Quantitative analysis of alloys and glasses by a calibration-free method using laser-induced breakdown spectroscopy

Space-, time- and spectrally resolved optical diagnostics of laser ablation plasma has provided the opportunity to realize calibration-free analyses of solid materials. In general, this variant of optical emission spectroscopy of pulsed plasma allows the plasma matrix effects to be overcome, yielding satisfactorily precise and accurate quantitative results on elemental composition of materials without using calibration curves, certified reference materials, and internal standards. Such analysis is very close to be nondestructive due to the minimum possible ablated mass, a feature which is very important in many applications, especially for unique museum exhibits and jeweler samples. In this paper, the use of the method for the analysis of elements in bronze, brass and gold alloys, glass samples, and archaeological findings is demonstrated. The results presented confirm the suitability of the approach for routine applications of our instrumentation, while at the same time simplifying the overall analytical procedure.     

Application of calibration-free laser-induced breakdown spectroscopy to radially resolved spectra from a copper-based alloy laser-induced plasma

In this work, the Calibration-Free approach for Laser-Induced Breakdown Spectroscopy (CF-LIBS) was applied for the first time to radially resolved spectra emitted by a laser-induced plasma. The radial profiles of plasma temperature and electron number density were used to calculate the local relative concentration of the elements of interest. We analyzed a set of profiles of the local spectral emission coefficient obtained previously by means of spatial deconvolution of the spectra from a copper-based alloy (Cu 93, Fe 5, Mn 1, Ni 1 wt.%) laser-induced plasma. A spatially integrated spectrum of the same plasma was also analyzed for comparison purpose. The relative abundance of the minor components Fe, Mn and Ni was calculated. The results obtained from the central region of the plasma were closer to the nominal concentrations than those obtained from the spatially integrated spectrum. However, an increasing deviation was observed towards the plasma edge. It is proposed that this deviation could be the result of a gradual departure from Local Thermodynamic Equilibrium (LTE) conditions due the significant decrease of the electron density at the external shells of the plasma.     

Laser-induced breakdown spectroscopy: Time-resolved spectrochemical applications

We have added time resolution to laser-induced breakdown spectroscopy in two forms, by gating an optical multichannel analyzer (OMA) and by time-resolving the output of a photomultiplier with a boxcar amplifier. Spectra were obtained for temporal segments of 25 to 100 ns, from 25 ns to 50 µs after initiation of the breakdown. OMA spectra of oxygen illustrate the power of this technique for survey purposes. The photomultiplier-boxcar arrangement was used to detect phosphorus atoms from diisopropylmethyl phosphonate in air, and also to detect chlorine in air, both in real time. In the former experiments we detected 690 ppm (w/w) of phosphorus and project a limit of detection with our current apparatus of 15 ppm (w/w). For chlorine, we observed signal from 120 ppm (w/w) and project a limit of detection of 60 ppm (w/w).     

Correction of self-absorption effect in calibration-free laser-induced breakdown spectroscopy by an internal reference method

A simplified procedure for correcting self-absorption effect was proposed in calibration-free laser-induced breakdown spectroscopy (CF-LIBS). In typical LIBS measurement conditions, the plasma produced is often optically thick, especially for the strong lines of major elements. The selection of self-absorption lines destroys the performance of CF-LIBS, and the familiar correction method based on the curve of growth is complex in implementation. The procedure we proposed, named internal reference for self-absorption correction (IRSAC), first chose an internal reference line for each species, then compared other spectral line intensity of the same species with the reference line to estimate the self-absorption degrees of other spectral lines, and finally achieved an optimal correction by a regressive algorithm. The self-absorption effect of the selected reference line can be ignored, since the reference line with high excitation energy of the upper level is slightly affected by the self-absorption. Through the IRSAC method, the points on the Boltzmann plot become more regular, and the evaluations of the plasma temperature and material composition are more accurate than the basic CF-LIBS.     

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.     

Laser-induced breakdown spectroscopy: Time-integrated applications

Spectral analysis of the plasma produced by laser-induced breakdown was demonstrated to be an effective real-time technique for the detection of atomic constituents in gases and gas-entrained particulates. The time-integrated technique, LIBS (laser-induced breakdown spectroscopy), was applied to the detection of sodium and potassium in a coal gasifier product stream, of airborne beryllium, and of phosphorus, sulfur, and chlorine in various organic molecules. In a companion paper (following) the time-resolved technique will be discussed.     

Direct determination of the nutrient profile in plant materials by femtosecond laser-induced breakdown spectroscopy

Femtosecond laser-induced breakdown spectroscopy (fs-LIBS) has been used for the first time for quantitative determination of nutrients in plant materials from different crops. A highly heterogeneous population of 31 samples, previously analyzed by inductively coupled plasma optical emission spectroscopy, covering a wide range of matrices was interrogated. To tackle the analysis, laser-induced plasmas under argon atmosphere of pellets prepared from sieved cryogenically ground leaves were studied. Predictive functions based on univariate and multivariate modeling of optical emissions associated to macro- (Ca, Mg, and P) and micronutrients (Cu, Fe, Mn and Zn) were designed. Hierarchical cluster analysis was performed to select representative calibration (n_cal = 17) and validation (n_val = 14) datasets. The predictive performance of calibration functions over fs-LIBS data was compared with that attained on spectral information from nanosecond LIBS (ns-LIBS) operating at different wavelengths (1064 nm, 532 nm, and 266 nm). Findings established higher accuracy and less uncertainty on mass fractions quantification from fs-LIBS, whatever the modeling approach. Quality coefficients below 20% for the accuracy error on mass fractions’ prediction in unknown samples, and residual predictive deviations in general above 5, were obtained. In contrast, only multivariate modeling satisfactorily handled the non-linear variations of emissions in ns-LIBS, leading to 2-fold decrease in the root mean square error of prediction (RMSEP) of Ca, Mg, P, Cu, Fe, Mn and Zn in comparison with the univariate approach. But still, an averaged quality coefficient about 35% and residual predictive deviations below 3 were found. Similar predictive capabilities were observed when changing the laser wavelength. Although predicted values by ns-LIBS multivariate modeling exhibit better agreement with reference mass fractions as compared to univariate functions, fs-LIBS conducts better quantification of nutrients in plant materials since it is less dependent on the chemical composition of the matrices.     

Review: Applications of single-shot laser-induced breakdown spectroscopy

As applications for laser-induced breakdown spectroscopy (LIBS) become more varied with a greater number of field and industrial LIBS systems developed and as the technique evolves to be more quantitative that qualitative, there is a more significant need for LIBS systems capable of analysis with the use of a single laser shot. In single-shot LIBS, a single laser pulse is used to form a single plasma for spectral analysis. In typical LIBS measurements, multiple laser pulses are formed and collected and an ensemble-averaged method is applied to the spectra. For some applications there is a need for rapid chemical analysis and/or non-destructive measurements; therefore, LIBS is performed using a single laser shot. This article reviews in brief several applications that demonstrate the applicability and need for single-shot LIBS.     

Multivariate analysis of remote laser-induced breakdown spectroscopy spectra using partial least squares,principal component analysis,and related techniques

Quantitative analysis with laser-induced breakdown spectroscopy traditionally employs calibration curves that are complicated by chemical matrix effects. These chemical matrix effects influence the laser-induced breakdown spectroscopy plasma and the ratio of elemental composition to elemental emission line intensity. Consequently, laser-induced breakdown spectroscopy calibration typically requires a priori knowledge of the unknown, in order for a series of calibration standards similar to the unknown to be employed. In this paper, three new Multivariate Analysis techniques are employed to analyze the laser-induced breakdown spectroscopy spectra of 18 disparate igneous and highly-metamorphosed rock samples. Partial Least Squares analysis is used to generate a calibration model from which unknown samples can be analyzed. Principal Components Analysis and Soft Independent Modeling of Class Analogy are employed to generate a model and predict the rock type of the samples. These Multivariate Analysis techniques appear to exploit the matrix effects associated with the chemistries of these 18 samples.     

Comparison of two laser-induced breakdown spectroscopy techniques for total carbon measurement in soils

The potentials of two advanced laser-induced breakdown spectroscopy (LIBS) techniques which are used to determine the total carbon content in soils have been examined. The first one is the combination of a single-pulse laser ablation with spark excitation of plasma plume triggering the gap between electrodes close to the target surface. The second one is a more conventional double-pulse LIBS. In both modes the calibration graphs have a nonlinear trend in the actual range of carbon contents and present a good R² value (0.97). In the combined laser-spark approach, using low-cost and portable laser instrumentation is possible, as well as inducing a micro-damage on the target surface. Certain regularities in the spectral line intensities of soil nutritious elements have been detected and appear to be connected to the total carbon content and to the soil origin.     

Parametric study of a fiber-optic laser-induced breakdown spectroscopy probe for analysis of aluminum alloys

In the present work we demonstrate a fiber-optic laser-induced breakdown spectroscopy (FO LIBS) system for delivering laser energy to a sample surface to produce a spark as well as to collect the resulting radiation from the laser-induced spark. In order to improve the signal/background (S/B) ratio, various experimental parameters, such as laser energy, gate delay and width, detector gain, lenses of different focal lengths and sample surface, were tested. In order to provide high reliability and repeatability in the analysis, we also measured plasma parameters, such as electron density and plasma temperature, and determined their influence on the measurement results. The performance of FO LIBS was also compared with that of a LIBS system that does not use a fiber to transmit the laser beam. LIBS spectra with a good S/B were recorded at 2-μs gate delay and width. LIBS spectra of six different Al alloy samples were recorded to obtain calibration data. We were able to obtain linear calibration data for numerous elements (Cr, Zn, Fe, Ni, Mn, Mg and Cu). A linear calibration curve for LIBS intensity ratio vs. concentration ratio reduces the effect of physical variables (i.e. shot-to-shot power fluctuation, sample-to-surface distance, and physical properties of the samples). Our results reveal that this system may be useful in designing a high-temperature LIBS probe for measuring the elemental composition of Al melt.     

Laser-induced breakdown spectroscopy analysis of minerals: Carbonates and silicates

Laser-induced breakdown spectroscopy (LIBS) provides an alternative chemical analytical technique that obviates the issues of sample preparation and sample destruction common to most laboratory-based analytical methods. This contribution explores the capability of LIBS analysis to identify carbonate and silicate minerals rapidly and accurately. Fifty-two mineral samples (18 carbonates, 9 pyroxenes and pyroxenoids, 6 amphiboles, 8 phyllosilicates, and 11 feldspars) were analyzed by LIBS. Two composite broadband spectra (averages of 10 shots each) were calculated for each sample to produce two databases each containing the composite LIBS spectra for the same 52 mineral samples. By using correlation coefficients resulting from the regression of the intensities of pairs of LIBS spectra, all 52 minerals were correctly identified in the database. If the LIBS spectra of each sample were compared to a database containing the other 51 minerals, 65% were identified as a mineral of similar composition from the same mineral family. The remaining minerals were misidentified for two reasons: 1) the mineral had high concentrations of an element not present in the database; and 2) the mineral was identified as a mineral with similar elemental composition from a different family. For instance, the Ca–Mg carbonate dolomite was misidentified as the Ca–Mg silicate diopside. This pilot study suggests that LIBS has promise in mineral identification and in situ analysis of minerals that record geological processes.     

Quantitative elemental analysis of nutritional,hazardous and pharmacologically active elements in medicinal Rhatany root using laser induced breakdown spectroscopy

Rhatany roots (RRs) have been used in indigenous systems of medicines to treat many common illnesses due to the presence of highly active astringent and antiviral biochemical constituents that possess strong therapeutic and pharmacological properties. Due to its widespread use, the accurate knowledge on the elemental composition of this medicinal plant can set a pharmacological research platform to investigate the effect of certain elements, and their ions in mediating the human metabolism and therapy. In this work calibration-free laser-induced breakdown spectroscopy (CF-LIBS) is used to detect the elements present in RRs sample, by analyzing the characteristic emission wavelengths and their respective intensities in the laser induced plasma, without the need for using any calibration standards or methods. Many nutritional elements, which are of human health significance and instrumental in mediating the established biological activities of RRs, were identified in a relative abundance. In addition to this, our analysis identified the trace level of a few toxic elements, whose overdose due to reckless intake wreaks havoc to human health and wellbeing. The reliability of qualitative and quantitative detection of the elements in RR by LIBS were validated by the standard inductively coupled plasma optical emission spectroscopy (ICP OES), the results of which are in good agreement with LIBS data with better relative accuracy. Also, in order to discriminate, and single out any two elements with the overlapping emission wavelength in LIBS, X-ray photoelectron spectroscopy was also carried out, which in its own right is in good agreement with the elemental analysis of LIBS in general.     

Depth-profiling study of a thermal barrier coated superalloy using femtosecond laser-induced breakdown spectroscopy

Depth-profiling of a thermal barrier coating (TBC) system was carried out using femtosecond laser-induced breakdown spectroscopy (fs-LIBS). The TBC system consisted of an outer 7% yttria stabilized zirconia (7YSZ) ceramic coating and an inner Pt-modified bond coat on a Ni-based superalloy single crystal substrate. In the absence of the ceramic layer, it was possible to qualitatively differentiate between the bond coat and the substrate, and also between the two layers of the bond coat. The spatial location of the interface between the two bond coat layers could also be obtained accurately from the spectral profile of W. In presence of the ceramic coating, it was also possible to easily and accurately determine the ceramic/bond coat interface from the spectral profile of Al. Potential use of fs-LIBS in combination with fs laser machining to produce superior quality cooling holes on thermal barrier coated superalloy components is discussed.     

Radiative models of laser-induced plasma and pump-probe diagnostics relevant to laser-induced breakdown spectroscopy

The paper describes past and present efforts in modeling of laser-induced plasma and overviews plasma diagnostics carried out by pump-probe techniques. Besides general information on existing plasma models, the emphasis is given to models relevant to spectrochemical analysis, i.e. models of radiating plasma. Special attention is paid to collisional-radiative (CR) and collisional-dominated (CD) plasma models where radiative processes play an important role. Also, calibration-free (CF) models are considered which may endow with the possibility for standardless spectroscopic analysis. In the diagnostic part, only methods based on the use of additional diagnostic tools (auxiliary lasers, optics, and probes) are described omitting those based on plasma own radiation. A short review is provided on image-based diagnostics (shadowgraphy, schlieren, and interferometry), absorption and fluorescence, Langmuir probe, and less frequently used cavity ringdown and Thomson scattering methods.     

Laser-induced breakdown spectroscopy of carbon in helium and nitrogen at high pressure

This work is devoted to the study of the gas pressure effect on the laser-induced breakdown spectroscopy signal intensity of carbon. Experiments are performed, using a 1064 nm Nd:YAG laser, with carbon solid samples placed inside a high pressure chamber filled with helium or nitrogen, the gas pressure varying from 1 to 80 atm. The signal intensity of the carbon line (247.86 nm) decreases with increasing pressure. As the plasma size strongly decreases with pressure, two collection optical setups are used, showing different raw results. To take into account the plasma size evolution with pressure, calculated corrections are applied to the collected light intensity. Carbon line emission is measured and corrected as a function of pressure in both gases. At 1 atm, the emission line is found to be greater in helium than in nitrogen by a factor of approximately 3, whereas the intensities in the two gases become close to each other at 80 atm.     

Comments on the paper: “Accurate quantitative analysis of gold alloys using multi-pulse laser-induced breakdown spectroscopy and a correlation-based calibration method”

The problem of finding new methods for the analysis of precious alloys has stimulated, in recent years, a number of different proposals for improving the analytical procedures introducing more robust calibration (or calibration-free) methods. In the paper “Accurate quantitative analysis of gold alloys using multi-pulse laser-induced breakdown spectroscopy and a correlation-based calibration method” by G. Galbács, N.Jedinski, G.Cseh, Z. Galbács and L. Túri [Spectrochimica Acta Part B, Volume 63, Issue 5, 591–597 (May 2008)] the authors proposed the use of multiple-pulse LIBS and a correlation-based method for building calibration curves for quantitative analysis of gold alloys. The method is proposed for gold alloys prepared using a fixed proportion of the alloying element. The general case where the relative concentration of the elements of the matrix is not a priori known is not discussed in the paper. In this communication, we will demonstrate that the method proposed is extremely fragile against matrix effects, and therefore cannot be usefully applied for the purpose of actual analytical measurements on gold alloys without a previous knowledge of the matrix composition.     

Comparison of double-pulse and single-pulse laser-induced breakdown spectroscopy techniques in the analysis of powdered samples of silicate raw materials for the brick-and-tile industry

Single- and double-pulse laser-induced breakdown spectroscopy techniques applied to the analysis of pressed pellets of silicate raw materials were compared in terms of precision, sensitivity and limits of detection. Two Nd:YAG lasers (1064 and 532 mm) in an orthogonal configuration with a reheating arrangement have been employed. The main factors influencing system performance were optimized, i.e. laser pulse energies and interpulse separation. The behaviour of plasma temperature was studied over a period of time and calibration curves for Mg were constructed for both the single and double-pulse setup. When comparing the single- and double-pulse techniques, limits of detection of Si and Mg for the double-pulse technique were found to be 10 times lower.     

Analysis of laser-induced breakdown spectroscopy spectra: The case for extreme value statistics

In most instances, laser-induced breakdown spectroscopy (LIBS) spectra are obtained through analog accumulation of multiple shots in the spectrometer CCD. The average acquired in the CCD at a given wavelength is assumed to be a good representation of the population mean, which in turn is implicitly regarded to be the best estimator for the central value of the distribution of the spectrum at the same wavelength. Multiple analog accumulated spectra are taken and then in turn averaged wavelength-by-wavelength to represent the final spectrum. In this paper, the statistics of single-shot and analog accumulated LIBS spectra of both solids and liquids were examined to evaluate whether the spectrum averaging approach is statistically defensible. At a given wavelength, LIBS spectra are typically drawn from a Frechet extreme value distribution, and hence the mean of an ensemble of LIBS spectra is not necessarily an optimal summary statistic. Under circumstances that are broadly general, the sample mean for LIBS data is statistically inconsistent and the central limit theorem does not apply. This result appears to be due to very high shot-to-shot plasma variability in which a very small number of spectra are high in intensity while the majority are very weak, yielding the extreme value form of the distribution. The extreme value behavior persists when individual shots are analog accumulated. An optimal estimator in a well-defined sense for the spectral average at a given wavelength follows from the maximum likelihood method for the extreme value distribution. Example spectra taken with both an Echelle and a Czerny–Turner spectrometer are processed with this scheme to create smooth, high signal-to-noise summary spectra. Plasma imaging was used in an attempt to visually understand the observed variability and to validate the use of extreme value statistics. The data processing approach presented in this paper is statistically reliable and should be used for accurate comparisons of LIBS spectra instead of arithmetic averaging on either complete or censored data sets.     

Polarization and fluence dependence of the polarized emission in nanosecond laser-induced breakdown spectroscopy

Several studies have appeared in the past two years reporting that the continuum emission produced by the laser ablation of solid materials is strongly polarized. In a paper that appears to conflict with these findings, Asgill et al. report that they did not observe a significant amount of polarization produced by nanosecond laser excitation of nitrogen gas and laser ablation of copper and steel ( M.E. Asgill, H.Y. Moon, N. Omenetto, D.W. Hahn, Investigation of polarization effects for nanosecond laser-induced breakdown spectroscopy, Spectrochim. Acta Part B (2010) xxx-xxx [7]). Here we show that the apparent discrepancy is resolved when laser fluence and polarization are taken into account. Using a 532 nm Nd:YAG laser to ablate Al samples in air, we find that the degree of polarization, P, of the continuum is greater for s- vs. p-polarized excitation and that P decreases with increasing fluence. We show that P would be < 10% under the conditions of Asgill et al., whereas P > 60% is obtained at low fluences with s-polarized excitation. We also confirm that at high fluence the polarization of the discrete emission is much smaller than that of the continuum.