We report the results of the application of Laser-Induced Breakdown Spectroscopy (LIBS) for the detection of some common military explosives and theirs precursors deposited on white varnished car’s external and black car’s internal or external plastic. The residues were deposited by an artificial silicon finger, to simulate material manipulation by terrorists when preparing a car bomb, leaving traces of explosives on the parts of a car. LIBS spectra were acquired by using a first prototype laboratory stand-off device, developed in the framework of the EU FP7 313077 project EDEN (End-user driven DEmo for CBRNe). The system operates at working distances 8-30 m and collects the LIBS in the spectral range 240-840 nm. In this configuration, the target was moved precisely in X-Y direction to simulate the scanning system, to be implemented successively. The system is equipped with two colour cameras, one for wide scene view and another for imaging with a very high magnification, capable to discern fingerprints on a target. The spectral features of each examined substance were identified and compared to those belonging to the substrate and the surrounding air, and those belonging to possible common interferents. These spectral differences are discussed and interpreted. The obtained results show that the detection and discrimination of nitro-based compounds like RDX, PETN, ammonium nitrate (AN), and urea nitrate (UN) from organic interfering substances like diesel, greasy lubricants, greasy adhesives or oils in fingerprint concentration, at stand-off distance of some meters or tenths of meters is feasible.
The ENEA Laser Application Section has participated to the European project ISOTREX (Integrated system for on-line
trace explosives detection in solid and vapor state), funded in the frame of the PASR 2006 with the main aim to exploit
different laser based techniques. Standard explosive compounds and their precursors have been investigated through an
atomic technique (LIBS; Laser-Induced Breakdown Spectroscopy), an absorption technique (LPAS; Laser Photoacoustic
Spectroscopy) and vibrational techniques (Laser Raman and SERS; Surface-Enhanced Raman Spectroscopy). LIBS and
SERS reached a sub ng level of detection, supported by a high rank of discrimination of the components via
chemometric analysis. This selectivity skill is also quite evident in the LPAS technique. These results assume particular
relevance due to the inclusion of interferents, such as dust, fingerprint oil and lubricant oil, into the investigated
compounds. The results of the measurements are presented in view of the possible integration of the three techniques in a
single device for trace detection, might contribute also to drastically limit the number of false positives.
In order to realize a compact instrument for detection of explosive at trace levels, LIBS was applied on residues from
different explosives and potentially interfering materials. The residues were simply placed on aluminum support and the
measurements were performed in air. Spectral line intensities from the characteristic atoms/molecules and their ratios,
are strongly varying from one sampling point to another. The reasons for such variations were studied and explained,
allowing establishing a suitable procedure for material recognition. Correct classification was always obtained for five
types of explosives, while for TATP, nitroglycerine, DNT and EGDN this occurred only for very thin residues. In all the
cases, the estimated detection threshold is between 0.1 ng and 1 ng.
LIBS is one of the most promising techniques for rapid, in-situ elemental analyses of artworks. It does not require sample preparation, it is almost non destructive (micro sampling) and information both about major and trace elements could be obtained simultaneously. LIBS has been used to recognize the elements present in different archaeological materials and has been also proposed for on-line monitoring during the object cleaning by lasers. Quantitative determination of the material composition can supply useful information to restorers and help the object cataloguing. However, the analytical LIBS measurements on the archaeological materials were rarely reported, mainly due to difficulties to obtain the corresponding matrix-matched standards, required for the initial calibration. Alternatively, Calibration-Free (CF) approach could be used on some class of materials if all the major sample elements are detected and if the laser plasma preserves the material stochiometry. The latter condition is sometimes missing, as in the case of bronzes under nanosecond pulse laser ablation. We have developed a theoretical model for laser ablation of quaternary copper alloys, which allows for correction of the missing plasma stochiometry in CF approach. The model also predicts the optimal calibration for this type of material. In our recent work, we also obtained quantitative LIBS results on marbles by realizing the calibration standards starting from doped CaCO3 powders and by applying the corrections on the plasma parameters, different for the laboratory standards and marbles. Semi-quantitative LIBS results have been also obtained on multi-layered renaissance ceramics by subtraction of the contribution to plasma of each ceramic layer.
The effects of energy and pulse duration in laser ablation experiments have been investigated in the case of copper based alloys. Experiments were carried out near 530nm by using two laser sources with different widths (8 ns and 250 fs). The craters generated by lasers were examined and their characteristics were related to different mechanisms involved in the ablation by laser pulses acting on different timescales. The optical emission of plasma produced was analyzed by LIPS, acquiring time resolved line intensities of the major elements contained in the samples, and determining plasma characteristics (temperature, electron density). Experiments demonstrated that fs and ns laser pulse must be modeled by different ablation regimes, the onset of which can be used to decrease the effect of fractionation on brasses and bronzes.
Laser induced plasma spectroscopy (LIPS) is nowadays recognized as a fast and accurate technique for elemental analysis of unknown samples. Analytical implications of the non-thermal equilibrium plasma generated at high pressure and electron temperature in 8000-15000K range, after the interaction of a sharply focused laser beam with a metals surface are investigated. On the basis of recent laboratory experiments, an attempt is made to defme the effects of non-equilibrium on the species population densities and on their spectroscopic emissions, which affect the achieved analytical accuracy. Experimental data on a steel sample are presented and discussed in combination with some theoretical modelling. The laser induced plasma is formed at the sample surface in air at atmospheric pressure, so that it is mainly constituted of light elements from air and evaporated atoms and ions from the sample surface. Species considered for modelling are metals such as iron and chromium with well known spectroscopic parameters, which are of special interest for analytical applications such as their characterization of ancient metal alloys, soil analysis and marine sediments analysis.