On-line laser-induced breakdown spectroscopy determination of magnesium coating thickness on electrolytically galvanized steel in motion

The application of laser-induced breakdown spectroscopy (LIBS) for online analysis of novel Zn based alloy coatings during continuous production of galvannealed steel has been demonstrated. Field trials were carried out at the ThyssenKrupp Steel (TKS) pilot plant in Dortmund, Germany. For this purpose, a portable LIBS demonstrator was constructed and evaluated, based on a dual-pulse Q-switched Nd:YAG laser, operated at 1064 nm. This system was used to generate plasmas on the moving sample surface after the annealing process, in order to control on-line the thickness of Mg on electrolytically galvanized steel. For variable Mg thicknesses (depending on strip speed of the pilot line, 100-1200 nm), and for steel sheets with a predetermined and constant Zn thickness (of 2 or 9 μm), a satisfactory agreement between plant LIBS measurements and data from laboratory chemical analysis (dissolution of the metallic coating and subsequent inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis) of Mg coating thicknesses has been obtained. The effects of environmental conditions on field measurements (strip temperature, mechanical vibrations, moisture on surface, etc.) have been demonstrated to be negligible, whereas minimal damage (crater diameters less than 150 μm) to the sample surface was caused.     

In situ semi-quantitative analysis of polluted soils by laser-induced breakdown spectroscopy (LIBS)

Time-saving, low-cost analyses of soil contamination are required to ensure fast and efficient pollution removal and remedial operations. In this work, laser-induced breakdown spectroscopy (LIBS) has been successfully applied to in situ analyses of polluted soils, providing direct semi-quantitative information about the extent of pollution. A field campaign has been carried out in Brittany (France) on a site presenting high levels of heavy metal concentrations. Results on iron as a major component as well as on lead and copper as minor components are reported. Soil samples were dried and prepared as pressed pellets to minimize the effects of moisture and density on the results. LIBS analyses were performed with a Nd:YAG laser operating at 1064 nm, 60 mJ per 10 ns pulse, at a repetition rate of 10 Hz with a diameter of 500 μm on the sample surface. Good correlations were obtained between the LIBS signals and the values of concentrations deduced from inductively coupled plasma atomic emission spectroscopy (ICP-AES). This result proves that LIBS is an efficient method for optimizing sampling operations. Indeed, "LIBS maps" were established directly on-site, providing valuable assistance in optimizing the selection of the most relevant samples for future expensive and time-consuming laboratory analysis and avoiding useless analyses of very similar samples. Finally, it is emphasized that in situ LIBS is not described here as an alternative quantitative analytical method to the usual laboratory measurements but simply as an efficient time-saving tool to optimize sampling operations and to drastically reduce the number of soil samples to be analyzed, thus reducing costs. The detection limits of 200 ppm for lead and 80 ppm for copper reported here are compatible with the thresholds of toxicity; thus, this in situ LIBS campaign was fully validated for these two elements. Consequently, further experiments are planned to extend this study to other chemical elements and other matrices of soils.     

Laser-induced breakdown spectroscopy (LIBS): a promising versatile chemical sensor technology for hazardous material detection

A series of laboratory experiments have been performed highlighting the potential of laser-induced breakdown spectroscopy (LIBS) as a versatile sensor for the detection of terrorist threats. LIBS has multiple attributes that provide the promise of unprecedented performance for hazardous material detection and identification. These include: 1) real-time analysis, 2) high sensitivity, 3) no sample preparation, and 4) the ability to detect all elements and virtually all hazards, both molecular and biological. We have used LIBS to interrogate a variety of different target samples, including explosives, chemical warfare simulants, biological agent simulants, and landmine casings. We have used the acquired spectra to demonstrate discrimination between different chemical warfare simulants, including those on soil backgrounds. A linear correlation technique permits discrimination between an anthrax surrogate and several other biomaterials such as molds and pollens. We also use broadband LIBS to identify landmine casings versus other plastics and environmental clutter materials. A new man-portable LIBS system developed as a collaborative effort between the U.S. Army Research Laboratory and Ocean Optics, Inc., is described and several other schemes for implementing LIBS sensors for homeland security and force protection are discussed.     

Multielemental chemical imaging using laser-induced breakdown spectrometry

Multichannel laser-induced breakdown spectrometry (LIBS) is used to generate selective chemical images for silver, titanium, and carbon from silicon photovoltaic cells. A 2.5 mJ pulsed nitrogen laser and a spectrometer using charge-coupled device detection were employed. LIBS images were acquired sequentially by moving the sample located on a motorized x-y translational stage step by step while storing the multichannel LIBS spectrum for each position of the sample, followed by computer-based reconstruction of two-dimensional selective images from intensity profiles at several wavelengths. Depth distributions of carbon impurities are also reported. Room temperature and atmospheric pressure operation as used here remove the restrictions on sample size exhibited by other surface analysis techniques used for imaging applications. Thus, the sample size in LIBS imaging is in principle unlimited. A LIBS experiment does not require a sample to be conductive. Therefore, virtually all materials can be imaged. Although LIBS is a typical example of destructive analytical technique, multichannel detection as demonstrated here confers the possibility to LIBS of obtaining multielement information from a given surface area. Lateral resolution of 80 μm and depth resolution of better than 13 nm were observed. The ultimate limitation to imaging the first layer of the surface in LIBS is the spectral signal-to-noise ratio as dictated by the ablation threshold of the material.     

Effective laser-induced breakdown spectroscopy (LIBS) detection using double pulse at optimum configuration

A short laser pulse is irradiated on a sample to create a highly energetic plasma that emits light of a specific peak wavelength according to the material. By identifying different peaks for the analyzed samples, their chemical composition can be rapidly determined. The characteristics of the laser-induced breakdown spectroscopy (LIBS) plasma are strongly dependent on the ambient conditions. Research aimed at enhancing LIBS intensity is of great benefit in advancing LIBS for the exploration of harsh environments. By using double-pulse LIBS, the signal intensity of Al and Ca lines was enhanced by five times compared to the single-pulse signal. Also, the angles of the target and detector are adjusted to simulate samples of arbitrary shape. We verified that there exists an optimal angle at which specific elements of a test sample may be detected with stronger signal intensity. We provide several optimum configurations for the LIBS system for maximizing the signal intensity for the analysis of a nonstandard aluminum sample.     

Spectrochemical microanalysis of aluminum alloys by laser-induced breakdown spectroscopy: identification of precipitates

Multielemental microanalysis of commercially available aluminum alloys has been performed in air by laser-induced breakdown spectroscopy (LIBS) by use of UV laser pulses with energies below 10 microJ. It is shown that the LIBS technique is capable of detecting the elemental composition of particles less than 10 microm in size, such as precipitates in an aluminum alloy matrix, by using single laser shots. Chemical mapping with a lateral resolution of approximately 10 microm of the distribution of precipitates in the surface plane of a sample was also carried out. Two main types of precipitate, namely, Mn-Fe-Cu (type I) and Mg-Cu (type II), were unambiguously distinguished in our LIBS experiments, in good agreement with x-ray microanalysis measurements. The relative standard deviations of emission of the main minor constituent elements (Cu, Mg, Mn) of the aluminum 2024 alloy range from 33% to 39% when laser shots on the precipitates are included in the analysis but decrease to a range from 5.3% to 7.4% when laser shots are taken only on the matrix material, excluding the precipitates.     

Carbon analysis for inspecting carbonation of concrete using a TEA CO2 laser-induced plasma

It has been demonstrated that a spectrochemical analysis of carbon using the laser plasma method can be successfully applied to inspect the carbonation of concrete by detecting carbon produced in aged concrete by a chemical reaction of Ca(OH)2 with CO2 gas in environmental air, turning into CaCO3, which induces degradation of the quality of building concrete. A comparative study has been made using a TEA CO2 laser (500-1000 mJ) and a Q-switched Nd-YAG laser (50-200 mJ) to search for the optimum conditions for carbon analysis, proving the advantage of the TEA CO2 laser for this purpose. Also, it was clarified that laser irradiation with suitable defocusing conditions is a crucial point for obtaining high sensitivity in the detection of carbon. Practical experiments on the inspection of carbonation were carried out using both a concrete sample that had been intentionally carbonated by exposure to high concentrations of CO2 gas and a naturally carbonated concrete sample. As a result, good coincidence was observed between the laser method and the ordinary method, which uses the chemical indicator phenolphthalein, implying that this laser technique is applicable as an in situ quantitative method of inspection for carbonation of concrete.     

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields

The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.     

Deep ablation and depth profiling by laser-induced breakdown spectroscopy (LIBS) employing multi-pulse laser excitation: application to galvanized steel

The potential of a multi-pulse (MP) laser excitation scheme for deep stratigraphy of electrolytically galvanized steel using laser-induced breakdown spectrometry (LIBS) has been evaluated. For this purpose, a commercial electro-optically (EO) Q-switched Nd:YAG laser was employed, where by reducing the delay between the Q-switch opening and the flash lamp, a train of pulses (up to 11) separated by approximately 7.40 μs was generated during one lamp flashing. Plasma emission after each individual laser pulse of the MP sequence was detected by a spectrograph equipped with an intensified charge-coupled device (iCCD) detector. With MP excitation, the ablation efficiency was increased ten-fold on iron sample and 22.5-fold on zinc material with respect to dual-pulse or single-pulse excitation. The LIBS signal generated by MP excitation shows an analogous enhancement. Although the total energy per shot delivered to samples was only 60 mJ, it was possible using LIBS to measure the sample stratigraphy up to depths of 90 μm on zinc-coated steel sheets. A satisfactory agreement between the Zn thickness determined by the MP-LIBS system and data from the manufacturer has also been obtained.     

Hydrogen leak detection using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is investigated as a technique for real-time monitoring of hydrogen gas. Two methodologies were examined: The use of a 100 mJ laser pulse to create a laser-induced breakdown directly in a sample gas stream, and the use of a 55 mJ laser pulse to create a laser-induced plasma on a solid substrate surface, with the expanding plasma sampling the gas stream. Various metals were analyzed as candidate substrate surfaces, including aluminum, copper, molybdenum, stainless steel, titanium, and tungsten. Stainless steel was selected, and a detailed analysis of hydrogen detection in binary mixtures of nitrogen and hydrogen at atmospheric pressure was performed. Both the gaseous plasma and the plasma initiated on the stainless steel surface generated comparable hydrogen emission signals, using the 656.28 Halpha emission line, and exhibited excellent signal linearity. The limit of detection is about 20 ppm (mass) as determined for both methodologies, with the solid-initiated plasma yielding a slightly better value. Overall, LIBS is concluded to be a viable candidate for hydrogen sensing, offering a combination of high sensitivity with a technique that is well suited to implementation in field environments.     

Standoff detection of chemical and biological threats using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time chemical and biological warfare agent detection in the field. We have demonstrated the detection and discrimination of the biological warfare agent surrogates Bacillus subtilis (BG) (2% false negatives, 0% false positives) and ovalbumin (0% false negatives, 1% false positives) at 20 meters using standoff laser-induced breakdown spectroscopy (ST-LIBS) and linear correlation. Unknown interferent samples (not included in the model), samples on different substrates, and mixtures of BG and Arizona road dust have been classified with reasonable success using partial least squares discriminant analysis (PLS-DA). A few of the samples tested such as the soot (not included in the model) and the 25% BG:75% dust mixture resulted in a significant number of false positives or false negatives, respectively. Our preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold. In addition, we have demonstrated that LIBS can distinguish five different organophosphate nerve agent simulants at 20 meters, despite their similar stoichiometric formulas. Finally, a combined PLS-DA model for chemical, biological, and explosives detection using a single ST-LIBS sensor has been developed in order to demonstrate the potential of standoff LIBS for universal hazardous materials detection.     

Unburned carbon measurement in fly ash using laser-induced breakdown spectroscopy with short nanosecond pulse width laser

The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N₂ and CO₂, while the CO₂ concentration varified to evaluate the CO₂ effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6?ns and 1?ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6?ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1?ns pulse width laser, and spectra had more stable background. By using 1?ns pulse width laser, the emission signal from surrounding CO₂ also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1?ns pulse width laser. It is demonstrated that 1?ns pulse width laser has the merits for fly ash flow measurement using LIBS.     

Laser-induced breakdown spectroscopy of bulk aqueous solutions at oceanic pressures: evaluation of key measurement parameters

The development of in situ chemical sensors is critical for present-day expeditionary oceanography and the new mode of ocean observing systems that we are entering. New sensors take a significant amount of time to develop; therefore, validation of techniques in the laboratory for use in the ocean environment is necessary. Laser-induced breakdown spectroscopy (LIBS) is a promising in situ technique for oceanography. Laboratory investigations on the feasibility of using LIBS to detect analytes in bulk liquids at oceanic pressures were carried out. LIBS was successfully used to detect dissolved Na, Mn, Ca, K, and Li at pressures up to 2.76 x 10(7) Pa. The effects of pressure, laser-pulse energy, interpulse delay, gate delay, temperature, and NaCl concentration on the LIBS signal were examined. An optimal range of laser-pulse energies was found to exist for analyte detection in bulk aqueous solutions at both low and high pressures. No pressure effect was seen on the emission intensity for Ca and Na, and an increase in emission intensity with increased pressure was seen for Mn. Using the dual-pulse technique for several analytes, a very short interpulse delay resulted in the greatest emission intensity. The presence of NaCl enhanced the emission intensity for Ca, but had no effect on peak intensity of Mn or K. Overall, increased pressure, the addition of NaCl to a solution, and temperature did not inhibit detection of analytes in solution and sometimes even enhanced the ability to detect the analytes. The results suggest that LIBS is a viable chemical sensing method for in situ analyte detection in high-pressure environments such as the deep ocean.     

Influence of Er:YAG and Nd:YAG wavelengths on laser-induced breakdown spectroscopy measurements under air or helium atmosphere

Laser-induced breakdown spectroscopy (LIBS) is widely dependent on the conditions of its implementation in terms of laser characteristics (wavelength, energy, and pulse duration), focusing conditions, and surrounding gas. In this study two wavelengths, 1.06 and 2.94 microm, obtained with Nd:YAG and Er:YAG lasers, respectively, were used for LIBS analysis of aluminum alloy samples in two conditions of surrounding gas. The influence of the laser wavelength on the laser-produced plasma was studied for the same irradiance by use of air or helium as a buffer gas at atmospheric pressure. We used measurements of light emission to determine the temporally resolved space-averaged electron density and plasma temperature in the laser-induced plasma. We also examined the effect of laser wavelength in two different ambient conditions in terms of spectrochemical analysis by LIBS. The results indicate that the effect of the surrounding gas depends on the laser wavelength and the use of an Er:YAG laser could increase linearity by limiting the leveling in the calibration curve for some elements in aluminum alloys. There is also a significant difference between the plasma induced by the two lasers in terms of electron density and plasma temperature.     

Development of size-selective sampling of Bacillus anthracis surrogate spores from simulated building air intake mixtures for analysis via laser-induced breakdown spectroscopy

Size-selective sampling of Bacillus anthracis surrogate spores from realistic, common aerosol mixtures was developed for analysis by laser-induced breakdown spectroscopy (LIBS). A two-stage impactor was found to be the preferential sampling technique for LIBS analysis because it was able to concentrate the spores in the mixtures while decreasing the collection of potentially interfering aerosols. Three common spore/aerosol scenarios were evaluated, diesel truck exhaust (to simulate a truck running outside of a building air intake), urban outdoor aerosol (to simulate common building air), and finally a protein aerosol (to simulate either an agent mixture (ricin/anthrax) or a contaminated anthrax sample). Two statistical methods, linear correlation and principal component analysis, were assessed for differentiation of surrogate spore spectra from other common aerosols. Criteria for determining percentages of false positives and false negatives via correlation analysis were evaluated. A single laser shot analysis of approximately 4 percent of the spores in a mixture of 0.75 m(3) urban outdoor air doped with approximately 1.1 x 10(5) spores resulted in a 0.04 proportion of false negatives. For that same sample volume of urban air without spores, the proportion of false positives was 0.08.     

Étude expérimentale d'une facade réalisée en panneaux porteurs préfabriqués

Résumé Cette étude donne la distribution et la valeur des contraintes dans un panneau porteur préfabriqué et sur la fa?ade réalisée par assemblage de ces panneaux. Le comportement mécanique d'un élément est semblable à celui d'une plaque perforée chargée dans son plan moyen. Les études comparées des différentes fa?ades obtenues en faisant varier les liaisons verticales entre panneaux montrent que la solution rendant possible la préfabrication avec joints verticaux libres est une solution cohérente puisqu'elle permet une meilleure utilisation du matériau béton qui est comprimé au lieu d'être fléchi. La technique expérimentale utilisée est la photoélasticimétrie sur maquette.

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Summary This study gives the distribution and the value of the stresses in a prefabricated bearing panel and on the fa?ade made by joining these panels. Apart from the dimensioning of these elements the problem to be solved is the determination of the impact of a vertical mechanical link between these. The mechanical behaviour of a panel is similar to that of a perforated plate loaded in its average plane. Curves give the main results in terms of a uniform mean stress. The comparative studies of the different fa?ades obtained by having the vertical links between panels vary show that the solution whereby prefabrication with free vertical joints is made possible is a coherent solution, as it allows a better use of the concrete materials which is compressed instead of being bent. The experimental technique used is photoelasticimetry on model which seems at the present time to be the only means of investigation for problems of this size in view of the relatively limited capacities of electronic calculators. The precision of this technique is of the order of approximately 5 to 6%. The technological details of manufacture and experimentation with the models are indicated, in particular the models are obtained by pouring araldite D in a cold state into a rubber mold.

     

Detection of lead in water using laser-induced breakdown spectroscopy and laser-induced fluorescence

Laser-induced breakdown spectroscopy (LIBS) is a well-known technique for fast, stand-off, and nondestructive analysis of the elemental composition of a sample. We have been investigating micro-LIBS for the past few years and demonstrating its application to microanalysis of surfaces. Recently, we have integrated micro-LIBS with laser-induced fluorescence (LIF), and this combination, laser ablation laser-induced fluorescence (LA-LIF), allows one to achieve much higher sensitivity than traditional LIBS. In this study, we use a 170 microJ laser pulse to ablate a liquid sample in order to measure the lead content. The plasma created was re-excited by a 10 microJ laser pulse tuned to one of the lead resonant lines. Upon optimization, the 3sigma limit of detection was found to be 35 +/- 7 ppb, which is close to the EPA standard for the level of lead allowed in drinking water.     

Fiber-optic laser sensor for mine detection and verification

What we believe to be a new optical approach for the identification of mines and explosives by analyzing the surface materials and not only bulk is developed. A conventional manually operated mine prodder is upgraded by laser-induced breakdown spectroscopy (LIBS). In situ and real-time information of materials that are in front of the prodder are obtained during the demining process in order to optimize the security aspects and the speed of demining. A Cr4+:Nd3+:YAG microchip laser is used as a seed laser for an ytterbium-fiber amplifier to generate high-power laser pulses at 1064 nm with pulse powers up to E(p) = 1 mJ, a repetition rate of f(rep.) = 2-20 kHz and a pulse duration of t(p) = 620 ps. The recorded LIBS signals are analyzed by applying neural networks for the data analysis.     

Sample presentation considerations in laser-induced breakdown spectroscopy in aqueous solution

Various sample presentation configurations for elemental analysis in aqueous media by laser-induced breakdown spectroscopy (LIBS) have been tested and analyzed. Direct and quantitative comparison between the two different sample presentation methods, plasma excitation within water bulk and on the surface in a water jet, has been carried out using the same LIBS system under the same experimental conditions. Temporal characteristics of light emitted from the plasma induced in both the water bulk and the jet surface containing calcium (Ca) were recorded and presented. Spectral data recorded under optimum detection gating conditions showed that the signal-to-noise ratio (S/N) for excitation in the water jet configuration is approximately 10 times higher than that in the bulk excitation, the actual values of enhancement being dependent on the element type. The typical spectra of aqueous samples containing sodium (Na), calcium (Ca), zinc (Zn), cadmium (Cd), and mercury (Hg) were detected and the signal-to-noise ratios were evaluated and compared for the sample presentation configurations under considerations. The results suggest that for better sensitivity of detection, a simple water jet sample presentation configuration could be designed and implemented for cost-effective commercial use of this technique for elemental analysis in a water environment.     

Scanning strategies and spacing effect on laser fusion of H13 tool steel powder using high power Nd:YAG pulsed laser

Layered manufacturing technologies have been used to produce complex parts of diversified materials through different physical/chemical manufacturing principles. Nevertheless only a few materials are commercially available to build parts suitable for engineering applications. In this paper, the powder fusion of H13 tool steel is investigated. A high power Nd:YAG pulsed laser source on a CNC machine was used to fuse the powder, layer by layer, building solid cubes for further analysis. Four different laser vector scanning strategies were evaluated by comparing the results of porosity and layer distortion. The complexity of the laser/powder interaction shows that a complex strategy must be used to avoid porosity and distortion.