Sequential-pulse laser-induced breakdown spectroscopy of high-pressure bulk aqueous solutions

Sequential-pulse (or dual-pulse) laser-induced breakdown spectroscopy (DP-LIBS) with an orthogonal spark orientation is described for elemental analysis of bulk aqueous solutions at pressures up to approximately 138 x 10(5) Pa (138 bar). The use of sequential laser pulses for excitation, when compared to single-pulse LIBS excitation (SP-LIBS), provides significant emission intensity enhancements for a wide range of elements in bulk solution and allows additional elements to be measured using LIBS. Our current investigations of high-pressure solutions reveal that increasing solution pressure leads to a significant decrease in DP-LIBS emission enhancements for all elements examined, such that we see little or no emission enhancements for pressures above 100 bar. Observed pressure effects on DP-LIBS enhancements are thought to result from pressure effects on the laser-induced bubble formed by the first laser pulse. These results provide insight into the feasibility and limitations of DP-LIBS for in situ multi-elemental detection in high-pressure aqueous environments like the deep ocean.     

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.     

On-line detection of heavy metals and brominated flame retardants in technical polymers with laser-induced breakdown spectrometry

The use of laser-induced breakdown spectrometry (LIBS) for the analysis of heavy metals and brominated flame retardants in end-of-life waste electric and electronic equipment (EOL-WEEE) pieces is investigated. Single- and double-pulse plasma excitation as well as the influence of detection parameters is studied to yield a parameter field with improved sensitivity and limits of detection. A LIBS analyzer was set up as an on-line measuring unit to detect heavy metals and brominated flame retardants in moving EOL-WEEE pieces in an automatic sorting line. An autofocusing unit with an adjustment range of 50 mm was incorporated to permit measurements of objects that pass by a LIBS analyzer with their surfaces at various distances from it. Tests with EOL-WEEE monitor housings on the conveyor belt of a pilot sorting system successfully demonstrated the capability of the LIBS analyzer to quantify the concentration of hazardous elements in real waste EOL-WEEE pieces.     

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.     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

等离子体光栅与光丝非线性耦合诱导击穿光谱

激光诱导击穿光谱技术（laser-induced breakdown spectroscopy,LIBS）是一种分析样品元素信息的有效工具,具有快速、简便、实时的优点,可以对固体、液体和气体中多元素成分进行定性分析和定量检测。但传统的LIBS以及光丝诱导击穿光谱技术（filament-induced breakdown spectroscopy,FIBS）受限于峰值功率钳制,灵敏度难以提高,导致在实际应用中具有一定的局限性,成为LIBS技术发明以来一直面临的重大技术瓶颈。本文从激发源的角度出发,讨论了LIBS和FIBS出现的问题,介绍了近年来优化LIBS技术的研究进展,将多光束耦合形成的等离子体光栅应用于LIBS,在保留其原有优点的基础上,有效克服了等离子体屏蔽效应、基体效应,突破了功率钳制,增强了谱线信号,提高了探测灵敏度和定量分析能力。这些研究将推进LIBS技术在各领域的实际应用,同时为飞秒激光等离子体与其他技术的结合提供了研究思路。     

Liquid steel analysis with laser-induced breakdown spectrometry in the vacuum ultraviolet

Laser-induced breakdown spectrometry (LIBS) with multiple pulse excitation has been applied for the multielemental analysis of liquid steel. The laser beam and the measuring radiation are guided inside a moveable lance to gain access to the melt surface from the top. Low-alloy steel grades were investigated with a focus on the light elements phosphorus, sulfur, and carbon by use of emission wavelengths in the vacuum ultraviolet. Calibration curves were determined for the elements carbon, phosphorus, sulfur, nickel, and chromium in steel melts of 100 kg. The estimated limits of detection for the light elements phosphorus, sulfur, and carbon are below 21 microg/g for direct analysis of liquid steel with LIBS. The results demonstrate the potential of the laser-based analysis to fulfill the requirements for a process integrated on-line analysis in the secondary metallurgy of steel works.     

Quantitative analysis of liquids from aerosols and microdrops using laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) is shown to be capable of low volume (90 pL) quantitative elemental analysis of picogram amounts of dissolved metals in solutions. Single-pulse and collinear double-pulse LIBS were investigated using a 532 nm dual head laser coupled to a spectrometer with an intensified charge coupled device (CCD) detector. Aerosols were produced using a micronebulizer, conditioned inside a concentric spray chamber, and released through an injector tube with a diameter of 1 mm such that a LIBS plasma could be formed ~2 mm from the exit of the tube. The emissions from both the aerosols and a single microdrop were then collected with a broadband high resolution spectrometer. Multielement calibration solutions were prepared, and continuing calibration verification (CCV) standards were analyzed for both aerosol and microdrop systems to calculate the precision, accuracy, and limits of detection for each system. The calibration curves produced correlation coefficients with R(2) values > 0.99 for both systems. The precision, accuracy, and limit of detection (LOD) determined for aerosol LIBS were averaged and determined for the emission lines of Sr II (421.55 nm), Mg II (279.80 nm), Ba II (493.41 nm), and Ca II (396.84 nm) to be ~3.8% RSD, 3.1% bias, 0.7 μg/mL, respectively. A microdrop dispenser was used to deliver single drops containing 90 pL into the space where a LIBS plasma was generated with a focused laser pulse. In the single drop microdrop LIBS experiment, the analysis of a single drop, containing a total mass of 45 pg, resulted in a precision of 13% RSD and a bias of 1% for the Al I (394.40 nm) emission line. The absolute limits of detection of single drop microdrop LIBS for the emission lines Al I (394.40 nm) and Sr II (421.5 nm) were approximately 1 pg, and Ba II (493.41 nm) produced an absolute detection limit of approximately 3 pg. Overall, the precision, accuracy, and absolute LOD determined for single microdrop LIBS resulted in a typical performance of ~14% RSD, 6% bias, and 1 pg for the elements Sr II (421.55 nm), Al I(394.40 nm), Mg II (279.80), and Ba II(493.41 nm).     

Monitoring uranium, hydrogen, and lithium and their isotopes using a compact laser-induced breakdown spectroscopy (LIBS) probe and high-resolution spectrometer

The development of field-deployable instruments to monitor radiological, nuclear, and explosive (RNE) threats is of current interest for a number of assessment needs such as the on-site screening of suspect facilities and nuclear forensics. The presence of uranium and plutonium and radiological materials can be determined through monitoring the elemental emission spectrum using relatively low-resolution spectrometers. In addition, uranium compounds, explosives, and chemicals used in nuclear fuel processing (e.g., tributyl-phosphate) can be identified by applying chemometric analysis to the laser-induced breakdown (LIBS) spectrum recorded by these spectrometers. For nuclear forensic applications, however, isotopes of U and Pu and other elements (e.g., H and Li) must also be determined, requiring higher resolution spectrometers given the small magnitude of the isotope shifts for some of these elements (e.g., 25 pm for U and 13 pm for Pu). High-resolution spectrometers will be preferred for several reasons but these must fit into realistic field-based analysis scenarios. To address the need for field instrumentation, we evaluated a previously developed field-deployable hand-held LIBS interrogation probe combined with two relatively new high-resolution spectrometers (λ/Δλ ~75,000 and ~44,000) that have the potential to meet field-based analysis needs. These spectrometers are significantly smaller and lighter in weight than those previously used for isotopic analysis and one unit can provide simultaneous wide spectral coverage and high resolution in a relatively small package. The LIBS interrogation probe was developed initially for use with low resolution compact spectrometers in a person-portable backpack LIBS instrument. Here we present the results of an evaluation of the LIBS probe combined with a high-resolution spectrometer and demonstrate rapid detection of isotopes of uranium and hydrogen and highly enriched samples of (6)Li and (7)Li.     

Laser-induced fluorescence-cued, laser-induced breakdown spectroscopy biological-agent detection

Methods for accurately characterizing aerosols are required for detecting biological warfare agents. Currently, fluorescence-based biological agent sensors provide adequate detection sensitivity but suffer from high false-alarm rates. Combining single-particle fluorescence analysis with laser-induced breakdown spectroscopy (LIBS) provides additional discrimination and potentially reduces false-alarm rates. A transportable UV laser-induced fluorescence-cued LIBS test bed has been developed and used to evaluate the utility of LIBS for biological-agent detection. Analysis of these data indicates that LIBS adds discrimination capability to fluorescence-based biological-agent detectors. However, the data also show that LIBS signatures of biological agent simulants are affected by washing. This may limit the specificity of LIBS and narrow the scope of its applicability in biological-agent detection.     

Capabilities of surface composition analysis using a long laser-induced breakdown spectroscopy spark

An apparatus has been investigated based on laser-induced breakdown spectroscopy (LIBS) for the rapid determination of the spatial distribution of elements on surfaces. Cylindrical optics are used to create a linear spark approximately 1 cm in length. Light emitted by atoms excited along the spark is collected and provides a spatial profile of elemental composition in the sample when analyzed with a spectrometer and gated charge-coupled device (ICCD) detector. Moving the spark across the sample surface as spectral data is recorded at regularly spaced intervals allows for the development of a three-dimensional elemental distribution map (emission intensity versus spatial distribution across an area). An analysis of the spatial resolution of this methodology is presented along with representative data from several sample types. Application of full-image analysis allowing for simultaneous investigations into the spatial distributions of multiple elements is also discussed and results are presented.     

Slag analysis with laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been applied for multi-elemental analysis of slag samples from a steel plant. In order to avoid the time-consuming step of sample preparation, the liquid slag material can be filled in special probes. After cooling of the liquid slag and solidification, the samples can be analyzed with LIBS. Chemical analysis of slag is an essential input parameter used for numerical simulations to control liquid steel processing. The relative variation range of element concentrations in slag samples from steel production can amount to up to 30%. A multivariate calibration model is used to take into account matrix effects caused by these varying concentrations. By optimizing the measuring parameters as well as the calibration models, an agreement between the standard X-ray fluorescence (XRF) analysis and LIBS analysis in terms of the coefficient of determination r2 of 0.99 for the main analytes CaO, SiO2, and Fetot of converter slag samples was achieved. The average repeatability of the LIBS measurement for these elements in terms of the relative standard deviation of the determined concentration is improved to less than 1.0%. With these results, the basis is established for future on-line applications of LIBS in the steel-making industry for slag analysis.     

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.     

Elemental analyses and determination of lead content in kohl (stone) by laser-induced breakdown spectroscopy

Elemental analyses of kohl (stone) samples collected from three different parts of the world were performed using laser-induced breakdown spectroscopy (LIBS). The analyses indicated that lead (Pb), copper (Cu), silver (Ag), iron (Fe), calcium (Ca), aluminum (Al), silicon (Si), and sodium (Na) were present in all the kohl samples. In addition to these elements, the sample from Madina, Kingdom of Saudi Arabia (KSA), contained the elements tin (Sn), zirconium (Zr), and antimony (Sb). The sample from Mount Toor, Egypt, also contained Sn. Also, quantitative analysis for lead was carried out by the standard addition method using the LIBS technique. The result showed the presence of 14.12 ± 0.28% by weight of Pb in the sample from Madina, which compares well with the measurement done using atomic absorption spectroscopy (AAS) (13.31 ± 0.46%). The standard addition method used three calibration curves drawn for three emission lines of the LIBS spectra of Pb. The limits of detection (LoD) for these calibration curves varied from 0.27% to 1.16% by weight. The lead contents of the samples from Mount Toor and the local market of Bangladesh were also measured by the AAS technique, and the results were 14.61 ± 0.48% and 8.98 ± 0.35% by weight, respectively. The reason for determining only the lead content in kohl, which may be used as an eye cosmetic, is the adverse effect that lead has on health.     

Determination of nitrogen in sand using laser-induced breakdown spectroscopy

The use of laser-induced breakdown spectroscopy (LIBS) to detect a variety of elements in soils has been demonstrated and instruments have been developed to facilitate these measurements. The ability to determine nitrogen in soil is also important for applications ranging from precision farming to space exploration. For terrestrial use, the ideal situation is for measurements to be conducted in the ambient air, thereby simplifying equipment requirements and speeding the analysis. The high concentration of nitrogen in air, however, is a complicating factor for soil nitrogen measurements. Here we present the results of a study of LIBS detection of nitrogen in sand at atmospheric and reduced pressures to evaluate the method for future applications. Results presented include a survey of the nitrogen spectrum to determine strong N emission lines and determination of measurement precision and a detection limit for N in sand (0.8% by weight). Our findings are significantly different from those of a similar study recently published regarding the detection of nitrogen in soil.     

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.     

Extracting coal ash content from laser-induced breakdown spectroscopy (LIBS) spectra by multivariate analysis

Laser-induced breakdown spectroscopy (LIBS) combined with partial least squares (PLS) analysis has been applied for the quantitative analysis of the ash content of coal in this paper. The multivariate analysis method was employed to extract coal ash content information from LIBS spectra rather than from the concentrations of the main ash-forming elements. In order to construct a rigorous partial least squares regression model and reduce the calculation time, different spectral range data were used to construct partial least squares regression models, and then the performances of these models were compared in terms of the correlation coefficients of calibration and validation and the root mean square errors of calibration and cross-validation. Afterwards, the prediction accuracy, reproducibility, and the limit of detection of the partial least squares regression model were validated with independent laser-induced breakdown spectroscopy measurements of four unknown samples. The results show that a good agreement is observed between the ash content provided by thermo-gravimetric analyzer and the LIBS measurements coupled to the PLS regression model for the unknown samples. The feasibility of extracting coal ash content from LIBS spectra is approved. It is also confirmed that this technique has good potential for quantitative analysis of the ash content of coal.     

Laser-induced breakdown spectroscopy detection and classification of biological aerosols

Laser-induced breakdown spectroscopy (LIBS) is examined as a potential method for detecting airborne biological agents. A spectrally broadband LIBS system was used for laboratory measurements on some common biological agent simulants. These measurements were compared to those of common, naturally occurring biological aerosol components (pollen and fungal spores) to determine the potential of LIBS for discriminating biological agents from natural background aerosols. A principal components analysis illustrates that linear combinations of the detected atomic lines, which are present in different ratios in each of the samples tested, can be used to discriminate biological agent simulants from other biological matter. A more sensitive, narrowband LIBS instrument was used to demonstrate the detection of single simulant (Bg) particles in the size range 1-5 microns. Ca, Mg, and Na, which are present in varying concentrations between 0.3 and 11% (by mass) in the Bg particles, were observed in single particles using LIBS.