Laser-Induced Breakdown Spectroscopy in open-path configuration for the analysis of distant objects

A review of recent results on stand-off Laser-Induced Breakdown Spectroscopy (LIBS) analysis and applications is presented. Stand-off LIBS was suggested for elemental analysis of materials located in environments where any physical access was not possible but optical access could be envisaged. This review only refers to the use of the open-path LIBS configuration in which the laser beam and the returning plasma light are transmitted through the atmosphere. It does not present the results obtained with a transportation of the laser pulses to the target through an optical fiber. Open-path stand-off LIBS has mainly been used with nanosecond laser pulses for solid sample analysis at distances of tens of meters. Liquid samples have also been analyzed at distances of a few meters. The distances achievable depend on many parameters including the laser characteristics (pulse energy and power, beam divergence, spatial profile) and the optical system used to focus the pulses at a distance. A large variety of laser focusing systems have been employed for stand-off analysis comprising refracting or reflecting telescope. Efficient collection of the plasma light is also needed to obtain analytically useful signals. For stand-off LIBS analysis, a lens or a mirror is required to increase the solid angle over which the plasma light can be collected. The light collection device can be either at an angle from the laser beam path or collinear with the optical axis of the system used to focus the laser pulses on the target surface. These different configurations have been used depending on the application such as rapid sorting of metal samples, identification of material in nuclear industry, process control and monitoring in metallurgical industry, applications in future planetary missions, detection of environmental contamination or cleaning of objects of cultural heritage. Recent stand-off analyses of metal samples have been reported using femtosecond laser pulses to extend LIBS capabilities to very long distances. The high-power densities achievable with these laser pulses can also induce self-guided filaments in the atmosphere which produce LIBS excitation of a sample. The first results obtained with remote filament-induced breakdown spectroscopy predict sample analysis at kilometer ranges.     

Fast analysis of complex metallic alloys by double-pulse time-integrated Laser-Induced Breakdown Spectroscopy

Results are reported on the application of double-pulse Laser-Induced Breakdown Spectroscopy (LIBS) for fast analysis of complex metallic alloys. The approach followed for the determination of the composition of the alloys is based on the time-integrated acquisition of LIBS spectra emitted by plasmas induced by collinear double-pulse laser excitation. The spectra are analysed using the Partial Least Squares method, which allows the determination of sample composition even in the presence of strong spectral interferences. The results shown indicate the possibility of measuring the composition of complex metallic alloys in very short times and using relatively cheap LIBS instrumentation.     

Online analysis of potassium fertilizers by Laser-Induced Breakdown Spectroscopy

Presently, online analysis in potassium fertilizers industry is performed by Natural Radioactivity Analyzers. Laser Distance Spectrometry (LDS) has tested, by laboratory scale, the possibility of Laser-Induced Breakdown Spectroscopy (LIBS) technique implementation for online fertilizers production control. The main advantage of the system comparing to the existing technique is the principal possibility to analyze all relevant elements, such as K, Na, Mg, and not only K40 isotope as done in natural radiation analytical systems. Good correlations between online LIBS results with chemical analysis data of K, Na and Mg impurities of samples from Russia, Belarus and Israel demonstrate that LIBS system is a perspective tool for online control of those elements in field conditions.     

Analysis of Roman-Hispanic Archaeological Ceramics using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a useful tool for characterization of archaeological samples because it involves little or no sample preparation, and it allows on-site rapid and nondestructive analysis. In this study, ceramics from the archaeological site of Labitolosa (Huesca) were analyzed qualitatively and quantitatively. Furthermore, a protocol is described for the determination of iron in archaeological materials by interpolating calibration graphs and standard addition. Moreover, the potential of LIBS for direct quantitative analysis of iron in real archaeological materials was evaluated, and optimum analytical conditions were determined. The reproducibility of LIBS spectra from different archaeological samples was investigated as a function of the number of laser shots. The results were compared with those of a reference material.     

Strategies for Mars remote Laser-Induced Breakdown Spectroscopy analysis of sulfur in geological samples

The key to understanding the sulfur history on Mars is to identify and determine sulfate and sulfide compositions and then to draw from them geologic clues about their environments of formation. To lay a foundation for use of remote LIBS to sulfur analysis in planetary exploration, we have undertaken a focused study of sulfur LIBS in geological samples in a simulated Mars atmosphere, with experimental parameters replicating the ChemCam LIBS instrument. A suite of twelve samples was selected, including rocks rich in minerals representative of sulfates and sulfides that might be encountered on Mars. Univariate analysis of sulfur emission lines did not provide quantitative information. Partial least squares (PLS) analysis was successful at modeling sulfur concentrations for a subset of samples with similar matrices. Sulfide minerals were identified on the basis of other siderophile or chalcophile peaks, such as those arising from Zn and Cu. Because the S lines are very weak compared to those of other elements, optimal PLS results were obtained by restricting the wavelength range to channels close to the most intense sulfur lines ~ 540-570 nm. Principal components analysis was attempted on the dataset, but did not differentiate the samples into meaningful groups because the sulfur lines are not strong enough. However, areas of the relatively weak S, H, and O peaks may be used to correctly classify all samples. Based on these outcomes, a flowchart that outlines a possible decision tree for identification and quantification of sulfur in remote LIBS analysis was constructed. Results suggest that LIBS data acquired under Mars conditions can meet the science requirements for the ChemCam instrument.     

Analysis of Pre-Hispanic Archaeological Samples Using Laser-Induced Breakdown Spectroscopy (LIBS)

Abstract

Laser-induced breakdown spectroscopy gave excellent results with respect to reproducibility, sensitivity, nondestructiveness, rapid elemental analisys, and depth-profile analisys. In this work, elements contained in pigments from pre-Hispanic ceramics of Cajamarquilla were identified. Also, some textile samples from the Uquira site, located in Asia Valley between Mala and Cañete (Perú), were analyzed. For this purpose, a Quantel Ultra CFR pulsed Nd/YAG laser was used.     

Quantitative Analysis of Roman Archeological Ceramics by Laser-Induced Breakdown Spectroscopy

In recent decades, numerous analytical techniques have been used for the analysis of archeological samples. Laser-induced breakdown spectroscopy (LIBS) is a promising technique due to its practically nondestructive nature and minimal sample preparation. In this work, LIBS was used for the qualitative and quantitative elemental analyses of pottery manufactured in ancient settlements of Rome. The qualitative study showed that the ceramics were composed of Fe, Ca, and Mg. For quantitative analysis, calibration curves of Fe, Ca, and Mg were constructed with reference samples of each element in a KBr matrix with zinc as an internal standard. The results obtained by LIBS were compared with values obtained by atomic absorption.     

激光诱导击穿光谱法分析焊丝样品

采用激光诱导击穿光谱法(LIBS)对焊丝样品中的硅、锰、磷、硫、铬及镍元素进行测定。采用扫描电子显微镜和激光共聚焦显微镜观察了样品经激光脉冲烧蚀后的形貌,并考察了各元素的谱线强度与激光脉冲个数之间的关系,从而确定了预剥蚀激光脉冲个数。通过分析标准样品建立了单个元素的谱线强度与其质量分数之间的标准曲线,相关系数在0.989~0.999之间。本方法对焊丝样品的分析结果与经典的电感耦合等离子体原子发射光谱法和高频红外硫碳分析方法的测定结果相吻合,其精密度稍差于经典的分析方法。     

A numerical study of expected accuracy and precision in Calibration-Free Laser-Induced Breakdown Spectroscopy in the assumption of ideal analytical plasma

Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) has been proposed several years ago as an approach for quantitative analysis of Laser-Induced Breakdown Spectroscopy spectra. Recently developed refinement of the spectral processing method is described in the present work. Accurate quantitative results have been demonstrated for several metallic alloys. However, the degree of accuracy that can be achieved with Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of generic samples still needs to be thoroughly investigated. The authors have undertaken a systematic study of errors and biasing factors affecting the calculation in the Calibration-Free Laser-Induced Breakdown Spectroscopy spectra processing. These factors may be classified in three main groups: 1) experimental aberrations (intensity fluctuations and inaccuracy in the correction for spectral efficiency of a detection system), 2) inaccuracy in theoretical parameters used for calculations (Stark broadening coefficients and partition functions) and 3) plasma non-ideality (departure from thermal equilibrium, spatial and temporal inhomogeneities, optical thickness, etc.). In this study, the effects of experimental aberrations and accuracy of spectral data were investigated, assuming that the analytical plasma is ideal. Departure of the plasma conditions from ideality will be the object of future work. The current study was based on numerical simulation. Two kinds of metallic alloys, iron-based and aluminum-based, were studied. The relative weight of the error contributions was found to depend on the sample composition. For the here-investigated samples, the experimental aberrations contribute to the overall uncertainty on the quantitative results more than theoretical parameters. The described simulation method can be applied to the Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of any other kind of sample.     

Archaeometric Analysis of Ancient Copper Artefacts by Laser-Induced Breakdown Spectroscopy Technique

Twelve archaeological copper objects from the burial site of “Fontino” cave, near Grosseto, (around 2500–2000 B.C.) were analysed using laser-induced breakdown spectroscopy. Qualitative results and a preliminary study of the samples’ composition are reported and used to make a quantitative estimate; based on these results, the samples were classified using principal components statistical analysis. The perspectives of using laser-induced breakdown spectroscopy for archaeometric analysis are also discussed.     

Calibration-Free Laser-Induced Breakdown Spectroscopy: State of the art

The aim of this paper is offering a critical review of Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS), the approach of multi-elemental quantitative analysis of LIBS spectra, based on the measurement of line intensities and plasma properties (plasma electron density and temperature) and on the assumption of a Boltzmann population of excited levels, which does not require the use of calibration curves or matrix-matched standards. The first part of this review focuses on the applications of the CF-LIBS method. Quantitative results reported in the literature, obtained in the analysis of various materials and in a wide range of experimental conditions, are summarized, with a special emphasis on the departure from nominal composition values. The second part is a discussion of the simplifying assumptions which lie at the basis of the CF-LIBS algorithm (stoichiometric ablation and complete atomization, thermal equilibrium, homogeneous plasma, thin radiation, detection of all elements). The inspection of the literature suggests that the CF-LIBS method is more accurate in analyzing metallic alloys rather than dielectrics. However, the full exploitation of the method seems to be still far to come, especially for the lack of a complete characterization of the effects of experimental constraints. However, some general directions can be suggested to help the analyst in designing LIBS measurements in a way which is more suited for CF-LIBS analysis.     

Artificial neural network for Cu quantitative determination in soil using a portable Laser Induced Breakdown Spectroscopy system

Laser Induced Breakdown Spectroscopy (LIBS) is an advanced analytical technique for elemental determination based on direct measurement of optical emission of excited species on a laser induced plasma. In the realm of elemental analysis, LIBS has great potential to accomplish direct analysis independently of physical sample state (solid, liquid or gas). Presently, LIBS has been easily employed for qualitative analysis, nevertheless, in order to perform quantitative analysis, some effort is still required since calibration represents a difficult issue. Artificial neural network (ANN) is a machine learning paradigm inspired on biological nervous systems. Recently, ANNs have been used in many applications and its classification and prediction capabilities are especially useful for spectral analysis. In this paper an ANN was used as calibration strategy for LIBS, aiming Cu determination in soil samples. Spectra of 59 samples from a heterogenic set of reference soil samples and their respective Cu concentration were used for calibration and validation. Simple linear regression (SLR) and wrapper approach were the two strategies employed to select a set of wavelengths for ANN learning. Cross validation was applied, following ANN training, for verification of prediction accuracy. The ANN showed good efficiency for Cu predictions although the features of portable instrumentation employed. The proposed method presented a limit of detection (LOD) of 2.3 mg dm^− 3 of Cu and a mean squared error (MSE) of 0.5 for the predictions.     

Classification of Chinese Herbal Medicine by Laser-Induced Breakdown Spectroscopy with Principal Component Analysis and Artificial Neural Network

Chinese herbal medicine has attracted increasing attention because of the unique and significant efficacy in various diseases. In this paper, three types of Chinese herbal medicine, the roots of Angelica pubescens, Codonopsis pilosula, and Ligusticum wallichii with different places of origin or parts, are analyzed and identified using laser-induced breakdown spectroscopy (LIBS) combined with principal component analysis (PCA) and artificial neural network (ANN). The study of the roots of A. pubescens was performed. The score matrix is obtained by principal component analysis, and the backpropagation artificial neural network (BP-ANN) model is established to identify the origin of the medicine based on LIBS spectroscopy of the roots of A. pubescens with three places of origin. The results show that the average classification accuracy is 99.89%, which exhibits better prediction of classification than linear discriminant analysis or support vector machine learning methods. To verify the effectiveness of PCA combined with the BP-ANN model, this method is used to identify the origin of C. pilosula. Meanwhile, the root and stem of L. wallichii are analyzed by the same method to distinguish the medicinal materials accurately. The recognition rate of C. pilosula is 95.83%, and that of L. wallichii is 99.85%. The results present that LIBS combined with PCA and BP-ANN is a useful tool for identification of Chinese herbal medicine and is expected to achieve automatic real-time, fast, and powerful measurements.     

Quantitative Analysis of Metallic Elements in Tobacco and Tobacco Ash by Calibration Free Laser-Induced Breakdown Spectroscopy

Cigarette smoking may be a substantial source of intake of some hazardous elements to the human health. In this work calibration-free laser-induced breakdown spectroscopy technique was employed to investigate the trace metallic elements (Fe, Ca, Al, Cu, K, Li, Mg, Mn, Na, Sr, Ti, and Zn) concentration in tobacco and tobacco ash. Moreover, variation of metallic elements mass concentration between tobacco and tobacco ash has been observed.     

Study of sub-mJ-excited laser-induced plasma combined with Raman spectroscopy under Mars atmosphere-simulated conditions

Laser-Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are complimentary techniques. LIBS yields elemental information while Raman spectroscopy yields molecular information about a sample, and both share similar instrumentation configurations. The combination of LIBS and Raman spectroscopy in a single instrument for planetary surface exploration has been proposed, however challenges exist for developing a combined instrument. We present LIBS and Raman spectroscopy results obtained using a diode pumped, intracavity doubled, Q-switched, Nd:YLF laser operating at 523 nm, which overcomes some of the difficulties associated with a combined instrument. LIBS spectra were obtained with 170 μJ per pulse at 4 Hz repetition rate in a low pressure Mars-simulated atmosphere and Raman spectra produced with 200 mW at 100 kHz. The Nd:YLF laser is switchable between LIBS and Raman spectroscopy modes only by a change in Q-switch repetition rate. Emissions from Ca, Ca II, Fe, Fe II, Mg, Na, and O atom were identified in the μ-LIBS spectrum of oolithic hematite. Evidence was found for a change in plasma dynamics between 7 and 5 Torr that could be explained as a decrease in plasma temperature and electron density below 5 Torr. This is relevant to future Mars exploration using LIBS as the mean surface pressure on Mars varies from 3.75 to 6 Torr. LIBS plasma dynamics should be carefully evaluated at the pressures that will be encountered at the specific Mars landing site.     

On-line monitoring of composite nanoparticles synthesized in a pre-industrial laser pyrolysis reactor using Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy (LIBS) was employed for on-line and real time process monitoring during nanoparticle production by laser pyrolysis. Laser pyrolysis has proved to be a reliable and versatile method for nanoparticle production. However, an on-line and real time monitoring system could greatly enhance the process optimization and accordingly improve its performances. For this purpose, experiments aiming at demonstrating the feasibility of an on-line monitoring system for silicon carbide nanoparticle production using the LIBS technique were carried out. Nanosecond laser pulses were focused into a cell through which part of the nanoparticle flux diverted from the production process was flowed for LIBS analysis purposes. The nanoparticles were vaporized within the laser-induced plasma created in argon used as background gas in the process. Temporally-resolved emission spectroscopy measurements were performed in order to monitor nanoparticle stoichiometry. Promising results were obtained and on-line Si/C_x stoichiometry was successfully observed. These results put forward the possibility of real time correction of the nanoparticle stoichiometry during the production process.     

Measurement of Stark broadening of Mn I and Mn II spectral lines in plasmas used for Laser-Induced Breakdown Spectroscopy

We present measurements of the Stark broadening of several Mn lines in the conditions of typical laser-induced plasmas. Single-and double-pulse Laser-Induced Breakdown spectroscopy (LIBS) configurations are studied on a series of Fe–Mn alloy samples with Mn concentration ranging from 6% to 30%. The effects of self-absorption on the measured line broadenings are discussed in detail. In particular, the experimental results evidence that self-absorption is much higher in laser-induced plasmas generated with double pulses, compared to the case of single pulse. After measurement of the electron density, the Stark coefficients of several neutral and ionic Mn lines are derived through the measure of the broadening in conditions of optically thin plasma. The results obtained for singly ionized Mn lines are compared with the theoretical and experimental data present in the literature. For the first time, experimental measurements of the Stark coefficient for several neutral Mn lines are also presented.     

Wavelength dependence on the elemental analysis of glass by Laser Induced Breakdown Spectroscopy

Laser Induced Breakdown Spectroscopy (LIBS) is presented as a tool for the elemental analysis of glass in forensic applications. Two harmonics of the Nd:YAG laser at 266 nm and 532 nm were used as the irradiation source for the analysis of several glass standards and soda–lime glass samples of interest to forensic scientists. Both lasers were kept at a constant energy of 20 mJ and focused using a 150 mm focal length lens. A series of experiments were also conducted to determine the importance of wavelength on lens-to-sample distance (LTSD) at each wavelength. It was determined that the optimal LTSD was found at ~ 1–2 mm focused into the surface for both wavelengths yet the crater depth resulting from the irradiation at 266 nm was significantly deeper (112 µm) than that from the 532 nm laser (41 µm). In addition, the analytical performance of LIBS on 5 NIST glasses and 6 automobile glasses at both wavelengths is reported. Good correlation for the quantitative analysis results for the trace and minor elements Sr, Ba and Al are reported along with the calibration curves, in most cases R² > 0.95, using absolute intensities at various emission lines. Although 266 nm resulted in more mass removal, the 532 nm produced greater emission intensities. A slightly higher plasma density was determined for irradiation by 532 nm using the Stark broadening technique in comparison to the 266 nm irradiation.     

Optimization of micro-Laser Induced Breakdown Spectroscopy analysis and signal processing

As a result of continuing instrumental development (Echelle spectrometer and ICCD detectors), micro-Laser Induced Breakdown Spectroscopy analysis may become an increasingly recognized analytical technique for determining elemental compositions of geologic materials. Best conditions of time resolution conditions (delay and time acquisition window) are estimated with respect to the collection geometry of optical plasma emission of our system. It turns out that the level of the Bremsstrahlung continuum emission is weak in the first tens of nanoseconds after the laser excitation pulse. The enlargement of the emission lines is identified in the first 100 ns but remains comparable to the spectral resolution of our system. Thus, results show that time-resolved conditions are not necessarily required to perform elemental analysis at the micrometric scale using LIBS, contrary to macro-LIBS. This suggests potential improvements of micro-LIBS analysis (sensitivity and spectral resolution) using non-intensified CCD connected with the laser pulse.     

On-line determination of nanometric and sub-micrometric particle physicochemical characteristics using spectral imaging-aided Laser-Induced Breakdown Spectroscopy coupled with a Scanning Mobility Particle Sizer

Laser-Induced Breakdown Spectroscopy has been employed to detect sodium chloride and metallic particles with sizes ranging from 40 nm up to 1 µm produced by two different particle generators. The Laser-Induced Breakdown Spectroscopy technique combined with a Scanning Mobility Particle Sizer was evaluated as a potential candidate for workplace surveillance in industries producing nanoparticle-based materials. Though research is still currently under way to secure nanoparticle production processes, the risk of accidental release is not to be neglected. Consequently, there is an urgent need for the manufacturers to have at their command a tool enabling leak detection in-situ and in real time so as to protect workers from potential exposure.In this context, experiments dedicated to laser-induced plasma particle interaction were performed. To begin with, spectral images of the laser-induced plasma vaporizing particles were recorded to visualize the spatio-temporal evolution of the atomized matter and to infer the best recording parameters for Laser-Induced Breakdown Spectroscopy analytical purposes, taking into account our experimental set-up specificity. Then, on this basis, time-resolved spectroscopic measurements were performed to make a first assumption of the Laser-Induced Breakdown Spectroscopy potentialities. Particle size dependency on the LIBS signal was examined. Repeatability and limits of detection were assessed and discussed. All the experiments carried out with low particle concentrations point out the high time delays corresponding to the Laser-Induced Breakdown Spectroscopy signal emergence. Plasma temperature temporal evolution was found to be a key parameter to explain this peculiarity inherent to laser/plasma/particle interaction.