Laser-induced Breakdown Spectroscopy Technique Research Articles

Development and optimization of experimental parameters for the detection of trace of heavy metal (Cr) in liquid samples using laser-induced breakdown spectroscopy technique

Detection of traces of impurities/elements present in liquid samples using laser-induced breakdown spectroscopy (LIBS) is challenging because the signal intensity is weaker than in the case of solid samples. The present paper deals with the optimization of experimental parameters for different phases of a liquid sample and the improvement of the limit of detection (LOD) in these LIBS experimental setups. LIBS spectra of chromium in the liquid sample have been recorded in three different configurations [laminar flow, i.e., liquid flowing through a small jet, frozen liquid (ice), and liquid deposited on a filter paper]. Experimental conditions for different phases were optimized to get a better signal-to-noise (S/N) ratio and signal-to-background ratio (S/B) in the LIBS spectra. The best S/N and S/B ratio is observed when LIBS spectra is recorded for the liquid deposited on the filter paper configuration. The spectral intensity of Cr is enhanced several folds (36 times) in the LIBS spectra recorded in the filter paper configuration as compared to the laminar flow. The calibration curve method is used to measure the LOD for three different configurations. The better LOD (9.7 ppm for 357.8 nm Cr line) is observed in the case of deposited liquid on the filter paper than the other liquid phase (laminar, 85.5 ppm and ice, 63.7 ppm) configurations.

Preliminary study of laser-induced breakdown spectroscopy for Mg/Ca investigation in cultured scallop shells

The element ratio of Mg/Ca is a primary proxy in studying the composition of seashells, and laser-induced breakdown spectroscopy (LIBS) is a practical technique for elemental analysis. In this work, we tried to apply LIBS to investigate the Mg/Ca ratio in cultured seashells of two scallop species (Chlamys farreri and Patinopecten yessoensis). It was found that LIBS was capable of tracking Mg/Ca variation on the cross section of the scallop shell (Chlamys farreri), and the variation trend was well verified by the measurement of SEM-EDS. Based on this, LIBS detection was carried out for Mg/Ca investigation on another species of the scallop shell (Patinopecten yessoensis), and the intensity ratio of Mg/Ca presented a similar variation to δ18O measurement under mass spectroscopy. The result also indicated that the LIBS technique is appropriate in the Mg/Ca investigation for scallop shells, and in cultured scallops, the Mg/Ca ratio would be equally useful for tracking temperature variations. In the future, LIBS is expected to be developed as a diagnostic tool in seashell cultivation, and more elemental compositions would be involved in this kind of investigation.

Effect of cavity-confinement and microwave-assistance on the sensitivity of LIBS for detecting Cu content in pig feed

The laser-induced breakdown spectroscopy (LIBS) is a potential tool for detecting heavy metals in pig feed. In order to implement a highly sensitive LIBS system required for the detection of Copper in pig feed and achieve better detection results, this work combines cavity-confinement and microwave-assistance with the LIBS system. Based on the characteristic spectral intensity of Cu I 324.75 nm, four factors including delay time, laser energy, cavity-confinement diameter, and microwave-assisted power are optimized by using single factor method and response surface methodology (RSM) for obtaining suitable experimental parameters and detection performance. The detection results show that as compared with the single factor method, the optimal experimental parameters obtained by using RSM increase the intensity of the characteristic spectral lines by 21.69% and the detection limit is as low as 0.878 mg/kg. As compared with the traditional LIBS techniques, the use of cavity-confinement combined with microwave-assistance increase the detection sensitivity of LIBS technique by 18.6-fold. The results show that RSM effectively analyzes the interactions between factors and their effects on the response values to effectively obtain better values for each factor. This enhances the LIBS spectra by cavity-confinement and microwave-assistance, and improves the sensitivity of detection of Cu in the pig feeds, which is suitable for realizing the quantitative detection of low concentration elements in pig feeds.

Using biospeckle and LIBS techniques with artificial intelligence to monitor phthalocyanine-gold nanoconjugates as a new drug delivery mediator for in vivo PDT

Photodynamic therapy (PDT) is a promising treatment method that generates reactive oxygen species by exciting a photosensitizer in the presence of light. The positive treatment outcome of PDT is enhanced by the photosensitizer’s ability to get out quickly from normal tissue. This study non-destructively evaluated the biocompatibility of novel synthesized phthalocyanine-gold nanoconjugates (Pc-Au NCs) as a drug delivery mediator for in vivo PDT. We first proposed a novel noncontact and safe biospeckle optical imaging technique to visualize the quantitative changes in the internal dynamics of biological tissue in response to PDT treatment. Second, laser-induced breakdown spectroscopy (LIBS) was utilized to emphasize the accumulation of gold nanoparticles (Au NPs) within the normal and cancerous tissue, which in turn verifies the cytotoxic and treatment effects of the Pc-Au NCs. The biospeckle images and LIBS spectra were obtained from tumor and normal tissues of male BALB/c malignancy-bearing mice before and after exposure to PDT. Then, they were statistically treated via different artificial intelligence techniques. For biospeckle analysis, the proposed method was established from the combination of second and higher-order statistical features extracted from the gray-level co-occurrence matrix (GLCM) and local contrast analysis (LCA), respectively, through principal component analysis (PCA). Regarding LIBS analysis, the powerful, most popular ensemble sequential learning algorithm Adaptive Boost was implemented on highly uncorrelated variables extracted from the raw spectra. The results, confirmed by the histological investigation, indicated the potential of the biospeckle and LIBS statistical properties in demonstrating the efficiency of the Pc-Au NCs as a drug delivery mediator for in vivo PDT.

Improving LIBS analysis of non-flat heterogeneous samples by signals mapping.

Heterogeneous material analysis by the laser-induced breakdown spectroscopy (LIBS) technique is challenging in real practice due to requirements for representative sampling and non-flat surfaces of the samples. Methods complementary to LIBS (plasma imaging, plasma acoustics, sample surface color imaging) have been introduced to improve zinc (Zn) determination in soybean grist material by LIBS. The detailed statistical study revealed that atomic/ionic lines emission and other LIBS signals were distributed normally except for acoustics signals. The correlation between LIBS and complementary signals was rather poor due to the large variability of the particle properties of soybean grist material. Still, analyte line normalization on plasma background emission was rather simple and effective for Zn analysis but required a few hundred spot samplings for representative Zn quantification. Non-flat heterogeneous samples (soybean grist pellets) were analyzed by LIBS mapping but it was demonstrated that the choice of sampling area is crucial for reliably analyte determination.

Evaluation of the nutrients in milk supplements suitable for children using LIBS and PAS technique

The presence of macro and micro nutrients essential for human health in four milk supplements is quickly identified using Laser-induced Breakdown Spectroscopy (LIBS) and Photoacoustic Spectroscopy (PAS). In addition to the minerals reported by the manufacturers, some additional spectral signatures of the elements like Na and K are also observed in the LIBS spectra which are not listed by some manufacturers. The ratio of various minerals like Ca/Mg, Na/Mg, etc. has been evaluated, and found a good correlation with the nutritional value listed on the product packaging. The organic elements C, H, N, and O together with the CN molecular band observed in the LIBS spectra of milk supplement indicate the presence of organic molecules. PAS is used for the identification of organic compounds such as Vitamins, Proteins, Fat, Taurine, and Sucrose in milk supplements. The results of the present study help in selecting milk supplements suitable for children suffering from chronic kidney diseases and diabetes. Based on the presence of minerals and their relative concentration, milk supplements are classified into three groups by applying Principal Component Analysis (PCA). Therefore, the results from the present work extend the application of LIBS and PAS together with PCA for the identification of the nutrients in milk supplements.

Chemometrics and Spectroscopic Analyses of Peganum harmala Plant’s Seeds by Laser-Induced Breakdown Spectroscopy

In the present work, the rapid identification of elements and their relative chemical composition in various Peganum harmala seed samples were investigated using a calibration-free laser-induced breakdown spectroscopy technique (CF-LIBS). A pulsed Nd:YAG laser-source with a 5 ns pulse-duration, and 10 Hz pulse repetition rates providing 400 and 200 mJ energy at 1064 and 532 nm wavelength, respectively, was focused on the Peganum harmala seed samples for ablation. A LIBS 2000+ spectrometer within the wavelength range (200 to 720 nm), emission-spectra were recorded. The measured spectra of the Peganum harmala sample gives spectral lines of Carbon (C), Magnesium (Mg), Lithium (Li), Sodium (Na), Calcium (Ca), Silicon (Si), Iron (Fe), Strontium (Sr), Copper (Cu), Potassium (K), and Lead (Pb). A CF-LIBS technique has been employed for the compositional study of the elements exist in the Peganum harmala seed samples. The measured results demonstrate that C, Mg, and Ca are found to be major elements in the Peganum harmala seed samples with compositions of ~36.64%, ~24.09%, and ~19.03%, respectively. Along with the major elements, the elements including Li, Na, Si, K, Fe, and Sr were identified as minor elements with compositions of ~2.87%, ~2.33%, ~3.72%, ~7.17%, ~2.83%, and ~1.14%, respectively. Besides Cu (~8.07 μg/g), and Pb (~1.10 μg/g) elements were observed as trace elements exist in the Peganum harmala seed samples. Furthermore, the electron number density including the plasma excitation-temperature were calculated using the stark-broadening line profile method and the Saha–Boltzmann plot method, respectively. The plasma parameters versus laser-irradiance and the distance from the sample were further investigated. Moreover, a principal component analysis (PCA) method was also utilized to the spectral data obtained by using LIBS to discriminate various seed samples with four classes, namely, α, β, γ, and Δ. Three principal-components (PCs) calculated from eigenvalues of score matrix described 87.6%, 4.6%, and 2.5% of total variance for PC1, PC2, and PC3, respectively. The LIBS spectral data variance covered by the initial 3 PCS was found as ~94.7% of total variance. The PCA results have successfully demonstrated the different classes of the Peganum harmala seed samples based on the different doping compositional ratios of the Zn element. This study confirmed the feasibility and ability of LIBS and PCA for the rapid analysis of Peganum harmala seed samples. Finally, the results achieved using CF-LIBS were incorporated with those obtained from the XRF and EDX analytical techniques.

Elemental study of Devarda’s alloy using calibration free-laser induced breakdown spectroscopy (CF‒LIBS)

In the present study, we present the compositional analysis of a Devarda’s alloy using the calibration–free laser-induced breakdown spectroscopy (CF-LIBS) technique. A nanosecond pulsed Q-switched Nd:YAG laser was focused on the target-sample under investigation to ablate its surface and the measured emission spectrum was registered by using a spectrometer (LIBS2000+) having the optical spectral within a range from 200–720 nm. The analysis of the measured optical spectra confirms the presence of three major elements Aluminum (Al), Copper (Cu), and Zinc (Zn) in the target sample. The emission intensity line profiles of Zn, Cu, and Al were utilized to estimate the plasma-parameters consisting of excitation temperature, and the plasma number density. The plasma excitation temperature was investigated using the Boltzmann-plot technique, which yields the temperature for Cu and Zn as 8547 ± 5% K and 8100 ± 5%, respectively, while the electron plasma density was calculated from the Stark-broadening of individual neutral emission lines of Al, Cu, and Zn. For the quantitative analysis of the elements that exist in the target sample, a CF-LIBS technique was employed by assuming the condition of optically thin plasma as well as local thermodynamics equilibrium. Using the CF-LIBS technique, the relative composition in the form of weight percentage was estimated to be Zn: 57%, Al: 39%, and Cu: 4%, whereas, the certified concentration of Devarda’s alloy was 50% for Zn, 45% for Al, and 5% for Cu. These measured results reveal that the elemental concentration utilizing CF-LIBS shows a reasonable agreement with standard estimates illustrated by the manufacturer. This study further suggests that the CF-LIBS technique opens up an opportunity for engineering and industrial usage of LIBS where a quantifiable study of the substance is exceedingly advantageous.

Comparison of light capturing approaches in Laser-Induced Breakdown Spectroscopy (LIBS) for multichannel spectrometers

LIBS technique requires the spectroscopic analysis of the light emitted by a laser-induced plasma plume. One challenge of the different approaches to capture the plasma light emission is the significant shot-to-shot variations of the plume inhomogeneities, position, and morphology. This is even more challenging when multichannel CCD spectrometers are used, because the light should be homogeneously divided among multiple capturing optical fibers (typically up to 8 fibers) with stable spectral efficiency for all channels. Otherwise, any further analysis of the atomic emission spectra involving multiple channels, such as line intensity ratios, Boltzmann plots, or calibration-free LIBS, could be compromised by the morphology-dependent spectral artifacts induced by the collection optics. In this work, we assess the performance of several collection optics in terms of overall capturing efficiency and channel-to-channel variations due to changes in plasma morphology. Results clearly show that this could be an issue even with the approaches with the best spatial homogenization, including optical fibers and Köhler optics.

Forty Years of Laser-Induced Breakdown Spectroscopy and Laser and Particle Beams

The laser-induced breakdown spectroscopy (LIBS) technique is one of the most promising laser-based analytical techniques. Coincidentally, the LIBS acronym was proposed by Radziemski and Loree in two seminal papers published in 1981, almost at the same time in which the Laser and Particle Beams journal started its publication. In this contribution, the evolution of the LIBS technique is discussed following a chronological collection of key papers in LIBS, some of which were in fact published on LPB.

Characteristics of Calcium Plasma Parameters by Laser-Induced Breakdown Spectroscopy Technique

In this work, the effect of laser energy on the properties of a calcium plasma generated by a Q-switched Nd: YAG laser at the fundamental wavelength was studied using spectroscopy. The Boltzmann plot and Stark broadening method were used to measure the main plasma parameters (electron temperature and electron density). The electron temperature ranged ( 0.169 -0.172 ) eV, the electron density ranged ( 2.10 – 2.63 ) for laser energy range of ( 400 – 700) mJ. Other basic plasma properties were also measured, including the Debye length, the number of particles in the Debye sphere, and the plasma frequency. Laser energy affects all plasma parameters, according to our results.

Quantitative analysis of heavy metals by laser-induced breakdown spectroscopy technique in association with temperature control and KCl additive

In order to avoid the spectral signal instability and low detection accuracy originating from “transient plasma” produced by laser-induced breakdown (LIBS) technology, the LIBS system based on three-dimensional scaffold was developed to optimize the optical fiber detection position. The spectral signal quality of samples with different contents of KCl additives (0%–100%) at 20–280 °C was analyzed. The double spectral line internal standard model of Cd and Pb elements in sediment was established. The experimental results show that the signal-to-noise ratio of Cd and Al in the sediment with 15% KCl additive at 180 °C is 1.78 and 1.39 times higher than that without additive at room temperature. The R2 of the calibration model for Cd and Pb in sediment with the double enhancement method increased from 0.938 08 and 0.948 80 to 0.963 41 and 0.952 61. The spectral line integral area bispectral line internal scale model R2 is improved to 0.990 34 and 0.988 83. The results showed that the sample heating combined with KCl additive could improve the quality of the spectral signal and increase the detection sensitivity.

Magnetic field confined laser-induced plasma: Improvement in sensitivity and repeatability

In this work, the transverse magnetic field confined laser-induced breakdown spectroscopy (MC-LIBS) technique was applied to investigate the signal sensitivity and repeatability of Al alloy plasma in the specially designed assembly. The emission signals from laser-induced plasma were recorded with varying laser energy (80 to 160 mJ) and magnetic field strength (1 to 3.5 kG). An emission signal enhancement up to 7 times was observed at 140 mJ laser energy and at 3 kG magnetic field strength. Compared to conventional LIBS, the magnetic field confined-LIBS improved the signal-to-noise ratio (SNR) by up to 5 times and the detection limit (LOD) from 86 to 26 ppm for zinc emission lines. Repeatability was evaluated using the relative standard deviation (RSD) of the Mg and Si emission lines, which was improved from 23 to 12% and 25 to 14%, respectively. Similarly, the measurement-to-measurement RSDs in electron temperature (Te) and electron number density (Ne) were improved from 12 to 7% and 1 3 to 5% when compared with conventional LIBS.

Milk’s inorganic content analysis via laser induced breakdown spectroscopy

In the present work, the inorganic content of different milk samples is investigated by Laser Induced Breakdown Spectroscopy (LIBS) technique. Milk samples of different animal origin, in liquid, lyophilized powder, and ashed forms were studied using both infrared (1064 nm) and visible (532 nm) laser excitation conditions and the optimum experimental conditions for the measurement of the inorganic elements present in low concentration, were determined. Spectral features of major (Ca, Na, Mg and K) and minor minerals (P, Zn, Cu and Si) were detected and identified. The LIBS results for the different milk samples were found to correlate perfectly with the results obtained from atomic absorption measurements, demonstrating the potential of LIBS technique for the fast and in-situ qualitative characterization of the inorganic content of different animal origin milk samples.

Understanding the Surface Condition of Silicone Rubber Nanocomposite Due to Corona Aging Using AFM Imaging and LIBS Spectroscopy

Silicone rubber insulators undergo degradation when they are exposed to corona discharges. The present study explores the surface degradation behavior of silicone rubber alumina nanocomposites due to corona aging using high-frequency ac voltage. The electric field distribution on the sample surface during corona aging is simulated using COMSOL. The change in silicone rubber functional group is studied using Fourier transform infrared (FTIR) spectroscopy analysis. The influence of corona aging time on the surface roughness recovery of silicone rubber nano alumina composites is studied using an atomic force microscopy (AFM). The elemental distribution behavior and change in the emission spectra persistence time are analyzed during recovery period of corona-aged silicone rubber nanocomposites using laser-induced breakdown spectroscopy (LIBS) technique. The addition of alumina nanofiller to the silicone rubber improves the resistance to aging due to corona discharge. Loss and recovery of hydrophobic property of silicone rubber due to corona aging were determined using static contact angle measurement. The results show a direct correlation between the contact angle and the surface roughness recovery characteristics. They indicate that the nanocomposite having a 5 wt% alumina content exhibits the highest resistance to corona discharge with least surface damage. Furthermore, fast recovery was observed in the first few hours after corona aging, and most samples eventually regained their properties after about 8 h of recovery time.

Emission enhancement in laser-induced breakdown spectroscopy from sample heating with a continuous-wave diode laser

In this work an affordable, energy-efficient technique to enhance the sensitivity of the laser-induced breakdown spectroscopy technique is presented. This method is based in applying the beam of a continuous wave (CW) diode laser into the spot where the focused beam of a pulsed Nd:YAG, high-power laser induces the plasma. With the present set-up the CW beam heats the sample's surface to 300 °C. It was found that pre-heating the target in this way increased the line intensity of both neutrals and ions by a factor of up to 4.5. It was also found that the plasma excitation temperature and the electron density of the heated sample did not differ significantly from those at room temperature. However the amount of ablated material did change significantly when the sample was heated, with the crater in the latter case as much as 8.9 times larger than that obtained at ambient temperature. We conclude that the intensification in the pre-heated-sample case stems not from increased electronic plasma density or temperature but rather from a larger plume size resulting from an increased amount of ablated material when the target is heated.

Application of quantitative analysis for aluminum alloy in femtosecond laser-ablation spark-induced breakdown spectroscopy using one-point and multi-line calibration

To achieve the quantitative elemental analysis of aluminum alloy samples, the one-point and multi-line calibration (OP-MLC) method and femtosecond laser-ablation spark-induced breakdown spectroscopy (fs-LA-SIBS) were combined in this study. An ultrafast Ti: sapphire laser with high single-pulse energy and high repetition–rate operation was used as the laser source to ablate the sample at a 1k Hz pulse repetition rate and achieve re-excitation using spark discharge. Subsequently, an enhanced spectral peak intensity was achieved, which increased the sensitivity and analytical capabilities of laser-induced breakdown spectroscopy (LIBS) technique. Considering only one confirmed sample, the OP-MLC method was used to calculate the concentration ratios of trace elements Mg, Cr, Cu, Mn and Zn in four aluminum alloy samples to the calibration samples by the integral intensity ratio of the elemental characteristic lines between the two samples. The average relative errors with the identified concentrations were 3.99%, 15.13%, 8.76%, 9.30% and 9.64%, respectively. The results showed that the combination of OP-MLC method and fs-LA-SIBS has good accuracy in the quantitative analysis of elements, and has the advantages of simplicity, rapidity and high sensitivity.

Study of spectral intensity of the laser ablated tungsten plasma and ablation mass at various laser spot sizes and laser fluence in vacuum environment

The ablation features such as spot size and crater volume are essential parameters for the in-situ laser-induced breakdown spectroscopy (LIBS) technique to be well applied in the quantitative diagnosis of the plasma-wall interaction (PWI) processes on the first wall or divertor in tokamak devices. The spectroscopic characteristics and the ablation features as well as the plume dynamics of laser-induced tungsten plasma in vacuum environment at various spot sizes were investigated in this work. A nano-second laser pulse (5 ns, 1064 nm) with a maximum energy of 200 mJ was focused on the tungsten target with different spot sizes (from 0.42 mm to 2.71 mm) by adjusting the distance from focal point to sample surface. A paraxially collective configuration (15°) and time-integrated spectral collection mode were applied, similar to the present in-situ LIBS system on Experimental Advanced Superconducting Tokamak (EAST). It was observed that the spot size affects the spectral intensity significantly, and the optimal spot size is 1.16 mm both for the observed tungsten ionic and atomic lines at the laser energy of 200 mJ. The electron temperature (Te), electron density (ne) and ablation mass also reach maxima at the optimal spot size. Temporally spectral dynamic investigation shows the tungsten line emission time at the optimal spot size lasts longer and decays more slowly than other sizes, and the fast photography investigation also verified this. It was found that the central intensity profile of the plasma plume image from the target surface along with the opposite direction of the laser incident at the early stage (< 8 ns, during the laser pulse) is stronger at the smaller spot size while it decays faster. This was attributed to the stronger plasma shielding effect leading to the relatively less total ablation mass. Moreover, the optimal spot size was found to increase with the laser energy in the energy range from 65 mJ to 200 mJ, and the corresponding optimal laser fluence almost keeps the constant value of ∼18 J/cm2, which provides valuable information for optimizing the current in-situ LIBS for quantitative diagnosis of the PWI processes such as fuel retention and impurity deposition in EAST.

Elemental Determination in Stainless Steel via Laser-Induced Breakdown Spectroscopy and Back-Propagation Artificial Intelligence Network with Spectral Pre-Processing

Minor elements significantly influence the properties of stainless steel. In this study, a laser-induced breakdown spectroscopy (LIBS) technique combined with a back-propagation artificial intelligence network (BP-ANN) was used to detect nickel (Ni), chromium (Cr), and titanium (Ti) in stainless steel. For data pre-processing, cubic spline interpolation and wavelet threshold transform algorithms were used to perform baseline removal and denoising. The results show that this set of pre-processing methods can effectively improve the signal-to-noise ratio, remove the baseline of spectral baseline, reduce the average relative error, and reduce relative standard deviation of BP-ANN predictions. It indicates that BP-ANN combined with pre-processing methods has promising applications for the determination of Ni, Cr, and Ti in stainless steel with LIBS and improves prediction accuracy and stability.

Domainal Investigation of a Quartz-Fluorite Composite Using Spectroscopic Techniques

The analysis of geological samples that have several chemically diffused zones which formed under certain physico-chemical condition is difficult to achieve. The quantitative estimations of the minerals in such samples are tedious. The present work demonstrates the application of LIBS for qualitative and quantitative analyses of a quartz-fluorite composite which was procured from an amygdaloidal basalt from Deccan Traps, India. The presence of weak emission lines of F in the spectral range of 200–900 nm makes it challenging to quantify the fluorine. This study has addressed a promising alternative to quantify the fluorine using electronic bands of CaF molecules observed in the Laser-induced Breakdown Spectroscopy (LIBS) spectrum. In addition to this spectroscopic technique, the authors also have used Photoacoustic Spectroscopy (PAS) and UV-VIS spectroscopy technique to obtain molecular information from the geological sample. Principal Component Analysis (PCA) was applied to a truncated spectral region of the CaF molecule, and it showed 99% variance. Further, the obtained results with these spectroscopic techniques were compared with the results that were obtained from X-ray diffraction and Electron Probe Micro Analyzer, and they show good agreement. Thus, the LIBS technique can be promising for in situ profile section (varies from few microns to centimeters size) studies without the sample’s destruction using the point detection capability of LIBS.