Double-pulse laser-induced breakdown spectroscopy for analysis of molten glass

A mobile double-pulse laser-induced breakdown spectroscopy system for industrial environments is presented. Its capabilities as a process analytical technique for the recovery of metals from molten inorganic wastes are investigated. Using low-melting glass doped with different amounts of additives as a model system for recycling slags, the optimum number of shots, laser inter-pulse and acquisition delay times are optimized for solid and liquid (1200 °C) glass. Limits of detection from 7 ppm (Mn) to 194 ppm (Zn) are achieved working at a distance of 75 cm from the sample. To simplify the quantification of molten samples in an industrial furnace, the possibility is examined of using solid standards for analysis of molten material.     

One-point calibration for calibration-free laser-induced breakdown spectroscopy quantitative analysis

We present a new method for improving the reliability of quantitative analysis by laser-induced breakdown spectroscopy (LIBS). The method can be considered as a variation of the calibration-free LIBS approach; although not completely standard-less, only one standard of known composition and similar matrix to the one to be analyzed is needed. On the other hand, the one-point calibration approach allows the empirical determination of essential experimental and spectroscopic parameters, whose knowledge is often imprecise or lacking; the result is a definite improvement of the trueness of LIBS analysis with respect to the traditional calibration-free approach.The characteristics and advantages of the proposed one-point calibration LIBS approach will be demonstrated on a set of copper-based samples of known composition.     

Forensic comparative glass analysis by laser-induced breakdown spectroscopy

Glass samples of four types commonly encountered in forensic examinations have been analyzed by laser-induced breakdown spectroscopy (LIBS) for the purpose of discriminating between samples originating from different sources. Some of the glass sets were also examined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Refractive index (RI) measurements were also made on all glass samples and the refractive index data was combined with the LIBS and with the LA-ICP-MS data to enhance discrimination. The glass types examined included float glass taken from front and side automobile windows (examined on the non-float side), automobile headlamp glass, automobile side-mirror glass and brown beverage container glass. The largest overall discrimination was obtained by employing RI data in combination with LA-ICP-MS (98.8% discrimination of 666 pairwise comparisons at 95% confidence), while LIBS in combination with RI provided a somewhat lower discrimination (87.2% discrimination of 1122 pairwise comparisons at 95% confidence). Samples of side-mirror glass were less discriminated by LIBS due to a larger variance in emission intensities, while discrimination of side-mirror glass by LA-ICP-MS remained high.     

All-fiber-coupled laser-induced breakdown spectroscopy sensor for hazardous materials analysis

An all-fiber-coupled laser-induced breakdown spectroscopy (LIBS) sensor device is developed. A passively Q-switched Cr⁴⁺Nd³⁺:YAG microchip laser is amplified within an Yb fiber amplifier, thus generating high power laser pulses (pulse energy E_p = 0.8 mJ, wavelength λ = 1064 nm, repetition rate f_rep. = 5 kHz, pulse duration t_p = 1.2 ns). A passive (LMA) optical fiber is spliced to the active fiber of an Yb fiber amplifier for direct guiding of high power laser pulses to the sensor tip. In front of the sensor a plasma is generated on the surface to be analyzed. The plasma emission is collected by a set of optical fibers also integrated into the sensor tip. The spectrally resolved LIBS spectra are processed by application of principal component analysis (PCA) and analyzed together with the time-resolved spectra with neural networks. Such procedure allows accurate analysis of samples by LIBS even for materials with similar atomic composition. The system has been tested successfully during field measurements at the German Armed Forces test facility at Oberjettenberg.

The LIBS sensor is not restricted to anti-personnel mine detection but has also the potential to be suitable for analysis of bulk explosives and surface contaminations with explosives, e.g. for the detection of improvised explosive devices (IEDs).     

Calibration free laser-induced breakdown spectroscopy of oxide materials

The quantitative determination of oxide concentration by laser-induced breakdown spectroscopy is relevant in various fields of applications (e.g.: analysis of ores, concrete, slag). Calibration free laser-induced breakdown spectroscopy and the multivariate calibration are among the methods employed for quantitative concentration analysis of complex materials. We measured the intensity of neutral and ionized atomic emission lines of oxide materials by laser-induced breakdown spectroscopy and we modified the calibration free laser-induced breakdown spectroscopy method to increase the accuracy. The concentration of oxides was obtained by using stoichiometric relations. Sample materials were prepared from oxide powder (Fe₂O₃, MgO, CaO) by mixing and pressing. The concentration was 9.8–33.3 wt.% Fe₂O₃, 7.6–33.3 wt.% MgO and 33.3–81.2 wt.% CaO for different samples. Nd:YAG laser (wavelength 1064 nm, pulse duration ≈ 6 ns) ablation was performed in air. The laser-induced plasma emission was measured by an Echelle spectrometer equipped with a sensitivity calibrated ICCD camera. The numerical calibration free laser-induced breakdown spectroscopy algorithm included the fast deconvolution of instrumental function, and the correction of self-absorption effects. The oxide concentration C_CF calculated from calibration free laser-induced breakdown spectroscopy results and the nominal concentration C_N were very close for all samples investigated. The relative error in concentration, |C_CF–C_N|/C_N, was < 10%, < 20%, and < 5% for Fe₂O₃, MgO, and CaO, respectively. The results indicate that this method can be employed for the analysis of major elements in multi-component technical materials.     

Quantitative analysis of slurry sample by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been employed for the analysis of slurry samples. Quantitative analysis of slurry samples is crucial and challenging. The problems associated with slurry samples include splashing, surface turbulence, and the difficulties of obtaining reproducible samples due to sedimentation. The LIBS analysis has achieved limited success due to inherent disadvantages when applied to slurry samples. In order to achieve improved measurement precision and accuracy, a spin-on-glass sampling method was evaluated. Five elements (Al, Ca, Fe, Ni, and Si) were examined in five slurry simulants containing varying amounts of each ion. Three calibration models were developed by using univariate calibration, multiple linear regression, and partial least square regression. LIBS analysis results obtained from the partial least square regression model were determined to be the best fit to results obtained from inductively coupled plasma optical emission spectroscopy analysis.     

Double-pulse standoff laser-induced breakdown spectroscopy for versatile hazardous materials detection

We have developed a double-pulse standoff laser-induced breakdown spectroscopy (ST-LIBS) system capable of detecting a variety of hazardous materials at tens of meters. The use of a double-pulse laser improves the sensitivity and selectivity of ST-LIBS, especially for the detection of energetic materials. In addition to various metallic and plastic materials, the system has been used to detect bulk explosives RDX and Composition-B, explosive residues, biological species such as the anthrax surrogate Bacillus subtilis, and chemical warfare simulants at 20 m. We have also demonstrated the discrimination of explosive residues from various interferents on an aluminum substrate.     

Towards quantitative laser-induced breakdown spectroscopy analysis of soil samples

A quantitative analysis of chromium in soil samples is presented. Different emission lines related to chromium are studied in order to select the best one for quantitative features. Important matrix effects are demonstrated from one soil to the other, preventing any prediction of concentration in different soils on the basis of a univariate calibration curve. Finally, a classification of the LIBS data based on a series of Principal Component Analyses (PCA) is applied to a reduced dataset of selected spectral lines related to the major chemical elements in the soils. LIBS data of heterogeneous soils appear to be widely dispersed, which leads to a reconsideration of the sampling step in the analysis process.     

Determination of silicon in plant materials by laser-induced breakdown spectroscopy

In spite of the importance of Si for improving the productivity of many important crops, such as those from the Poaceae family (e.g. sugar cane, maize, wheat, rice), its quantitative determination in plants is seldom carried out and restricted to few laboratories in the world. There is a survey of methods in the literature, but most of them are either laborious or difficult to validate in view of the low availability of reference materials with a certified Si mass fraction. The aim of this study is to propose a method for the direct determination of Si in pellets of plant materials by laser-induced breakdown spectroscopy (LIBS). The experimental setup was designed by using a Q-switched Nd:YAG laser at 1064 nm (5 ns, 10 Hz) and the emission signals were collected by lenses into an optical fiber coupled to an Echelle spectrometer equipped with an intensified charge-coupled device. Experiments were carried out with leaves from 24 sugar cane varieties, with mass fractions varying from ca. 2 to 10 g kg^− 1 Si. Pellets prepared from cryogenically ground leaves were used as test samples for both method development and validation of the calibration model. Best results were obtained when the test samples were interrogated with laser fluence of 50 J cm^− 2 (750 μm spot size) and measurements carried out at Si I 212.412 nm emission line. The results obtained by LIBS were compared with those from inductively coupled plasma optical emission spectrometry after oven-induced alkaline digestion, and no significant differences were observed after applying the Student's t-test at 95% confidence level. The trueness of the proposed LIBS method was also confirmed from the analysis of CRM GBW 07603 (Bush branches and leaves).     

Fast quantitative determination of platinum in liquid samples by laser-induced breakdown spectroscopy

The potential of laser-induced breakdown spectroscopy (LIBS) for the rapid determination of platinum in liquid silicone oils has been evaluated in the framework of on-line process control. A comparison of LIBS sensitivity between three setups designed for liquid analysis (static, liquid jet and flowing liquid) was performed using a 266 nm Nd/YAG laser irradiation. Best results were obtained using the flowing liquid setup and a similar limit of detection was obtained using the liquid jet. The effect of different buffer gases (Ar, He, N(2), etc.) on the signal sensitivity was studied in liquid jet analysis and best values were obtained with a nitrogen sheath gas. Detection limits were in the 100 mg/kg range for both setups. Quantitative determination of platinum in real liquid samples was also investigated using both liquid jet and flowing liquid setups. Calibration curves were plotted for Pt with the liquid jet and the flowing liquid setups under optimised temporal acquisition parameters (delay time and gate width). A normalisation using a silicon line was applied and recovery ranged from 3 to 15% for Pt in catalyst samples with both setups showing that LIBS is a sensitive and accurate method for on-line applications.     

Particle size limits for quantitative aerosol analysis using laser-induced breakdown spectroscopy: Temporal considerations

The temporal evolution of the Si atomic emission signal produced from individual silica microspheres in an aerosolized air stream was investigated using laser-induced breakdown spectroscopy (LIBS). Specifically, the temporal evolution of Si emission from 2.47 and 4.09-micrometer-sized particles is evaluated over discrete delay times ranging from 15 to 70 µs following plasma initiation. The analyte signal profile from the microspheres, taken as the silicon atomic emission peak-to-continuum ratio, was observed to follow the same profile of silicon-rich nanoparticles over the range of delay times. The ratio of analyte signals for the 2.47 and 4.09-micrometer particles was observed to be approximately constant with plasma decay time and less than the expected mass ratio, leading to the conclusion that further vaporization and enhanced analyte response do not continue with increasing delay times for these microsphere sizes. While recent research suggests that the temporal component of analyte response is important for quantitative LIBS analysis, the current study does confirm earlier research demonstrating an upper size limit for quantitative aerosol particle analysis in the diameter range of 2 to 2.5 µm for silica microspheres.     

The use of laser-induced breakdown spectroscopy for the determination of fluorine concentration in glass ionomer cement

The influence of He atmosphere and gate width in laser-induced breakdown spectroscopy (LIBS) determination of fluorine concentration was investigated in detail. The measurements were realized on two double pulse LIBS devices featuring different parameters. Calibration curves, describing the relationship between the fluorine concentration and the corresponding intensity of the LIBS signal, were constructed for both LIBS devices, with and without He flow, respectively. Detection limits achieved were in the range 1.18-0.47 wt.%. The best LOD value was obtained in He atmosphere. The LIBS measurement of fluorine content is influenced by different gate widths and the atmosphere in the working chamber. The proposed method was successfully applied to the determination of fluorine concentration in glass ionomer cements.     

Quantitative determination of element concentrations in industrial oxide materials by laser-induced breakdown spectroscopy

Calibration-free laser-induced breakdown spectroscopy (CF-LIBS) method is employed for quantitative determination of oxide concentrations in multi-component materials. Industrial oxide materials from steel industry are laser ablated in air, and the optical plasma emission is collected by spectrometers and gated detectors. The temperature and electron number density of laser-induced plasma are determined from measured LIBS spectra. Emission lines of aluminium (Al), calcium (Ca), iron (Fe), manganese (Mn), magnesium (Mg), silicon (Si), titanium (Ti), and chromium (Cr) of low self-absorption are selected, and the concentration of oxides CaO, Al₂O₃, MgO, SiO₂, FeO, MnO, TiO₂, and Cr₂O₃ is calculated by CF-LIBS analysis. For all sample materials investigated, we find good match of calculated concentration values (C _CF) with nominal concentration values (C _N). The relative error in oxide concentration, e _r = |C _CF − C _N|/C _N, decreases with increasing concentration and it is e _r ≤ 100% for concentration C _N ≥ 1 wt.%. The CF-LIBS results are stable against fluctuations of experimental parameters. The variation of laser pulse energy over a large range changes the error by less than 10% for major oxides (C _N ≥ 10 wt.%). The results indicate that CF-LIBS method can be employed for fast and stable quantitative compositional analysis of multi-component materials.     

Investigation of polarization effects for nanosecond laser-induced breakdown spectroscopy

The spectral and temporal polarization dependencies of nanosecond laser-induced plasmas are explored for analysis of gaseous and solid samples using various experimental configurations. Plasma emission measurements were resolved into vertical and horizontal polarization components, and the ratio of the two polarization-resolved measurements was calculated for each sample and configuration. For the solid target, measurements were recorded with the sample oriented both normal to the incident laser beam as well as at oblique angles of incidence. The results for the breakdown of a pure, nitrogen gaseous sample revealed no degree of polarization in either the continuum or atomic emission, with the ratios of the horizontally-to-vertically resolved plasma emission showing values equal to unity when resolved both temporally and spectrally. The analysis of both copper and steel solid samples also showed no polarization dependency in the spectral and temporal data when the laser was incidentally normal to the sample surface. For oblique angles of incidence, some polarization (< 10%) was observed within the first tens of nanoseconds of plasma lifetime. The polarization was manifested as a slight reduction in the horizontal component of plasma emission, but significantly, the observed polarization was found to be spectrally flat, with no difference observed between continuum and atomic emission features. The small polarization effect was found to diminish with plasma residence time, effectively vanishing by about 1 μs following breakdown. The transient polarization is hypothesized to arise from reflection effects (i.e. Fresnel reflectivity) between the plasma light and the solid target surface present with oblique angles of incidence for reflected light, with temporal effects due to the dynamic nature of the plasma development and plasma–surface interactions. Overall, no evidence was found to support any inherent anisotropy or polarization specific to the plasma continuum or the atomic emission for the transitions studied.     

Preliminary evaluation of laser-induced breakdown spectroscopy for tissue classification

Laser-induced breakdown spectroscopy (LIBS) is an on-line, real-time technology that can produce immediate information about the elemental contents of tissue samples. We have previously shown that LIBS may be used to distinguish cancerous from non-cancerous tissue. In this work, we study LIBS spectra produced from chicken brain, lung, spleen, liver, kidney and skeletal muscle. Different data processing techniques were used to study if the information contained in these LIBS spectra is able to differentiate between different types of tissue samples and then identify unknown tissues. We have demonstrated a clear distinguishing between each of the known tissue types with only 21 selected analyte lines from each observed LIBS spectrum. We found that in order to produce an analytical model to work well with new sample we need to have representative training data to cover a wide range of spectral variation due to experimental or environmental changes.     

Femtosecond laser-induced breakdown spectroscopy of sea water

The composition of the line and band spectra of the plasma induced by a femtosecond laser pulse on the surface of sea water is determined. The temporal behaviors of the intensity of the continuum and the Ca II, Mg II and Na I lines are investigated. It is shown that the time dependence of the intensity of the Na I line is described by a monoexponential function. The characteristic decay times of the line intensities of Mg II and Na I were used to estimate the three-body recombination times. Using these values, we estimate the electron number density and the feasibility of Local Thermodynamic Equilibrium (LTE) criterion. A method involving excitation rate constants is proposed for the comparison of detection limits. For a plasma generated on a liquid surface, the following relation among detection limits will be obtained: LOD(Na) < LOD(K) < LOD(Ca) < LOD(Al) < LOD(Mg) < LOD(Zn).     

Effects of experimental parameters in quantitative analysis of steel alloy by laser-induced breakdown spectroscopy

Calibration curves for the elements Cr, Ni and Mn in commercial steel alloy samples were obtained employing the Internal Standardization method in atmospheric air pressure. A number of experimental parameters have been optimized in order that the produced calibration curves showed good linearity over a broad range of concentrations covered by the alloy elements. Emphasis was given to the development of the method for field analysis and for the on-line control of production processes.     

Time-resolved ultraviolet laser-induced breakdown spectroscopy for organic material analysis

Ultraviolet pulses (266 nm) delivered by a quadrupled Nd:YAG laser were used to analyze organic samples with laser-induced breakdown spectroscopy (LIBS). We present characteristics of the spectra obtained from organic samples with special attentions on the emissions of organic elements, O and N, and molecular bonds CN. The choice of these atomic or molecular species is justified on one hand, by the importance of these species to specify organic or biological materials; and on the other hand by the possible interferences with ambient air when laser ablation takes place in the atmosphere. Time-resolved LIBS was used to determine the time-evolution of line intensity emitted from these species. We demonstrate different kinetic behaviors corresponding to different origins of emitters: native atomic or molecular species directly vaporized from the sample or those generated through dissociation or recombination due to interaction between laser-induced plasma and air molecules. Our results show the ability of time-resolved UV-LIBS for detection and identification of native atomic or molecular species from an organic sample.     

A semi-quantitative standard-less analysis method for laser-induced breakdown spectroscopy

We report on recently developed analytical software to model laser-induced breakdown spectroscopy emission spectra and predict sample composition using a proposed calibration-free algorithm. The model uses a database of atomic emission lines to create a theoretical emission spectrum for selected elements using defined plasma parameters. The resulting theoretical spectrum is fitted to experimental data obtained from a laser-induced breakdown spectroscopy instrument comprising of four compact spectrometers that image the plasma emission. Elemental concentrations are obtained by comparing observed and predicted spectra while varying the plasma temperature and relative elemental concentrations. The use of the model for analysis of major elements in bauxites, brass and mineral samples as well as the analysis of laboratory air is demonstrated. For the majority of elements investigated agreement within 25% is achieved between estimated and certified values.