J. Javier Laserna

Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces

The detection and characterization of energetic materials at distances up to 45 m using stand-off laser induced breakdown spectroscopy (LIBS) has been demonstrated. A field-portable open-path LIB spectrometer working under a coaxial configuration was used. A preliminary study allowed choosing a single-pulse laser source over a double-pulse system as the most suitable source for the stand-off analysis of organic samples. The C2 Swan system, as well as the hydrogen, oxygen and nitrogen emission intensity ratios were the necessary parameters to identify the analyte as an organic explosive, organic non-explosive and non-organic samples. O/N intensity ratios of 2.9 and 2.16 with relative standard deviations of 4.03% and 8.36% were obtained for 2,4-dinitrotoluene and aluminium samples, respectively. A field test with known samples and a blind test were carried out at a distance of 30 m from the sample. Identification of energetic compounds in such conditions resulted in 19 correct results out of 21 samples.

Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy. Atoms are excited from the lower energy level to high energy level when they are in the high energy status. The conventional excitation energy source can be a hot flame, light or high temperature plasma. The excited energy that holds the atom at the higher energy level will be released and the atom returns to its ground state eventually. The released energy is welldefined for the specific excited atom, and this characteristic process utilizes emission spectroscopy for the analytical method. LIBS employs the laser pulse to atomize the sample and leads to atomic emission. Compared to the conventional flame emission spectroscopy, LIBS atomizes only the small portion of the sample by the focused laser pulse, which makes a tiny spark on the sample. Because of the short-life of the spark emission, capturing the instant light is a major skill to collect sufficient intensity of the emitting species. Three major parts of the LIBS system are a pulse laser, sample, and spectrometer. Control system is usually needed to manage timing and the spectrum capturing. Figure 1 illustrates those three major components and a computer in the conventional LIBS.

Simultaneous Raman Spectroscopy-Laser-Induced Breakdown Spectroscopy for Instant Standoff Analysis of Explosives Using a Mobile Integrated Sensor Platform

A novel experimental design combining Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in a unique integrated sensor is described. The sensor presented herein aims to demonstrate the applicability of a hybrid dual Raman-LIBS system as an analytical tool for the standoff analysis of energetic materials. Frequency-doubled 532 nm Nd:YAG nanosecond laser pulses, first expanded and then focused using a 10x beam expander on targets located at 20 m, allowed simultaneous acquisition of Raman-LIBS spectra for 4-mononitrotoluene (MNT), 2,6-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), C4 and H15 (plastic explosives containing 90% and 75% of RDX by weight, respectively), and Goma2-ECO (Spanish denominated dynamite class high explosive mainly composed of ammonium nitrate, nitroglycol, and dinitrotoluene among other compounds), sodium chlorate, and ammonium nitrate. With the use of a Cassegrain telescope, both Raman and LIBS signals from the same laser pulses were collected and conducted through a bifurcated optical fiber into two identical grating spectrographs coupled to intensified charge-coupled device (iCCD) detectors. With the use of the appropriate timing for each detection mode, adjustment of the laser power on the beam focal conditions is not required. The ability of the present single hybrid sensor to simultaneously acquire, in real time, both molecular and multielemental information from the same laser pulses on the same cross section of the sample at standoff distances greatly enhances the information power of this approach.

Mapping of platinum group metals in automotive exhaust three-way catalysts using laser-induced breakdown spectrometry.

The use of laser-induced breakdown spectrometry for spatial distribution analysis of platinum, rhodium, and palladium in car catalytic converters is discussed. Fresh converters were extracted from the car exhaust system, cut in pieces of an appropriate size, and analyzed for mapping purposes. Spectral detection, pulse energy, and beam focal conditions were optimized according to the ablation behavior of the material. Difficulties in distribution analysis caused by the complex elemental composition of the sample were overcome by an extensive spectral analysis using appropriate internal standards. Data on the spatial distribution of the active metals in both the axial and radial directions of the catalytic structures are presented.

Multielemental chemical imaging using laser-induced breakdown spectrometry

Multichannel laser-induced breakdown spectrometry (LIBS) is used to generate selective chemical images for silver, titanium, and carbon from silicon photovoltaic cells. A 2.5 mJ pulsed nitrogen laser and a spectrometer using charge-coupled device detection were employed. LIBS images were acquired sequentially by moving the sample located on a motorized x-y translational stage step by step while storing the multichannel LIBS spectrum for each position of the sample, followed by computer-based reconstruction of two-dimensional selective images from intensity profiles at several wavelengths. Depth distributions of carbon impurities are also reported. Room temperature and atmospheric pressure operation as used here remove the restrictions on sample size exhibited by other surface analysis techniques used for imaging applications. Thus, the sample size in LIBS imaging is in principle unlimited. A LIBS experiment does not require a sample to be conductive. Therefore, virtually all materials can be imaged. Although LIBS is a typical example of destructive analytical technique, multichannel detection as demonstrated here confers the possibility to LIBS of obtaining multielement information from a given surface area. Lateral resolution of 80 μm and depth resolution of better than 13 nm were observed. The ultimate limitation to imaging the first layer of the surface in LIBS is the spectral signal-to-noise ratio as dictated by the ablation threshold of the material.

Three-dimensional distribution analysis of platinum, palladium and rhodium in auto catalytic converters using imaging-mode laser-induced breakdown spectrometry

Abstract Laser-induced breakdown spectrometry (LIBS) is reported here as an effective technique to describe the volume distribution of platinum, rhodium and palladium in catalytic converters installed in motor vehicles. Using the second harmonic output of a Nd:YAG laser and a CCD-based atomic emission spectrometer, LIBS is used in multielemental, imaging-mode to permit the simultaneous analysis of the several elements present in the converter, including the internal standard. The data are reported with a lateral resolution of 1.75 mm over a fresh catalytic structure which is 128 mm long. The concentrational variability of the platinum group metals (PGMs) varies in the range ∼3–23% relative standard deviation depending on the element, the substrate and the direction investigated. The causes of the dispersion observed are discussed.

New Raman–Laser-Induced Breakdown Spectroscopy Identity of Explosives Using Parametric Data Fusion on an Integrated Sensing Platform

The principal goal of sensors for the detection of explosives is to establish the identity of the interrogated target as a key to threat assessment and decision making. Despite the fact that both Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) have shown their capability in standoff detection of explosives, such techniques are not exempt from certain limitations, in terms of sensitivity and selectivity, to carry out this purpose when they are used individually. For this reason, the idea for the fusion of data reported by these orthogonal techniques, Raman and LIBS, has been around for a while. The present manuscript proposes an approach for the combination of the spectral outputs of Raman and LIBS sensors (data fusion strategy) in order to obtain knowledge about the identity of compounds better than that achieved when each technique acts alone. After simple mathematical treatment, a new pattern of identification (two-dimensional, 2D, image) of several compounds (explosives, confusants, and supports) was generated from the assembly of their Raman and LIBS spectra. The efficiency of this strategy was evaluated by comparing the results obtained for differentiation between compounds using simple correlation coefficient values from the 2D images and those achieved using Raman and LIBS spectra separately. Additionally, the effect of two spectral pretreatments (autoscaling and normalization) on the generation of the 2D image was evaluated. The results derived from this study demonstrate that the 2D image improves the identification of compounds, mainly in those critical situations in which it is not easy to differentiate them when Raman spectroscopy or LIBS is used separately.

Surface and tomographic distribution of carbon impurities in photonic-grade silicon using laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been tested as a method for surface analysis of photonic-grade silicon. A pulsed nitrogen laser (337.1 nm) was used to create a microplasma on the silicon, the light from which was dispersed and detected by a charge-coupled device (CCD) detector. A total area of 3×2.1 mm2 was analysed with a lateral resolution of 70 µm and depth resolution of about 0.16 µm. Factors affecting lateral and depth resolution of the approach were studied. Two- and three-dimensional distribution maps of carbon contamination on silicon are presented. Mapping by LIBS seems to be a powerful tool to use for the 3D characterisation of solid samples combining in a single step, surface and subsurface analysis capabilities.

Preliminary studies on stand-off laser induced breakdown spectroscopy detection of aerosols

Laser induced breakdown spectroscopy (LIBS) was demonstrated in stand-off mode as a detection technique for polydisperse liquid aerosols. The goal of this work was to assess the possibility of producing optical breakdown in an aerosol at 10 meters of distance, and to detect the analytical signal resulting from the optical discharge. The doublet characteristic of Na in the spectral region 588.6 nm – 589.5 nm was examined. For data analysis a method is proposed based on the study of the standard deviation of the acquired signal resulting from successive laser shots on the aerosol. This method rests on the high variability of the LIBS signal characteristic in aerosols systems (the percentage of spectra collected with analytical signal was around 30%). The standard deviation method was compared with the more traditional methods of ensemble-average and single-shot data analysis. The limit of detection (L.O.D.) was estimated for each method using a linear approximation, resulting in a value of 55 ppm for standard deviation method while in the case of the ensemble-average method calculation was unsuccessful due to the large uncertainty of the data. The principal advantage of the standard deviation method is that it allows the real-time elemental analysis from highly variable LIBS signals without the need of a priori knowledge of the sample which it is particularly suited for aerosol analysis in stand-off mode of detection.

Stand-off analysis of moving targets using laser-induced breakdown spectroscopy

This work investigates the capabilities of stand-off LIBS for the analysis of moving samples. The effects of sample speed, repetition rate and incidence angle of the laser beam on the sample have been studied. Stainless steel samples tilted from 0° to 45° at speeds up to 24 cm min−1 have been analyzed at a distance of 10 m from the instrument. The spectral intensity of the spectra observed is related to the amount of sample surface overlapped by successive laser shots, and this can be controlled both by changing the laser repetition rate and the sample speed. As a proof of concept, a semi-quantitative analysis of stainless steel samples has been carried out under simulated field conditions—10 m from the laser source, incidence-collection angle of 40° and sample speed of 24 cm min−1. Samples corresponding to five different stainless steel grades were sorted according to their content of Ni, Mo and Ti.