Normally, the explosives, regardless of whether they lead to an explosion or a detonation, are characterized by the presence of chemical groups capable of generating effective self-redox reactions. Oxidizing and reducing groups are often both present in the same molecule of the explosive substance, or possibly in the same mixture. Many explosives contain nitrate, chlorate, perchlorate and permanganate groups, which are very rich in oxygen and are therefore strong oxidants. Furthermore, the nitrogen–oxygen bond (in nitrates, nitrogroups and nitroamines) has a lower splitting energy than the bonds between carbon and oxygen, leading to a great gain in terms of chemical transformation.
However, the substances used by criminal groups are often not produced specifically as explosive materials, but they are added with other commonly used materials. The presence of natural compounds in the fragments collected after a blast could give an indication of an explosive of illicit nature, and so homemade crated. An example can be the presence of sucrose mixed with potassium nitrate or an unusual presence of fuel oil mixed with ammonium nitrate.
If each of these substances can be easily analyzed and identified separately, when they are mixed, the analyses are more complex due to the fact that they have often completely different nature and chemical–physical properties. For example, in the Raman spectrum of a mixture of substances one of which exhibits strong fluorescence, the peaks and bands of those of interest can be masked, and they are difficult to recognize. Indeed, also the analysis of not explosive components of an explosive device can be useful for a forensic analysis.
In order to simulate this complexity, different kind of samples were selected as the main components of an explosive and of the debris of a blast (Fig. 1): solid urea, organic fertilizer with equine dejections, two different nitrate-based detergents (detergent_1 was liquid, deter-gent_2 was solid), solid ammonia nitrate, two different plasticizers (plasticizer_1 was thin, plasticizer_2 was thick), plasticizer with conductive wire, an example of plastic case and two samples of commercial tape (scotch and Teflon).
We selected these materials, because they contain precursors of explosive constituents .
Two different kind of analytical techniques have been used for samples analyses: Raman spectroscopy to verify its capability to be used directly on-site on the crime scene and CN analysis off-site to better characterize the samples already analyzed by through Raman spectroscopy. All the analyses have been performed in the Laboratory of the ENEA research center in Bologna (FSN-SICNUC-TNMT).
Raman spectroscopy is a powerful tool that can be useful in both situations, particularly because it can be used for a very large number of samples, regardless on their physical state and chemical nature. This technique can be applied on solids, crystalline and amorphous materials, polymers, liquids and mixtures. Raman spectroscopy is a vibrational spectroscopic technique. It is widely used in the study of materials, solid, liquid and even gaseous. It is a fast, non-destructive technique that often requires no sample preparation. In the Raman spectroscopy, the radiation emitted by a sample stimulated by a laser, which can be visible, IR or UV, is measured. In the spectrum of diffuse radiation, the components with different energies can be distinguished. Rayleigh radiation represents almost all of it, but having the same energy as the incident radiation, it has negligible use for identification of substances. However, there are also radiations whose energy is different and depend only on the material, so that the obtained signal represents a “chemical footprint” of the substance. In this way, a material can be identified and characterized from the point of view of its chemical–physical properties. In recent years, this technique has also been used in biology and medicine, as well as in material science and in many others fields. In the present work, Raman spectrum of different materials, mixtures or blends were acquired by a BWTEK i-Raman plus spectrometer equipped with a 785 nm laser. This instrument can be used in combination with a BAC151B video microscope or with a portable Raman probe for in situ analysis. The measurement parameters, acquisition time, number of repetitions, laser energy, have been selected for each sample in order to maximize the signal to noise ratio. For each spectrum, a reference acquisition with the same parameters was previously carried out to subtract the instrumental background.
Elemental analysis of all the samples can reveal the typical composition of the explosive compounds. We used an elemental combustor (Vario Max Cube, Elementar Gmbh, Frankfurt, Germany), based on the Dumas combustion reaction, in order to determine the total carbon and nitrogen content of the samples and its ratio. The elemental combustor is equipped with steel crucibles to avoid the cross-contamination of the samples and to let the recovery of any non-combusted fraction of the sample. The combustion tube was loaded according to manufacturer’s specifications with silvered cobaltous/cobaltic oxide, chromium oxide, and quartz wool and operated at 900 °C. The post-combustion tube maintained at 900 °C was packed with copper oxide, zinc, and quartz wool. The quartz reduction tube, maintained at 830 °C, was packed with reduced copper wire filled between the bottom 40 mm of the tube and the top 30 mm of the tube to maintain a high temperature throughout the copper. Resultant water was removed by an anidride trap, and CO2 gases were trapped in active charcoal column at 40 °C. Adjustable helium (99,9995% purity) flux was used as carrier. N2 gases passed through the circuit without being trapped and the thermo-conductivity detector directly measured them. After the N2 analysis has been completed, the trapping column for CO2 was heated up to 230 °C to release the adsorbed gas that was then measured by the thermo-conductivity detector . Each different kind of samples were weighed into standard reusable stainless steel crucibles without any pre-treatment, following the different analysis method previous selected and described in Table 1. Regardless the weight, all the samples were analyzed and compared with a reference material consisting of aspartic acid (36% C, 10% N), which was used to calculate the daily factor of the instrument and to normalize C and N data of the sample to a reference sample.
Forensics is a scientific discipline concerning with scientific analysis of physical, biological, behavioral, and documentary evidences in the frame of civil, criminal, or international law. The goal of Forensics is to assess linkages among people, places, things and events. Forensics contributes to the process for identifying the material origin, pathways, or perpetrators associated with an event, relying on the mixture of information from multiple sources, including forensics, law enforcement investigations, and intelligence. The process of identification of the material origin is of paramount importance in case of seizing of illicit explosive materials or in case of explosions due to malevolent acts. Both seized illicit explosive materials and debris from an explosion should be analyzed in a laboratory, and then they should be compared with a database of known explosive materials (this could be a National Explosives Forensic Library) in order to search for a possible match. Such a database could include all type of known explosives and their precursors and also materials suitable to be used as explosive shell. The search for a possible match is possible using the following method:
Observations matrix Arrange the database in a mxn matrix in which the m rows are the observations and the n columns are the characteristic parameters for each observation. The columns are the n elements of the parameter vector P of the observations (Fig. 2a). Each observable is a point in the n-dimensional space of the parameters Sp.
Vector Pu of useful parameters Select a parameters vector Pu = [P1, P3, P6,…, Pq] (q ≤ n) with the parameters useful to assess the possible match. If the dimension n of the space Sp is small, the identification of the useful parameters is possible based on the researcher’s expertise. If the dimension n of the space Sp is quite large (n ≥ 30), it could be useful to reduce the dimension finding the most significant variables through advanced statistical techniques like, for example, a principal component analysis.
Investigation matrix The application of the Pu vector to the observation matrix will result in an investigation matrix mxq (q ≤ n) (Fig. 2b). The Pu vector should be also applied both to the seized-explosive and the explosion-debris. Then the possible match between the seized-explosive/explosion-debris and the library could be assessed using a chromatic code: Green = positive match; Red = negative match; White = impossible match (Fig. 2c).
Result matrix Excluding observations with at least one red value, the investigation matrix is reduced to a result matrix rxs (r ≤ m; s ≤ n) (Fig. 2d).
Most likely, not all the observations will have all green fields, and therefore it will be possible to assess a match percentage between the library and the seized-explosive/explosion-debris.