Comparisons between LIBS and ICP/OES

In the framework of the development of new techniques, the ability of laser-induced breakdown spectroscopy (LIBS) to analyse remotely complex aqueous solutions was investigated. The jet configuration with a collimated gas stream was chosen because it appeared to be the most promising method for the LIBS probe, particularly in terms of sensitivity and repeatability. For emission collection, the echelle spectrometer offers a simultaneously recorded wavelength range from the UV to the near IR and is interesting for multielemental analysis for LIBS and also for inductively coupled plasma (ICP) optical emission spectroscopy (OES). The importance of parameters influencing the quantitative results of LIBS such as multispecies analysis, sheath gas, use of an internal standard and temporal parameters for analysis is described. LIBS quantitative data have been directly compared with results from the more standard ICP/OES technique.     

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.     

Multielement trace determination in SiC powders: assessment of interlaboratory comparisons aimed at the validation and standardization of analytical procedures with direct solid sampling based on ETV ICP OES and DC arc OES

The members of the committee NMP 264 “Chemical analysis of non-oxidic raw and basic materials” of the German Standards Institute (DIN) have organized two interlaboratory comparisons for multielement determination of trace elements in silicon carbide (SiC) powders via direct solid sampling methods. One of the interlaboratory comparisons was based on the application of inductively coupled plasma optical emission spectrometry with electrothermal vaporization (ETV ICP OES), and the other on the application of optical emission spectrometry with direct current arc (DC arc OES). The interlaboratory comparisons were organized and performed in the framework of the development of two standards related to “the determination of mass fractions of metallic impurities in powders and grain sizes of ceramic raw and basic materials” by both methods. SiC powders were used as typical examples of this category of material. The aim of the interlaboratory comparisons was to determine the repeatability and reproducibility of both analytical methods to be standardized. This was an important contribution to the practical applicability of both draft standards. Eight laboratories participated in the interlaboratory comparison with ETV ICP OES and nine in the interlaboratory comparison with DC arc OES. Ten analytes were investigated by ETV ICP OES and eleven by DC arc OES. Six different SiC powders were used for the calibration. The mass fractions of their relevant trace elements were determined after wet chemical digestion. All participants followed the analytical requirements described in the draft standards. In the calculation process, three of the calibration materials were used successively as analytical samples. This was managed in the following manner: the material that had just been used as the analytical sample was excluded from the calibration, so the five other materials were used to establish the calibration plot. The results from the interlaboratory comparisons were summarized and used to determine the repeatability and the reproducibility (expressed as standard deviations) of both methods. The calculation was carried out according to the related standard. The results are specified and discussed in this paper, as are the optimized analytical conditions determined and used by the authors of this paper. For both methods, the repeatability relative standard deviations were <25%, usually ~10%, and the reproducibility relative standard deviations were <35%, usually ~15%. These results were regarded as satifactory for both methods intended for rapid analysis of materials for which decomposition is difficult and time-consuming. Also described are some results from an interlaboratory comparison used to certify one of the materials that had been previously used for validation in both interlaboratory comparisons. Thirty laboratories (from eight countries) participated in this interlaboratory comparison for certification. As examples, accepted results are shown from laboratories that used ETV ICP OES or DC arc OES and had performed calibrations by using solutions or oxides, respectively. The certified mass fractions of the certified reference materials were also compared with the mass fractions determined in the interlaboratory comparisons performed within the framework of method standardization. Good agreement was found for most of the analytes.     

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.     

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.     

Artificial neural network for on-site quantitative analysis of soils using laser induced breakdown spectroscopy

Nowadays, due to environmental concerns, fast on-site quantitative analyses of soils are required. Laser induced breakdown spectroscopy is a serious candidate to address this challenge and is especially well suited for multi-elemental analysis of heavy metals. However, saturation and matrix effects prevent from a simple treatment of the LIBS data, namely through a regular calibration curve. This paper details the limits of this approach and consequently emphasizes the advantage of using artificial neural networks well suited for non-linear and multi-variate calibration. This advanced method of data analysis is evaluated in the case of real soil samples and on-site LIBS measurements. The selection of the LIBS data as input data of the network is particularly detailed and finally, resulting errors of prediction lower than 20% for aluminum, calcium, copper and iron demonstrate the good efficiency of the artificial neural networks for on-site quantitative LIBS of soils.     

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.     

Analysis of Roman-Hispanic Archaeological Ceramics using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a useful tool for characterization of archaeological samples because it involves little or no sample preparation, and it allows on-site rapid and nondestructive analysis. In this study, ceramics from the archaeological site of Labitolosa (Huesca) were analyzed qualitatively and quantitatively. Furthermore, a protocol is described for the determination of iron in archaeological materials by interpolating calibration graphs and standard addition. Moreover, the potential of LIBS for direct quantitative analysis of iron in real archaeological materials was evaluated, and optimum analytical conditions were determined. The reproducibility of LIBS spectra from different archaeological samples was investigated as a function of the number of laser shots. The results were compared with those of a reference material.     

Quantitative Analysis of Roman Archeological Ceramics by Laser-Induced Breakdown Spectroscopy

In recent decades, numerous analytical techniques have been used for the analysis of archeological samples. Laser-induced breakdown spectroscopy (LIBS) is a promising technique due to its practically nondestructive nature and minimal sample preparation. In this work, LIBS was used for the qualitative and quantitative elemental analyses of pottery manufactured in ancient settlements of Rome. The qualitative study showed that the ceramics were composed of Fe, Ca, and Mg. For quantitative analysis, calibration curves of Fe, Ca, and Mg were constructed with reference samples of each element in a KBr matrix with zinc as an internal standard. The results obtained by LIBS were compared with values obtained by atomic absorption.     

Method development for the determination of manganese, cobalt and copper in green coffee comparing direct solid sampling electrothermal atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry

A method has been developed for the determination of cobalt, copper and manganese in green coffee using direct solid sampling electrothermal atomic absorption spectrometry (SS-ET AAS). The motivation for the study was that only a few elements might be suitable to determine the origin of green coffee so that the multi-element techniques usually applied for this purpose might not be necessary. The three elements have been chosen as test elements as they were found to be significant in previous investigations. A number of botanical certified reference materials (CRM) and pre-analyzed samples of green coffee have been used for method validation, and inductively coupled plasma optical emission spectrometry (ICP OES) after microwave-assisted acid digestion of the samples as reference method. Calibration against aqueous standards could be used for the determination of Mn and Co by SS-ET AAS, but calibration against solid CRM was necessary for the determination of Cu. No significant difference was found between the results obtained with the proposed method and certified or independently determined values. The limits of detection for Mn, Cu and Co were 0.012, 0.006 and 0.004 μg g⁻¹ using SS-ET AAS and 0.015, 0.13 and 0.10 μg g⁻¹ using ICP OES. Seven samples of Brazilian green coffee have been analyzed, and there was no significant difference between the values obtained with SS-ET AAS and ICP OES for Mn and Cu. ICP OES could not be used as a reference method for Co, as essentially all values were below the limit of quantification of this technique.     

Quantitative analysis of oxide materials by laser-induced breakdown spectroscopy with argon as an internal standard

Laser-induced breakdown spectroscopy (LIBS) is demonstrated as a quantitative technique for geochemical analysis. This study demonstrates the applicability of LIBS to multielemental analysis of minerals using argon as an internal standard. Laser-induced breakdown spectroscopy has been applied to measure elements in oxide form. In the present study, the contents of several oxides, such as Fe₂O₃, CaO and MgO, in geological samples from the Tierga Mine (Zaragoza, Spain) were analyzed by LIBS. An argon environment was used to eliminate interference from air at atmospheric pressure. Furthermore, argon was used as an internal standard. The result was enhanced signal and enhanced linearity of the calibration curves. The Fe₂O₃, CaO and MgO concentrations determined by LIBS were compared with the results obtained using another analytical technique, inductively coupled plasma optical emission spectrometry (ICP-OES). The concentrations found using LIBS were in good agreement with the values obtained by ICP-OES.     

Particulate matter (PM10) from São Carlos-SP (Brazil): spectroanalytical techniques to evaluate and determine chemical elements

Chemical composition of PM10 was studied during the period of 2014–2015 in the city of São Carlos, Brazil (‘Dos Voluntários’ Square). PM10 samples were directly analysed by wavelength dispersive X-ray fluorescence – WD-XRF (Al, Ca, Ce, Cl, Fe, K, Mg, P, S, Si, Ti, V, and Zn), and by laser-induced breakdown spectroscopy – LIBS (Ba, Ca, Fe, K, Mg, Na, Si and Ti) both for qualitative purposes. For quantitative analysis of Al, Ba, Ca, Fe, K, Na, S, and Zn, the analytes were extracted from samples of PM10 collected, in filters of glass fibre, with an extractive acid solution (HNO₃ and HCl) and determined by inductively coupled plasma optical emission spectrometry – ICP OES. Spatial variations of elemental concentrations (ng m^–3) were significantly higher in winter Al (19.0), Ba (13.6), Ca (20.0), Na (27.0), S (37.1), and Zn (9.5), autumn showed the highest concentration of Na (26.4), spring showed the highest concentration of Fe (11.6), K (13.1) and also S (25.3) and summer did not show a high concentration in the comparison between the seasons at the site. Using principal component analysis (PCA), as a data interpretation tool, with the data obtained by the WD-XRF and LIBS it was not possible to obtain a good correlation, but with the data of ICP OES, it was possible to verify correlations between identified and determinate elements, with samples collected in the autumn, winter, spring and summer seasons in the city of São Carlos. These associated analytical techniques were excellent tools in environmental monitoring, through the analysis of PM10 samples, presenting reliable and efficiency strategy, and based on the PCA and the EF equation was possible to draw the profile of the possible origins of these elements in the city.     

Artificial neural network for Cu quantitative determination in soil using a portable Laser Induced Breakdown Spectroscopy system

Laser Induced Breakdown Spectroscopy (LIBS) is an advanced analytical technique for elemental determination based on direct measurement of optical emission of excited species on a laser induced plasma. In the realm of elemental analysis, LIBS has great potential to accomplish direct analysis independently of physical sample state (solid, liquid or gas). Presently, LIBS has been easily employed for qualitative analysis, nevertheless, in order to perform quantitative analysis, some effort is still required since calibration represents a difficult issue. Artificial neural network (ANN) is a machine learning paradigm inspired on biological nervous systems. Recently, ANNs have been used in many applications and its classification and prediction capabilities are especially useful for spectral analysis. In this paper an ANN was used as calibration strategy for LIBS, aiming Cu determination in soil samples. Spectra of 59 samples from a heterogenic set of reference soil samples and their respective Cu concentration were used for calibration and validation. Simple linear regression (SLR) and wrapper approach were the two strategies employed to select a set of wavelengths for ANN learning. Cross validation was applied, following ANN training, for verification of prediction accuracy. The ANN showed good efficiency for Cu predictions although the features of portable instrumentation employed. The proposed method presented a limit of detection (LOD) of 2.3 mg dm^− 3 of Cu and a mean squared error (MSE) of 0.5 for the predictions.     

Self-absorption model in quantitative laser induced breakdown spectroscopy measurements on soils and sediments

Application of laser induced breakdown spectroscopy (LIBS) in the quantitative analysis of elemental composition of soils with different origins and Antarctic marine sediments has been considered. The analytical method followed includes the usual plasma modeling at local thermal equilibrium (LTE) based on average temperature and electron density values, as well as spectra normalization, introduced in order to reduce the effects related both to the substrate optical and thermal properties and to the influence of laser parameters on quantitative data. The computational algorithm takes into account only atomic species and their first ionization states, which is sufficient at the plasma temperature measured in the experiments. Calibration curves are finally generated for each element of interest measured on certified samples with different provenience and matrix composition. In this paper a model is developed which takes into account the effects responsible for non-linearities in the relationship between line intensity and elemental concentration. The model properly includes line re-absorption and contributions from space regions with different plasma densities. Its application permits us to obtain the correlation coefficients between the LIBS measured and certified concentration of each element analyzed. These coefficients, specific for a given experimental layout and atomic lines data base, are successively applied in analytical LIBS measurements allowing for the direct determination of a single element concentration in any sample, regardless of its unknown matrix composition. The LIBS method presented here was tested on a priori unknown samples, and gave uncertainties in concentration varying from 15 to 40% over a large concentration range covering several orders of magnitude. The measuring error depends on element type, on the concentration value and also on the number of certified samples used for the initial calibration. The present results are already significant for some field application, such as on-board marine sediment analysis where a significant matrix variation with layer depth is common.     

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.     

Quantitative LIBS analysis of vanadium in samples of hexagonal mesoporous silica catalysts

The method for the analysis of vanadium in hexagonal mesoporous silica (V-HMS) catalysts using Laser Induced Breakdown Spectrometry (LIBS) was suggested. Commercially available LIBS spectrometer was calibrated with the aid of authentic V-HMS samples previously analyzed by ICP OES after microwave digestion. Deposition of the sample on the surface of adhesive tape was adopted as a sample preparation method. Strong matrix effect connected with the catalyst preparation technique (1st vanadium added in the process of HMS synthesis, 2nd already synthesised silica matrix was impregnated by vanadium) was observed. The concentration range of V in the set of nine calibration standards was 1.3-4.5% (w/w). Limit of detection was 0.13% (w/w) and it was calculated as a triple standard deviation from five replicated determinations of vanadium in the real sample with a very low vanadium concentration. Comparable results of LIBS and ED XRF were obtained if the same set of standards was used for calibration of both methods and vanadium was measured in the same type of real samples. LIBS calibration constructed using V-HMS-impregnated samples failed for measuring of V-HMS-synthesized samples. LIBS measurements seem to be strongly influenced with different chemical forms of vanadium in impregnated and synthesised samples. The combination of LIBS and ED XRF is able to provide new information about measured samples (in our case for example about procedure of catalyst preparation).     

Quantitative analysis of pharmaceutical products by laser-induced breakdown spectroscopy

In this paper, the capabilities of laser-induced breakdown spectroscopy (LIBS) for rapid analysis of multi-component pharmaceutical tablets are illustrated using several examples. The atomic line emission from an element present only in a particular component of the tablet (for instance, emission of phosphorus from the drug, or of magnesium from the lubricant) enables the quantitative analysis of that component. It is also demonstrated that simple schemes can significantly improve the analytical performance of LIBS in this context. In particular, internal standardization with a carbon line was found to enable the correction of a matrix effect, apart from improving the precision of measurement. Furthermore, an improvement in the linearity of calibration was observed when the plasma continuum emission was used as internal standard. Finally, in the case of drugs containing halogen species (e.g. F or Cl), producing the plasma in a helium atmosphere caused a seven to eight-fold increase of the signal-to-background ratio, thus improving sensitivity. These data illustrate the strengths of LIBS for fast at-line assessment of the reliability of pharmaceutical manufacturing processes.     

Arsenic speciation in marine organisms: from the analytical methodology to the constitution of reference materials

An analytical procedure for total arsenic and arsenic species quantification in marine organisms has been developed. Fresh materials are freeze-dried and reduced to powders before analysis. Arsenic is determined either by energy dispersive X-ray fluorescence (EDXRF) directly or by inductively coupled plasma optical emission spectrometry (ICP/OES) after microwave digestion. Arsenic speciation is performed on the extracted sample using liquid chromatography coupled to ICP/OES for arsenobetaine and arsenocholine determination and to the hydride generation-quartz furnace atomic absorption spectrometric technique for arsenite, arsenate, monomethylarsonic and dimethylarsinic acids quantification. Special precautions are taken to avoid losses or contaminations as well as to prevent analytical errors during the quantification stage. Other methods are applied and the corresponding results compared for each step of the procedure. The method is finally validated by means of intercomparison studies within the Measurements and Testing Programme of the European Community (formely BCR).     

Arsenic speciation in marine organisms: from the analytical methodology to the constitution of reference materials

An analytical procedure for total arsenic and arsenic species quantification in marine organisms has been developed. Fresh materials are freeze-dried and reduced to powders before analysis. Arsenic is determined either by energy dispersive X-ray fluorescence (EDXRF) directly or by inductively coupled plasma optical emission spectrometry (ICP/OES) after microwave digestion. Arsenic speciation is performed on the extracted sample using liquid chromatography coupled to ICP/OES for arsenobetaine and arsenocholine determination and to the hydride generation-quartz furnace atomic absorption spectrometric technique for arsenite, arsenate, monomethylarsonic and dimethylarsinic acids quantification. Special precautions are taken to avoid losses or contaminations as well as to prevent analytical errors during the quantification stage. Other methods are applied and the corresponding results compared for each step of the procedure. The method is finally validated by means of intercomparison studies within the Measurements and Testing Programme of the European Community (formely BCR).     

Calibration model of simultaneous multielement atomic-emission analysis using analytical line groups of each determined element

A calibration model of multielement methods for simultaneous determination of micro- and macro-concentrations of elements by computing the arc atomic-emission spectra has been developed. A calibration procedure for the analytical line group of the elements to be determined is offered. It allows the lower and upper limits of the concentration range for each line of the determined element to be calculated by means of the least-square method (LSM) and the Weibull distribution law is used to extend the concentration region. The calibration model was successfully tested for different arc optical emission spectroscopy (OES) methods. Received: 17 June 1997 / Revised: 3 November 1997 / Accepted: 7 November 1997