitate the analyzes and to have reliable results. In this way, by reconstructing the dynamics of the event, it is possible to verify any criminal liability related to the presence or absence of flammable liquids or solids extraneous to the scene. In order to make the scientific result easily accessible to non-experts, the verbal scale of the likelihood ratio (LR) was introduced by the ENFSI in 2010. During the work a simulation was performed by using a Molotov device made with a diesel-sulfuric acidic mix (3:1). A cloth was used as a stopper separating a bag of paper containing potassium chlorate. When the bottle breaks, the mixture comes into contact with the salt giving the formation of a colorless flame that starts the reaction. We have planned experiments with 16 fireboxes, with negative controls for a total of 120 samples. After the creation of the chemometric model on the basis of the simulation results, data relating to a real case of a malicious nature were used in the model to verify whether or not the considered flammable materials had been used. The insertion of the data in the classification model returned an ambiguity of the results, in fact 2 out of the 15 collected samples were incorrectly assigned to the kerosene class while the other 13 samples were assigned to the class of IED. In this case the uncertainty in the evaluation is not attributable to a defect in the model but rather to the liquid used to make the illicit act. From the investigations carried out by the competent authorities, the suspect used gasoil, an oil derivative such as kerosene but belonging to a different class and present in the ratio 3:1 with sulfuric acid of studied IED.