Evaluation of laser induced breakdown spectroscopy for the determination of macronutrients in plant materials

Laser induced breakdown spectroscopy (LIBS) has become an analytical tool for the direct analysis of a large variety of materials in order to provide qualitative and/or quantitative information. However, there is a lack of information for LIBS analysis of agricultural and environmental samples. In this work a LIBS system has been evaluated for the determination of macronutrients (P, K, Ca, Mg) in pellets of vegetal reference materials. An experimental setup was designed by using a Nd:YAG laser operating at 1064 nm and an Echelle spectrometer with ICCD detector. The plasma temperature was estimated by Boltzmann plots and instrumental parameters such as delay time, lens-to-sample distance and pulse energy were evaluated. Certified reference materials as well as reference materials were used for analytical calibrations of P, K, Ca, and Mg. Most results of the direct analysis of plant samples by LIBS were in reasonable agreement with those obtained by ICP OES after wet acid decomposition.     

Comparison of univariate and multivariate calibration for the determination of micronutrients in pellets of plant materials by laser induced breakdown spectrometry

The application of laser induced breakdown spectrometry (LIBS) aiming the direct analysis of plant materials is a great challenge that still needs efforts for its development and validation. In this way, a series of experimental approaches has been carried out in order to show that LIBS can be used as an alternative method to wet acid digestions based methods for analysis of agricultural and environmental samples. The large amount of information provided by LIBS spectra for these complex samples increases the difficulties for selecting the most appropriated wavelengths for each analyte. Some applications have suggested that improvements in both accuracy and precision can be achieved by the application of multivariate calibration in LIBS data when compared to the univariate regression developed with line emission intensities. In the present work, the performance of univariate and multivariate calibration, based on partial least squares regression (PLSR), was compared for analysis of pellets of plant materials made from an appropriate mixture of cryogenically ground samples with cellulose as the binding agent. The development of a specific PLSR model for each analyte and the selection of spectral regions containing only lines of the analyte of interest were the best conditions for the analysis. In this particular application, these models showed a similar performance, but PLSR seemed to be more robust due to a lower occurrence of outliers in comparison to the univariate method. Data suggests that efforts dealing with sample presentation and fitness of standards for LIBS analysis must be done in order to fulfill the boundary conditions for matrix independent development and validation.     

Evaluation of laser induced breakdown spectroscopy for cadmium determination in soils

Cadmium is known to be a toxic agent that accumulates in the living organisms and present high toxicity potential over lifetime. Efforts towards the development of methods for microanalysis of environmental samples, including the determination of this element by graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma optical emission spectrometry (ICP OES), and inductively coupled plasma-mass spectrometry (ICP-MS) techniques, have been increasing. Laser induced breakdown spectroscopy (LIBS) is an emerging technique dedicated to microanalysis and there is a lack of information dealing with the determination of cadmium. The aim of this work is to demonstrate the feasibility of LIBS for cadmium detection in soils. The experimental setup was designed using a laser Q-switched (Nd:YAG, 10 Hz, λ = 1064 nm) and the emission signals were collimated by lenses into an optical fiber coupled to a high-resolution intensified charge-coupled device (ICCD)-echelle spectrometer. Samples were cryogenically ground and thereafter pelletized before LIBS analysis. Best results were achieved by exploring a test portion (i.e. sampling spots) with larger surface area, which contributes to diminish the uncertainty due to element specific microheterogeneity. Calibration curves for cadmium determination were achieved using certified reference materials. The metrological figures of merit indicate that LIBS can be recommended for screening of cadmium contamination in soils.     

Evaluation of grinding methods for pellets preparation aiming at the analysis of plant materials by laser induced breakdown spectrometry

It has been demonstrated that laser induced breakdown spectrometry (LIBS) can be used as an alternative method for the determination of macro (P, K, Ca, Mg) and micronutrients (B, Fe, Cu, Mn, Zn) in pellets of plant materials. However, information is required regarding the sample preparation for plant analysis by LIBS. In this work, methods involving cryogenic grinding and planetary ball milling were evaluated for leaves comminution before pellets preparation. The particle sizes were associated to chemical sample properties such as fiber and cellulose contents, as well as to pellets porosity and density. The pellets were ablated at 30 different sites by applying 25 laser pulses per site (Nd:YAG@1064 nm, 5 ns, 10 Hz, 25 J cm⁻²). The plasma emission collected by lenses was directed through an optical fiber towards a high resolution echelle spectrometer equipped with an ICCD. Delay time and integration time gate were fixed at 2.0 and 4.5 μs, respectively. Experiments carried out with pellets of sugarcane, orange tree and soy leaves showed a significant effect of the plant species for choosing the most appropriate grinding conditions. By using ball milling with agate materials, 20 min grinding for orange tree and soy, and 60 min for sugarcane leaves led to particle size distributions generally lower than 75 μm. Cryogenic grinding yielded similar particle size distributions after 10 min for orange tree, 20 min for soy and 30 min for sugarcane leaves. There was up to 50% emission signal enhancement on LIBS measurements for most elements by improving particle size distribution and consequently the pellet porosity.     

Simultaneous optimization by neuro-genetic approach for analysis of plant materials by laser induced breakdown spectroscopy

A simultaneous optimization strategy based on a neuro-genetic approach is proposed for selection of laser induced breakdown spectroscopy operational conditions for the simultaneous determination of macro-nutrients (Ca, Mg and P), micro-nutrients (B, Cu, Fe, Mn and Zn), Al and Si in plant samples. A laser induced breakdown spectroscopy system equipped with a 10 Hz Q-switched Nd:YAG laser (12 ns, 532 nm, 140 mJ) and an Echelle spectrometer with intensified coupled-charge device was used. Integration time gate, delay time, amplification gain and number of pulses were optimized. Pellets of spinach leaves (NIST 1570a) were employed as laboratory samples. In order to find a model that could correlate laser induced breakdown spectroscopy operational conditions with compromised high peak areas of all elements simultaneously, a Bayesian Regularized Artificial Neural Network approach was employed. Subsequently, a genetic algorithm was applied to find optimal conditions for the neural network model, in an approach called neuro-genetic. A single laser induced breakdown spectroscopy working condition that maximizes peak areas of all elements simultaneously, was obtained with the following optimized parameters: 9.0 µs integration time gate, 1.1 µs delay time, 225 (a.u.) amplification gain and 30 accumulated laser pulses. The proposed approach is a useful and a suitable tool for the optimization process of such a complex analytical problem.     

Laser-induced breakdown spectroscopy for determination of uranium in thorium-uranium mixed oxide fuel materials

Laser-induced breakdown spectroscopy (LIBS) has been developed for determining the percentage of uranium in thorium-uranium mixed oxide fuel samples required as a part of the chemical quality assurance of fuel materials. The experimental parameters were optimized using mixed oxide pellets prepared from 1:1 (w/w) mixture of thorium-uranium mixed oxide standards and using boric acid as a binder. Calibration curves were established using U(II) 263.553 nm, U(II) 367.007 nm, U(II) 447.233 nm and U(II) 454.363 nm emission lines. The uranium amount determined in two synthetic mixed oxide samples using calibration curves agreed well with that of the expected values. Except for U(II) 263.553 nm, all the other emission lines exhibited a saturation effect due to self-absorption when U amount exceeded 20 wt.% in the Th-U mixture. The present method will be useful for fast and routine determination of uranium in mixed oxide samples of Th and U, without the need for dissolution, which is difficult and time consuming due to the refractory nature of ThO₂. The methodology developed is encouraging since a very good analytical agreement was obtained considering the limited resolution of the spectrometer employed in the work.     

Preliminary studies of laser-induced breakdown spectrometry for the determination of Ba,Cd, Cr and Pb in toys

The performance of laser-induced breakdown spectrometry (LIBS) for the determination of Ba, Cd, Cr and Pb in toys has been evaluated by using a Nd:YAG laser operating at 1064 nm and an Echelle spectrometer with intensified charge-coupled device detector. Samples were purchased in different cities of São Paulo State market and analyzed directly without sample preparation. Laser-induced breakdown spectrometry experimental conditions (number of pulses, delay time, integration time gate and pulse energy) were optimized by using a Doehlert design. Laser-induced breakdown spectrometry signals correlated reasonably well with inductively coupled plasma optical emission spectrometry (ICP OES) concentrations after microwave-assisted acid digestion of selected samples. Thermal analysis was used for polymer identification and scanning electron microscopy to visualize differences in crater geometry of different polymers employed for toy fabrication. Results indicate that laser-induced breakdown spectrometry can be proposed as a rapid screening method for investigation of potentially toxic elements in toys. The unique application of laser-induced breakdown spectrometry for identification of contaminants in successive layers of ink and polymer is also demonstrated.     

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.     

Strategies for the identification of urinary calculus by laser induced breakdown spectroscopy

The present work studies two different strategies to identify urinary calculus. On one hand, (linear or parametric and rank or non-parametric) correlation methods using a μ-LIBS system are studied. On the other hand, elemental ratios of reference materials are determined by using a higher-energy laser and an Echelle spectrograph with an ICCD camera, although without microscope. A data-treatment method was applied for each system and real samples of kidney stones - previously analyzed by IR spectroscopy - were used for reliable evaluation of two identification strategies.     

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.     

Laser-induced breakdown spectroscopy for simultaneous determination of Sm, Eu and Gd in aqueous solution

This paper reports studies on the determination of trace levels of samarium, europium and gadolinium in aqueous samples by laser-induced breakdown spectroscopy (LIBS). In this work, a membrane-based filter paper was used as a sample support for the liquid samples. The laser-induced plasma was produced in air at atmospheric pressure, using a pulsed Nd:YAG laser. Calibration standards and synthetic mixtures of these lanthanides were prepared using solutions prepared from respective high purity oxides. Linear calibration was obtained for Sm, Eu and Gd by normalizing the intensities of lanthanides emission lines with respective to C(I) 193.029 nm emission line. The concentrations of Sm, Eu and Gd were then determined in a solution containing a mixture of these lanthanides. The concentrations of individual lanthanides were obtained within 5% of the expected values. Limits of detection were found to be 1.3 ppmw (Sm), 1.9 ppmw (Eu) and 2.3 ppmw (Gd).     

Absolute characterization of laser-induced breakdown spectroscopy detection systems

A quantitative comparison of the performance of four different laser-induced breakdown spectroscopy detection systems is presented. The systems studied are an intensified photodiode array coupled with a Czerny–Turner spectrometer, an intensified CCD coupled with a Czerny–Turner spectrometer, an intensified CCD coupled to an Echelle spectrometer, and a prototype multichannel compact CCD spectrometer system. A simple theory of LIBS detection systems is introduced, and used to define noise-equivalent spectral radiance and noise-equivalent integrated spectral radiance for spectral detectors. A detailed characterization of cathode noise sources in the intensified systems is presented.     

Optimization of collinear double-pulse femtosecond laser-induced breakdown spectroscopy of silicon

This paper investigates the optimization of double-pulse collinear femtosecond laser-induced breakdown spectroscopy (FLIBS) for silicon. Double-pulse FLIBS signal enhancements were observed over an extended range of sample focal plane position compared to single pulse FLIBS. The FLIBS signal intensity was studied as a function of pulse energy, inter-pulse delay (0 ps‑80 ps) and sample position. Correlation between crater volume and signal intensity was measured over a limited range of the sample focal plane position. It was found that double-pulse FLIBS is superior to single pulse for certain focal plane positions.     

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.     

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.     

Characterization of coal fly ash components by laser-induced breakdown spectroscopy

The high sensitivity of laser-induced breakdown spectroscopy (LIBS) for the detection of most of the fly ash components enables the analysis of these residues produced during the combustion of coal. Fly ash consists of oxides (SiO₂, Al₂O₃, Fe₂O₃, CaO…) and unburnt carbon which is the major determinant of combustion efficiency in coal fired boilers. For example, an excessive amount of residual carbon dispersed in the fly ash means a significant loss of energy (Styszko et al., 2004 [1]). Standard methods employed for the analysis of fly ash make not possible a control of boiler in real time. LIBS technique can significantly reduce the time of analysis, in some cases even an online detection can be performed. For this reason, some studies have been addressed in order to demonstrate the capability of the laser-induced breakdown spectroscopy technique for the detection of carbon content in high pressure conditions typical of thermal power plants (Noda et al., 2002 [2]) and for the monitoring of unburnt carbon for the boiler control in real time (Kurihara et al., 2003[3]).In particular, the content of unburnt carbon is a valuable indicator for the control of fly ash quality and for the boiler combustion. Depending on this unburnt carbon content, fly ash can be disposed as an industrial waste or as a raw material for the production of concrete in the construction sector. In this study, analyses were performed on specimens of various forms of preparation. Pressed pellets were prepared with two different binders. Presented results concern the nature and amount of the binder used to pelletize the powder, and the laser-induced breakdown spectroscopy parameters and procedure required to draw calibration curves of elements from the fly ash. Analysis “on tape” was performed in order to establish the experimental conditions for the future “online analysis”.     

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.     

Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting

Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed.     

Fast vacuum slag analysis in a steel works by laser-induced breakdown spectroscopy

Samples taken from the liquid slag layer in a vacuum degasser station of a steel works are analyzed after solidification by laser-induced breakdown spectroscopy (LIBS) without any further sample preparation. The mass fractions of the major components of the vacuum slags are in the range of 50–60% for CaO, 0.5–12% for SiO₂ and 20–40% for Al₂O₃. The species are distributed heterogeneously in the solid samples having diameters of 35 mm. Furthermore the color and structure of the samples is varying significantly. A fast spatial averaging of representative sample areas is realized by spatial laser beam shaping. Multivariate calibration and its validation is carried out with calibration and validation sets of production samples which are analyzed by X-ray fluorescence measurements or as borate beads for reference. The laser-induced breakdown spectroscopy instrument is installed in the steel works at a distance of about 10 m from the vacuum degasser. The laser-induced breakdown spectroscopy analysis runs automatically after the sample placement and it takes 80 s including data transfer to the host computer of the steel works. Operational tests are carried out to demonstrate the feasibility of a fast slag analysis in the harsh environment of the vacuum degasser plant.     

Vacuum ultraviolet laser-induced breakdown spectroscopy analysis of polymers

Laser-induced breakdown spectroscopy (LIBS) in the vacuum ultraviolet range (VUV, λ < 200 nm) is employed for the detection of trace elements in polyethylene (PE) that are difficult to detect in the UV/VIS range. For effective laser ablation of PE, we use a F₂ laser (wavelength λ = 157 nm) with a laser pulse length of 20 ns, a pulse energy up to 50 mJ, and pulse repetition rate of 10 Hz. The optical radiation of the laser-induced plasma is measured by a VUV spectrometer with detection range down to λ = 115 nm. A gated photon-counting system is used to acquire time-resolved spectra. From LIBS measurements of certified polymer reference materials, we obtained a limit of detection (LOD) of 50 µg/g for sulphur and 215 µg/g for zinc, respectively.The VUV LIBS spectra of PE are dominated by strong emission lines of neutral and ionized carbon atoms. From time-resolved measurements of the carbon line intensities, we determine the temporal evolution of the electronic plasma temperature, T_e. For this, we use Saha–Boltzmann plots with the electron density in the plasma, N_e, derived from the broadening of the hydrogen H-α line. With the parameters T_e and N_e, we calculate the intensity ratio of the atomic sulphur and carbon lines at 180.7 nm and at 175.2 nm, respectively. The calculated intensity ratios are in good agreement with the experimentally measured results.