Combined remote LIBS and Raman spectroscopy at 8.6m of sulfur-containing minerals, and minerals coated with hematite or covered with basaltic dust

Combined remote laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy investigations at a distance of 8.6m have been carried out in air and under a simulated Martian atmosphere of 933Pa (7Torr) CO(2) on calcite (CaCO(3)), gypsum (CaSO(4).2H(2)O), and elemental sulfur (S), and LIBS investigations on chalcopyrite (CuFeS(2)) and pyrite (FeS(2)). Both Raman and LIBS techniques have also been used sequentially in air on hematite-coated calcite crystals and on a sample of anhydrite covered with basaltic dust. These experiments demonstrate that by using a frequency-doubled Nd:YAG pulsed laser co-radiating 1064 nm and 532 nm laser beams with a 5x beam expander, it is possible to measure simultaneously both the Raman and LIBS spectra of calcite, gypsum and elemental sulfur by adjusting the laser power electronically. The spectra of calcite, gypsum, and elemental sulfur contain fingerprint Raman lines; however, it was not possible to measure the remote Raman spectra of pyrite and chalcopyrite because of low intensities of Raman lines. In the cases of CuFeS(2), FeS(2), and elemental sulfur, S atomic emission lines in the LIBS spectra were detected only in 7Torr of CO(2) pressure and not in air. No S atomic emission lines were detected for gypsum in air or in CO(2). In the case of coated/dusted minerals, it was possible to remove the coating or dust with the focused LIBS laser and measure the Raman spectra of subsurface minerals with a 532 nm laser excitation. The complementary nature of these two techniques is highlighted and discussed.     

Study of sub-mJ-excited laser-induced plasma combined with Raman spectroscopy under Mars atmosphere-simulated conditions

Laser-Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are complimentary techniques. LIBS yields elemental information while Raman spectroscopy yields molecular information about a sample, and both share similar instrumentation configurations. The combination of LIBS and Raman spectroscopy in a single instrument for planetary surface exploration has been proposed, however challenges exist for developing a combined instrument. We present LIBS and Raman spectroscopy results obtained using a diode pumped, intracavity doubled, Q-switched, Nd:YLF laser operating at 523 nm, which overcomes some of the difficulties associated with a combined instrument. LIBS spectra were obtained with 170 μJ per pulse at 4 Hz repetition rate in a low pressure Mars-simulated atmosphere and Raman spectra produced with 200 mW at 100 kHz. The Nd:YLF laser is switchable between LIBS and Raman spectroscopy modes only by a change in Q-switch repetition rate. Emissions from Ca, Ca II, Fe, Fe II, Mg, Na, and O atom were identified in the μ-LIBS spectrum of oolithic hematite. Evidence was found for a change in plasma dynamics between 7 and 5 Torr that could be explained as a decrease in plasma temperature and electron density below 5 Torr. This is relevant to future Mars exploration using LIBS as the mean surface pressure on Mars varies from 3.75 to 6 Torr. LIBS plasma dynamics should be carefully evaluated at the pressures that will be encountered at the specific Mars landing site.     

Combined Raman spectrometer/laser-induced breakdown spectrometer for the next ESA mission to Mars

Among the different instruments that have been pre-selected to be on-board the Pasteur payload on ExoMars is the Raman/laser induced breakdown spectroscopy (LIBS) instrument. Raman spectroscopy and LIBS will be integrated into a single instrument sharing many hardware commonalities. An international team under the lead of TNO has been gathered to produce a design concept for a combined Raman spectrometer/LIBS elegant bread-board (EBB). The instrument is based on a specially designed, extremely compact, spectrometer with high resolution over a large wavelength range, suitable for both Raman spectroscopy and LIBS measurements. Low mass, size and power consumption are the main drivers of the instrument's design concept. In this paper, science objectives for the combined instrument are detailed. Background information on Raman spectroscopy and LIBS are presented, focussing on the synergy of these two techniques. In the last section, the instrument concept resulting from the assessment of the feasibility of the combined Raman/LIBS EBB is presented.     

Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS

The authenticity of objects and artifacts is often the focus of forensic analytic chemistry. In document fraud cases, the most important objective is to determine the origin of a particular ink. Here, we introduce a new approach which utilizes the combination of two analytical methods, namely Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS). The methods provide complementary information on both molecular and elemental composition of samples. The potential of this hyphenation of spectroscopic methods is demonstrated for ten blue and black ink samples on white paper. LIBS and Raman spectra from different inks were fused into a single data matrix, and the number of different groups of inks was determined through multivariate analysis, i.e., principal component analysis, soft independent modelling of class analogy, partial least-squares discriminant analysis, and support vector machine. In all cases, the results obtained with the combined LIBS and Raman spectra were found to be superior to those obtained with the individual Raman or LIBS data sets.     

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.     

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).     

基于多光谱特征融合技术的面粉掺杂定量分析方法

提出了一种基于拉曼光谱技术(Raman)和激光诱导击穿光谱技术(LIBS)的多光谱特征融合技术(MFFT),利用拉曼光谱中分子组分信息和激光诱导击穿光谱中原子组分信息之间的互补特性,采用自适应小波变换(AWT)-竞争性自适应加权(CARS)-偏最小二乘回归(PLS)建模技术,获取了面粉体系更为全面的特征信息。在多光谱特征融合技术中,首先采用AWT-CARS方法分别提取拉曼光谱和激光诱导击穿光谱中的特征变量,然后将两者的特征变量融合为一个向量,采用PLS方法构建MFFT模型,实现了面粉掺杂物的定量分析。通过对二氧化钛、硫酸铝钾等面粉掺杂体系建模分析,考察MFFT模型的有效性。结果表明,与单一拉曼光谱技术或激光诱导击穿光谱技术建立的预测模型相比,MFFT模型显著提升了模型的预测性能,二氧化钛和硫酸铝钾预测模型的线性相关系数分别从相对较差的Raman模型的0.884、0.877提升到0.981、0.980,其预测均方根误差分别从相对较差的Raman模型的0.151、0.154降低到0.069、0.068。表明多光谱特征融合技术可以准确提取Raman光谱中的分子信息和LIBS光谱中的元素信息,使其互为补充、互为校正,进而有效克服面粉基质对掺杂组分定量分析的干扰,显著提高模型的预测精度。     

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.     

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.     

Calibration Measurements in laser-induced breakdown spectroscopy using nanosecond and picosecond lasers

The influence of laser pulse duration on laser-induced breakdown spectroscopy (LIBS) calibration curves is investigated in the present work. Two Nd:YAG lasers providing pulses of 35 ps and 5 ns, respectively, both operating at 1064 nm, have been used to create plasmas on aluminium, manganese, iron, and silicon targets and on prepared stoichiometric samples of these metals in a matrix. The time-resolved, space-averaged plasma temperatures have been deduced using Boltzmann plots, while the electron number density has been determined from the broadening of spectral lines. The effect of laser pulse duration on the plasma characteristics is discussed, and comparisons are made with previously reported data measured under similar experimental conditions. The optimum experimental conditions (i.e., time delay, gate width, laser energy) have been determined for reliable use of LIBS for quantitative analysis for both pulse durations. For each of the metals of interest, calibration curves have been constructed for concentrations ranging up to 2%.     

On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy

The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode microspectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.     

The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.     

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.     

Double-pulse laser-induced breakdown spectroscopy analysis of scales from petroleum pipelines

Pipeline scales from the Campos Bay Petroleum Field near Rio de Janeiro, Brazil have been analyzed by both Raman spectroscopy and by laser-induced breakdown spectroscopy (LIBS) using a double-pulse, calibration-free approach. Elements that are characteristic of petroleum (e.g. C, H, N, O, Mg, Na, Fe and V) were detected, in addition to the Ca, Al, and Si which form the matrix of the scale. The LIBS results were compared with the results of micro-Raman spectroscopy, which confirmed the nature of the incrustations inferred by the LIBS analysis. Results of this preliminary study suggest that diffusion of pipe material into the pipeline intake column plays an important role in the growth of scale. Thanks to the simplicity and relative low cost of equipment and to the fact that no special chemical pre-treatment of the samples is needed, LIBS can offer very fast acquisition of data and the possibility of in situ measurements. LIBS could thus represent an alternative or complementary method for the chemical characterization of the scales by comparison to conventional analytical techniques, such as X-ray diffraction or X-ray fluorescence.     

Influence of pulse-to-pulse delay for 532 nm double-pulse laser-induced breakdown spectroscopy of technical polymers

Laser induced breakdown spectroscopy (LIBS) is an emerging technique for fast and accurate compositional analysis of many different materials. We present a systematic study of collinear double-pulse LIBS on different technical polymers such as polyamide, polyvinyl chloride, polyethylene etc. Polymer samples were ablated in air by single-pulse and double-pulse Nd:YAG laser radiation (8 ns pulse duration) and spectra were recorded with an Echelle spectrometer equipped with an ICCD camera. We investigated the evolution of atomic and ionic line emission intensities for different delay times between the laser pulses (from 20 ns to 500 μs) at a laser wavelength of 532 nm. We observed double-pulse LIBS signals that were enhanced as compared to single-pulse measurements depending on the delay time and the type of polymer material investigated. LIBS signals of polymer materials that are enhanced by double-pulse excitation may be useful for monitoring the concentration of heavy metals in polymer materials.     

Comparison of laser induced breakdown spectroscopy and spark induced breakdown spectroscopy for determination of mercury in soils

Mercury is a toxic element found throughout the environment. Elevated concentrations of mercury in soils are quite hazardous to plants growing in these soils and also the runoff of soils to nearby water bodies contaminates the water, endangering the flora and fauna of that region. This makes continuous monitoring of mercury very essential. This work compares two potential spectroscopic methods (laser induced breakdown spectroscopy (LIBS) and spark induced breakdown spectroscopy (SIBS)) at their optimum experimental conditions for mercury monitoring. For LIBS, pellets were prepared from soil samples of known concentration for generating a calibration curve while for SIBS, soil samples of known concentration were used in the powder form. The limits of detection (LODs) of Hg in soil were calculated from the Hg calibration curves. The LOD for mercury in soil calculated using LIBS and SIBS is 483 ppm and 20 ppm, respectively. The detection range for LIBS and SIBS is discussed.     

Laser-induced breakdown spectroscopy in water: Improvement of the detection threshold by signal processing

Laser-induced breakdown spectroscopy (LIBS) has been performed on immersed solid samples with different grades of surface roughness and material homogeneity and on bulk water solutions. The underwater plasma was produced by applying double-pulse excitation at 1064 nm, with different sets of laser pulse energies. LIBS spectra were recorded separately for each couple of laser pulses in order to monitor shot-to-shot plasma behavior and to apply signal post processing. The latter was aimed at improving the detection limits for elemental analyses. Except in the case of flat homogeneous solid samples at high laser pulse energies, the measurements were affected by strong shot-to-shot signal oscillations. Automatic elimination of low intensity spectra reduced the detection limit up to a factor of seven. The optimum level for spectral filtering depends strongly on sample properties. For bulk water, a poor correlation was observed between the peak line intensities and the plasma continuum emission, making the peak-to-background ratio unsuitable for internal standardization purposes. The analytical performance of LIBS for bulk liquid was also affected by the spatial fluctuations of the breakdown location, a phenomenon known as “moving breakdown” in the literature, which was responsible for the signal depletion in the detection region. In preliminary measurements on water solutions, the detection limit of 0.2 mg/l for magnesium has been obtained after applying data post processing.     

Comparative analysis of automotive paints by laser induced breakdown spectroscopy and nonparametric permutation tests

Laser-induced breakdown spectroscopy (LIBS) has been investigated for the discrimination of automobile paint samples. Paint samples from automobiles of different makes, models, and years were collected and separated into sets based on the color, presence or absence of effect pigments and the number of paint layers. Twelve LIBS spectra were obtained for each paint sample, each an average of a five single shot “drill down” spectra from consecutive laser ablations in the same spot on the sample. Analyses by a nonparametric permutation test and a parametric Wald test were performed to determine the extent of discrimination within each set of paint samples. The discrimination power and Type I error were assessed for each data analysis method. Conversion of the spectral intensity to a log-scale (base 10) resulted in a higher overall discrimination power while observing the same significance level. Working on the log-scale, the nonparametric permutation tests gave an overall 89.83% discrimination power with a size of Type I error being 4.44% at the nominal significance level of 5%. White paint samples, as a group, were the most difficult to differentiate with the power being only 86.56% followed by 95.83% for black paint samples. Parametric analysis of the data set produced lower discrimination (85.17%) with 3.33% Type I errors, which is not recommended for both theoretical and practical considerations. The nonparametric testing method is applicable across many analytical comparisons, with the specific application described here being the pairwise comparison of automotive paint samples.     

Detection of heavy metals in Arabian crude oil residue using laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) was applied for the elemental analysis of Arabian crude oil residue samples. The spectra due to trace elements such as Ca, Fe, Mg, Cu, Zn, Na, Ni, K and Mo were recorded using this technique. The dependence of time delay and laser beam energy on the elemental spectra was also investigated. Prior to quantitative analysis, the LIBS system was calibrated using standard samples containing these trace elements. The results achieved through this method were compared with conventional technique like inductively coupled plasma.     

Direct Determination of Aluminum in Archaeological Clays by Laser‐Induced Breakdown Spectroscopy

Abstract

Instrumental techniques that allow the direct analysis of solids with little or no sample preparation are particularly important for the evaluation of samples that are difficult to analyze such as refractory or geological materials. Laser‐induced breakdown spectroscopy (LIBS) is a promising technique for the direct, rapid analysis of elements in solid materials with minimal sample preparation. The main advantages over wet techniques are virtual nondestructiveness and analysis speed. The goal of this work is the direct determination of aluminum of archaeological pieces using laser‐induced breakdown spectroscopy. The corresponding signals of metals were interpolated from calibration graphs of different salts of the metals. The matrix effects from the direct determination of these elements were thoroughly investigated. The potential of this technique for direct quantitative analysis of real archaeological materials (from Department of Ancient Science, University of Zaragoza) was evaluated, and the reproducibility of LIBS spectra from different archaeological samples was measured as a function of the number of laser shots. Finally, the results from LIBS are compared with those obtained by laser ablation inductively coupled plasma mass spectrometry.