Laser-induced Breakdown Spectroscopy Intensity Research Articles

Quantitative elemental mapping of biological tissues by laser-induced breakdown spectroscopy using matrix recognition

The present study demonstrates the importance of converting signal intensity maps of organic tissues collected by laser-induced breakdown spectroscopy (LIBS) to elemental concentration maps and also proposes a methodology based on machine learning for its execution. The proposed methodology employs matrix-matched external calibration supported by a pixel-by-pixel automatic matrix (tissue type) recognition performed by linear discriminant analysis of the spatially resolved LIBS hyperspectral data set. On a swine (porcine) brain sample, we successfully performed this matrix recognition with an accuracy of 98% for the grey and white matter and we converted a LIBS intensity map of a tissue sample to a correct concentration map for the elements Na, K and Mg. Found concentrations in the grey and white matter agreed the element concentrations published in the literature and our reference measurements. Our results revealed that the actual concentration distribution in tissues can be quite different from what is suggested by the LIBS signal intensity map, therefore this conversion is always suggested to be performed if an accurate concentration distribution is to be assessed.

Analysis of Ti-inclusions in high carbon chromium bearing steel by laser-induced breakdown spectroscopy

The presence of Ti-inclusions has a significant impact on the fatigue life of bearing steel. Laser-induced breakdown spectroscopy (LIBS) is a useful tool for analyzing Ti-inclusions, despite the fact that the effective excitation area of LIBS is actually larger than the pre-set spot diameter. The area affected by the LIBS spot ranges from 150 μm to 376.6 μm. A new parameter L has been introduced for better understanding the characterization of Ti-inclusions detected in SEM-EDS and LIBS measurements, When the L threshold falls below the value of 188.3 μm, the SEM-EDS and LIBS data are obtained from the same Ti-inclusion. The study demonstrates that there is a linear relationship between LIBS intensity and Ti-inclusions area, which was tested both theoretically and practically. The analysis of six samples of rolling GCr15 bearing steel with a Cr element content of 1.5% shows that one regression line is enough for analyzing the same type of inclusion processed under the same conditions. This method can significantly reduce the workload and analysis time during the actual analysis process.

Chemical and physical studies of metallic alloy-based old Indian coins with LIBS coupled with multivariate analysis

The present work aims to demonstrate the capabilities of Laser-induced Breakdown Spectroscopy (LIBS) coupled with a multivariate technique for rapid quantification and classification of old Indian coins made of various alloys. Thirteen old Indian coins in different years of circulation, (1922–1986) were selected for the study. The concentrations were determined by Calibration free LIBS (CF-LIBS) method. The concentration of cuprum (Cu) is negligible, and aluminum (Al) is maximum in the first five coins, and vice-versa in the remaining eight coins. Two different multivariate methods, Principal Component Analysis (PCA) and Soft Independent Modelling of Class Analogy (SIMCA) have been used to classify and identify the coins. PCA classified all thirteen samples into four main alloy categories. The discernment of unknown samples to their probable class membership of alloy was performed using SIMCA. The surface hardness (Brinell hardness number) is linearly correlated with the plasma temperature and LIBS intensity ratios. The sample surface of the first and fifth coin belongs to Al-alloy, having the least surface hardness, and it became harder for Cu–Ni alloy, Ni-brass alloy, and bronze alloy. The hardness of the surface is more for bronze sample twelve. It is also observed that the plasma temperature increases monotonically with the Brinell hardness number. This analysis provides valuable information on fabrication methodology and explains large diversification in the elementary composition of old coins.

The multi-dimensional characterization of Al-doped CuO thin films by laser-induced breakdown spectroscopy (LIBS) with nanosecond and picosecond lasers

Glasses coated with various optical thin film materials are widely used in people's life and scientific research. In this study, a series of aluminum-doped (Al-doped) copper oxide (CuO) thin films with different sputtering pressures and powers were deposited on soda-lime glass substrates by RF-magnetron sputtering to explore the effect of doping element on the optical properties of the thin films. However, laser-induced breakdown spectroscopy (LIBS) with nanosecond and picosecond lasers was employed to achieve the multi-dimensional analysis of the Al element. With ns-LIBS, the rapid quantitative analysis was performed by drawing the calibration curves of LIBS intensity ratio of Al/Cu versus concentration ratio under different sputtering pressures and powers. Furthermore, the 2D chemical mappings were drawn to provide a more complete and detailed relative spatial distribution of the Al element according to the LIBS intensity ratio of Al/Cu. Meanwhile, the thin film thickness was determined by the picosecond laser pulse number since the depth ablated by a single laser pulse was assessed as nearly 46.75 ± 4.25 nm. Moreover, the variation trends of optical band gaps with sputtering parameters were similar to that of LIBS intensity ratios with sputtering parameters, as suggested by analyzing the transmission spectra of thin films. The results demonstrated that the multi-dimensional characterization (quantitative analysis, 2D chemical mapping, thin-film thickness estimation, and optical properties analysis) of Al-doped CuO thin films prepared by RF-magnetron sputtering can be effectively realized by LIBS.

Prediction of heavy metal Cd and stress on minerals in rice by analysis of LIBS spectra.

To predict the nutrition and safety of agricultural products by laser-induced breakdown spectroscopy (LIBS), heavy metal Cd in rice was selected as an analytical target. Mature rice grain samples from 40 growing geographical areas around Poyang Lake were picked on-site and processed by grinding to obtain the edible rice. The content of Cd in rice samples was determined by graphite furnace atomic absorption spectrometry, and the rice pellets were detected by LIBS. The risk intake was estimated by the target hazard quotient and Chinese National Standard. Moreover, the samples were classified as clean, slight, and severe ones according to evaluation. The content of Cd was predicted by analyzing LIBS spectra coupled with the partial least square (PLS) model. The correlation coefficients (R2) reached 0.9036 and 0.9771 for the training and prediction sets, respectively, and the root mean square errors were 0.0487 and 0.027, respectively. It denotes that the PLS model has a higher prediction ability especially after LIBS spectra were processed by smoothing and multiplicative scatter correction. For the clean, slight, and severe rice samples, the LIBS intensity ratio between minerals Mg, K, Na, Si, and Mn to Ca was compared. The ratio was decreased in all samples as Cd stress increased. Correlation analysis results show that Mn displayed a highly significant negative correlation with Cd stress, while Mg, K, and Na displayed a significant negative correlation with Cd stress. The relationship between Si and Cd did not reach a significant level. This work indicated that it was feasible to use LIBS combined with a suitable data process to predict Cd content and the effect of Cd stress on minerals in rice. It is promising to evaluate the nutrition and safety of food products by analyzing LIBS spectra.

Impact of DBDS and Silver Sulfide on the Performance of Thermally Aged Mineral oil Impregnated Pressboard Material

The corrosive sulfur compounds present in the mineral oil causes the formation of dibenzyl disulfide (DBDS) complex and it can also erode the on-load tap changing selector contacts leading to generation of silver sulfide (AgS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ).The present work reports the effect of DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S concentrations in mineral oil on thermally aged pressboard material. The results demonstrate that surface discharge inception voltage (SDIV) is higher for the aged pressboard specimen. The higher concentrations of DBDS showed a reduction of more than 50% in the magnitude of SDIV compared to its effect with the addition of Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S. The surface potential analysis indicates a higher initial potential for the aged specimens with an increased half lifetime. In addition, only deep traps observed on the aged specimens with a slight right shift in its trap energy level. The mechanical strength of aged pressboard specimens with DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S is understood using tensile testing and simultaneous analysis is captured using Digital Image Correlation (DIC) technique to understand its strain percentage at different areas on the pressboard material during its elongation. The laser induced breakdown spectroscopy (LIBS) analysis is performed to identify the elements responsible for thermal ageing such as carbon, copper and sulfur on the surface of pressboard material. The plasma temperature calculated from sulfur peaks is higher for DBDS aged specimen compared to AgS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and a linear correlation is observed on the LIBS intensity with ageing duration. Further, the Principal Component Analysis (PCA) is used for its classification on the degradation of pressboard due to the diffusion of both copper and silver sulfide from the mineral oil. The two principal components (PC1 and PC2) provided a higher variance of 99.8% with a clear classification observed between the unaged and aged pressboard specimens due to the addition of both DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S in mineral oil.

Effect of crater volume on laser-induced plasma lasers and Laser-Induced Breakdown Spectroscopy intensity

Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Induced Plasma Lasers (LIPL) in elongated Laser-Induced Plasma (LIP) are investigated. It is shown that LIBS and LIPL behavior versus ablation laser pulses are about the same, though LIBS emission is proportional to the atoms in excited states, but LIPL generation is proportional to the atoms in the ground state. Furthermore, two peaks are found in LIBS/LIPL intensities dependencies on laser pulses. The first peak is well-known in LIBS. The second, observed at large ablation pulses number (≥104 pulses in our experimental conditions), may be attributed to the sample's thermal properties, especially thermal capacity and thermal diffusivity.

Data fusion of laser induced breakdown spectroscopy (LIBS) and infrared spectroscopy (IR) coupled with random forest (RF) for the classification and discrimination of compound salvia miltiorrhiza

As an important traditional Chinese medicine, compound salvia miltiorrhiza has many functions, such as reducing blood pressure, promoting blood circulation and relieving pain, nourishing heart and tranquilizing mind, etc. Therefore, it is significant important to monitor the quality of compound salvia miltiorrhiza. In this work, a novel strategy based on data fusion of LIBS and IR coupled with RF was constructed for the classification and discrimination of compound salvia miltiorrhiza. The LIBS and IR spectra collection of compound salvia miltiorrhiza samples was carried out. In data fusion section, normalization was used for eliminating orders of magnitude difference between the LIBS and IR spectral data intensity, variable importance (VI) and variable importance in the projection (VIP) were applied to select the feature variables in the LIBS and IR spectral data, and variable importance threshold was optimized based on out of bag (OOB) error estimation. Then, the optimized feature variables was fused by a feature layer data fusion. Finally, the RF discrimination model was constructed based on LIBS-IR fusion data, and the obtained result was compared with the RF discrimination models based on LIBS and IR spectra. The results show that the RF discrimination model based on LIBS-IR fusion data selected by VI can achieve the best predictive performance for the quality analysis of compound salvia miltiorrhiza with the sensitivity ​= ​0.9333, the specificity ​= ​0.9667 and the accuracy ​= ​0.9619 for testing set. The result fully certificates the feasibility of the quality analysis of compound salvia miltiorrhiza via data fusion based on LIBS-IR spectra proposed in the work, and it will provide a new technology for the identification and analysis of Chinese medicinal materials.

Impact of microstructural properties on hardness of tungsten heavy alloy evaluated by stand-off LIBS after PSI plasma irradiation

The key issue in tokamak is change of physical properties of plasma facing materials, such as tungsten due to extreme conditions and long pulse operation. Practically, it is difficult to analyze hardness changes online through conventional methods. Hence, an online monitoring technique is much desired which measures hardness changes in fusion devices. In this study, laser induced breakdown spectroscopy (LIBS) is used as in-situ monitoring tool to measure hardness of tungsten heavy alloy (97W–2Ni–1Fe) samples after exposure to different irradiation of plasma ranging from 0.108 to 1.00 MW/m2. The exposure of PSI plasma beam is a key factor which give rise to pronounced change in crystallographic estimation, microstructural properties and surface hardness. The X–ray diffraction study revealed modification in the structure caused, by variation in the peak intensity, crystal size, dislocation line density and micro-strains upon plasma irradiation. The change in crystallographic measurements and surface hardness are well correlated with irradiated PSI power densities. Moreover, the hardness influence on LIBS intensity of tungsten heavy alloy (WHA) was carried out. The LIBS calibration curves were formed by utilizing the ratio of ionic to atomic line intensities of tungsten versus the Vickers hardness. The ratio of ionic to atomic spectral emission line intensity and plasma electron temperature (Te), significantly increases with hardness. The obtained Pearson correlation coefficient (R2) values (in case of calibration curve and plasma temperature) in measuring hardness of all investigated samples have good approximation which indicate the preciseness of LIBS approach. The achieved results in the present study by in-situ LIBS system confirmed its potential ability to estimate the hardness of WHA more efficient than other conventional method.

Laser-induced breakdown spectroscopy enhancement of glass by few-layer graphene coating

We present, for the first time, the use of graphene layers on the surface of transparent samples (glass) to improve laser-induced breakdown spectroscopy (LIBS) performance. Our results show that the presence of few-layer graphene allows the optical emission of glass to be observed at lower irradiance than conventional LIBS. This effect is discussed in the frame of the absorption of the laser pulse by the graphene and the heat transfer to the beneath the sample. It is reported that the LIBS intensity increases with the number of graphene layers when it ranges from 6 to 15 layers. The surface of the sample after LIBS revealed that the graphene was almost completely removed after one pulse without causing apparent visible damage to the sample. The proposed method to improve LIBS in order to obtain a compelling characterization of transparent samples is both easy and cheap to implement.

Exploring LIBS for simultaneous estimation of Sr, Ba and La in LiCl-KCl salt

In this work, we report the simultaneous estimation of Sr, Ba and La in LiCl-KCl salt using laser induced breakdown spectroscopy (LIBS). The samples are prepared in aqueous medium and the solution is spread over a filter paper. The laser (532 nm) is focussed on the paper to produce plasma in air at atmospheric pressure. Using Yb (369.42 nm) as an internal standard, linear calibration curves are obtained from 50−800 ppm for Sr (421.55 nm), 50−1000 ppm for Ba (455.40 nm) and 300−4000 ppm for La (394.91 nm) with a detection limit of 21, 19 and 68 ppm, respectively in 12.5 % salt matrix. Salt effect on LIBS intensity is studied and found to be negligible over 5–20 % of salt. Low value of correlation uncertainty (5–11 %) illustrates that LIBS technique has a great potential for monitoring the quality of pyro-processing salt.

Rapid micro-analysis of Al-In-Sn-O thin film using laser induced breakdown spectroscopy with picosecond laser pulses

Laser induced breakdown spectroscopy (LIBS) based on picosecond laser pulses was used to achieve rapid micro-analysis of Al-In-Sn-O (AITO) thin films deposited by radio frequency (RF) magnetron sputtering at various sputtering powers. We calculated the plasma temperature of 5063 ± 150 K and electron density of 4.6 × 1016 cm−3 with single ablation crater diameter of 46 ± 1 μm. The content ratio of Al/(Al+In+Sn) in AITO thin film was closely related to the optical band gap of the film, and the ratio was exhibited by the spectral line intensity of LIBS for each corresponding element. The quantitative analysis of Al/(Al+In+Sn) with LIBS intensity ratio was achieved by plotting calibration curves according to the value of energy dispersive X-ray spectroscopy (EDS). It was found that the optical band gaps calculated were decreasing as the increase of the sputtering power in the exhibition of the transmittance spectra of AITO thin films deposited at various sputtering powers. In addition, we found the consistency of the evolutions of LIBS target line intensity ratio and optical band gap of AITO thin films. Therefore, the utilization of LIBS based on picosecond laser pulses is available and efficient in the rapid micro-analysis of AITO thin film.

Detection of K in soil using time-resolved laser-induced breakdown spectroscopy based on convolutional neural networks

One of the technical bottlenecks of traditional laser-induced breakdown spectroscopy (LIBS) is the difficulty in quantitative detection caused by the matrix effect. To troubleshoot this problem, this paper investigated a combination of time-resolved LIBS and convolutional neural networks (CNNs) to improve K determination in soil. The time-resolved LIBS contained the information of both wavelength and time dimension. The spectra of wavelength dimension showed the characteristic emission lines of elements, and those of time dimension presented the plasma decay trend. The one-dimensional data of LIBS intensity from the emission line at 766.49 nm were extracted and correlated with the K concentration, showing a poor correlation of = 0.0967, which is caused by the matrix effect of heterogeneous soil. For the wavelength dimension, the two-dimensional data of traditional integrated LIBS were extracted and analyzed by an artificial neural network (ANN), showing = 0.6318 and the root mean square error of validation (RMSEV) = 0.6234. For the time dimension, the two-dimensional data of time-decay LIBS were extracted and analyzed by ANN, showing = 0.7366 and RMSEV = 0.7855. These higher determination coefficients reveal that both the non-K emission lines of wavelength dimension and the spectral decay of time dimension could assist in quantitative detection of K. However, due to limited calibration samples, the two-dimensional models presented over-fitting. The three-dimensional data of time-resolved LIBS were analyzed by CNNs, which extracted and integrated the information of both the wavelength and time dimension, showing the = 0.9968 and RMSEV = 0.0785. CNN analysis of time-resolved LIBS is capable of improving the determination of K in soil.

Determination of Pb in soils by double-pulse laser-induced breakdown spectroscopy assisted by continuum wave-diode laser-induced fluorescence.

Laser-induced breakdown spectroscopy (LIBS) has attracted a lot of attention due to its potential to rapidly identify and quantify any chemical element with minimal sample preparation. Despite continuous improvements, the sensitivity of this technique still remains a challenge. In order to increase LIBS intensity, a laser-induced fluorescence (LIF) system can be coupled with LIBS to re-excite a transition of the element in the plasma by employing very expensive optical parametric oscillators (OPO). In this work, a homemade tunable continuum wave-diode laser (CW-DL) has been developed and coupled to a double pulse (DP) LIBS system to enhance the sensitivity of Pb detection in a soil sample at the transition 6s26p2-P32→6s26p7s-P31 at 405.78 nm. Before sample analysis, the production of no scattered light by the plasma was ascertained, and the optimal temperature of 10,000 K was estimated for this transition, feasible to be achieved in DP-LIBS systems. An increase of approximately 100% for the Pb I transition at 405.78 nm was obtained by DP-LIBS-CW-DL-LIF with respect to the DP-LIBS system alone. This result opens a new promising line of research to improve LIBS sensitivity using the CW-DL approach.

Improving the accuracy of spectroscopic identification of geographical origins of agricultural samples through cooperative combination of near-infrared and laser-induced breakdown spectroscopy

As a versatile strategy to improve accuracy for identification of the geographical origin of agricultural samples, milk vetch root samples in this study, both near-infrared spectroscopy (NIRS) and laser-induced breakdown spectroscopy (LIBS) have been cooperatively combined. The motivation was based on the potential of accuracy improvement by utilization of these two methods providing complementary spectral information, compositions of organic compounds and elements. For initial evaluation, NIRS and LIBS were separately employed to discriminate domestic milk vetch root samples from imported ones using support vector machine (SVM). The near-infrared (NIR) information in a full spectral range was used for the analysis, while in LIBS spectra, the intensities of 35 selected discrete element peaks were used. The use of NIR information providing organic compositions of the samples resulted in a discrimination accuracy of 91.5%, better than that of using LIBS elemental peak intensities (73.1%). Next, to utilize both sets of spectral data for discrimination, support vector regression (SVR) was used to represent NIR spectral feature of a sample as a SVR coefficient, and then it was merged with the existing discrete LIBS intensity data; accuracy was improved to 95.8%. The cooperative combination of information on organic and elemental composition of the samples was the root of improvement.

Investigation of laser-induced plasma characteristics in bulk water under different focusing arrangements.

Laser-induced breakdown spectroscopy (LIBS) has drawn more attention as a new technique for in situ detection of seawater, especially for hydrothermal areas. In order to evaluate the focusing geometry effect on laser-induced plasma in bulk water, four focusing arrangements were tried out with a single lens as well as with a double-lens combination. We demonstrated that, for the same transmission distance in water, the double-lens combination with shorter effective focal length generated more condensed plasma, as shown by the spectroscopic and fast imaging results. Accordingly, the moving breakdown phenomenon significantly decreased with well-improved LIBS intensity, signal-to-noise ratio, and stability. The plasma emissions evidently attenuated along the laser transmission due to the strong absorption effect of the water medium. Based on the acquired results, the performance of a practical detection window was evaluated by combining the regular lens with a customized lens-shaped optical window, and a positive outcome was also reached. The obtained results suggested that improved LIBS detection could be easily achieved via settling another lens window to LIBS system, which is considered helpful for better in situ submarine application.

Determination of Lead and Copper in Ligusticum wallichii by Laser-Induced Breakdown Spectroscopy

ABSTRACTLaser-induced breakdown spectroscopy was used for the analysis of the Chinese traditional medicine, Ligusticum wallichii. The laser energy and delay time were optimized to obtain best spectral quality. The limits of detection for lead and copper were 15.7 and 6.3 µg g−1, respectively. Multiple linear regression models between the laser-induced breakdown spectroscopy intensity and the mass fraction of lead and copper were constructed. Good agreement was observed between the actual concentrations and predicted values obtained by the models. These results demonstrate that the laser-induced breakdown spectroscopy coupled with multiple linear regression is suitable for the determination of heavy metals in Chinese traditional medicine.

Double pulse laser induced breakdown spectroscopy of a solid in water: Effect of hydrostatic pressure on laser induced plasma, cavitation bubble and emission spectra

There is a growing interest in the development of sensors use in exploration of the deep ocean. Techniques for the chemical analysis of submerged solids are of special interest, as they show promise for subsea mining applications where a rapid sorting of materials found in the sea bottom would improve efficiency. Laser-Induced Breakdown Spectroscopy (LIBS) has demonstrated potential for this application thanks to its unique capability of providing the atomic composition of submerged solids. Here we present a study on the parameters that affect the spectral response of metallic targets in an oceanic pressure environment. Following laser excitation of the solid, the plasma persistence and the cavitation bubble size are considerably reduced as the hydrostatic pressure increases. These effects are of particular concern in dual pulse excitation as reported here, where a careful choice of the interpulse timing is required. Shadowgraphic images of the plasma demonstrate that cavitation bubbles are formed early after the plasma onset and that the effect of hydrostatic pressure is negligible during the early stage of plasma expansion. Contrarily to what is observed at atmospheric pressure, emission spectra observed at high pressures are characterized by self-absorbed atomic lines on continuum radiation resulting from strong radiative recombination in the electron-rich confined environment. This effect is much less evident with ionic lines due to the much higher energy of the levels involved and ionization energy of ions, as well as to the lower extent of absorption effects occurring in the inner part of the plasma, where ionized species are more abundant. As a result of the smaller shorter-lived cavitation bubble, the LIBS intensity enhancement resulting from dual pulse excitation is reduced when the applied pressure increases.

Determination of heavy metal chromium in pork by laser-induced breakdown spectroscopy

Meat can be polluted by heavy metals during the feeding of livestock and processing of meat. It is necessary to monitor the concentration of toxic metals in meat. The element chromium (Cr) in pork was selected as a determination target for this work. Fresh pork was polluted in a Cr solution to create a different content level, then dried and pressed into pellets to eliminate the effect of water and improve the stability and sensitivity of laser-induced breakdown spectroscopy (LIBS). The spectra of pressed pellets were collected at optimized LIBS experimental parameters. After that, the real content of samples was obtained by atomic absorption spectroscopy. Characteristic lines Cr I 425.43, Cr I 427.48, and Cr I 428.97 were verified, and a model comparing the LIBS intensity of the line peak and the actual concentration of Cr was constructed. The results showed that the model has better predicted precision and accuracy, especially applying the line Cr I 425.43 for calibration. This work makes it obvious that LIBS has food safety potential for detecting residue of heavy metals in meat.

Note: A portable laser induced breakdown spectroscopy instrument for rapid sampling and analysis of silicon-containing aerosols.

A portable instrument has been developed for measuring silicon-containing aerosols in near real-time using laser-induced breakdown spectroscopy (LIBS). The instrument uses a vacuum system to collect and deposit airborne particulate matter onto a translatable reel of filter tape. LIBS is used to analyze the deposited material, determining the amount of silicon-containing compounds present. In laboratory testing with pure silica (SiO2), the correlation between LIBS intensity for a characteristic silicon emission and the concentration of silica in a model aerosol was determined for a range of concentrations, demonstrating the instrument's plausibility for identifying hazardous levels of silicon-containing compounds.