Tomás Delgado

Primary and recombined emitting species in laser-induced plasmas of organic explosives in controlled atmospheres

The main difficulties in spectral interpretation of laser-induced plasmas from species containing C, N, O or H rely on the crossroad concerning their origin: direct release from native bonds of the molecule or recombination of atoms and molecular fragments with ambient constituents. In order to add further insight into the issue, this paper presents the influence of the surrounding atmosphere (gas type and pressure) on the spectra of energetic nitro compounds (TNT and PETN). The study was completed with coincidental detection at high vacuum of both optical emission and mass spectra originated from the same laser event. We have proposed probable fragmentation pathways for the compounds taking into account the reactions that prevail over other routes depending on experimental conditions: laser fluence, pressure and the surrounding atmosphere. The suggested routes are supported by identification of many non-emitting, reactive species present in the plasma using the corresponding mass spectra.

Laser-induced plasma spectroscopy of organic compounds. Understanding fragmentation processes using ion–photon coincidence measurements

Coincidence detection is of interest to get as much information as possible about transient events occurring in laser induced plasmas. The present work is focused on coincidence ion–photon detection of laser plasmas of high-energy organic compounds (TNT and DNT) in a condensed phase irradiated with UV laser pulses using an advanced instrument for simultaneous monitoring of both types of chemical species generated. The optical emission spectrum is acquired from atoms, atomic ions and diatomic molecules, whereas the mass spectrum derives from fragment ions of the molecule. These fragments result from direct ionization or may be formed through indirect pathways. Fluence-resolved experiments showed the evolution of the main optical-mass signals in the acquired spectra for a limited energetic range, showing the different stages of lifetime of plasma: the rising thresholds and extinction of the different atomic and molecular studied species, besides the breakage of the aromatic ring and the later excitation of ionic species at a higher fluence level.

Condensed‐phase laser ionization time‐of‐flight mass spectrometry of highly energetic nitro‐aromatic compounds

RATIONALE Analysis of explosive compounds represents an interesting field of work due to the obvious social relevance of these compounds. Direct laser ionization allows the analysis of these high internal energy compounds without sampling or preparation procedures. We have studied nitro-aromatic compounds to understand their mass spectra when directly ionized in the condensed phase, different from the gas-phase studies commonly conducted. METHODS Direct condensed-phase laser ionization time-of-flight mass spectrometry of high energy density materials has been performed using a 5 ns width quadrupled Nd:YAG laser. No matrix assistance was used. Fine control of the laser energy allowed the study of the fragmentation processes from values close to the ionization threshold to ones where atomic-only mass spectra were recorded. RESULTS The influence of the variation of extraction conditions on the recorded mass spectra was investigated. For low extraction width pulses, ions with low m/z values were mainly observed, whereas, at higher widths, higher mass fragment ions were also detected while the total ion current was maintained. Therefore, the mass spectra can be modulated to obtain mass spectra containing molecular or atomic information. The onset of ion generation for the different fragment ions was also studied, yielding information that can help to understand the processes involved in the fragmentation pathways of the molecule and in the dissociation mechanisms. Two sampling procedures allowed the prospective use of LIMS as a screening technique for nitro-aromatic-based highly energetic explosives. CONCLUSIONS Direct analysis of explosive compounds has been performed by laser ionization. A large dependence of the resultant spectra on the laser energy was observed that might be useful for studies of fragmentation pathways. For forensic applications, two sampling procedures might allow the use of LIMS as a screening technique.

Distinction strategies based on discriminant function analysis for particular steel grades at elevated temperature using stand-off LIBS

This work presents experimental discrimination strategies based on advanced chemometric tools for the differentiation of steel grades using stand-off laser-induced breakdown spectroscopy. The experiments are aimed at simulating the conditions existing in the continuous casting line of a steel factory. For this purpose, measurements of the as-cast samples provided by steelmakers have been carried out at a distance of 4.5 m, with samples in motion heated at temperatures up to 900 °C. Coaxial double-pulse laser excitation was used. The main challenge of the present study derives from the formation of an oxide layer on the surface when the steel sample is heated in contact with air. The chemical composition of such a layer deviates significantly from that of the bulk material, and not in a linear way. Moreover, complete removal of the scale layer from the surface with a single laser shot is not feasible. Therefore, quantitative analysis or calibration-based methods have to be discarded and specific chemometric strategies for identification purposes were developed. Discriminant Function Analysis (DFA) has been chosen as the semi-quantitative statistical method for the differentiation of steel grades from the corresponding scale layers. The statistical parameters derived from DFA and discriminant functions are presented with good correlation between real and predicted steel grades.

Pressure effects in laser-induced plasmas of trinitrotoluene and pyrene by laser-induced breakdown spectroscopy (LIBS).

The influence of the ambient atmosphere on the dynamics of plasma expansion, besides the interaction between excited plasma and gas molecules, has been studied for specific organic aromatic compounds. To analyze the influence of air on the formation pathways of atomic and molecular species inside the plasma plume, the spectral emissions in laser-induced breakdown spectroscopy (LIBS) of 2,4,6-trinitrotoluene (TNT) and pyrene were compared at different pressure environments, from high vacuum to atmospheric pressure. Pelletized samples of the compounds were introduced in a vacuum chamber for excitation with the fourth harmonic output of an Nd:YAG laser (266 nm). The optical emission signal was collected with an optical fiber connected to a spectrograph fitted with a intensified charge-coupled device detector. Results from LIBS spectra indicate that changes in pressure level affect the kinetics of the characteristic excited species and their spatial distribution inside the plasma plume.

Acting Role of Background Gas in the Emission Response of Laser-Induced Plasmas of Energetic Nitro Compounds

This study focuses on the analysis of the optical emission response obtained by laser-induced breakdown spectroscopy from energetic nitro compounds in condensed phase sampled in atmospheres of variable composition. The influence of different background gases was evaluated from the characteristic emissions of the excited species coexisting in the plasma plume and conclusions concerning the main pathways involved in the generation of such emission species were extracted. Different reactive (O2, N2, H2) and inert (Ar, He) gases were tested to establish the comparative emission features of organic compounds.

Isomer discrimination in condensed phase by laser-induced breakdown spectrometry and laser-ionization mass spectrometry using a tailored paired-pulse excitation scheme

A new excitation scheme for direct analysis of organic compounds in condensed phase is described. The approach involves the combined use of two laser-ablation based techniques (laser ionization mass spectrometry (LIMS) and laser induced breakdown spectrometry (LIBS)) within a common experimental setup to simultaneously obtain atomic and molecular information capable of discriminating organic isomers. An initial laser pulse tailored to induce gentle excitation with limited fragmentation is followed by a second pulse at a specific fluence above the plasma formation threshold. Under a simultaneous ion-photon detection scheme, this configuration allows the recording of optimized optical emission and mass spectra. Despite the consecutive excitation, the low fluence conditions of the first pulse assure the sample integrity for the LIBS analysis of the successive pulse. The improved capabilities of the new excitation scheme have been demonstrated in the analysis of three dinitrotoluene isomers (2,3-, 2,4- and 2,6-), where the information provided by LIMS and LIBS is processed by a discriminant function analysis method to establish discrimination criteria for the three nitro aromatic isomers based on differences in spectral features.