Laser-induced Breakdown Spectroscopy Measurements Research Articles

Laser-Induced Breakdown Spectroscopy (LIBS) characterization of granular soils: Implications for ChemCam analyses at Gale crater, Mars

The Curiosity rover has been characterizing mineralogical and chemical compositions of Gale crater soils on Mars since 2012. Given its sub-millimeter scale of analysis, the ChemCam instrument is well suited to study the composition of soil constituents. However, the interpretation of LIBS data on soils in the martian environment is complicated by the large diversity of particle sizes (from dust to sand), combined with the unknown physical arrangement of their mineral constituents (i.e., the type of grain mixtures). For example, martian soils contain a significant amount of X-ray amorphous materials whose physical form remains unclear. In this study, we reproduced martian soil analyses in the laboratory to understand how the LIBS technique can provide specific insights into the physical and chemical properties of granular soils. For this purpose, different types of samples were studied with various ranges of grain sizes, mimicking two possible mixtures that may occur in martian soils: mechanical mixtures of two populations of grains made of distinct chemical compositions; and material forming a compositionally distinct coating at the surface of grains. Our results, also supported by in situ ChemCam data, demonstrate that both the sizes and the type of mixture of soil particles have a strong influence on the LIBS measurement. For mechanical mixtures of two populations of grains larger than 125–250 μm, the scatter of the data provides information about the chemical composition of the end-members. On the other hand, the chemistry recorded by LIBS for grains with surface coatings is fully dominated by the outer material for grains smaller than 500 μm in diameter. This is due to the small penetration depth of the laser (~0.3–1.5 μm per shot), combined with the ejection of small grains at each shot, which leads to a constant replenishment of fresh material. This experimental work will thus improve our understanding of martian soils analyzed by ChemCam, and more broadly, will benefit LIBS studies of granular materials.

Discrimination of automotive glass by conjoint Raman and laser-induced breakdown spectroscopy and multivariate data analysis

Conjoint laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy and high level data fusion were used to discriminate sixteen different glass samples by principal component analysis (PCA). LIBS provides information on the elemental composition of a sample and Raman spectroscopy gives information about the molecular composition. The results were compared to single LIBS and Raman measurements as well as to μ-XRF and SEM-EDS which are also frequently used in glass analysis. The direction dependent Mahalanobis distance between the sample point groups in the space of the most significant principle components was evaluated as a criterion for potential discrimination of the different samples. Very good discrimination (up to 99%) was achieved with conjoint LIBS and Raman and high level data fusion, as well as LIBS alone. The discrimination is based primarily on different contents of trace elements (e.g., Fe, Ti, Ba, Sr), which was confirmed by the μ-XRF and SEM-EDS and differences in fluorescence (detected by Raman). Besides the similar discrimination power of LIBS and conjoint LIBS and Raman, the combination and fusion of LIBS and Raman leads to greater distances between the data in the principal component space due to the additional information added by Raman. This should result in a lower misclassification rate of unknown samples. Results show that conjoint LIBS and Raman spectroscopy can be an alternative in the forensic analysis of glass samples.

Quantitative 1-D LIBS measurements of fuel concentration in natural gas jets at high ambient pressure

Fuel concentration information is critical to the development of high-efficiency combustion system of direct-injection natural gas engines. In this study, a one-dimensional (1-D) measurement approach based on laser induced breakdown spectroscopy (LIBS) is developed to quantitatively measure 1-D fuel concentration distributions in gas jets at high ambient pressures. Except for the spectral dimension, another dimension of the intensified charge coupled device (ICCD) is employed for the spatial resolution of measurements. The effects of laser energy and ambient pressure on plasma dynamics are investigated to explore the feasibility of 1-D LIBS in concentration measurements at high ambient pressure. Since the peak intensity ratio (PIR) of H656/N746 shows significant spatial variations in the plasma volume, a 1-D calibration strategy is developed by establishing respective calibration curves at different spatial positions to quantify the fuel concentration distributions. The measurement uncertainties are evaluated, including both the systematic and random errors. The temporal variations of the 1-D fuel concentration distributions in methane jets are presented, and the shot-to-shot fluctuations are analyzed in detail. The results show that the coefficient of variation of equivalence ratio is lowest at the end of injection (EOI), suggesting that ignition at the EOI could enable a relatively stable flame kernel formation. Along the radial direction of the jet, a high ignitable mixture percentage appears at approximately 3.0 mm from the jet axis at the EOI, suggesting an optimal ignition position. The results reveal that the 1-D LIBS could be a powerful tool for calibration of numerical models and optimal designs of combustion engines taking advantage of high-pressure gas jets.

Hydrogen content in divertor baffle tiles in Wendelstein 7-X

Abstract Plasma–wall interactions play a crucial role for the performance of fusion devices and the lifetime of plasma-facing components. In this work the results of some plasma–wall interaction processes, namely the erosion/deposition pattern and the fuel content of divertor baffle tiles exposed in Wendelstein 7-X in the initial phase of island divertor, are presented. Net-deposition of carbon with co-deposited oxygen and hydrogen is determined on the graphite tiles after about 1 hour of plasma operation in hydrogen and helium in operation phase OP 1.2a. C is predominately a result of net-erosion of the graphite target plates and oxygen is the strongest intrinsic impurity in Wendelstein 7-X in OP 1.2a. The hydrogen content distribution on a set of tiles exposed in equivalent positions in three of the five modules of the stellarator was quantified for the first time. Ex-situ performed laser-induced breakdown spectroscopy measurements show the depth-resolved fuel content in deposited layers as well as implantation and diffusion in the base material. Complementary, gas analysis after laser-induced ablation offers quantitative hydrogen content determination in the deposited layers up to 1018 hydrogen atoms/cm 2 that is non-uniformly distributed over the 95 mm × 125 mm sized tiles. The results show a toroidal asymmetry with 60% more hydrogen in the stellarator module 2 with respect to module 1 as well as a slight top-down asymmetry for the baffle tiles in upper and lower half modules. A clear dependence of the hydrogen content on the surface temperature during plasma exposition in stellarator discharges in standard magnetic divertor configuration with edge transformation 5/5 was not observed.

In situ laser-induced breakdown spectroscopy measurements during laser welding of superalloy.

Laser-induced breakdown spectroscopy (LIBS) has been utilized for in situ diagnostics of the laser welding process. The influence of different weld spot areas (melt pool, solid weld) on LIBS signals and plasma properties has been studied in detail. Liquid metal sampling and high target surface temperature of the melt enhance LIBS plasma intensity and increase plasma temperature. The influence of laser welding process parameters on LIBS measurements has been studied in order to differentiate optimal and defective laser welding. In case of defective laser welding, the melt pool was intensively boiling, so we have observed greater LIBS signals but poor reproducibility. For the first time, the LIBS technique was demonstrated to detect defective laser welding during in situ measurements utilizing atomic and ionic line comparison by paired sample t-test hypotheses testing.

Remote identification and ranking of Sodium in bulk liquid under high pressure condition

Laser Induced Breakdown Spectroscopy (LIBS) based spectral and temporal measurements are carried out to identify and rank the Sodium in bulk liquid under high pressure condition. The experiments are carried out by varying the incident laser irradiance (4 to 5 * 1011 W/cm2), ambient pressure condition (1 to 8 atm) and stand-off collection distance (0.6 to 2 m). The concentration of Sodium Chloride (NaCl) in the sample solution is varied between 250 and 1500 ppm. From the spectral data, the Na I emission line (589 nm) is visualized for all experimentation condition. Emission due to singly ionized atom and doubly ionized atom are not observed, due to the requirement of high ionization energies. It is observed that the Na I (589 nm) peak intensity decreases with increase in ambient pressure condition, due to the plasma condensation effect. It is also observed that the Na I (589 nm) peak intensity increases with increase in incident laser irradiance. Temporal measurement based quantification of Sodium in bulk liquid is carried out. It is observed that the plasma emission time period (Na I at 589 nm) increases with increase in NaCl concentration. This is related to the re-excitation of neutral atoms (Na I) in the plasma state. In the stand-off distance analysis, it is observed that the plasma (Na I) emission peak intensity and time period of emission decreases with increase in stand-off collection distance. This might be due to the low coupling of plasma emission to the measurement system, at longer stand-off distances. The Na detection limit of about 10 ppm is achieved for a laser irradiance of 3 * 1011 W/cm2. The above specified detection limit is obtained for an ambient pressure condition of 1 atm and stand-off collection distance of 0.6 m. On a summary, remote LIBS with temporal measurement is highly suitable to identify and rank the Sodium concentration in bulk liquid under high pressure condition. However, the calibration of LIBS measurements at different experimental condition is required to utilize the instrument in real-time environment.

Crosstalk effects of the surface layers of metallic samples on laser-induced breakdown spectroscopy measurements

Material ablation driven by focused laser pulses is an important aspect of laser-induced breakdown spectroscopy (LIBS). Schematics of the studied measuring object as model artefact for non-representative surface layers.

Online and in situ laser-induced breakdown spectroscopy for laser welding monitoring

The laser induced breakdown spectroscopy (LIBS) technique was suggested for online and in situ detection of laser welding process failures. For the first time, the LIBS technique was demonstrated to detect defective laser welding during in situ measurements. Three sensing procedures were verified to indicate defective laser welding: thermal emission spectroscopy of the welding melt pool induced by a continuous wave (cw) laser (passive sensing); online LIBS sampling of a solidified hot weld; in situ LIBS measurements of the melt pool surface. The passive sensing failed to detect problems of the laser welding process. Online LIBS measurements of a hot solidified weld did not succeed as well. However, in situ weld melt pool LIBS measurements revealed that defective and optimal welding can be distinguished with atomic/ionic lines signals comparison utilizing hypothesis testing by paired sample t-test.

Al and Si quantitative analysis in aqueous solutions by LIBS method

In the paper results of Laser-Induced Breakdown Spectroscopy (LIBS) measurements of Al and Si content in aqueous solutions are presented. Calibration curves exhibit good linearity and satisfying R2 coefficients of determination (over 0.99). The calibration procedures were next used for the determination of Al and Si concentration in biological material transferred into water solutions of unknown Al and Si content. Obtained data were verified by the AAS method (atomic absorption spectrometry). Measurement results showed good conformity. The LIBS spectra were also processed using Factorial Analysis (FA). It was found for Si samples that instead of a classic approach based on the selection of a particular spectral line for the calibration the FA makes possible the use of entire spectra to obtain quantitative information on the investigated material.

Mechanism of signal uncertainty generation for laser-induced breakdown spectroscopy

Relatively large measurement uncertainty severely hindered wide application for laser-induced breakdown spectroscopy (LIBS), therefore it is of great importance to understand the mechanism of signal uncertainty generation, including initiation and propagation. It has been found that the fluctuation of plasma morphology was the main reason for signal uncertainty. However, it still remains unclear what mechanism leads to laser-induced plasma morphology fluctuation. In the present work, we employed three fast-imaging cameras to capture three successive plasma images from a same laser-induced Titanium alloy plasma, which enables us to understand more clearly of the plasma evolution process especially for the early plasma evolution stage when plasma and surrounding gases interact drastically. Seen from the images, the plasma experienced an increasing morphological fluctuation as delay time increased, transforming from a “stable plasma” before the delay time of 100 ns to a “fluctuating plasma” after the delay time of 300 ns. Notably, the frontier part of plasma showed a significant downward motion from the delay time of 150 ns to 200 ns and crashed with the lower part of the plasma, making the plasma flatter and later even splitting the plasma into two parts, which was considered as a critical process for the transformation of “stable plasma” to “unstable plasma”. By calculating the correlation coefficient of plasma image pairs at successive delay times, it was found that the higher the similarity between two plasma at early stage, the more similar at later stage; this implied that the tiny plasma fluctuation earlier than the critical delay time (150–200 ns) was amplified, causing a large plasma fluctuation at the later stage as well as LIBS measurement uncertainty. The initiation of slight fluctuation was linked with Rayleigh-Taylor Instability (RTI) due to the drastic material interpenetration at the plasma-ambient gas interface at earlier stage (before 50 ns). That is, the uncertainty generation of LIBS was proposed as: plasma morphology fluctuation was inevitably trigged by RTI at the early stage and the tiny fluctuation was amplified by the back pressed downward process of plasma frontier material, leading to severe morphology fluctuation as well as LIBS signal uncertainty.

High enhancement factor in low-power unipolar discharge arc assisted laser induced plasma spectroscopy

The enhanced laser induced breakdown spectroscopy is highly desirable for the quantitatively detection with low limit of detection (LOD) and high accuracy. Higher enhancement factor is always obtained from discharge assisted laser-induced breakdown spectroscopy with enough higher energy injection in comparison to the normally standard laser induced breakdown spectroscopy (LIBS), which means dangerous with high voltage and elevated power. Here, a low power (10 W) injected unipolar discharge arc is proven to greatly enhance the LIBS with a large enhancement factor up to 13.8 when the discharged electric arc followed by a laser induced plasma are simultaneously applied on a silicon wafer, which is comparative or larger than those results with high-power (kW) and high voltage (kV) being applied. Furthermore, the enhanced technique is applied on an actual qualitative detection of cadmium in soil. The lines of Cd II (214.44 nm) from soil is enhanced as compared to a LIBS measurement. Accordingly, the LOD of the typical emission line is reduced to 23.5 ppm by the unipolar discharge arc assisted laser induced breakdown spectroscopy (UD-LIBS), which is 2.45-fold lower than that from the normal LIBS. The pre-ionization of the environment and the surface by the modulated discharge arc to directly make the Inverse Bremsstrahlung (IB) absorption with higher absorption coefficient dominate the initial stage of plasma formation rather than the multi-photon absorption process, thus make both electron density and the temperature of plasma raising are the main mechanism of spectral enhancement, which is further proved by the evolution dynamics images of two plasma overlapping processes under an in-situ intensified charge-coupled device (ICCD) based photographic system. The simple, portable, commercial highly enhancement way is greatly benefit to the actual applications.

Analysis of Ilmenite Slag Using Laser-Induced Breakdown Spectroscopy

The feasibility of using laser-induced breakdown spectroscopy (LIBS) for the compositional analysis of ilmenite slag was explored. The slag was obtained from a pilot-scale ilmenite smelting furnace. The composition of major oxides TiO2, FeO, and MgO are determined by the calibrated LIBS method. LIBS measurements are done under normal atmosphere and temperature. A Q-switched Nd:YAG laser operating at 355 nm was used to create a plasma on an ilmenite slag sample. The characteristic lines based on the NIST database of Fe, Mg, and Ti can be identified on the normalized LIBS spectra for the slag samples. The spectral range chosen for the study is 370 to 390 nm. Calibration curves were plotted using the data collected from various industrial ilmenite samples of varying compositions of TiO2, FeO, and MgO. The univariate simple linear regression technique was used to do the analysis and the prediction accuracy was checked by the root mean square error (RMSE). To validate the application of LIBS, both qualitative and quantitative analysis is done and compared to the analytical ICP-OES results. The model predicts the magnesium content with the highest accuracy and gives good prediction for iron and titanium content. This study demonstrates the capability of using LIBS for the surface analysis of the ilmenite slag sample.

Quantification of trace elements in molten aluminum with randomized impurity concentrations using laser-induced breakdown spectroscopy

An industrial molten metal chemical analyzer based on laser-induced breakdown spectroscopy (LIBS) has been used to perform concentration analysis of important trace elements (Fe,Si,Cr,Mn,Ti, 20-3000 ppm) in liquid aluminum. In order to rule out accidental correlations between different elements in the spectral analysis, the impurity concentration of the measured samples was fully randomized. Reference concentration measurements were performed using arc-spark optical emission spectroscopy (spark-OES) on solid samples cast from the full volume of the LIBS-analyzed melt. For elements Fe, Cr, Mn and Ti, correlation coefficients and prediction uncertainty of LIBS measurements, using a linear correlation model, are shown to be determined mainly by random measurement error, which was of the order of 1% for both the LIBS analysis and the spark-OES analysis for concentrations above 100 ppm for all of the investigated elements. In the case of silicon, we postulate that inhomogeneous solidification is leading to a reduced absolute accuracy of spark-OES reference measurements and a corresponding increase in the minimum prediction uncertainty. The results confirm that LIBS analysis of molten aluminum can be used for on-line process control in the aluminum industry by providing measurement accuracy comparable of current industry-standard laboratory analysis methods.

Identification of type of cement through laser-induced breakdown spectroscopy

The composition of concrete determines its resistance to various degradation mechanisms such as ingress of ions, carbonation or reinforcement corrosion. Knowledge of the composition of the hardened concrete is therefore helpful to assess the remaining service life of an existing structure or evaluate the damage observed during inspections. For example, for most existing concrete structures the type of cement originally used is not known and must therefore be determined afterwards. This paper presents a preliminary study on the application of laser-induced breakdown spectroscopy (LIBS) to identify the type of cement. For this purpose, ten different types of cement were investigated. For every type, three cement paste prisms were produced: (i) prisms dried, ground and pressed into tablets, (ii) prisms dried and (iii) prisms untreated. LIBS measurements were performed with a diode-pumped low energy laser (1064 nm, 3 mJ, 1.5 ns, 100 Hz) in combination with two compact spectrometers which cover the UV and NIR spectral range. A reduced subset of spectral features was used to build a classification model based on linear discriminant analysis. The results show that the classification of homogenized pressed cement powder samples provides a high accuracy, however, factors such as a different sample matrix and moisture content can affect the accuracy of the classification. The study demonstrates that LIBS is a promising tool to identify the type of cement.

Determination of Stark Shifts and Widths Using Time Resolved Laser-Induced Breakdown Spectroscopy (LIBS) Measurements.

Stark broadening parameters have been estimated for resonant lines of Al(I) using time resolved measurements. The relation between the various emission line characteristics at different phases of opacity have been utilized to obtain the value of plasma temperature and Stark width parameters from the experimental data. The observed value of the center line intensity and Lorentzian component of the line width are compared against a simulated value of these parameters for optically thin case. The plasma temperature and Stark broadening parameters are obtained for the best fit condition by matching the experimentally observed and the simulated values of intensity and line widths. The time resolved measurements result in much better estimates for Stark parameters by allowing multiple points for fitting keeping the number of variables limited. The Stark shift parameters are also obtained from the slope of the plot of observed central wavelength shifts versus observed electron number density which is measured as a function of time. Hence, a method utilizing multiple-time observation data to obtain the Stark broadening parameters for lines showing self-absorption has been demonstrated.

Soft classification scheme with pre-cluster-based regression for identification of same-base alloys using laser-induced breakdown spectroscopy

In this study, a novel soft classification scheme is proposed for metal scrap identification with laser-induced breakdown spectroscopy (LIBS) measurements. LIBS provides unique spectra for different metals that can be utilized for classifying metal scraps in real time. Despite its potential, LIBS-based metal classification is not yet fully implemented in practice due to the large shot-to-shot variation and non-linear relationships between spectral intensities and elemental concentrations. Particularly for recycling metal alloys of the same base, learning all candidate types is infeasible, and conventional classification approaches exhibit limited performance in classifying unknown samples of untrained types due to the variability and non-linearity in LIBS measurements. To overcome the limitations of LIBS-based metal scrap classification, the proposed scheme employs pre-cluster-based regression (PCBR) analysis. PCBR takes advantage of the joint relationships between the elemental concentration variations of the pre-clusters (p-clusters), which are pre-determined by prior information of the probability distributions. The variance of the regression of individual p-clusters can be significantly reduced compared to global regression by jointly taking into account common relationships between the elemental concentrations within a particular p-cluster. By combining the layered regression results with their estimated statistics, soft multi-label classification and extraction of the likelihood values of trained classes is possible even for samples of untrained types.For performance evaluation, a list of reference alloys from the National Institute of Standard and Technology (NIST) and Brammer databases was divided into finite sets of p-clusters based on the relationships of elemental concentrations, in particular, four p-clusters for Cu-based alloy tests with Cu and Zn concentrations. Then, PCBR were trained with LIBS-captured spectra of 35 certified reference materials for all and four individual p-clusters. The partial least squares (PLS) and random forest (RF) regression methods were employed, and the root mean square error (RMSE) of the estimation and soft classification measures was investigated. The evaluation results of same-base alloy regression revealed that the proposed PCBR reduced the RMSE of the major element concentration estimation compared to conventional regression schemes. In addition, the accuracy of the soft classification of same-base alloys by PCBR for untrained types was tangibly improved compared to that of prior approaches, such as PLS discriminant analysis and soft independent modeling of class analogy (SIMCA).

A predictive model for elemental carbon, organic carbon and total carbon based on laser induced breakdown spectroscopy measurements of filter-collected diesel particulate matter

A predictive model for elemental carbon, organic carbon and total carbon based on laser induced breakdown spectroscopy measurements of filter-collected diesel particulate matter

Multipulse laser ablation of BiSn eutectic: Noncongruent element transfer and morphology analysis

The surface of a eutectic BiSn target was ablated by several series of pulses at different spots using an excimer laser with a wavelength of 351 nm and a pulse duration of 20 ns. Morphology of ablation craters formed under a different number of pulses was analyzed with a scanning electron microscope, an optical profilometer, and an energy dispersion spectroscope (EDS). Laser-induced breakdown spectroscopy (LIBS) measurements were taken for each crater with the last pulse in the series of pulses. The ratio of peak intensities (Bi I peak at 472.25 nm and Sn I 563.16 nm) of LIBS spectra for craters formed by different series of pulses decreased from maximum at a single pulse to a stable magnitude after a series of 70 pulses. EDS mapping of crater areas for different series of pulses shows a decrease in Bi atoms abundance, in correlation with the decrease in normalized intensity of Bi peak LIBS spectra. The change of the chemical composition in the ablation area is explained based on the Knudsen evaporation model. For the first time, pronounced hydrodynamic effects resulting in the formation of 50 μm-high rims were observed. The unusual morphology of the ablated area is explained on the basis of hydrodynamic effects and recoil pressure.

LIBS measurements inside the FTU vessel mock-up by using a robotic arm

Abstract The amount of tritium (T) retained in plasma facing components (PFCs) and in-vessel structures has to be carefully monitored in ITER because of the limits fixed by safety issues. Laser-Induced-Breakdown-Spectroscopy (LIBS) is one of the eligible techniques for this task because it does not require any sample pre-treatment (or removal), it can work both in atmospheric pressure or in vacuum, with different background gases. LIBS can be set up both on a robotic arm entering the vacuum vessel or outside it, for analyses during shut-down periods or in-between plasma discharges. In this paper we describe a new, compact LIBS system, mounted on the Frascati Tokamak Upgrade (FTU) robotic arm, to perform measurements on ITER-relevant samples placed in the FTU mock-up. The samples, composed of thin layers (few μm) of W, Al, D, simulate the co-deposition of Be with T on ITER divertor tiles (Be and T are replaced by Al and D in the samples). W, Al, D emission lines has been clearly detected, both as major constituents or in trace quantities, with different background gases using a single laser pulse.

Determination of Lead and Chromium in Aloe Vera Pulp and Aloe Vera-Based Cosmetics by Laser-Induced Breakdown Spectroscopy (LIBS)

This study focuses on the application of laser-induced breakdown spectroscopy (LIBS) to determine lead and chromium in pristine aloe vera and aloe vera based beauty soaps using the second harmonic (532 nm) of a Q-switched Nd:YAG laser. The optimal experimental conditions were evaluated to improve the sensitivity of the detection system by a parametric dependence study. The atomic transition lines at 405.7 nm and 425.4 nm were used as the analytical lines to determine lead and chromium, respectively. The LIBS system was calibrated for these toxic elements and the samples under analysis included 8.00–15.00 ppm by mass of lead and 5.00–12.00 ppm by mass of chromium, which are far above the safe permissible levels of these elements (i.e., 0.50 ppm for Pb and 1.00 ppm for Cr). The quantitative results were checked before and after normalization with the background and better outcomes were obtained when the spectrum was normalized with the background. The LIBS results were compared to the outcomes of selected samples by a standard analytical method, inductively coupled plasma – optical emission spectroscopy (ICP-OES). Both outcomes were in outstanding conformity implying the reliability of the LIBS measurements. The current study is attractive for the general evaluation of human health and specifically for the analysis of aloe vera based cosmetics.