Laser induced breakdown spectroscopy for bulk minerals online analyses

The purpose of the work was to prove the ability of LIBS to provide on-line analyses for raw ores in field conditions. An industrial LIBS machine was developed and successfully tested for on-belt evaluation of phosphate measuring Mg, Fe, Al, Bone Phosphate Lime (BPL), Insoluble phase and Metal Impurity Ratio (MER) and of coal measuring its ash content. The comparison of LIBS on-line data with control analyses revealed good correlation, which corresponds to the required detection limits and accuracy. With frequent elemental data from a LIBS system, process engineers have the tools to best optimize the process. These processes could be minerals blending and separation to meet customer specifications, monitoring and controlling the efficiency of a minerals process, or a minerals accounting function.     

Laser induced breakdown spectroscopy on soils and rocks: Influence of the sample temperature,moisture and roughness

ExoMars, ESA's next mission to Mars, will include a combined Raman/LIBS instrument for the comprehensive in-situ mineralogical and elemental analyses of Martian rocks and soils. It is inferred that water exists in the upper Martian surface as ice layers, “crystal” water or adsorbed pore water. Thus, we studied Laser Induced Breakdown Spectroscopy (LIBS) on wet and dry rocks under Martian environmental conditions in the temperature range − 60 °C to + 20 °C and in two pressure regimes, above and below the water triple point. Above this point, the LIBS signals from the rock forming elements have local minima that are accompanied by hydrogen (water) emission maxima at certain temperatures that we associate with phase transitions of free or confined water/ice. At these sample temperatures, the plasma electron density and its temperature are slightly lowered. In contrast to powder samples, a general increase of the electron density upon cooling was observed on rock samples. By comparing the LIBS signal behavior from the same rock with different grades of polishing, and different rocks with the same surface treatment, it was possible to distinguish between the influence of surface roughness and the bulk material structure (pores and grains). Below the triple point of water, the LIBS signal from the major sample elements is almost independent of the sample temperature. However, at both considered pressures we observed a hydrogen emission peak close to − 50 °C, which is attributed to a phase transition of supercooled water trapped inside bulk pores.     

Spark discharge assisted laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy is combined with a spark discharge to operate in a laser triggered spark discharge mode. This spark discharge laser induced breakdown spectroscopy (SD-LIBS) is evaluated for Al and Cu targets in air under atmospheric pressure. Significant enhancement in the measured line intensities and the signal-to-background ratios, which depend on the spark discharge voltage and the laser fluence, is observed in spark discharge laser induced breakdown spectroscopy when compared to laser induced breakdown spectroscopy alone for similar laser conditions. The measured line intensities increase with the applied voltage for both targets, and the ratio of the measured line intensity using spark discharge laser induced breakdown spectroscopy to that using laser induced breakdown spectroscopy is found to increase as the laser fluence is decreased. For Al II 358.56, such intensity enhancement ratio increases from 50 to 400 as the laser fluence is decreased from 48 to 4 J/cm² at an applied voltage of 3.5 kV. Thus, spark discharge laser induced breakdown spectroscopy allows for using laser pulses with relatively low energy to ablate the studied material, causing less ablation, and hence less damage to its surface. Moreover, applying spark discharge laser induced breakdown spectroscopy gives up to 6-fold enhancement in the S / B ratio, compared to those obtained with laser induced breakdown spectroscopy for the investigated spectral emission lines.     

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.     

Analysis of fresco by laser induced breakdown spectroscopy

The laser-based techniques have been shown to be a very powerful tool for artworks characterization and are used in the field of cultural heritage for the offered advantages of minimum invasiveness, in situ applicability and high sensitivity. Laser induced breakdown spectroscopy, in particular, has been applied in this field to many different kinds of ancient materials with successful results. In this work, a fragment of a Roman wall painting from the archaeological area of Pompeii has been investigated by LIBS. The sample elemental composition resulting from LIBS measurements suggested the presence of certain pigments. The ratio of the intensities of different lines related to some characteristic elements is proposed as an indicator for pigment recognition.     

Utilization of laser induced breakdown spectroscopy for investigation of the metal accumulation in vegetal tissues

We report on the development and implementation of analytical methodology for investigating elemental accumulation in different layers within plant leaves, with in-situ spatial resolution mapping, exploiting the technique of LIBS. The spectrochemical analysis of lead-doped leaf samples is demonstrated to develop a real time identification procedure in order to complement other analytical techniques not lending themselves for spatial resolution analysis. Our findings suggest that with elevated levels of Pb within the plants transportation and storage of some nutrition elements is changed.     

Laser induced breakdown spectroscopy of soils, rocks and ice at subzero temperatures in simulated martian conditions

We applied Laser Induced Breakdown Spectroscopy (LIBS) on moist soil/rock samples in simulated Martian conditions. The signal behavior as a function of the surface temperature in the range from + 25 °C to − 60 °C was studied at pressure of 7 mbar. We observed the strong signal oscillations below 0 °C with different negative peaks, whose position, width and magnitude depend on the surface roughness. In some cases, the signal was reduced for one order of magnitude with consequences for the LIBS analytical capability. We attribute such a signal behavior to the presence of supercooled water inside the surface pores, which freezing point depends on the pore size. On a same rock samples with different grades of the surface polishing, the signal has different temperature dependence. Its decrease was always registered close to 0 °C, corresponding to the freezing/melting of normal disordered ice, which can be present inside larger pores and scratching. An amount of the signal reduction at the phase transition temperatures does not seem to change with the laser energy density in the examined range. Comparative measurements were performed on a frozen water solution. A large depression, for two orders of magnitude, of the LIBS intensity was observed close to − 50 °C. The same negative peak, but with a smaller magnitude, was also registered on some rock/soil samples. Ablation rates and plasma parameters as a function of the sample temperature are also discussed, and their consequences for in-situ analyses.     

Ultrafast laser induced breakdown spectroscopy for high spatial resolution chemical analysis

Femtosecond laser induced breakdown spectroscopy (LIBS) was used to identify the spatial resolution limitations and assess the minimal detectable mass restrictions in laser-ablation based chemical analysis. The atomic emission of sodium (Na) and potassium (K) dopants in transparent dielectric Mica matrices was studied, to find that both these elements could be detected from 450 nm diameter ablation craters, full-width-at-half-maximum (FWHM). Under optimal conditions, mass as low as 220 ag was measured, demonstrating the feasibility of using laser-ablation based chemical analysis to achieve high spatial resolution elemental analysis in real-time and at atmospheric pressure conditions.     

Forensic comparative glass analysis by laser-induced breakdown spectroscopy

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

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.     

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.     

Laser-induced breakdown spectroscopy applied on low-alloyed zinc samples

The use of laser-induced breakdown spectroscopy for chemical analysis in the single-shot regime has been investigated for low-alloyed zinc samples. Several parameters that are important for plasma formation has been investigated and their importance for accurate and precise single-shot measurements are discussed. The standard deviation is compared for measurements performed on a day-to-day basis during four days and for a measurement series consisting of five measurements in one day. It was found that the spread is not larger for a measurement series performed on a single day compared to day-to-day basis. The influence of local spatial inhomogeneities of the alloy elements in the sample concentrations is discussed in this context and the reference samples have been investigated with a scanning electron microscopy and light optical microscopy to verify this. It is found that the relative standard deviation of the signal depends to large extent on the sample homogeneity at low concentrations. The importance of spatial averaging for LIBS when doing calibrations is established in this case. The relative error for single-shot measurements will depend on the slope of the analytical curve and increase at lower concentrations.     

Analysis of explosive and other organic residues by laser induced breakdown spectroscopy

With the aim of realizing a compact instrument for detection of energetic materials at trace levels, laser induced breakdown spectroscopy was applied on residues from nine explosives in air surroundings. Different potentially interfering organic materials were also analyzed. The residues were not uniformly distributed on an aluminum support and single-shot discrimination was attempted. For a single residue type, large shot-to-shot fluctuations of the line intensity ratios characteristic for organic samples were observed, which made material classification difficult. It was found that both atomic and molecular emission intensities, as well as their ratios, are strongly affected by an amount of the ablated support material, which mainly determines the plasma temperature. With respect to the spectra from the clean support, emission intensities of atomic oxygen and nitrogen are always reduced in the presence of an organic material, even if its molecules contain these elements. This was attributed to chemical reactions in a plasma containing carbon or its fragments. Hydrogen atomic emission depends strongly on the local humidity above the sampled point and its line intensity shows shot to shot variations up to 50%, also on a homogeneous sample. It is argued that shock waves generated by previous spatially and/or temporally close laser pulses blow away a relatively heavy water aerosol, which later diffuses slowly back towards the sampled point. C₂ and CN exhibit a peak emission behavior with atomic Al emission, and their variable ratio indicates an existence of different formation or removal mechanisms from the plasma, depending on the plasma parameters and on the composition of the organic residue. On the basis of these observations, an attempt is made to establish a suitable procedure for data analysis and to determine the optimal experimental conditions, which would allow for discrimination of explosives from other, potentially interfering, residues.     

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.     

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.     

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.     

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.