Patricia Lucena

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.

Depth Profiling of Phosphorus in Photonic-Grade Silicon Using Laser-Induced Breakdown Spectrometry

Laser-induced breakdown spectrometry (LIBS) has been evaluated for depth profiling of phosphorus doping in silicon. Laser plasmas were formed by focusing a Nd:YAG laser (operating in the second harmonic, 532 nm) on the sample surface. Plasma emission was collected, dispersed, and detected with the use of a charge-coupled device (CCD). Experimental parameters, such as delay time and sample position relative to the laser focal point, were optimized to improve the signal-to-background ratio of phosphorus line emission. Diffusion profiles by LIBS of samples with different phosphorus diffusion steps are shown. Crater depth per pulse and ablated mass per pulse were measured to be 1.2 μm pulse−1 and 50 ng pulse−1, respectively. The knowledge of depth per pulse permitted the estimation of thickness of the P diffusion layer.

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.

Compositional Mapping of Poisoning Elements in Automobile Three-Way Catalytic Converters by Using Laser-Induced Breakdown Spectrometry

The use of laser-induced breakdown spectrometry (LIBS) for generating two-dimensional (2D) multielemental distribution maps of poisoning agents in automobile three-way catalysts (TWCs) is discussed. A complete in-depth analysis of P, Zn, and Pb at different locations along the central channel of a used TWC in the gas flow direction was performed. The distribution maps for the metals in charge of the catalytic action and for the stabilizer of these metals—i.e., Pt, Pd, and Ce—are also presented. The 2D maps provide a fast, useful, and intuitive tool to visualize the selective distribution of poisoning elements in a used TWC. This study may help explain the deterioration processes of a catalyst with a lifetime of about 30 000 km.

Standoff LIBS detection of explosive residues behind a barrier

Development studies reveal that explosives detection is possible behind a barrier placed between a target and a standoff LIBS sensor. Barriers such as polymethylmethacrylate (PMMA) and a variety of glasses have been tested. Results for the detection of organic (DNT, TNT, C4 and H15) and inorganic (sodium chlorate) explosive residues placed up to 30 m from the sensor are presented. The standoff experiments have been evaluated in the emission (studies on laser beam transmission sent by the sensor) and collection (studies on plasma light emission collected by the sensor) channels. These experiments open wide perspectives for the detection of IEDs (improvised explosive devices) through windows located, for instance, inside vehicles, industrial warehouses and buildings where explosive residues have been produced, handled, stored or prepared.

Evaluating the use of standoff LIBS in architectural heritage: surveying the Cathedral of Málaga

Laser-induced breakdown spectroscopy (LIBS) is a cutting-edge technology which offers appealing features for its application in the field of the cultural heritage. It is a proven technology for the fast and simultaneous detection of major and trace elements with minimal destructiveness, using easily compactable instrumentation into movable platforms for the in situ and standoff chemical analysis of objects in real time. In the present work, a standoff LIBS sensor has been used for surveying the Cathedral of Malaga. The spectroscopic measurements were gathered in situ although from an averaged distance of 35 m. A comprehensive characterization of the materials composing the main facade as well as identification of the noticeable pollutants at their surfaces has been performed. The standoff LIBS results have fitted neatly with the mineralogical analysis of all the stones assayed. The large emissions of Si, Al, Ca and Mg have confirmed that the structure was almost entirely built using sandstone. In turn, the sensitivity to carbonate chemistry has demonstrated the capability of standoff LIBS for coherently classifying different marbles, thus allowing the identification of their origins. Standoff LIBS has also allowed the detection of pollutants such as Si, Ca, Mg, Fe, Al, Ba and Sr, originating from natural sources such as the transport of re-suspended dust and atmospheric particulate matter related to marine aerosols. In addition, trace elements such as Ti, Pb and Mn from exhausts of gasoline and diesel engines are also involved in the pollution triggering of materials. To obtain all these findings, scaffolding or other intrusive facilities have not been required.

Spatial distribution of catalytically active elements and deactivants in diesel-engine automobile converters by laser-induced plasma spectrometryElectronic Supplementary Information available: spectra corresponding to the washcoat and cordierite. See http://www.rsc.org/suppdata/ja/b2/b200975g

Laser-induced plasma spectrometry was used as a procedure for the comparative study of diesel-engine exhaust converters. Three-dimensional images were obtained and compared from “0 km” and “80 000 km” converters to generate semi-quantitative chemical images of the Pt load and of the poisoning elements P and Zn. The data revealed that a significant progressive depletion of Pt occurred from the converter region closest to the engine to the outer part of the catalyst. Deactivant elements were deposited along the full catalyst on usage for 80 000 km.

Selective Sampling and Laser-Induced Breakdown Spectroscopy (LIBS) Analysis of Organic Explosive Residues on Polymer Surfaces

A method for selective sampling and analysis of explosive residues on solid surfaces based on laser-induced breakdown spectroscopy (LIBS) is presented. Organic explosives are difficult to analyze when present as residues on organic materials. Under these circumstances LIBS suffers from the limitations imposed by the limited spectroscopic information available for the analysis. Since ablation and subsequent plasma formation are sensitive to the beam focal conditions and the pulse energy deposited on the surface, the choice of an appropriate set of experimental conditions increases the surface sensitivity of the analysis and hence a selective inspection of the residue in the absence of spectral contribution from the organic support analyzed. 2-Mononitrotoluene (MNT), 2,6- dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) are used as model residues, whereas nylon and Teflon are used as illustrative surfaces of daily life objects. The results demonstrate that selective sampling is successfully achieved in all cases when the plasma formation threshold of the residues and the object is substantially different. Plasma imaging demonstrates that the species distribution along the plume changes with beam focal conditions, which is exploited here to further increase the selectivity of the approach.