Laser-Induced Breakdown Spectroscopy - ACS Publications

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Review pubs.acs.org/ac

Laser-Induced Breakdown Spectroscopy Francisco J. Fortes, Javier Moros, Patricia Lucena, Luisa M. Cabalín, and J. Javier Laserna*

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Department of Analytical Chemistry, University of Málaga, 29071 Málaga, Spain

CONTENTS

General Information: Books, Reviews, and Conferences Fundamentals Interaction of Laser Beam with Matter Factors Affecting Laser Ablation and LaserInduced Plasma Formation Influence of Target on the Laser-Induced Plasmas Influence of Laser Parameters on the LaserInduced Plasmas Laser Wavelength (λ) Laser Pulse Duration (τ) Laser Pulse Energy (E) Influence of Ambient Gas on the Laser-Induced Plasmas LIBS Methods Double Pulse LIBS Femtosecond LIBS Resonant LIBS Ranging Approaches Applications Surface Inspection, Depth Profiling, and LIBS Imaging Cultural Heritage Industrial Analysis Environmental Monitoring Biomedical and Pharmaceutical Analysis Security and Forensics Analysis of Liquids and Submerged Solids Space Exploration and Isotopic Analysis Space Exploration Isotopic Analysis Conclusions and Future Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

Early in 1963, spectra produced solely by laser excitation were recognized to result in fairly reproducible quantitative relationships among the various elemental constituents of the sample.2 Many additional advantages of LIBS as an analytical tool were recognized in the pioneering works of the 1960s. By 1966, timeresolved LIBS was reported,3 and the electron temperature and number density were calculated on a hydrogen plasma. Other diagnostics studies included pressure and pulse width dependence of the breakdown threshold; absorption, scattering, and reflection characteristics of the plasma; Doppler-shift of scattered light from the luminous front; and expansion of the shock wave initiated by the spark. This fundamental knowledge in combination with the appealing features of LIBS as an analytical tool fostered an extraordinary interest among the scientific community on a technique that nowadays is still in continuous expansion. Today, LIBS is deployed and working at the surface of Mars, on an impressive demonstration of experimental maturity.4,5 This article is the first Analytical Chemistry Review on LIBS. It focuses on developments in LIBS over the years 2008−2012. No attempt has been made to exhaustively quote all literature published in this period. Instead, an ample, critical selection of the most important contributions is presented. After introducing the general information sources of LIBS, the paper discusses the advancement in the understanding of fundamental principles of LIBS and the excitation strategies based on dual and multipulse schemes, resonant LIBS, and ultrafast lasers. Approaches to the analysis of distant objects follow. Applications are presented on the basis of a broad selection of new areas of research and innovative uses of LIBS. The Conclusions and Future Outlook briefly summarize recent advances and provides a prospective of short-term developments in LIBS. No specific section has been devoted to instrumentation as this issue is covered in each of the specific sections. In the last 5 years, a vast amount of experimental and theoretical work has been developed in the area of laser ablation (LA). Since this process is extremely versatile, its applications have proliferated over a broad front of disciplines, ranging from science to engineering. Although many features of laser ablation are relevant to LIBS, there is no specific coverage for LA in the present paper.

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GENERAL INFORMATION: BOOKS, REVIEWS, AND CONFERENCES During the period covered by this article, a monograph on LIBS has been published.6 The book provides an extensive coverage of fundamental principles, experimental parameters, and plasma dynamics, with a chapter devoted to modeling of plasma emission.

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aser-induced breakdown spectroscopy (LIBS) has experienced a spectacular growth in the past decade. From the time LIBS was studied only in a few laboratories around the world to the present, nearly fifty years have elapsed during which a striking progress has been accomplished. The first experiments on LIBS were reported early after the demonstration of laser action. In 1962, Brech and Cross used a ruby laser to produce vapors which were excited by an auxiliary spark source to analyze metallic and nonmetal materials by atomic emission spectroscopy.1 © 2012 American Chemical Society

Special Issue: Fundamental and Applied Reviews in Analytical Chemistry 2013 Published: November 8, 2012 640

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and calibration strategies are revisited. Burakov et al. reviewed the analysis of soils.22 The efficiency of double-pulse LIBS has been demonstrated in solving a number of environmental problems such as the determination of heavy and toxic metals in soil and the detection of sulfur in coal. Detection of explosives residues has been reviewed.23 Recent advances in laboratory instrumentation, standoff systems, and data analysis techniques are discussed. Gaudiuso and co-workers reviewed the uses of LIBS for elemental analysis in environmental, cultural heritage, and space applications.24 Two review articles discuss biomedical applications of LIBS.25,26 Developments on fieldable LIBS instrumentation have been reviewed.27 New trends in the implementation of LIBS systems were presented, with particular emphasis on portable analyzers, remote instruments, and standoff systems. Several specialized conferences and symposia are completely devoted to LIBS. The International Conference on LIBS was recently held in Luxor (Egypt). The meeting is held biennially since 2000, with the 2014 conference scheduled for China. Two symposia on LIBS are held every other year in the US and Europe alternating with the international conference. The next North American Symposium on LIBS will be organized within the framework of SCIX 2013. The Euromediterranean Symposium on LIBS of 2013 is to be held in Bari (Italy). LIBS has recently also appeared prominently in China with the organization of the Chinese Symposium on LIBS. The first event held in 2011 had an attendance of about 40 conferees, whereas the second symposium held in March 2012 had over 140 participants. In addition to the above-mentioned books and review articles, themed issues of the journals Spectrochimica Acta, Part B Atomic Spectroscopy and Applied Optics are usually published with papers presented in the specific LIBS conferences and symposia.

Several chapters provide a broad treatment of LIBS applications in analysis of metal alloys, nonconducting materials, surfaces and interfaces, and a final chapter devoted to industrial applications. Although out of the period covered by this Review, it is worth mentioning a number of books published since 2000. The book by Lee, Song, and Sneddon7 provides an excellent overview of LIBS achievements during the past century. The instrumentation required and options available are covered in the first part. The second part discusses fundamental studies of the laser plasma, and the third part deals with applications. The book edited by Miziolek, Palleschi, and Schechter8 provides an extensive account of LIBS fundamentals and applications until approximately 2005. Another book, authored by Cremers and Radziemski, uses a combination of tutorial discussions ranging from basic principles up to more advanced descriptions of equipment, methods, and techniques.9 The second edition of the handbook is to be published early in 2013. Singh and Thakur edited a monograph intended for analytical chemists and spectroscopists and also for graduate students and researchers engaged in the fields of combustion, environmental science, and planetary and space exploration.10 A number of useful review articles published since 2008 summarize the developments and applications of LIBS. In a series of two articles, Hahn and Omenetto provided a comprehensive overview of LIBS. In the first paper, basic diagnostics and plasma-particle interactions are covered.11 The second paper focuses on instrumentation, methodology for material analysis, and applications.12 Aragón and Aguilera focused on the progress achieved in the determination of the physical parameters characteristic of the plasma such as electron density, temperature, and densities of atoms and ions.13 Optically thin spatially integrated measurements and local measurements characterized by nonoptically thin conditions are discussed. The review article by Gornushkin and Panne describes modeling of laser-induced plasmas and overviews plasma diagnostics carried out by pump−probe techniques.14 The emphasis is given to models relevant to spectrochemical analysis, with special attention to collisional-radiative and collisionaldominated plasma models where radiative processes play an important role. An overview of spectroscopic diagnostics techniques for low temperature plasmas has been presented with an emphasis to electron number density measurement.15 The attention is drawn to techniques used for line intensity and line profile measurement, which are often overlooked in experimental work. The so-called calibration-free method for multielemental quantitative analysis has been reviewed by the first proposers of this approach.16 An extensive list of applications and a discussion on the weak points of the method are presented. The authors ask for systematic studies involving large numbers of samples and variable experimental settings in order to confirm and enlarge the knowledge gathered. Michel reviewed the applications of single-shot laser-induced breakdown spectroscopy.17 A review focuses on what has been reported about the performance of LIBS in reduced pressure environments as well as in various gases other than air.18 Cremers and Chinni presented an overview of LIBS as an analytical method, discussing its many advantages and some important limitations and how these relate to potential applications.19 Applications of LIBS in specific areas have also been reviewed in some useful articles. A review assesses the applications of LIBS for chemical analysis mainly centered in biomaterials and plants.20 Analysis of plant materials has been further reviewed by Santos and co-workers.21 Sample preparation procedures

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FUNDAMENTALS Interaction of Laser Beam with Matter. Interaction of a focused laser beam with matter is a complex and not yet fully understood phenomenon, which is still under intensive investigation. When a high-power laser pulse impacts on the surface of any material, the irradiation at the focal spot leads to some material removal (ablation phenomenon). The ablated material compresses the surrounding atmosphere and leads to the formation of a shock wave. During this process, a wide variety of phenomena including rapid local heating, melting, and intense evaporation is involved. Then, the evaporated material expands as a plume above the sample surface, and because of the high temperature, a plasma is formed. This plasma contains electrons, ions, and neutral as well as excited species of the ablated matter, whose light emission constitutes the analytical signal measured by LIBS. However, what authors seek to face in this section is not the analytical strength, which can be drawn from the plasma, but the factors affecting its generation, its dynamics, and its significant parameters. Notwithstanding, the goal of this section is to convey to the potential readers some highlights of physics of the plasma plume from a review of the most recent LIBS literature. It will not be the subject of this section, a special attention to the complex expressions that are given to refer on the diagnostic characterization of the complex scenario of physical−chemical processes leading to the formation and expansion of the plasma plume, in particular on the theoretical assumptions made and the approaches used for its modeling, and on physical parameters used within LIBS literature regarding plasma diagnostics. 641

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Figure 1. Schematic diagram of those changing variables affecting the produced plasma, together with the physical parameters of both target and the generated plume that may be altered.

Accordingly, considerations of this section will only refer to the newest research. Moreover, for the sake of clarity and ease of comprehension, the section will be divided into subsections, each focusing on the particular effects that each input variable, namely, the targeted sample, laser parameters (wavelength and pulse duration, as well as fluence), and surrounding atmosphere, has on the parameters of the resulting plume. Although authors will try to provide a critical review of progress made, the reader should not forget that the high dependence of the values of physical parameters on the experimental conditions complicates the comparison of results from different experiments. Influence of Target on the Laser-Induced Plasmas. Aguilera et al.28 investigated (Nd:YAG, 1064 nm, 100 mJ, 4.5 ns) plumes from Fe−Ni, Cu−Ni, and Al−Ni binary matrixes in air at atmospheric pressure. After a complete description of plasma emissions detected in the 3−4 μs time window using a set of parameters (electron temperature, Te; electron density, Ne; total number density in the plasma, N; the length of the plasma along the lineof-sight, l; the perpendicular radiating area of the plasma, β), their results reveal the existence of a weak matrix effect that leads to a variation of these plasma parameters, as only metallic samples are considered. Nevertheless, it is expected that variation of plasma parameters will be larger for materials having bigger differences on their physical properties. Viskupt and collaborators29 have reported on LA (Nd:YAG, 1064 nm, 100 mJ, 6 ns // KrF, 248 nm, 50 mJ, 20 ns) in air and Ar gas background of FeO targets prepared under differing forms, namely, nanopowder, pressed powder pellets, and sintered ceramics. As demonstrated, spectroscopy of the different targets was comparable to each other and qualitatively independent of target morphology. In contrast, the different formats featured by the targets strongly influence the processes occurring at the surface of the treated material. Hence, plume dynamics as well as particles ejected from the irradiated material seem to significantly

Readers with no substantial prior knowledge on this subject are invited to thoroughly read the review of Hahn and Omenetto12 as well as the references contained herein. Additionally, if expert readers find that this section lacks detailed information on those topics, the present Review will be also a very thorough supplement. Factors Affecting Laser Ablation and Laser-Induced Plasma Formation. As exemplified in Figure 1, the nature and characteristics of laser-induced plasmas are strongly affected by the laser operating conditions, i.e., laser wavelength (λ), pulse duration (τ), and energy (E). At the same time, it should be recalled that, while specific mechanisms governing laser energy absorption in the target depend on the type of material, the surrounding atmosphere, both in composition and pressure, plays an important role because it is the surrounding medium where the plasma evolves. The effects of each of those factors on the plasma plume in itself, through objective parameters that characterize it, have been extensively discussed in the literature. The review article13 presented by Aragón and Aguilera focuses on the progress achieved from 1980 to 2007 on characterization of plasmas. A full and quite updated description of the experiments carried out and the procedures developed to apply the different characterization methods in order to determine accurate values of the plume parameters is documented there. Also, the review manuscript presented by Konjević et al.15 may help the reader to learn more on theoretical and experimental procedures used to determine plasma electron density and temperature. In contrast, the review article of Gornushkin and Panne14 provides important information on the diversity of plasma processes and means of how these processes can be modeled or derived from experiments. Besides general information on existing plasma models, the emphasis is given to models relevant to spectrochemical analysis, i.e., models of radiating plasma. 642

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way that the plasma expands and thermalizes by transferring the energy to its surroundings. Influence of λs (1064, 532, and 266 nm) on emissions from Sn plasmas have been also investigated, specifically focusing on the changes in debris generation, in an effort to develop efficient (and debris free) light sources at 13.5 nm for the next-generation extreme ultraviolet (EUV) lithography.34 Deposition studies have shown 266 nm to generate a larger amount of atomic particles, consistent with mass-ablation estimates. In contrast, their kinetic energy profiles are found to be broader with decreasing λ; a fact attributed by the authors to the enhanced three-body recombination in dense plasmas produced by shorter λ. Results lead to the conclusion that, from a perspective of development of a EUV source, 1064 nm is the best option of the λs studied, owing to its higher 13.5 nm conversion efficiency and lower atomic debris. Campos35 and Harilal36 have also explored the effects of λ on plasmas produced from planar Sn targets using 1.06 μm Nd:YAG (6 ns) and 10.6 μm CO2 (30 ns) lasers. Several striking differences in the features of the two plumes have been noticed. The major difference involves the spatial−temporal evolution of Ne. Some light has shed on the nature of the hydrodynamic expansion of plasmas, revealing that Nd:YAG plasma forms a forward biased jet whereas the CO2 plasma expands almost spherically, in clear agreement with the less amount of debris per pulse that it emits. The analysis of craters created also confirmed a larger mass ablation rate (3.6 times higher) for Nd:YAG plasmas compared to that produced with CO2 lasers. A significant difference in Te between CO2 plasmas and Nd:YAG plumes has been also observed. From all these findings, the use of 10.6 μm CO2 as a EUV source has been suggested. In order to increase the sensitivity of LIBS, Coons et al.37 have compared single-pulse (SP) plasmas with dual-pulse (DP) plasmas generated using 1.06 μm Nd:YAG laser and reheated by a 10.6 μm CO2 laser. Their results conclude that the Nd:YAGCO2 laser combination improves the sensitivity by the effective reheating effect resulting from efficient inverse bremsstrahlung absorption of the longer λ laser. It is thus clear that a proper choice of the λ allows one to produce a plume with better properties for analytical applications. To push more in this direction, Ma and co-workers have investigated the ablation of Al targets in one-bar Ar background using nanosecond UV (355 nm) or IR (1064 nm) laser pulses.38 Differences in absorption rate between UV and IR radiations leading to different propagation behavior of the produced plasma have been demonstrated. While for UV ablation the background gas is principally evacuated by the expansion of the vapor plume, for IR ablation the background gas is effectively mixed to the ejected vapor during at least hundreds of nanoseconds after the initiation of the plasma. As a consequence, higher Te and Ne are observed for UV ablation than for IR ablation. These parameters confirm a hotter, confined Al plasma for UV ablation, whereas for IR ablation, a larger axially extended Al vapor plume with a better homogeneity is observed. Their observations suggest descriptions by “lasersupported combustion wave” and by “laser-supported detonation wave” for the propagations of plasma produced by UV and IR lasers, respectively. Laser Pulse Duration (τ). Irrespective of their duration, laser pulses usually reach the required conditions for ablation of targets since the rate of energy deposition greatly exceeds the rate of energy redistribution and dissipation, thus resulting in extremely high temperatures in those regions where energy absorption occurs. However, as a consequence of the different mechanisms of energy dissipation in the sample, differences in pulse duration

depend on the degree of target compaction. Also, in this direction, the relationship between sample hardness and the laserinduced (Nd:YAG, 532 nm, 30 mJ, 15 ns) plasma parameters has been investigated for aluminum−lithium alloys (Al−Sc−Li, Al− Mg−Li, Al−Cu−Li) and lithium ferrites.30 Differences in plasma temperature for ferrites are not related to changes in sample composition but are caused by their different physical properties and structure. For aluminum alloys, variation of their composition induced changes in hardness and both factors influenced plasma properties. It has been also proved that the ablated mass is in inverse proportion with hardness. Schmitz et al.31,32 have given new experimental insights into the characteristics of atmospheric-pressure ablation of molecular solids with respect to analytical MALDI (matrix-assisted laser desorption/ionization) applications. The early processes, material release and formation and expansion of hemispherical shock waves, in ablation (nitrogen, 337 nm, 235 μJ, 4 ns) of different common MALDI matrixes, namely 2,5-dihydroxybenzoic acid, a-cyano-4-hydroxycinnamic acid, and sinapinic acid, as well as anthracene, have been studied. For a given fluence, similar crater shapes of the same dimension for MALDI matrixes have been observed as compared with the consistently smaller, and of remarkable “clear−cut” form, crater of anthracene. Furthermore, for ablation at whatever energy regime, smallest expansions at any given time were observed for anthracene as contrasted with those from MALDI matrixes. The pronounced differences between anthracene and the striking similarity of MALDI matrixes have been justified by authors through the release CO2 from these last targets by laser-induced decarboxylation. Influence of Laser Parameters on the Laser-Induced Plasmas. The laser is likely to be by far the most delicate, and the most important variable affecting the characteristics of the plasma since the effects of its parameters are 2-fold: first, during its interaction with the targeted sample and, then, with the plasma plume itself. In general, photons are coupled within the available electronic, or vibrational, states in the material depending on its wavelength. During this coupling, the material is heated to a particular temperature depending on the mechanism of interaction of the laser pulse with that, and the onset of ablation (either thermal or photochemical) occurs if the fluence is above a particular threshold. Once the plasma plume is generated, its density may obstruct (‘‘plasma shielding’’) partially or entirely laser radiation, depending on the laser wavelength and pulse length. Consequently, not the full energy is transferred from the laser pulse to the original material. With all these key aspects affecting the whole chain of possible events occurring during plasma formation, authors have no other aim than to put in situation to the reader on how different laser parameters may affect the physics of the plasma plume. To ensure a more user-friendly and understandable framework, influences of laser parameters are addressed separately as follows. Laser Wavelength (λ). A general outcome of the effect of λ on plasma parameters may be extracted from the work of Hanif et al.33 where the spatial evolution of Cu plumes produced by 1064 nm (Nd:YAG, 400 mJ, 5 ns, 10 Hz) and 532 nm (Nd:YAG, 200 mJ, 5 ns, 10 Hz) wavelengths has been investigated. Again, Te calculated for 1064 nm (15600 K) is slightly larger than that for 532 nm (14750 K), whereas Ne in the case of 1064 nm (2.50 × 1016 cm−3) is somewhat smaller than in the case of 532 nm (2.60 × 1016 cm−3). Both works also reveal that Te and Ne decrease along the direction of plume propagation due to the rapid conversion of thermal energy into kinetic energy, in such a 643

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when using a relatively long pulse is underscored. Additionally, an interesting physical phenomenon for 100 ns pulse (but not for 200 ns pulse) LA has been observed: while the plasma bottom is detached from the ablated target surface shortly after laser pulse ends, this plasma bottom grows backward toward the target. In short, due to the much longer ns laser pulse duration, distribution of plasma radiation intensity and propagation of its front have different physical features from those produced by much shorter ns laser pulses. A suggestive investigation has been made by examining some aspects in the plasma plumes from Cu plates immersed in water when using different pulse widths, namely, 19, 90, or 150 ns, of a Nd:YAG laser.44 As compared to a short pulse, a minimized damage and efficient heating of the plume due to the direct absorption of the later part of the long pulse has been observed. According to these authors, long ns pulses are more favorable for this LIBS application due to the relatively slow heating of the plasma, causing a larger and less-dense plume and, therefore, fairly intense and less broadened emission lines. Also, a weaker continuum has been observed. Some comparative studies, even shuffling different experimental variables, seem to be in clear concordance with the improved analytical performance of DP-excitation mode, due to the much lower density in its plasmas and to the temperature growth as well as to the increase of both plasma lifetime and dimensions of the plume along the observation line. A detailed comparative study of collinear SP and DP-LIBS (KrF excimer-dye laser, 248 nm, 450 fs) on the basis of emission lifetime, Te, and Ne, for fs ablation in ambient air of brass (Cu−Zn), has been carried out.45 As expected, the DP arrangement yields hotter and longer-lived plasmas and, in consequence, a significant enhancement of the intensity and reproducibility of the optical emission signals. Furthermore, its lower Ne at early times together with higher Te, lead to DP spectra with a slightly higher signal-to-background ratio (although its broad background is still higher) and a considerable decrease of line broadening. As a result, an overall improvement in spectral resolution is observed even using a nongated data acquisition mode. Besides, the DP arrangement reduces the threshold fluence for plasma formation by about a factor of 2, allowing the acquisition of good analytical spectra at lower fluence values while minimizing damage of the original surface. Another comparative study involves the use of 1064 and 532 nm beams from two Nd:YAG lasers for generating Al plasmas.46 As in the previous case,45 after an optimization of interpulse delay time [≠f(λ)] and pulse energy ratio [=f(λ)], the DP approach yielded an emission signal enhancement of over 300-fold as compared to the single pulse instance. This signal enhancement is also underpinned by thermal reheating of the plasma plume. Higher values for Te and Ne have been also calculated. The authors suggest to keep the pulse energy of the second laser larger than that of the first one for a better experimental realization of DP-LIBS. In another turn of the screw toward deeper knowledge, plasmas from Fe generated by SP and cross-beam DP configurations using Nd:YAG (1064 nm, 39 mJ, 0.33 Hz) and CO2 (10.6 μm, 75 mJ, 0.33 Hz) lasers have been investigated.47 An enhanced signal when the CO2 pulse interacts with the sample before the Nd:YAG pulse has been observed. The CO2 pulse heats the target during the first 700 ns before the arrival of the Nd:YAG pulse without melting the sample and without any noticeable deformation of the surface. Then, a few microseconds after the arrival of the CO2 pulse, heavy and slow moving particles are extracted and ejected from the target

result in fundamental differences of the ablation process. Indeed, interaction of nanosecond (ns) pulses with materials are substantially different from those of femtosecond (fs) pulses since the rate of energy deposition is significantly shorter in this last instance. Thus, for ns pulses, the material undergoes transient changes in the thermodynamic states from solid, through liquid, into a plasma state. Furthermore, the leading edge of the laser pulse creates plasma, and the remaining part of the pulse heats the plasma instead of interacting with the target. In the case of ultrashort laser pulses, at the end of the laser pulse, only a very hot electron gas and a practically undisturbed lattice are found. In order to get a better insight of the physical mechanisms involved in LA using ultrashort pulses, plumes (Ti:sapphire, 800 nm, 1 mJ, 100 fs, 1 kHz) from both Cu and f used silica targets have been compared.39 These investigations reveal a sole main component (‘‘fast’’) in the plume from f used silica. Contrarily, two components in the case of Cu have been found: a ‘‘slow’’ component of high intensity that evolves close to the target surface and a less intense, but ‘‘fast’’, component observed farther away. Also, the component of f used silica plumes is 3 times faster than the fastest component of Cu plasmas. Moreover, while nanoparticles (NPs) represent a large fraction of matter ablated from Cu, they are not observed during ablation of f used silica. Spatial−temporal maps of ionic, neutral, and molecular species generated from planar graphite targets irradiated by fs laser (Ti:sapphire, 800 nm, ∼87.5 J cm−2, 40 fs, 10 Hz) under varying ambient N2 gas pressures have been reported.40 Space-time contours for those species within the plume have revealed that, in the presence of an ambient gas, the distribution of each species is related to its next ionized level, whereas the molecular species spatial extension at the early stages of plasma life is directly correlated to the excited ions. Gacek et al.41 presented simulations on the fundamentals of plume splitting at atomistic level under the influence of shock wave during the early stage of its propagation (up to 2 ns). According to their findings, at the very beginning of ablation, two distinguishable density peaks at the plume emerge and quickly disappear due to the spread-out of the slower moving part. While the front peak (coming from the faster moving of atoms and smaller particles) propagates out against time and experiences strong constraint from the ambient gas, the second peak (originating from the larger clusters) moves slower and experiences very little constraint, eventually picking up their velocity during the early evolution. Regarding the ambient, the larger the pressure, the earlier is the plume splitting, that occurs at a distance closer to the target surface. In contrast, when the ambient pressure is reduced, the plume splitting becomes weak and barely visible. Furthermore, under stronger laser fluence irradiation, the plume splitting also happens earlier. At the other end of the temporal regime, Zhou et al.42,43 have experimentally studied plasmas induced during ablation (SPI G3.0 laser, 1064 nm, ∼0.2 to 0.4 mJ) of polished Ti and Al targets in air at atmospheric pressure using relatively long (200 ns) laser pulses. A rapid growth of the plasma size with time during the laser pulse (from 50 to 200 ns) has been observed. Subsequently, the radiation intensity of the plasma region increases, but it is not uniform. Plume again shows two distinguishable regions: a bright spot located just above the target surface and other highradiation-intensity region behind the expanding plasma front. Later, the region just above the target surface disappears first moving upward and, then, through merging with the other region, thus forming a single high-intensity zone. The important role of laser−plasma interaction during the plasma evolution 644

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surface was observed. Hence, while Ne is maximum near the target surface and decreases at larger distances from the target surface due to the recombination with ions, Te slightly decreases at both the plasma edge, due to the higher radiative cooling at this area (larger emitting surface as the plasma expands) and the rapidly conversion of thermal energy into kinetic energy, and close to the target surface, because thermal conduction from the plasma toward the target considering the equipartition time for energy transfer from electrons to ions. The early stage structure and dynamics of plumes induced (Nd:YAG, 1064 nm, 0.974 J, 5.1 ns, 10 Hz) from Al, Ti, and Fe samples (cleaned, but not polished, surface) has been investigated.52 The influence of irradiance on plasma expansion has been highlighted. At high irradiance, the plume, containing more ablated matter and having higher internal energy, is capable of pushing the surrounding air far enough in front of itself in order to expand into a hemispherical shape; whereas at lower irradiances, the ablation plume has less energy, so its expansion in the radial direction prevails over that in the longitudinal direction, so the plume core remains “attached” to the sample surface, having a more disk-like shape. Influence of Ambient Gas on the Laser-Induced Plasmas. As a result of the target evaporation, the plume, with short temporal existence, transient in nature and containing particles of the ablated matter, expands at supersonic velocity toward the surrounding atmosphere in front of the target. The interaction of the plume with the ambient gas is a complex gas dynamic process due to the rise of new physical processes, including deceleration, thermalization of the ablated species, interpenetration of gas components into the plasma, radiative recombination, formation of shock waves, and clustering. In this respect, not only is the composition of the surrounding atmosphere but also the pressure under which such plume is evolving, combined with the crucial requirements of LA to be performed in background gas with residual pressure typically for pulsed-laser deposition (PLD), perhaps, are the reasons which have led many groups to investigate on this issue in the past few years. In this section, we will look at how ambient conditions affect the dynamics of the plasma plume; in short, the role of the nature and pressure of various ambient environments on the spatial and temporal evolution of the plasma as well as on its parameters (see Figure 2). Regrettably, many important details of plasma expansion are highly difficult to obtain experimentally. Hence, numerical simulations from theoretical models play an important role in getting more detailed information about what could happen in the process. Despite its common goal, several theoretical studies identified in the literature are based on different assumptions according to the several aspects of LA that the model intends to describe. Thus, González and collaborators53 have used the particle-in-cell (PIC) computational method to study the Coulomb interaction between the particles of the initial states of Al plasmas (Nd:YAG, 1064 nm, 500 mJ, 9 ms, 10 Hz) expansion in vacuum. An ideal model assuming that the plasma is in a local thermal equilibrium as well as the ablated particles have a fixed temperature, and a constant evaporation flux (J) from the target surface has been considered. In contrast, considering a wide range of ambient pressure conditions and for a high vacuum ambience, Antony and co-workers54 used an approach based on hydrodynamic equations to model dynamics of plume temperature and plume velocity from LIBS experiments (Nd:YAG, 355 nm, 90 mJ, 5−10 ns, 10 Hz) on Al, Cu, and ZnO targets.

surface, thus providing fuel for subsequent plasma creation by the Nd:YAG laser. To close this matter, a comparative study of effects of SP as well as both collinear and orthogonal preablation DP configurations on plasma emissions from Ag samples using two Nd:YAG lasers operating at 532 and 1064 nm has been presented.48 As expected, DP configurations yield up to 12 times (collinear) and 6 times (orthogonal) signal enhancement as compared to the SP method. For these enhancements, the optimum value of the interpulse delay time between both pulses is independent from DP configuration, whereas the optimized value of pulses energy ratio depends on the DP configuration. A shorter wavelength pulse for generating plasma, and a delayed pulse of longer wavelength for plume reheating, is recommend for better DP-LIBS. Laser Pulse Energy (E). Ablation and plasma formation are largely affected by the laser pulse energy, E (closely connected with pulse duration). In fact, for LIBS, the energy per unit area that can be delivered to the target is more important than the absolute value of E. Thus, the primary energy-related parameters influencing the laser−matter interaction are usually termed either as irradiance (energy per unit area and time, W cm−2) or as f luence (energy per unit area, J cm−2). Experimental determination of irradiance or fluence requires a careful evaluation of the spot size over which the laser beam is focused. However, despite that change on focusing distance introduces a degree of flexibility in the energy frame to be able to reach identical irradiance/ fluence levels, this possibility is a dangerous double-edged sword, because plasma plumes generated using different pulse energies and focusing distances show a spatial and temporal scaling. Investigations on the temporal history (from 0 to 8 μs) of physical parameters of plasmas produced by focusing laser pulses (Nd:YAG, 1064 nm, 200 mJ, 7 ns, 1 Hz) of variable energies (50, 70, and 95 mJ) have been reported by Sarkar and collaborators.49 Vanadium oxides (VO, V2O3, VO2, and V2O5) in air at atmospheric pressure were studied. Their findings are in good agreement with the predicted increase of Te with E. Besides, its temporal profile also reveals faster decay of Te with increasing E. The rate of this decay, following power law, is assumed to be independent of the nature of the target. The same decay holds true for Ne but faster with reduction in E; this is the reverse behavior of that observed for Te. Since the amount of ablated material increases with E, Ne can be sustained for a longer time; hence, the rate of decay of Ne decreases with increasing ablation. In contrast, the effect of changing laser (1064 nm, 5 ns) incident irradiance by changing the working distance on plasma parameters has been studied by Abdelhamid and collaborators.50 Au thin films deposited onto Cu substrates were studied. Despite slight fluctuations attributed to high reflectivity of metals in the layered material, Ne values nearly independent of the working distance, and subsequently of the irradiance, have been observed. Similarly, no measurable change has been found in Te, so the range of irradiance values studied by these authors seem not enough for modifying the plume parameters. Both Ne and Te on transient and elongated plumes have been also evaluated when focusing pulses at seven different incident irradiances (3.7, 6.3, 9.7, 12.4, 15.9, 19.0, and 21.9 GW cm−2) of the laser (Nd:YAG, 1064 nm, 19.7 ns, 1 Hz) onto Al alloys in air, at atmospheric pressure.51 An increase was detected for Ne (varying from 2.45 × 1017 to 3.15 × 1017 cm−3) and Te (ranging from 6085 to 7498 K) with the raise of the irradiance (from 3.7 to 21.9 GW cm−2). Furthermore, spatial evolution of these plume parameters from 0.1 to 4.0 mm axial heights above the target 645

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Figure 2. Pictorial diagram of the influence of pressure on the spatial−temporal dynamics of the plasma plume.

Using both adiabatic expansion and kinetic models, Kumar et al.55 have made an attempt to estimate the lengths of plumes and to investigate plasmas expansion rates across the laser beam axis for the ablation (Nd:YAG, 1064, 532, and 238 nm, 5−10 ns, beam radius at focal point 0.2−0.4 mm, Gaussian beam profile) of Ti thin foils under different ambient gas pressures. Alternatively, a theoretical thermal model consisting of equations of conservation of mass, momentum, and energy for studying the target heating, plasma formation, and plume expansion during interaction with ns laser pulses has been presented and experimentally corroborated on Al plumes (Nd:YAG, 1064 nm, 450 mJ, 10 ns), by Mościcki and co-workers.56 The work from Bogaerts’ research group offers an overview of their modeling activities in ns- and fs-LA.57 On the other hand, from a more experimental perspective, influence of the surrounding ambient is limited toward specific diagnostics of physical parameters defining the state of the plasma as well as its morphology profiles with respect to the spatial and temporal evolution of the plume. In this connection, in a thorough manner, temporal evolution of those plumes under a wide range of pressures (from 103 to 10−4 mbar) has been characterized.58 A general decrease in Te and Ne values is observed with decreasing ambient pressure. However, the most striking feature is the step change in the plasma behavior, between ambient pressures of ≥10 mbar and ≤1 mbar, despite that in the early stages of plasma formation (110 ns) plasmas generated are comparable in terms of size and luminosity whatever the ambient pressure. Then, as pressure decreases, a lower

confinement and a larger acceleration on the plasma dispersion, and subsequently highly distinguishable plumes, have been observed. Also, the role of air in the dynamical evolution and thermodynamic state of polished Al plumes (Nd:YAG, 1.06 μm, 70 mJ, 20 ns) has been investigated,59 and a strong presence of air in the plasma core was detected. Thus, while a very limited presence of Al species is found in the region farther from the target, where mostly air emissions are observed, a prevalence of air species in the plume composition, even in the region closer to the target, is noticed. Similar investigations on Al plasmas (Nd:YAG, 532 nm, 100 mJ, 8 ns) have been carried out under the influence of both vacuum and air at atmospheric pressure.60 In this case, overall behaviors of spatial−temporal variations for Te have been observed almost the same. Contrarily, for Ne, a highest value has been observed close to the target for both different ambient conditions, as well as posterior decreasing at both longer times and distances from the target. To gain a deeper insight, timing waveforms and characteristic sizes of the radiating area in Al plasmas at different residual air pressures, ranging from 6.7 to 133.3 Pa, and different irradiances, varying in the range (3.8−4.8) × 108 W cm−2, have been also studied.61,62 A decrease of intensity of the Al plasma with raising pressure has been observed. Such drop is justified from the fact that, under low pressure, the plume interacts less strongly with ambient molecules (less transfer of part of the energy to them), thus decreasing the number of recombining ions. In contrast, when the ambient gas prevents its expansion, recombination 646

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over the Al plasma core, having quite uniform distributions of Ne and Te but with a large amount gas of mixed within it, has been observed. In contrast, Mendys et al. applied the Thomson scattering method to quantify Ne and Te at different instants of the evolution, at atmospheric pressure, of a plume core, first induced (Nd:YAG, 532 nm, 2.0 kJ cm−2, 4.5 ns, 10 Hz) and then perturbed by a probe laser pulse (Nd:YAG, 532 nm, 40 J cm−2, 6 ns, 10 Hz).69 An identical rate for temporal decay of Te and Ne up to about 1.5 μs has been found. Then, a significant deceleration on the decay rate for Te (cooling governed rather by gas dynamics and energy losses than by the equilibrium state of the plasma) as compared with that of Ne (directly related to the rate of the plume expansion) has been reflected. Regarding He ambient, spatial−temporal dependencies of plasma emissions from ablation (266 nm, 10 ns) of Cu, under various both pressure (100, 500, and 760 Torr) and irradiance (0.5, 0.7, and 1 GW cm−2), have been investigated by Mehrabian et al.70 As expected, at specified irradiance, the increasing of He pressure leads to the more compression of Cu and He atoms (stronger shockwaves with edges closer to the target surface), and Ne becomes more whereas Te has less spatial expansion. Influences of Ar and He changing pressures (from 10−5 to 3 Torr) on dynamics of laser (Nd:YAG, 1064 nm, 1.6 J, 8 ns) blow-off plasma plume from multilayered LiF−C thin films have also been evaluated.71 A deep dependence of intensity, size, and shape of the plume on the nature and composition of the ambient gas has been observed. Hence, while velocity of the plume has been found to be higher in He ambient, intensity enhancement is greater in an Ar environment. Finally, the effects of Ar, He, and air under different filling pressures (from 5 to 760 Torr) on Cu72 and Cd73 plasmas (Nd:YAG, 1064 nm, 200 mJ, 10 ns) have been investigated. A strong influence of the pressure and nature of the ambient gases, due to their differences in density, mass, ionization potential, and thermal characteristics, on optical emission intensity, Te, and Ne has been revealed. Surface morphological changes on the irradiated target have been observed. Together with all above references, those readers interested in deeper knowledge on this subject are invited to check the review of Effenberger and Scott18 as well as the references contained herein. The article focuses on compiling an understanding of LIBS phenomena that have been gained through the various pressure dependence and atmospheric composition studies. There is no doubt that more references related to evaluation of physics of laser-induced plumes are available in the scientific literature. However, the goal of authors here was simply to raise awareness about some basic knowledge on this topic to the reader through some subtle accents. Moreover, this section seeks to bring the reader into a deep thought about the complexity within the complete sequence of events that take place during the LA, depending on the variables by which it is governed. Thus, despite the large number of scientific and practical applications of LIBS that will be reflected below, the study of its basic mechanisms is still a great challenge, since there is no universal model to completely describe this phenomenon.

processes proceed faster since energy exchange between particles inside the plume becomes more efficient. The expansion of brass plasmas also in various air pressures (0.01, 10, and 105 Pa), after ablation (Nd:YAG, 266 nm, 60 mJ, 4 ns, 10 Hz), has been studied by Patel et al.63 As before, a larger confinement of the plume near the target surface, with a decreasing on the velocity of the plume front, has been noticed as the pressure increases. This confinement also reported emission intensity longer sustained in time because of enhanced collisional processes. Decreasing of Te and Ne is larger when the pressure decreases because expansion of the plasma becomes faster, thus resulting in faster cooling. The expansion dynamics of different plumes after LA, under other different background atmospheres than air, have been also evaluated by several research groups. Hence, the influence of O2 gas pressure on propagation of plumes from complex oxides (LaAlO3 and LaGaO3) has been investigated64 to elucidate and discuss the role of the surrounding background on characteristics of ablation (KrF excimer, 2.0 J cm−2, 25 ns) plume and, in turn, address consequent effects on the growth of interfaces for producing high quality oxides thin films. As with air atmosphere, also similar regimes of influence have been noticed: almost free plume expansion at low pressures; plume splitting at middle pressures; and the braking effect of the gas on plume propagation along the normal to the target surface at high pressures. Besides, pressure only slightly affects the plume at early delays after the laser pulse. At larger delays, despite similar plume elongation, whatever the pressure, the interaction with the background gas starts affecting the plume spatial characteristics. Investigations on dynamics of the plasma and its properties under the influence of some inert gases as Ar and He, how they change with pressure as well as critical comparison with those evolutions observed under air ambient, have been also made. On considering Ar atmospheres, hydrodynamic expansion features of Al plumes (Nd:YAG, 1064 nm, 6 ns) at atmospheric pressure have been investigated by Harilal et al.65 In their experiments, the main temporal regimes for plasma expansion under Ar gas, namely, the initial-stage asymmetric expansion, the internal shock-wave-like plume structure, and the vortical motion, have been observed. In contrast, dynamics of the Fe and graphite plumes (Q-switched Nd:YAG, 532 nm, ∼30 J cm−2, ∼8 ns) during its propagation under different pressures (from 2 × 10−4 to 20 mbar) have been also reported.66 Identical dependence on pressure has been observed for plume expansion. However, different temporal expansion regimes have been noticed: an initial expansion up to ∼100 ns, whatever the gas pressure is; an intermediate plume expansion where the temporal elongation is pressure dependent; and finally, a spherical expansion for all operating pressures after several tens of μs. Besides, the increased pressure reduces not only the kinetic energy of species within the plume but also the velocity of the expanding plume front. Additionally, the plume splitting phenomenon for Fe and Al plasmas, as they expand through different pressures (0.5 and 1.0 mbar for Fe, and 0.2 mbar for Al) of Ar gas, has been more deeply investigated.67 Unlike for lower and higher gas pressures, at which plasma plume expansion does not show any visible frontal edge, at moderate gas pressures the plume splitting is observed. Furthermore, the pressure regime resulting in plume splitting is target dependent. Also, a detailed comparison of the interplay between plumes (Nd:YAG laser, 1064 nm, 50 mJ, 5 ns, 10 Hz) from Al alloys (Al 89.5%; Si 8.39%; Fe 0.999% and some traces) from high pressure to atmospheric pressure of Ar gas has been described by Ma and co-workers.68 A soft confinement by Ar

■

LIBS METHODS Double Pulse LIBS. The original purpose of DP-LIBS was the improvement of the observed signal in an attempt to increase the analytical sensitivity. However, the advantages of this approach and their combined and multiple optical configurations lead also to improvements in LIBS applications such as underwater analysis (see section on Analysis of Liquids and Submerged 647

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Table 1. Collinear Double-Pulse LIBS Applications sample

collinear configurationa

enhancement factorb/remarks

arsenic (mine tailing soils)

Nd:YAG @ 1064 nm, width 6 ns, combined 90 mJ, dt 1−180 μs

chromium (dyed wool fabric) crop plants (K, P, Mg, Ca) explosives residues food (potatoes) geomaterials carbonates (mineral and rock) natural fluorite (mineral), silicate (rocks and soils) graphite metallic alloys (steel) metallic alloys (Pb)

Nd:YAG @ 1064 nm, width 10 ns, 90 mJ pulse−1, dt 7 μs Nd:YAG @ 1064 nm, width 12 ns, 65−78mJ pulse−1, dt 7 μs Nd:YAG @ 1064 nm, width 5 ns, 335 mJ pulse−1 Nd:YAG @ 1064 nm, width 5 ns, 10 mJ pulse−1, dt 100 ns Nd:YAG @ 1064 nm, width 7 ns, 320 mJ pulse−1, dt 1 μs

metallic aqueous solutions (Cr) multielement aqueous solutions (Fe, Pb, and Au) multielement aqueous solutions (Ca, Ba, Sr,and Mg) Ti solid solid samples (brass, Fe, Si, BaSO4, Al)

Nd:YAG @ 532 nm, 20 mJ pulse−1, 36° incident angle Nd:YAG @ 1064 nm, 60 mJ pulse−1, dt 1 μs Nd:YAG, width 8 ns, combined 40 mJ, dt 7.4 μs first pulse @ 532 nm; second pulse @ 1064, 532, and 355 nm Nd:YAG @ 532 nm, width 5 ns, combined 300 mJ, dt 2−3 μs quasi-collinear first pulse: Nd:YAG @ 266 nm, width 7 ns, 32 mJ pulse−1 second pulse: Nd:YAG @ 1064 nm, width 7 ns, 200 mJ pulse−1, dt 2−3 μs−1 Nd:YAG @ 532 nm, width 3−5 ns, dt 50 ns 2 × 65 mJ (aerosol), 2 × 35 mJ (microdrop) Nd:YAG @ 532 nm, width 8 ns, 20 mJ pulse−1, dt 0.5 μs excimer−dye @ 248 nm, width 450 fs, 15 mJ pulse−1

intensity 13% SNR 165% LOD 5−10mg kg−1 intensity 2−3× intensity 2× no advantages for sample classification except in the case of soils

intensity 5× intensity 10× LOD 10× all wavelength combinations best LOD (0.0017%) for 532 nm/355 nm ablated mass ≥3.5× LOD 10× LOD 10×

intensity 1.5−2.5× (aerosol) intensity 4× (microdrop) intensity 5× intensity 3−10×

ref. 80 81 82 74 88 78

87 84 92 76 89

90 79 97

dt corresponds to the delay time between pulses. bEnhancement factor of the analytical figures of merit produced by double-pulse LIBS with respect to single-pulse LIBS.

a

Table 2. Orthogonal Double-Pulse LIBS Applications sample

orthogonal configurationa −1

Al (metal)

ablation laser: Nd:YAG @ 532 nm, width 8 ns, 7 mJ pulse prespark laser: Nd:YAG @ 1064 nm, width 10 ns, 26 mJ pulse−1, dt 0−100 μs archaeometallurgical objects cleaning laser: Nd:YAG @ 1064 nm, width 4 ns; 60 mJ pulse−1 reheating laser: Nd:YAG @ 532 nm, width 5 ns, 60 mJ pulse−1 ash (coal fired power plant) Nd:YAG @ 1064 nm, width 6 ns, 100 mJ pulse−1 (Ba, K, Mg, Ti, Fe, Ca, Al) Nd:YAG @ 1064 nm, width 6 ns, 300 mJ pulse−1, dt 1 μs ceramics (powder) ablation laser: Nd:YAG: @ 1064 nm, width 6,5 ns, 30 mJ pulse−1 reheating laser: Nd:YAG @ 532 nm, width 5 ns, 45 mJ pulse−1, dt 500 ns ceramic tiles ablation laser: Nd:YAG: @ 1064 nm, width 6,5 ns;50 mJ pulse−1 reheating laser: Nd:YAG @ 532 nm, width 5 ns, 40 mJ pulse−1, dt 500 ns concrete (Cl, Ca) ablation laser: Nd:YAG @ 532 nm, 2.5 mJ, dt 10 μs prespark laser in He gas: Nd:YAG @ 1064 nm, width ns, 110 mJ copper-based alloys ablation laser: Nd:glass @ 527 nm, width 250 fs reheating laser: Nd:YAG @ 532 nm, width 7 ns, dt (1−200 μs) deuterium in zircalloy ablation laser: Nd:YAG, @ 1064 nm, width 20 ps, 26 mJ pulse−1 prespark laser: Nd:YAG @ 1064 nm, width 8 ns, 80 mJ pulse−1, dt 1 μs Gd (oxide) ablation laser, 2 mJ: Nd:YAG, width 10 ns, @ 532 nm or Ti: sapphire, 100 fs prespark or reheating: Nd:YAG @ 532 nm, 10 ns, 30 mJ pulse−1 fossil (snake) ablation laser: Nd:YAG, @ 266 nm, 10 mJ pulse−1 reheating laser: Nd:YAG @ 266 nm, 90 mJ pulse−1, dt 500 ns Ni-based super alloys ablation laser and reheating laser: Ti:sapphire @ 775 nm, 150 fs, Emax 800 μJ pulse−1, dt 0−10.36 ns

enhancement factorb/remarks

ref.

decrease Al I signal and increase Al II signal

96

improvement in the depth resolution irradiated area 4× unequal ablation could be partially surpassed

75

comparable in analytical performance

83

intensity 2×

93

LOD (Cl) of 80 ppm

94

R2 (0.998−0.999)

77

LOD 20 μg g−1

91

intensity 25×

95

85

86 reduction of the plasma threshold 100×

98

dt corresponds to the delay time between pulses. bEnhancement factor of the analytical figures of merit produced by double-pulse LIBS with respect to single-pulse LIBS.

a

(perpendicular to its surface) and the second pulse (parallel to the sample surface) is sent either before to form a preablation spark or after in order to reheat the plasma generated by the first pulse. Additional combinations have been proposed using different beam geometries, pulse width, laser beam wavelength, interpulse delay, relative energy of the pulses, etc. For instance, in collinear DP-LIBS, several laser wavelength combinations

Solids), which would not be feasible without the benefits of double-pulse excitation. Recent applications of DP-LIBS in collinear and orthogonal configurations are summarized in Tables 1 and 2, respectively. The collinear configuration, in which the two laser beams have the same propagation pathway, is the simplest but less versatile approach. In the orthogonal approach, a pulse ablates the sample 648

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Recent advances in instrumentation have allowed incorporation of the DP-LIBS configuration into portable sensors of application, for instance, in cultural heritage (where the use of microinvasive techniques is mandatory), and standoff analysis. For in situ LIBS applications, a portable laser system weighing as little as 3 kg has been designed and constructed.102 This low-cost and compact laser even permits the use of nonintensified chargecoupled device (CCD) detection. Commercial and mobile DP laser instruments have been used for numerous purposes.84,103−105 The setting of these sensors is usually collinear DP at 1064 nm with an energy per pulse in the range of 50−120 mJ at a maximum repetition rate of 10 Hz. For LIBS measurements on bronze alloys, a mobile DP laser instrument has been used.103 A set of reliable LIBS parameters for the quantitative analysis of Cu, Sn, and Zn has been identified. The sensor caused minimal damage to the sample surface. Also, fast and reliable quantitative analysis of complex metallic alloys (steel, in this case) has been performed with a cheap experimental setup.84 Mn, Fe, Ni, and Zn oxides in molten glass were quantified.104 Intensity enhancement factor in the range of 1.5−3 and double reproducibility were obtained with a mobile dual-pulse system. LODs between 7 and 194 ppm have been achieved for these metals in liquid glass. A portable DP-LIBS instrument for a rapid qualitative analysis of materials of interest in cultural heritage has been used.105 A comparison with micro-X-ray fluorescence (XRF) analysis was performed. In DP-LIBS arrangement, the enhancement in signal intensities leads one to expand the use of the CCDs. Hence, CCDs require the accumulation of a larger number of laser shots, but it can be advantageous for trace element detection as in the case of P in an FeO matrix where the estimated LOD for P(I) line at 214.91 nm is 10 ppm.106 This fact is due to the low noise level recorded by the CCDs which produces an improvement in the LOD of minority elements. Otherwise, iCCDs are more versatile and provide better results. Applications of DP-LIBS for the analysis of alloys,77,84,91,92,98 ceramics,83,93 inorganic78,80,107 and organic74,85,87,107−109 materials, liquids (water solutions,76,89,90 samples underwater75,110), and aerosols111 have been reported. DP-LIBS has been applied for the determination of heavy and toxic metals (Pb) in soils and to improve the detection of S in coal.107 In the analysis of polymers, such as polyamide, polyvinyl chloride, and polyethylene, the LIBS signal enhancement has been found to depend on the delay time and the type of polymeric material investigated when compared to single-pulse measurements.108 In a recent report, different DP-LIBS approaches have been evaluated for the analysis of macro- and microelement concentrations in algal biomass.109 The purpose of this research was to improve the analytical performance (selectivity and sensitivity) for the detection of toxic heavy metal and elements with biological significant (K, Mg, Ca, Na) for potential industrial biotechnology applications. The efficiency of the DP-LIBS method for the analysis of liquids was optimized by studying the influence of the excitation energy in the LIBS response.110 An enhancement in the LOD of Mg in water of about 1 order of magnitude was obtained by decreasing the energy of both pulses. The authors suggested that the energy for the first pulse should be close to the plasma threshold where the gas bubble has its maximum lateral expansion and the secondary plasma is still well-localized. DP-LIBS has been evaluated for aerosol analysis. Asgill et al.111 investigated the potential of LIBS to discriminate between the

(532/1064, 532/532, and 532/355 nm) have been tested for determining Pb in metal alloys.92 In all cases, the DP approach improves the limit of detection (LOD) by 1 order of magnitude as compared to the single pulse method. Although a dependence of Pb intensity with the matrix was found for all combinations, a better correlation coefficient of the calibration curves is reported for the combination of 532/355 nm. An orthogonal DP-LIBS method for in-depth characterization of ceramic multilayer samples has been presented.93 In this study, a first defocused pulse (1064 nm, 50 mJ) was used to ablate the material and a second one (532 nm, 40 mJ) to excite and reheat the vaporized sample. The results demonstrate that the signal-to-background ratio (SBR) was improved by almost 2-fold as compared to the single-pulse approach. In addition, a lower ablation rate and a better depth resolution (with a reproducibility of twice better) was found. In the orthogonal DP-LIBS configuration, the ability to use lower energy during the ablation process results in smaller crater sizes compared with SP. This means that the orthogonal arrangement is a useful tool for almost a nondestructive analysis. In this sense, a crater diameter of 10 μm has been observed in the surface of zircalloy samples.91 A prespark in helium at atmospheric pressure, sent 1 μs before a ps ablation laser at an output energy of 26 mJ, was used. A LOD of deuterium in zircalloy very low (20 μg g−1) was found. A similar arrangement has been reported for the analysis of Cl in concrete.94 In this study, a prespark in He with a delay time between pulses of 10 μs allowed the use of ablation energies of only 2.5 mJ per pulse. Experimental approaches using various pulse widths in orthogonal DP-LIBS have been tested. For the analysis of gadolinium oxide, ns or fs pulses were used for the sample ablation and ns pulse was employed as prepulse (prespark in air) or reheating pulse.95 For prepulse mode, no intensity enhancement was observed compared with SP-LIBS mode. For reheating mode, intensive emissions have been obtained for the fs−ns combination of pulse widths. In prespark orthogonal DP-LIBS, the signal enhancement has been attributed to the increase in the plasma temperature.96 In addition, an increase in the intensity of ionic emissions and a corresponding decrease in the atomic emissions (which is related to the role of Saha equilibrium) have been observed. The selective prolongation of emission lifetime only for the enclosed part of the plasma in a rarefied region by preablation spark has been also noted. DP-LIBS methods describing several combinations of laser beams have also been published.99−101 An UV Nd:YAG laser was used for standoff analysis at 55 m, whereas a second pulse from a CO2 laser at 10.6 μm was simultaneously delivered to the target.99 This study reports that the signal enhancement factor (100×) for the targets assayed (metals, ceramics, and plastics) is caused by the higher temperature (several thousands of degrees) of the plasma as compared with that achieved using SP. The same laser combination was used for the analysis of polystyrene film on a Si substrate and for trinitrotoluene (TNT) residues.100 Also, a Nd:YAG laser at 1064 nm (8 ns, 120 mJ pulse−1) synchronized with a CO2 laser (200 ns, 1.5 J pulse−1), in He at atmospheric pressure, have been used for the determination of H in zircalloy samples, an analysis difficult to perform using a conventional LIBS system.101 In this case, the CO2 laser was guided perpendicular to the sample surface, and the ablation laser was focused at an incidence angle of 45°. 649

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ambient gas due to the propagation of the semispherical shock wave generated by the first laser was demonstrated to be responsible for the enhanced material ablation and enlarged plasma dimension for the second laser pulse. An enhancement factor up to 10-fold could be achieved at optimum irradiance conditions. The temporal evolution of Si laser plumes in air at different pressures58 has been reported. The morphology of the plume has been observed under a range of low pressures, and it seems to be strongly dependent on the ambient pressure. The density and temperature of the plasma have also been demonstrated that vary critically with plasma morphology. Three ambient pressure regimes have been identified where the plasma evolution has been observed to differ markedly. The effect of the atmosphere surrounding the plasma in collinear DP-LIBS has been also studied for brass samples.118 An increase in the spectral intensities of several lines has been observed in DP-LIBS in Ar compared to DP-LIBS in air. The enhancement in spectral intensity dropped as the pressure was reduced. A numerical model, describing laser−solid interaction, vapor plume expansion, plasma formation, and laser−plasma interaction, has been developed to describe the effects of DP-LIBS of Cu in He ambient gas at 1 atm.119 The results of DP-LIBS were compared with the SP-LIBS measurements with the same total energy. The DP configuration might be more efficient because the target remains for a longer time in the molten state although the temperature is a bit lower. The mass ablated rate was higher in DP configuration since the total laser absorption in the plasma was clearly lower (reduced plasma shielding). In orthogonal prespark DP-LIBS, mass removal mechanisms at different fluence regimes have been studied by Cristoforetti.120 Results indicate that the air pressure strongly drives the laser shielding effect and that the enhancement of intensity and mass removal were produced when the laser shielding was lower. Also, orthogonal DP laser ablation of Al-based alloy has been investigated.121 In this study, the effect of the sample heating by the preablation pulse and the mechanisms for the mass removal increasing were discussed. De Giacomo et al.122 have experimentally and theoretically compared SP and DP modes. The two laser beams were used in collinear configuration with an incidence angle of 45°. The authors indicated that SP-LIBS has a marked recombination character and it is affected by chemical reactions with the surrounding air. The DP-LIBS, expanding in the first pulse induced plasma, keeps its energy for longer times which turns in a higher ionization degree, better fulfillment of the local thermodynamic equilibrium condition and in a more stable signal. The analysis of the results demonstrated that the most important feature of DP-LIBS, as far as analytical applications are concerned, is the possibility to increase the detection time window and the emission volume, thus obtaining a more stable and intense emission signal. In reference to the LIBS analysis of solid targets in water, the dynamic of the plasma induced by DP has also been studied.123 This work has examined the role of the first and second laser pulses; it was found that the first pulse produces the bubble (without produces plasma), and the second pulse induces the mild plasma in the bubble. Recently, the problem of the poor reproducibility of the DP-LIBS analysis of samples underwater has been investigated, and laser ablation process in water has been discussed (see section on Analysis of liquids and submerged solids).124 In multipulse excitation or MP-LIBS (more than two sequential pulses), only a few papers have been published. A homemade MPQ-switched Nd:YAG laser system has been presented.125 The laser

particulate and gaseous forms of carbon species in an aerosol. The C response was improved in DP configuration, and the use of the ratio of the DP to SP responses has been proposed as an analytical tool. An improvement in the analytical performance is not guaranteed as a result of the double pulse effect in LIBS. A critical study of the experimental parameters must be done before any new application. For example, calcified tissues analysis by DP-LIBS was based on the measurement of relative intensities of ionic and atomic lines for Mg and Ca.112 Compared to SP, the intensity ratio of Mg was similar and the intensity ratio of Ca was worse in the DP arrangement. DP was also tested to classify geomaterials (carbonates as mineral and rock, natural fluorite as mineral, and silicate as rocks and soils) using chemometric tools.78 The DP-LIBS system did not provide any advantage for sample classification over the SP-LIBS system, except in one of the cases tested (soil sample). Also, DP-LIBS has been employed to detect key elements in aqueous solutions at high pressures to simulate the chemistry of the deep ocean.113 Each element (Na, Mg, Ca, Mn, K) was found to have a unique optimal set of parameters for detection. The results suggest that the plasma lifetime is very short (around 500 ns) in high pressure aqueous solutions and LOD did not improve in the DP experiment compared with the SP configuration. Several models to achieve a better understanding of the physical processes arising in DP-LIBS and their effectiveness have been reported. The main cause of improvement in the signal LIBS in the collinear configuration is an increase of the mass ablated with the second pulse. A theoretical model has been published by Rai et al.114 This work indicates that the second laser pulse increases the mass ablated more than 3.5 times in comparison with single pulse mode. The enhancement factor produced by the DP configuration depends on the square of plasma density, its volume, and the fraction of second laser pulse absorbed by the plasma of the first laser pulse (inverse bremsstrahlung absorption due to electron−ion collisions). The increase in plasma temperature causes a greater volume, but this fact only has enhanced effect through enhanced ablation. The role of the matrix composition on the emission enhancement in collinear DP-LIBS has been studied for pure metals by Cristoforetti et al.115 This study attempts to relate optical properties of metals with the correlation between enhancement of ablated atomized mass and the increased plasma temperature. As a conclusion, among all the optical properties of the metals studied, authors have reported that only a relation with the metal diffusivity exists. Nagli et al.116 have studied the first 500 ns of the plasma lifetime in collinear DP configuration on Si and Al targets. When the second pulse arrives, the increase of the plasma volume is higher than the increase of ablated mass. Consequently, the electron density decreases, the number of collisions with freeelectron is reduced, and the plasma lifetime increases. Furthermore, free electrons absorb the remaining energy of the second pulse so that the energy of the free electron increase, favoring the concentration of ionized species in the plume. Thus, for the double pulse arrangement, lower continuous radiation and narrower lines implying a lower electron density should be expected, the relative intensity of ionized species being higher than for neutral species. The expansion dynamics of the plume in SP and DP excitation regimes was studied on glass samples.117 In reference to the double pulse dynamics in collinear mode, local rarefaction of the 650

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atmosphere showed noticeable line broadening particularly at high ambient pressure. Although typically performed using ns lasers, some recent investigations used fs lasers for surface diagnostics in materials analysis: surface inspection, depth profiling, interface characterization, and thin film analysis. Fs laser ablation was applied to the chemical analysis of Si with the aim of determining the crater size from which spectral emission could be measured.135 A Ti:sapphire laser delivering 100 fs pulses was used. The beam in the far-field was able to produce subμm craters and in the nearfield resulted in the formation of sub-30-nm craters. Depth profiling with an average ablation rate of 15 nm per pulse has been achieved.136 The fs LIBS plasmas were generated using a pulsed laser at 795 nm with pulse energy and duration of 200 μJ and 130 fs, respectively. A study of depth-profiling and interface characterization have been performed using fs LIBS.137 The authors used a Ti:sapphire laser working at 780 nm, with 150 fs pulses and a maximum energy of 720 μJ. Small individual circular craters with approximate diameter of 30 μm without cracking or melting were observed. However, ablation-induced surface roughening was noticed. In another work, spectrochemical analysis of microcracks and their propagation on the surface with lateral resolution of 2 μm have been studied.138 Fs LIBS has been used for the detection of heavy metal dopants in porous thin films.139 The benefit of selective ablation and better control of material removal may overcome the additional expense of ultrafast lasers for thin film analysis. Other applications of fs LIBS include the analysis of euro coins140 and of animal tissues141 and the monitoring of silver transport through silicon layers of fuel particles for a high temperature gas reactor.142 Also, fs LIBS has been evaluated for the detection of explosive residues.143 A comparison of LIBS using ns and fs pulses for analysis of trinitrotoluene (TNT) residues on an Al substrate showed that the molecular bands (CN and C2) are more intense with fs pulses. The spectrum with ns pulses was dominated by elemental lines. However, De Lucia et al.144 suggested that several advantages attributed to fs pulses are not realized at higher laser fluence for the analysis of explosive traces. Ultrashort laser pulses have been also studied in DP arrangements. Piñoń et al.97 employed a collinear geometry and have studied the effect of the interpulse delay time. Compared with fs SP-LIBS, the energy of one pulse divided into two pulses led to improve the optical emission by 1 order of magnitude at a delay time comprised between 50 and 1000 ps. The fs DP mode increases the plasma temperature and the electronic density resulting in an increase of plasma lifetime, signal intensity, and signal reproducibility.45 For lines with higher energy levels, the intensity enhancement has been more noticeable than for low energy levels. A reduction of the plasma threshold by a factor of 2 minimizes the surface damage. Surface effects have also been studied in Ni-based superalloys.98 The use of fs DP-LIBS in these samples reduced the crater depth to less than 60 nm compared with the SP mode (depth of 200 nm). Crater depth on Ag foil by fs laser pulses has been measured in vacuum at different fluences.145 The effect of the second collinear fs pulse and the delay time has been studied. Results suggest that the laser ablation depth has a logarithmic dependence with the beam fluence. Combinations of fs and ns pulses in orthogonal DP-LIBS experiments have been developed. For the analysis of Cu-based alloys, excellent linear regression coefficients (0.998−0.999) were obtained by integrating all emission intensity data along the whole interpulse delays used.77 For the analysis of gadolinium

basically consisted of a train of pulses (up to 6) and width in the range of 20−30 ns with average energy per pulse of 28 mJ (total energy about 170 mJ). MP-LIBS has been applied to the analysis of steel.126 In this case, a train of pulses (up to 11) of lower energy and separated by a few μs was obtained by reducing the delay between the Q-switch opening and the flash lamp. A crater depth of 90 μm in Zn suggests that MP-LIBS is a suitable arrangement for the online analysis and quality control of layered materials. MP-LIBS has been also evaluated to the quantitative analysis of gold alloys.127 Good results in terms of accuracy and precision were achieved. Twenty spectral lines from eleven elements were investigated under MP-LIBS (up to 6 pulses, interpulse delay of 20−40 μs). The improvement found in the LOD compared with DP has been discussed.128 In this context, the processes involved in signal enhancement have been reviewed by Galbács et al.129 The enhancement in MP-LIBS is caused by the increased material ablation but also because the reheated plume provides long excitation for certain species. As a conclusion, MP-LIBS causes a lower breakdown threshold and consequently an improvement in the ablation efficiency. Femtosecond LIBS. This section reviews recent LIBS work using ultrashort pulses. Under this approach, the beam does not interact with the resulting plasma. The interaction of fs pulses with the material provides special features to the ablation process as a lower threshold and a larger efficiency which are different for longer pulses. Other advantages include the smaller heat affected zone, a better depth resolution, a faster broadband-background decay, and matrix-independent sampling. Progress in fs technology is occurring quickly and new applications of short-pulsed ablation are expected to grow. However, for this to happen, the ultrafast laser should be more affordable. The principles and uses of LIBS, including coherent Raman spectroscopy (CARS) and terahertz (THz) spectroscopy using ultrafast lasers, have been reviewed.130 The influence of pulse duration for LIBS analysis using ns pulses or fs pulses has been studied in the analysis of bronzes.131 By comparing single-shot LIBS with fs and ns laser pulses, the fs spectrum was well-resolved and presented a very low background emission, allowing signal accumulation for a large number of pulses. For fs-laser pulses, plasma emission was found to vary much more rapidly with time than in the case of ns-laser-produced plasma. This plasma behavior using fs pulses results in a significant reduction of the continuum as mentioned. In this sense, plasmas produced by ultrashort laser seem to reach Local thermodynamic Equilibrium (LTE) after its formation. Spectra of depleted U metal obtained using ns Nd:YAG (1064 nm) and fs Ti:sapphire (800 nm) laser pulses have been compared.132 Fs pulses generate cooler plasmas, and the lines are short-lived compared with longer pulses. Neutral U atoms lines appear immediately after excitation. However, the line intensities are generally higher in ns LIBS whereas the sensitivity of ns LIBS is better than that of fs LIBS. The detection power of fs LIBS however may change drastically when different samples are analyzed. For instance, a Ti:sapphire laser (40 fs) was used to generate plasmas on the surface of aqueous solutions. LOD values for Al, Cu, Fe, K, and Zn in water were 6.4−200 times lower compared with ns laser excitation.133 A study on the influence of buffer gas (air, Ar, and He) and the pressure on the LIBS spectrum has been reported for UV fs pulses on solid samples (brass, Cu, Al, and Si).134 Even though the maximum emission intensities were measured when Ar gas was used, He led to a higher signal-to-noise ratio (SNR) and less broadening of the emission lines (attributed to the Stark effect) resulting in an enhancement of the spectral resolution. Air or Ar 651

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for the ablation step is Gaussian (2−3-fold) and when the heating pulse was directed transversely to the plume (6−12-fold). The application of resonant LIBS has also been tested for the analysis of polymers. The enhancement observed when the laser wavelength was tuned to the vibrational transition of the polymer was studied by Khachatrian et al.154 The degree of enhancement depends on the particular vibrational mode excited. Significant enhancements were found for the C−H stretch fundamental vibrational transitions. Resonant LIBS in the mid-infrared (midIR) spectral region have been discussed for the detection of residues on surfaces. However, this approach seems to be more appropriate to bulk samples.

oxide cited above, ns or fs lasers were used to the ablation and a ns laser was used as prepulse or reheating pulse.95 In the reheating mode, the signal intensity enhancement was 25-fold for the fs−ns combination. In addition, applications of ultrashort LA, namely, fs pulse thin films deposition and fs/ns pulses in DP-LIBS for elemental analysis have been discussed.146 One of the most important features of the plasma generated with ultrashort laser pulses is the large presence of nanometric-sized particles. Resonant LIBS. Resonant LIBS (RELIBS) is based on the photoresonant or selective excitation of species in the plume. Suppression of background and improvement in sensitivity have been demonstrated using this method. A requirement for the method to work is the prior knowledge of the sample. Vadla et al.147 have studied the role of energy in resonant excitation of cesium vapor generated by a continuous laser tuned to a resonance transition. The loss of energy in exothermic collisions of laser excited atoms was concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas. RELIBS provides the capability to selectively enhance atom generation by careful selection of the ablation laser wavelength and incident energy. The application of RELIBS to quantitative analysis has also been evaluated.148 Resonant laser ablation (RLA) was assessed for steel analysis. The signal from an element at a wavelength coinciding with the selected wavelength improves over the signals of other elements. This fact supports the hypothesis that desorption induced by electronic transitions is involved in the enhancement process. Detection of Pb at low concentration in contaminated water was improved when a second resonant laser was delivered to the sample.149 The plume produced by one laser pulse at 266 nm (170 μJ pulse−1) was re-excited after a short delay with a ns laser pulse tuned to a specific transition of Pb. In this case, the LOD for Pb in water was found to be approximately 60 ppb when 1000 shots were accumulated. Detection of Pb traces on brass and on water was also tested in DP mode.150 The LOD of Pb improved when a second laser tuned at a selected λ transition in lead was used. In this work, the configuration of DP-LIBS was named LIBS-LIF because the selected λ was used to excite the traces of the element. The DP setup consisted of a Q-switched Nd:YAG laser and a ns optical parametric oscillator (OPO) laser. The LODs for Pb in both solid and water samples were 2 orders of magnitude with respect to LIBS. For RELIBS analysis where the second pulse excites the matrix, the result was similar to the LIBS response. In an attempt to improve the LOD of trace elements (Mg and Si) in Al alloys, a Nd:YAG laser pulse (1064 nm, 7 ns) was used for ablation and a OPO laser pulse (tuned at 396.15 nm, 7 ns) was used to resonantly excite the aluminum host atoms.151 The LODs achieved using RELIBS were comparable to LIBS, but a much lower amount of matter could be ablated from the sample. In other study, RELIBS was investigated for enhancement of the LOD of Pb in Cu alloys.152 An optical parametric oscillator laser was used to ablate the sample, and it was tuned to the Pb(I) 283.31 nm line. The Stokes direct-line fluorescence signal at 405.78 nm was recorded. The LOD for Pb (8 ppm over 500 laser shots) was 1 order of magnitude better than using regular LIBS. Aluminum alloys have been ablated by a laser pulse, and the expanding plume was photoresonantly reheated by a dye laser pulse.153 Cheung et al. discussed the enhancement in the signalto-noise ratio (SNR) of Mg, Pb, Si, and Cu, when the laser beam

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RANGING APPROACHES Field analytical instrumentation is an attractive option for fast chemical response (industrial, military, and security applications), on-site measurement ability (geological exploration and environmental monitoring), and in those cases where the object/ material cannot be transported to the lab (archeological and cultural heritage applications). In other words, in situ analysis is needed in those applications where access to the sample is difficult or in situations that may severely affect the human health (for example, in radioactive environments). Due to its versatility, LIBS is an excellent candidate for use as a field sensor. In the past few years, continuous advances in reducing the size and weight while increasing the performance of lasers, spectrographs, and detectors make possible the development of compact and rugged instrumentation. In especial, the use of fiber lasers, compact spectrometers, and fiber optics for guiding the plasma emission offers greater flexibility while reducing the risk of instrument failure. Fortes and Laserna reported a full revision of fieldable LIBS instruments and applications.27 On the basis of this report, we categorize the ranging approach of LIBS as portable, remote, and stand-off configurations. In a portable system conf iguration, that is, a man-portable sensor for field analysis, the operator and the sensor are close to the target. The number of portable instruments designed by different research laboratories has considerably increased in the past few years. Due to the proven reliability and ruggedness of solid-state lasers, the majority of works reported in the literature used a Nd:YAG laser working at 1064 nm. The specific application (environmental, industrial, geological, cultural heritage, and security, among others) determines the use of detectors with an analytical response in the entire spectral range. This fact requires a compromise between the spectral resolution of the detector and the requirements for a miniature instrument. Most portable systems were based on a sampling probe (containing the laser head) and a main unit (mainly a suitcase or an aluminum case containing the laser power supply, the spectrograph, the detector, and the laptop). Cuñat et al.155 designed a man-portable laser system in which both the spectrometer and the computer are enclosed in a specially adapted backpack. This prototype was successfully evaluated for geochemical analysis of karstic formations,155 for determination of Pb in road sediments,156 and for the inspection of oil spill residues in the coast.157 The analyzer was a technological evolution of the original instrument proposed by the same authors. A similar configuration was also used by Munson et al.158 for the detection of indoor biological hazards. A novel compact and portable pulsed laser system was developed for in situ SP- and DP-LIBS applications102 in the IESL-FORTH. In this case, Goujon and co-workers demonstrated the versatility of the instrument for identification purposes within a broad variety 652

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Figure 3. Schematic diagram of (A) remote LIBS system and (B) standoff LIBS instrument specially designed for field measurements: (1) laser, (2) optical module, (3) laser power supply, (4) personal computer, and (5) detector and spectrograph.

Sea. This demonstration opens a new front of applications to the LIBS technology in the area of submarine research. In cases where a large area must be analyzed, an open-path LIBS configuration should be used. In this mode of operation, named stand-off LIBS (ST-LIBS), both the laser radiation and the returning light from the plasma are transmitted through the atmosphere. As demonstrated, the use of a laser system with a good beam quality factor is mandatory for laser-induced plasma formation at long distances.165 One of the major problems associated with ST-LIBS is the attenuation of light by the atmosphere.166 However, nowadays, the range of ST-LIBS applications has considerably increased with interesting demonstrations in the area of forensic science and in analysis of explosives (see Security and Forensics).167 Although LIBS is essentially an elemental analysis technique, it has been successfully tested as a powerful method for detection and identification of residues and bulk explosives.23,74,168−173 Recently, the possibility of combining LIBS with other spectroscopic techniques has been explored. Particularly well suited technologies for the combination with LIBS are Raman spectroscopy and molecular fluorescence spectroscopy as these techniques use essentially the same instrumentation and also the three of them are stand-off technologies. In a series of three papers, Moros et al. explored the Raman-LIBS sensor fusion as a powerful approach for detection of explosives and related materials.174−176 The information extracted from the same single spot when both sensors work side-by-side was first demonstrated.174 Later, the strong and weak points of each technique were analyzed.175 Finally, the ability for Raman-LIBS detection and discrimination of explosives residues at 20 m was demonstrated. Using the 2D image generated from preprocessed molecular and atomic data, a wide range of compounds was easily distinguished from each other by simple linear correlation.176 The potential of hyphenation of these spectroscopic methods has been also demonstrated in the authentication of inks.177 On the other hand, the combination LIBS/LIF also yields valuable information since it combines the atomic information provided by LIBS with the molecular data provided by LIF. Furthermore, LIF has been widely used in the analysis of vegetation, pollutants, and cultural heritage samples.

of materials, such as pigments in paintings and icons, metals, ceramics, etc. Most recently, Rakovský et al.159 designed a compact portable instrument for geochemically recognizing tephra layers in lacustrine sediments and fossilization processes in ammonites. It should be noted that, whether it be a commercial or homemade portable instrument, results obtained using this technology are in good agreement with those reported in laboratory.160,161 Several options can be considered when LIBS measurements at a distance are needed. In this context, some confusion exists regarding the use of the terms remote and stand-off when referring to the analysis of distant objects. For the purposes of the present article, we distinguish both approaches in Figure 3. In a remote system (Figure 3A), the laser and the signal are transmitted through a fiber-optic cable, whereas in a stand-of f system (Figure 3B) both the laser and the signal are transmitted along an open path. In both cases, the operator is located far from the target. Laser technology and optical fibers play an increasingly important role in the design and construction of LIBS sensors and measuring systems for remote analysis. The integration of fiber-optic cables is a solution for applications in which the target is not directly accessible or is located in extreme environments. When fiber-optics (FO) is used within LIBS instrumentation, one or more optical fibers can be used. These FO devices are capable of transmitting laser radiation in the range of MW cm−2 without any damage to the fiber. Furthermore, there exists the possibility of collecting the plasma light using the same fiber employed to transmit the laser beam. Most remote LIBS applications have been specifically focused on soil analysis and environmental monitoring. Bousquet et al.162 designed a mobile system based on remote LIBS technology for the in situ analysis of polluted soils at 10 m. The authors used independent optical fibers for transmitting the laser radiation and for collecting the plasma light. A single lens at the end of the fiber was used to focus the laser beam. In contrast, Dumitrescu et al.163 tested a movable fiber probe for gas-phase LIBS. The authors evaluated the associated effects of delivery fiber curvature on LIBS signal in order to improve the fiber to laser coupling. Most recently, Guirado et al.164 demonstrated the capability of LIBS for the recognition and identification of archeological materials submerged in seawater at depth of 30 m in the Mediterranean 653

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and number of laser pulses.187 Compared with nanosecond laser pulses, fs excitation provided lower damage thresholds to the targets, although higher damage thresholds to the near field scanning optical microscope (NSOM) probes were at the wavelength studied. In the conventional applications of LIBS discussed in this Review, laser pulse energies range between 10 and 100 mJ. In this energy regime, typical laser focal spot sizes are on the order of a few micrometers. However, improved lateral resolution and surface sensitivity can be obtained by decreasing the pulse energy down to hundreds of μJ. Godwal et al.188 have integrated a LIBS system with a microfluidic platform demonstrating the sensitive detection of Na in a microdroplet. These same authors149 have generated 2D maps of a fingerprint on a Si wafer using 5 μJ ultraviolet laser pulses. Here, the detection sensitivity has the limiting factor in the amount of material ablated at very low laser powers. In these circumstances, increasing the excitation efficiency by resonant ablation is largely beneficial. Laser-induced plasma formation followed by resonant excitation by a second pulse tuned to a specific transition of Pb has resulted in a limit of detection of approximately 60 ppb for Pb in water. A setup has been proposed for analyzing artworks using a microscope.189 Best working conditions in order to obtain the least amount of material removal during analysis have been investigated. In order to solve the problem of sensitivity at very low ablation rate, a new method of signal processing on two-dimensional echelle images has been developed.190 This method is based on the comparison of two 2D images and the identification of pixels groups (particles) that can be considered as representative of actual signals. In this methodology, the majority of noise peaks is eliminated, and identification of weak signals is possible without altering the intensity ratio between emission lines. It has the advantage of requiring only two LIBS data acquisition sequences, which is very important when working on very small samples. Ablation sampling down to the sub-30 nm range using fs near field apertured laser processing has been demonstrated.135 No LIBS signals have been reported from such a limiting amount of ablated material. Cultural Heritage. In the past decade, LIBS has become a valuable analytical tool for cultural heritage. No sample preparation, minimally destructive, fast analytical response, depthprofiling analysis, and the capability for in situ analysis are the main features that make LIBS a very attractive technique for the characterization and conservation of archeological samples, artworks, and other important materials.24,191 The chemical composition extracted from a LIBS analysis gives archeologists additional information to better understand our history. Thus, a close collaboration between the LIBS community and the archeology world is required. Important assets such as sculptures, stone, ceramics, metallic artifacts, wood, and painted artworks, among others, have been analyzed by LIBS. Harmon et al.192 identified the provenance of obsidian samples (a volcanic glass used by ancient peoples as a raw material for producing tools) using partial-least-squares discriminant analysis (PLS-DA) of LIBS data. Although information extracted from this analysis allows archeologists to better understand artifact production and past trade patterns, LIBS still requires further improvements in data processing analysis for discrimination analysis. Bronze materials are one the samples receiving more attention in cultural heritage applications. Recently, Gaudiuso et al.193 proposed a new procedure for determination of plasma excitation temperature in copper alloys. This method is based on local thermodynamic equilibrium and consisted of inverting the

APPLICATIONS Surface Inspection, Depth Profiling, and LIBS Imaging. During the last decades, the potential of LIBS as a surface characterization tool for spot analysis, line scan, depth-profiling, area analysis, and compositional mapping with a single instrument in air at atmospheric pressure has been demonstrated. As compared to other techniques of surface analysis, LIBS presents the advantages of sampling flexibility in terms of size and shape of the analyzed specimen in combination with a fairly good lateral and in-depth resolution and surface sensitivity. Some interesting examples include trace elements in teeth,178 accumulation of heavy metal in vegetal tissues,179 characteristic elements in an engine valve,180 dopants in transparent dielectric,181 and spatial distribution of elements in speleothems.182,183 Furthermore, LIBS has been combined with optical catapulting (OC-LIBS) and applied for the first time as a new developing technique for the analysis of explosive residues in human fingerprints deposited on glass.184 Results obtained by OC-LIBS corroborated the absence of spectral interferences when compared with conventional LIBS and the freedom from spectral contribution of the substrate where the sample was deposited. The effect of surface topography on LIBS signal during point analysis and acquisition of chemical maps has been studied.185 The analyzed samples consisted of stainless steel with different surface finishes. The authors concluded that differences on the surface state yield changes on the LIBS signal that can thus lead to a misinterpretation of the results. However, this effect was minimized when enough pulses were delivered on the same position, and the intensity detected was similar on all the analyzed samples regardless of the initial state of the surface. The development of LIBS as an accurate depth profile analysis technique for coatings has been worked out for many years. Different LIBS approaches have been demonstrated for this application.186 For instance, Galmed et al.136 have evaluated the ability of fs-LIBS to generate depth profiling for a Ti thin film of thickness 213 nm deposited on a silicon (100) substrate before and after thermal annealing. An average ablation rate (AAR) of 15 nm per pulse was achieved. The depth profiling was verified with a theoretical simulation model, and a very good agreement with the experimental results was obtained. In other significant studies, an orthogonal DP-LIBS arrangement was applied for first time to the depth characterization of ceramic multilayered materials as an alternative to the SP approach that used defocused beams to decrease the power density and consequently the AAR.93 The depth resolution with the double pulse configuration was improved by almost 2-fold as compared to the regular single-pulse approach whereas the reproducibility in the description of depth profiles was also twice better. The potential of MP laser excitation scheme to in-depth characterization of electrolytically galvanized steel has been presented by Cabaliń et al.126 Here, bursts of 11 laser pulses with interpulse gaps of 7.4 μs were produced from a commercial electro-optically Q-switched Nd:YAG laser containing a KDP* crystal. With MP excitation, the ablation efficiency was increased 10-fold on iron and 22.5-fold on zinc with respect to dual pulse or singlepulse excitation. The results demonstrated that it was possible to obtain the sample stratigraphy up to depths of 90 μm on zinccoated steel sheets, using a single burst with a total energy of only 60 mJ. Sundaram et al. have performed a systematic study to understand the size and shape of nanopatterns generated on selected semiconducting (Si and Ge) and metallic (Cr, Cu, and Ag) targets under different laser pulse durations, laser energies, 654

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Figure 4. (A) The diver working at a 30 m depth (Reprinted with permission from ref 164. Copyright 2012 Elsevier) and (B) LIBS spectra of a bronze material acquired at 30 m depth.

all these cases, XRF was used as a validation method. Results were in good agreement with LIBS data. In addition, the hyphenation of LIBS with Raman spectroscopy has broadened the range of applications of both techniques. As discussed above, sensor fusion yields complementary information about the sample under examination (see Ranging Approaches). Sharma et al.204 integrated a combined Raman-LIBS system for the chemical characterization of minerals. Here, Raman spectroscopy yields information concerning the minerals and their polymorphs while LIBS is sensitive to minor/trace elements in the elemental composition of the sample. In addition, Osticioli et al.205 also demonstrated the versatility of a Raman-LIBS instrument for giving information on the chemical composition of different artworks such as fresco and terracotta samples and a bronze figure from the Ghibertis̀ “Porta di Paradiso”. At present, continuous advances in mobile technology have resulted in the construction of portable LIBS instruments capable for on-site analysis in museums, art galleries, caves, or archeological excavations. Hence, transport of the artwork to the laboratory is eliminated, thus reducing the total analysis time and the risk of irreversible damage to the object. Of particular interest in contemporary archeology is the in situ characterization of archeological materials from the marine environment. Certainly, extraction of submarine assets is quite often not practical and/or not permitted, thus making the development of analytical technology for submersed material a particularly appealing activity. Recently, Guirado et al.164 provided a new solution for this problem and demonstrated for the first time the analysis of submarine archeological materials using a remote LIBS instrument. Figure 4A shows a picture taken during the onsite trials on the Mediterranean Sea. The chemical signatures of ceramic materials, metallic samples, and precious metals were acquired at depths of 30 m during a field campaign performed in the Mediterranean Sea (Figure 4B). Industrial Analysis. LIBS has been extensively investigated for industrial process control in the steelmaking industry. Significant applications include characterization of hot and molten metals, inspection of surface dust on steel sheets, online measurement of coating thickness and composition, classification of metals and alloys, sorting of steel parts, analysis of slag, and classification of scrap.8,206,207 Robustness, stability, reliability, analysis speed, and operational availability are important performance characteristics that make LIBS an important tool in the steel factory.208,209

calibration-free (CF) approach, overcoming some of the disadvantages of CF-LIBS and calibration curves. Also, only one standard is required for the analysis (no matrix-matching is necessary). The protocol was performed in a set of archeological bronzes. High precision data comparable to those obtained with CF-LIBS and calibration curves were reported. One of the most important problems regarding the analysis of archeological materials arises from the fact that most of these pieces are structured in different layers, either from the manufacturing process or due to degradation phenomena. When compared to other analytical techniques, LIBS is capable of describing the surface and subsurface composition of a material while preserving the sample integrity. This capability is largely advantageous when attempting the discrimination between genuine archeological findings and modern counterfeits. Consequently, depth profiling analysis has been performed in a broad variety of materials. Significant examples include Roman wall paintings from the archeological area of Pompeii,194 characterization of iron Age pottery from Turkey,195 analysis of frescoes,196 authentication studies of unglazed earthenware artifacts,197 and chemical analysis of multilayered painted surfaces198 and metallic artifacts.199 Recently, LIBS has emerged as a promising technique for the analysis of bioarcheological materials such as calcified tissues, namely, teeth and bones. Harith et al.200 evaluated the influence of biological degradation and environmental effects on the interpretation of archeological bone from three different ancient Egyptian dynasties. Authors demonstrated that post-mortem effects must be taken into consideration on studying dietary habits. Also, a clear correlation between the degradation of the tissues in the archeological bones with the absence of CN and C2 molecular band in the LIBS spectra was found. Rusak and coworkers201 used calcium-to-fluorine ratios as indicators of bone preservation quality. Thus, a value for this ratio of 5.70 could be used to distinguish well-preserved and poorly preserved bones, regardless of the species considered. LIBS has been extensively used in combination with multivariate statistical approaches such as linear discriminant analysis (LDA), principal component analysis (PCA), and soft independent modeling of class analogy (SIMCA), for establishing the geographical origin of historical building materials.202 Classification of silver Roman denarii203 and the characterization of calcareous and refractory materials from the ancient GreekRoman theater of Taormina105 have been also demonstrated. In 655

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Figure 5. Photograph of the LIBS demonstrator as installed during the field trials developed in TKS: (1) optical system, (2) laser heads, (3) optical fiber, (4) spectrometer, (5) pulse generators, (6) power supplies, (7) steel strip, and (8) deflection pulley. Reproduced with permission from ref 211. Copyright 2010 Society for Applied Spectroscopy.

Noll et al.210 have demonstrated versatile high-speed steel analyzers that use a Paschen−Runge spectrometer, photomultiplier tubes (PMTs), and multichannel integrator electronics capable for the simultaneous detection of a wide variety of elements with measuring frequencies of up to 1 kHz. On the opposite side, the University of Málaga has designed and constructed a portable LIBS system based on high resolution miniature spectrometers using integrated linear CCD arrays exhibiting a spectral resolution of 0.08 nm. This instrument was designed for online production control of the thickness of Mg on electrolytically galvanized steel.211 Figure 5 shows a photograph of the LIBS demonstrator as mounted in the field trials carried out at ThyssenKrupp Steel (TKS) pilot plant in Dortmund, Germany. For variable Mg thicknesses (depending on strip speed of the pilot line), a satisfactory agreement between plant LIBS measurements and data from laboratory chemical analysis by ICP-OES of Mg coating thicknesses was found. Additional investigations have been conducted in order to assess the potential of LIBS for sorting and recycling of scrap materials. In this case, the greatest constraints arise from the irregular sample geometry, the presence of surface debris, matrix effects, and interferences, all of which result in poor reproducibility and repeatability. To overcome these difficulties, several approaches have been developed. For instance, Werheit et al.212 proposed the use of a 3D scanning LIBS setup, where the focal point can be rapidly changed to improve analytical performance. Their system successfully classified and automatically separated Al postconsumer scrap charges, consisting of wrought and cast alloys. The application of LIBS to control the process of precious metal recovery and recycling has been also evaluated.213 The results demonstrated that LIBS can be considered as a viable alternative to inductively coupled plasma optical emission spectrometry (ICP-OES) and XRF for the determination of recovered precious metals. Recently, LIBS together with discriminant function analysis (DFA) has been utilized for identification and classification of six

groups of the most used polymers in manufacturing and packaging of materials.214 The spectral line ratios of CN(388.3)/C(247.85), H(656.28)/C(247.85), and Cl(837.59)/C(247.85) were used as input variables in DFA. Results show that LIBS/DFA was capable of the correct classification of 99% of the polymers. In a recent work, Matiaske et al.104 demonstrated the capability of a mobile DP-LIBS system for the analysis of mineral melts. Solid standards and a calibration transfer approach have been evaluated for this application. Environmental Monitoring. Soils and slurries constitute the environmental workspace most affected by heavy and toxic metals from many anthropogenic sources. Numerous LIBS investigations have focused on the in situ semiquantitative and quantitative characterization of these metals.22,24 The most recent studies related to this topic are summarized in Table 3.80,107,156,157,162,215−227 Macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Cu, Mn, Zn, B, Mo, Ni, and Cl) play a decisive role in plant nutrition and can affect crop yields when not present in appropriate concentration levels in soils. LIBS seems a suitable in situ and real-time technique to determine nutrient distribution in soils, although soil heterogeneity and matrix effects are two factors that make this application difficult. Trevizan et al.228,229 have determined macro- and micronutrients in pellets of plant materials using an approach based on univariate calibration with certified reference materials (CRMs). Some discrepant results were observed, which indicates that matrix effects and differences in particle size distribution of CRMs and samples are relevant for the analysis. Recently, LIBS possibilities and drawbacks in the quantification of the total elemental concentration of soils under laboratory-controlled conditions have been studied.230 Other interesting applications of LIBS have focused on the investigation of the metal accumulation in vegetal tissues.231 Galiová et al. demonstrated the capability of LIBS for mapping Ag and Cu distribution directly in plant leaves of Helianthus Annuus L.232 Environmental challenges from different types of industries are associated with wastewater generation contaminated with toxic 656

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657

soil

roadside materials rock, sandstone, beach sand soil

slurry slurry

soil slurry

soil

qualitative determination of oil spill residues using a man portable LIBS system toxic metals in Gulf War oil spill contaminated soil

online monitoring the remediation process of soil contaminated with chromium metal development of a mobile system and a classification of the samples by PCA quantitative multielemental method for contaminant determination in soil under sewage sludge application multivariate and univariate analysis to determine total C concentration quantitative analysis of slurry samples using univariate calibration, multiple linear regression, and partial least-squares regression quantification of metal applying univariate and multivariate calibration analysis of simulant slurry samples used in the vitrification process of liquid radioactive wastes quantitative determination of Pb using a man portable analyzer

soil soil soil soil and sludge

soil

in situ semiquantitative analysis of polluted soils quantification of Pb in soil using DPcomparison of As detection by the SP and DP quantitative and qualitative determination of heavy metal

soil

issue analyzed by LIBS

quantification of Cr, Cu, Pb, V, and Zn; determination of anthropogenic index (AI) for Cr (AICr) and Zn (AIZn) screening of cadmium contamination in soils

soil

sample

C, CN, and C2 bands H, N, O, Mg, Na, Fe, and V and Ca, Si, and Al Al, Ba, Ca, Cr, Fe, Mg, Na, S, Sr, Ti, V, and K

Pb

Mg, Si, and Fe Al, Fe, Si, Na, and Li

total C Al, Ca, Fe, Ni, and Si

Ba, Co, Cu, Mn, Ni, V, and Zn

Fe, Ca, Na, Si, and Al

Cu, Pb, Fe Pb As Al, Ca, Cr, Cu, Fe, Mg, Mn, Pb, Si, Ti, V, and Zn in soil Cr

Cd

Cr, Cu, Pb, V, and Zn

elements

Table 3. Summary of LIBS Applications to Analysis of Heavy and Toxic Metals in Soils and Sludge −1

limit of detection −1

−1

Al (12 ppm), Ba (3 ppm), V (2 ppm), Ti (6 ppm), Sr (7 ppm), S (7 ppm), Fe (12 ppm), Ca (14 ppm), Mg (9 ppm), Cr (2 ppm), K (10 ppm), Na (7 ppm)

Pb (190 mg kg−1)

Ba (8.01 mg kg−1), Co (9.33 mg kg−1), Cu (9.94 mg kg−1), Mn (114 mg kg−1), Ni (7.86 mg kg−1), V (46.9 mg kg−1), Zn (30.7 mg kg−1)

Cr (2 mg kg−1)

Cr (17 mg kg ), Cu (61 mg kg ), Pb (20 mg kg ), V (29 mg kg ), Zn (55 mg kg−1) Cd II 214.441 nm (1.3 μg g−1) Cd II 226.502 nm (3.6 μg g−1) Cd I 228.802 nm (4. 0 μg g−1) Pb (200 ppm), Cu (80 ppm) Pb (20 ppm) As (85 mg kg−1)

−1

227

157

156

225 226

221−223 224

220

162

219

217 107 80 218

216

215

ref.

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Table 4. Biomedical Applications of LIBS applications analysis of carious tooth decay

samples teeth

in dentistry

teeth

analysis of nails in gastroenterology

nails gallbladder stone

laser ablation of stones and kidney stones, urinary application in stones nephrology analysis of body fluids in diagnosis

glucose and NaCl malignant tissues

tissue classification analysis of bones

brain, lung, spleen, liver, kidney, and skeletal muscle calcified bones

analysis of blood

blood samples

biodiagnostic

pathogens and viruses on substrates salt and soil

analysis of biological specimens

elements

issue studied by LIBS

Ca, Na, and Sr

spectroscopic investigation of carious tooth decay

Ca, Mg, Cu, Zn, Sr, Ti, C, P, H, O, Na, and K Ca, Mg, P, and Zn Cr, Co, and I Ca, Na, and K C, Ca, Cu, Fe, K, Mg, Mn, N, Na, O, P, Sr, and Zn C2 swan bands and CN violet bands Ca, Mg, Cu, Fe, Zn, Sr, Na, K, C, H, N, O, P, S, Cl, etc. C, H, N, O, Ca, Mg, Na, Sr, K, and Pb

rapid identification of caries and the roles of the presence of metal elements for caries formation microleakage in dentistry; microleakage between infrastructure and veneer materials in dentures

qualitative and quantitative analysis of different kinds of kidney 253, 254 stones, urinary stones and cross-sectional study

analysis of organic matter such as glucose and metal elements spectrochemical analysis to identify and characterize some types of human malignancies Ca, Al, Fe, Cu, Na, Zn, Cr, Mg, K, characterization of different normal animal tissues P, C, Li, Ni, Mo, Sn, Sc

255 256 257

biological degradation and environmental effects in bone 200 samples qualitative analysis of blood and other liquid organic compounds 258 differentiation of pathogens and viruses on substrates

Ca, Mg, Na, K, Al, Si, Sr, Ti, Zn, analysis of common edible salts, soils and heterogeneous Fe, S, C, H, N, and O, C, and N biological samples

metals such as Ni, Cr, Pb, etc. In this field, LIBS is particularly interesting for on-site and remote monitoring of the concentration of these toxic metals in liquid environmental samples such as wastewater from industrial plants and in seawater. As discussed in the section devoted to the analysis of submerged solids, plasma formation in liquid media is a complex phenomenon dominated by a continuum background and consequently a low emission signal. Generally, direct analysis of liquid samples by LIBS is implemented by focusing a laser pulse on the surface or in the bulk liquid or on the surface of a liquid jet. Inherent drawbacks of such a method include splashing and formation of surface ripples in the liquid produced by laser-induced shock waves. However, several interesting results have been reported in this area.233,234 For example, Hussain and Gondal have developed a LIBS system for the analysis of Ca, Mg, P, Si, Fe, Na, and K in wastewater from a dairy product plant.235 To overcome the problem of splashing of water on optical components, a special cell was designed. In addition, relatively high energy laser pulses (100 mJ) were used to enhance the emission intensity. This approach provided accurate analytical results that were in good agreement with those obtained with ICP-OES. Determination of Cr in industrial wastewater from electroplating industries has been also conducted.236 In this case, a liquid jet was used. LIBS results were in good agreement with atomic absorption spectrometry data. Rai et al.237 also explored the effect of additional elements like Cd and Co in the Cr contaminated water. Recently, a similar configuration was evaluated for analysis of Pb and Cd.238 LIBS has been extensively used for continuous and in situ monitoring of gas and particle emissions (heavy metals) originating from exhaust stacks (incinerators, industry, foundries, etc.). Approaches reported in the literature include the focalization of the laser on particles collected on a filter or direct analysis of the aerosol cloud.239−241 Gallou et al.242 have compared these approaches for quantification of Cu particles with sizes ranging from 1 μm to 7 μm.

248, 249

quantitative analysis of normal and pathological nails 250 qualitative and quantitative analysis of different kinds of 251, 252 gallstones and variations in the elemental composition across their width

Ca and Mg

Sr, Ba, Al, Pb, and CN and C2 swan bands O, H, C, N, Fe, Mg, Ca, K, and Na

ref 246, 247

259 260

Indirect analysis appears to be significantly more efficient than direct analysis. For direct LIBS analysis, the minimum detectable concentration of CuSO4 particles in air was 15 μg m−3 for an analysis time of 1 min. The second approach consisted of analyzing quartz fiber filters enriched with the same CuSO4 particles. The LIBS LOD was 60 μg m−3 for a similar measurement time. LIBS has shown its potential as a valuable technique for unburned carbon (UC) analysis in fly ash from furnaces of pulverized-coal-fired power plants.85,243,244 Under optimized conditions, the UC content in fly ash is in the range of 2−5 wt %, while this value may be up to 20 wt % under nonoptimized operating conditions. Online monitoring of UC by LIBS could provide a close-up monitoring operation that could help in optimizing the combustion process through an adjustment of the air−coal ratio. Zhang et al.245 have developed a LIBS system for online analysis of unburned carbon in fly ash without being affected by the type of coal burned. A suction capacity of 1.4 m3·min−1 was suggested to enhance the stability and reliability of the quantitative analysis. For minimizing matrix effects, a second-order polynomial multivariate inverse regression method was considered. The accuracy observed using the C line at 247.86 nm was 0.26%, whereas the average relative error was 3.81%. Biomedical and Pharmaceutical Analysis. Several review papers20,25,26 (cited in the General Information: Books, Reviews, and Conferences) have been published summarizing the effort related to the fundamentals, instrumentation, and applications of LIBS for the analysis of biomaterials. The characterization of biomaterials is interesting for obtaining clinical diagnostic information, which may better guide treatment and may also be helpful in optimizing the therapeutic technique. According to Rehse et al.,25 biomedical applications of LIBS can be broadly classified into two categories: (1) analysis of human clinical specimens (including hard tissue such as teeth and bones and soft tissue such as human hair and skin or tissue samples, human blood, or other fluid samples) and (2) 658

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et al.271 prepared model tablets containing nicardipine hydrochloride as API (pale yellow) and excipients (all of white color). Visual inspection, FT-IR mapping and LIBS analysis were compared. These authors demonstrated that LIBS analysis was the simplest and the fastest method for migration monitoring. Recently, aerosolized drug delivery methods have gained popularity due to their improved efficiency in administration of nano-sized drug particles to the specific target organs. In this context, the viability of LIBS for chemical analysis of carboncontaining aerosolized drugs has been demonstrated.272 Relative elemental ratios of carbon to various trace elements in different drugs were examined. Three different carbon-based powdered vitamin drugs with different carbon contents and trace elements such as Fe, Ca, and Mg were evaluated, and the elemental ratios of [C]/[Mg], [C]/[Fe], and [C]/[Ca] were calculated from the LIBS spectral data analysis on each of the drugs. An excellent agreement with the expected stoichiometric ratios based on their chemical formulas was found. Security and Forensics. A considerable amount of literature has been published in this area. In the security field, an assessment of the application of LIBS to the detection of explosive residues has been published.23 Methods to improve the sensitivity and selectivity of LIBS for detection of explosives such as the use of chemometric tools, double-pulse arrangements, resonant LIBS, and ultrashort laser pulses are discussed. The field of forensics can greatly benefit from the analytical performance of LIBS, in particular from the single-shot LIBS approach due to the often small sample size available for analysis. Two other important areas that could benefit from single-shot LIBS are explosive detection and military applications. Such applications require rapid detection thus using a minimum number of laser shots. Applications of single-shot LIBS have been reviewed by Michel.17 A recent article reviews the information over the last 5 years related to the advances in explosive detection techniques: innovative detection approaches, improvement of existing techniques, instrumentation developments (miniaturization, portability, fieldruggedness, improvements in standoff distances), and selectivity and sensitivity enhancements.273 The authors summarize spectroscopic approaches for explosive detection that could compete with LIBS, including: ion mobility spectroscopy; mass spectrometry; terahertz spectroscopy; Raman spectroscopy; and cavity ring down spectroscopy. A critical review of laser-based methods for standoff detection of explosives including LIBS has also been presented.171 Recently, forensic applications of LIBS have also been discussed, and they enclose the analysis of glass, gunshot residues, papers and inks, paints, and soil samples.12 A significant effort has been performed in developing explosive detection using LIBS in combination with chemometrics tools. The efficacy of chemometric techniques such as linear correlation, PCA, and PLS-DA for the identification of explosive residues has been evaluated.74 The use of the full spectra or the intensities and ratios specific to explosives in the chemometric model has been discussed. The combined use of the two models seems to find better results.172 In this regard, the importance of variable selection for maximizing residue discrimination with PLS-DA has been highlighted.274 Moreover, LIBS combined with PLS-DA has been tested to discriminate explosives from plastics.275 Dingari et al. have proposed the use of support vector machines (SVMs) for discrimination based on LIBS measurements applied to forensic area.276

analysis and detection of pathogenic microorganisms (e.g., bacteria, pollen, virus, fungus, yeasts) that can infect human subjects and cause disease. Table 4200,246−260 summarizes the most recent LIBS biomedical applications. LIBS has been successfully used for discrimination, classification, and identification of bacterial specimens on the basis of their atomic composition.261,262 For this purpose, advanced computerized chemometric methods in combination with highresolution and broadband echelle spectrometers using sensitive CCD and iCCD detectors have been employed. A method based on LIBS and neural networks (NNs) for rapid identification and discrimination of specific bacteria strains (Pseudomonas aeroginosa, Escherichia coli, and Salmonella typhimurium) has been reported.263 The effect that adverse environmental and metabolic stresses have on LIBS identification of bacterial specimens has been evaluated.264 All bacteria were correctly identified regardless of their metabolic state, and the LIBS-based diagnostic retained its selectivity and sensitivity. An interesting review focused on the analysis of plant materials by LIBS has been recently published.21 Applications to fresh or dried surface of leaves, roots, fruits, vegetables, wood, and pollen are quoted. From the perspective of consumer safety and human health hazardous contaminant, the potential of LIBS to determine the concentration of toxic elements in four different lipstick brands sold at local markets in Saudi Arabia has been examined.265 Important findings of this study are that the concentration of some of the toxic species like Pb, Cr, and Cd was much higher than the safe permissible limits for human use and could lead to serious health problems. Godoi et al. evaluated LIBS for the identification of Ba, Cd, Cr, and Pb in plastic toys. The LIBS signal correlated well with ICP-OES concentrations.266 LIBS could be then used as a simple and fast screening method capable of discriminating toys that offer potentially toxic effects from safe products. Several applications of LIBS for monitoring the distribution of active pharmaceutical ingredients (API), drugs, lubricants, and other components used in the formulation of powder blends and tablets have been reported.267 Doucet et al.268 have reported that, using selective molecular bands such as CN, CH, and C2, the atomic lines of C, H, and Mg and two ionic lines of Ca, coupled with chemometric tools, it is possible the complete and simultaneous qualitative and quantitative prediction of all ingredients present in a complex matrix such as a pharmaceutical formulation. Similarly, two chemometric algorithms, namely, PCA and SIMCA, have been implemented for the classification and discrimination of pharmaceutical tablets of brufen, brufen (coated), glucosamine, glucosamine (coated), paracetamol, and vitamin C.269 While PCA was a valuable tool for recognizing similarities between sample types, SIMCA was employed to assign class groups to the tested tablets. A set of 30 test samples were discriminated by SIMCA with an average rate of approximately 94% correct classification. LIBS has been evaluated for studying the distribution of macroand micronutrients in multielement tablets.270 The results obtained for Ca, Mg, P, Cu, Fe, Mn, and Zn were compared with the analysis of the corresponding acid digests by ICP-OES. In general, elemental concentrations measured by LIBS were in good agreement with those obtained by ICP-OES, although differences due to matrix effects in powder blends were observed. Another important application of LIBS involves the study of migration in coated tablets and hard capsules of drugs from the interior to the surface. To evaluate drug migration, Yokoyama 659

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A number of fundamental studies attempted to explain, from a theoretical point of view, the mechanisms and processes inside liquids, with particular emphasis on bubble dynamics. Peel et al.281 provided a comprehensive overview of the dynamics of laser-induced cavitation. In fact, authors generated plasmas in bulk water in order to investigate the rate of bubble expansion and to estimate the maximum bubble diameter and bubble lifetime prior to the unavoidable final collapse. The total duration of the oscillating cavitation bubble was obtained using two techniques, namely, pump−probe beam deflection and highspeed photography. The mean value measured was ∼800 μs. Most recently, Lazic et al.282 studied light transmission through a laser formed bubble during the ablation of a metallic target inside water. Due to its importance in many applications, the authors investigated optical properties of the vapor cavity formed under these conditions, observing that inhomogeneous water vapor clustering inside the cool expanded bubble further perturbs the light transmission and induces irregular ablation by the successive laser pulse. Other general analytical studies have been performed using the single pulse approach. Recently, Sakka and co-workers44 evaluated the effect of pulse duration on the laser ablation of a Cu target in water. Shadowgraphy images revealed that long pulses (150 ns) were more favorable for LIBS analysis. Under these conditions, the relatively slow heating of the plume causes a larger and less dense plume, and consequently, the spectral emission shows less broadening and a weaker continuum. On the basis of this report, the same authors performed a compositional analysis of a Cu/Zn alloy in water using a 150 ns pulse.283 Different ablation efficiency for Cu and Zn was observed, suggested by a significantly Zn-rich plume compared with the target Zn content. Schechter et al.284 investigated the earlier stages of optical breakdown in water and alcohols using nanosecond pulses of 1064 nm. Authors observed the discrete structure of the laser spark column, consisting of abundant micro plasma balls in the tested liquid. Also, they found that the discrete nature of the plasma column lasts up to 100 ns. On the other hand, ultrashort laser pulses have been also used to evaluate the spectral and temporal characteristics of plasmas produced in seawater.132,285 The LODs for Al, Ba, Ca, Cu, Fe, K, Mg, Na, and Zn in water were calculated as 0.19, 0.08, 0.01, 0.78, 3.4, 0.006, 1, 0.0009, 2.5 mg L−1 respectively. Moreover, SP-LIBS have also been used in the determination of Ca and Mg in aqueous solution,286 quantitative determination of Pt in liquid samples287 and chemical characterization of crude oil.157 Several attempts to improve the sensitivity in liquids using different instrumental developments have been presented. As known, sample introduction is a weak point in LIBS analysis of liquids. In order to overcome this drawback, Yalçin et al.288 designed a continuous flow hydride generation LIBS system for the determination of Sn in aqueous environment. Although results were quite satisfactory, the method still requires further advances for the determination of toxic elements (As, Cd, and Pb) in aquatic systems at level traces (μg L−1). Most recently, the same authors improved the LODs (enhancement factor of 2−3-fold) for toxic elements using an ultrasonic nebulizationsample introduction system for LIBS analysis.289 Rai et al.237 investigated the matrix effects for Cr in liquid jets and observed that the introduction of additional elements such as Cd and Co in the Cr decreases the detection sensitivity in binary and tertiary matrixes. In addition, Feng and co-workers290 evaluated the influence of experimental parameters on the LIBS signal of Pb in a Pb(NO3)2 aqueous solution. The LOD was calculated in

Other strategies have been designed to improve the discrimination of explosives from nonexplosive substances.277−279 Concerning the analysis of explosive residues, selective sampling and analysis of explosive residues on solid surfaces has been successfully tested. As demonstrated, the substrate contribution could be minimized under certain experimental conditions.279 Other attempts to improve the detection of explosive residue by LIBS include the use of ultrashort laser ablation and resonant LIBS, which have been cited above (see LIBS methods, fs-LIBS and RELIBS). The combination of LIBS with Raman spectroscopy has been presented as an alternative for the detection of explosives. The successful integration of both sensing technologies in a single hybrid sensor capable of simultaneously acquiring, in real time, both multielemental and molecular information from the same laser pulses on the same cross section of the sample at standoff distances, has been displayed.174 Additionally, strong and weak points of LIBS and Raman spectroscopy, in terms of selectivity, sensitivity, and throughput, have been critically examined, discussed, and compared for assessing the options on the fusion of the responses of both sensing technologies.175 Furthermore, how to combine the spectral outputs of LIBS and Raman for generating a new pattern of identification (2D image) and achieve synergy for obtaining knowledge about the identity of compounds better than that achieved when each technique acts alone has also been proved.176 New strategies for the detection of explosive residues have been tested as the detection of explosive behind a barrier (polymethylmethacrylate and glass) placed between a target and a standoff LIBS sensor.170 Analysis of explosive residues in human fingerprints using a laboratory scale instrument has been described by optical catapulting-LIBS (OC-LIBS).184 Finally, the potential of LIBS to detect chemical and biological threats has been studied.169,280 Simulant samples such as Bacillus atrophaeus spores, Escherichia coli, MS-2 bacteriophage, á-hemolysin from Staphylococcus aureus, 2-chloroethyl ethyl sulfide, and dimethyl methylphosphonate were tested and correctly classified by LIBS together with chemometric models based on PLS-DA. Analysis of Liquids and Submerged Solids. Applications of LIBS in aqueous media are of interest in laser medicine (e.g., ophthalmic microsurgery, laser lithotripsy, and angioplasty) and archeology. In recent years, a number of interesting ideas concerning the study in liquids and its applications have emerged. Among all the applications inside liquids, this Review focuses in liquid samples and submerged targets. Table 5 provides an overview of the different configurations reported in the literature for this application. When a laser pulse is focused into a liquid, it produces a rapid heating of the liquid followed by its explosive expansion, bubble, and shock waves formation. As a result, the lifetime of the plasma generated is very short due, among other factors, to the increased occurrence of electron-ion recombination processes. Direct consequences are a relatively poor signal in the conventional single pulse LIBS approach, and spectral emission is characterized by a broad spectral continuum. Under these conditions, the accumulation of several laser pulses turns into the unique alternative for improving the signal-to-noise ratio. Nevertheless, the most important mechanical effect when a laser pulse is focused on a liquid is the formation of a cavitation bubble since its characterization allows the understanding of the laser−matter interaction inside a liquid and provides an explanation of the phenomena observed in the DP-LIBS configuration. 660

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Table 5. Overview of the Different Configurations Performed in the Literature for the Analysis of Liquids and Submerged Solids laser

λ (nm)

τ (ns)

f (Hz)

E (mJ)

LIBS configuration

collection geometrya S.V. (90°)

Nd:YAG

1064

10

140

SP-

Nd:YAG

1064

6.5

210

SP-

Nd:YAG

19/90/150

Nd:YAG

1064

150

Nd:YAG

1064

6

6.6 20−150

Ti:sapphire

800

40 fs

100

Nd:YAG

532

16

1−10

1.1

samples

LOD

observations

ref.

water bulk

dynamic of laser-induced cavitation

281

Al

light transmission through a laser formed bubble

282

SP-

S.V. (90°)

Cu

effect of pulse duration on LA in liquid 44

SP-

S.V. (90°)

Cu/Zn

effect of fractionation

283

liquids

time-dependent structure of optical breakdown using interferometric techniques

284

SP-

SP-

S.V. (90°)

seawater

SP-

S.V. (45°)

Ca

1.9 μg mL−1

determination of elements in seawater 132, 285

Mg

3.2 μg mL−1 100 mg kg−1

chemical analysis

286

chemical analysis

287

Nd:YAG

266

3

20

SP-

C.V.

Pt

Nd:YAG

1064

4

20

50

SP-

S.V. (45°)

crude oil

Nd:YAG

532

10

10

150

SP-

S.V. (90°)

Sn

Nd:YAG

532

10

10

45−150

SP-

S.V. (90°)

metals in liquids

Nd:YAG

532

4

10

425

SP-

S.V. (45°)

Cr

1.1−2 ppm

LIBS for liquid jet analysis

Nd:YAG

532

10

10

200

SP-

S.V. (90°)

Pb

60 ppm

LIBS for liquid jet analysis

290

2 Nd:YAG

532

5

10

300

DP-

C.V.

Cr

120 ppb

temporal dependence in the signal enhancement of Cr by DP-LIBS

76

1

DP-

S.V. (90°)

Al, Cu

synergetic effects of DP for the formation of mild plasma in water

123

Al, Au

effects of experimental parameters on the laser-induced cavitation bubble

124

LIBS of metals covered by water droplets

292

2 Nd:YAG 2 Nd:YAG

18/16 1064

25 ps/20 ns

10

1064

4−6

20 20

DP-

0.3 mg L−1

in situ detection of oil spill residues

157

flow hydride generation LIBS system

288 289 224

532 2 Nd:YAG

1064

13

2 Nd:YAG

266 1064

7 7

2 Nd:YAG

532

6

2 Nd:YAG

1064

Nd:YAG

a

1064

DP-

S.V. (90°)

metals

32 250

DP-

C.V.

Fe Pb Au

8 ppm 6 ppm 3.5 ppm

chemical analysis

293

20

30−100

DP-

S.V. (45°)

B Li

0.8 ppm 0.8 ppb

chemical analysis

89

5

200

DP-

C.V.

Na

50 ppm

LIBS at oceanic pressure 113 (2.76 × 107Pa); not improved LODs

Mn Ca

1000 ppm 500 ppm

Na

50 ppm

Mn Ca

500 ppm 50 ppm

5

400 35

60

SP-

C.V.

LIBS at oceanic pressure (2.76 × 107Pa)

294

S.V. (side view); C.V. (collinear view to the incoming laser beam).

workers evaluated the temporal dependence in the signal enhancement of Cr emission lines by DP-LIBS.76 Authors indicate that the increase in ablated material and subsequent signal enhancement may be due to the rarefied gas density inside the region enclosed by the shock wave produced by the first laser pulse. In addition, the LOD of Cr in aqueous solution was measured in 120 ppb with DP-LIBS, 1 order of magnitude better than in conventional LIBS. Recently, the synergetic effects of DP-LIBS for the formation of low-density plasma in water were investigated by Sakka et al.123 Authors demonstrated that pulse interval and pulse energies of 15−30 μs and ∼1 mJ, respectively, were appropriate for the acquisition of narrow emission spectral lines (lowdensity plasma) without time-gated detection. It must be noted that the pulse energy employed here is much smaller than those typically used in DP-LIBS. They emphasized that this unprecedented finding suggests a new type of mechanism for laserinduced breakdown in water. Most recently, Cristoforetti et al.124

60 ppm, still requiring a large effort to achieve trace level performance in liquid samples. Analysis using the single-pulse approach is limited by a number of shortcomings (splashing, limited lifetime of the plasma, large background, poor excitation efficiency...) that affect the sensitivity and analytical precision of LIBS. Although nowadays it is fully established in the LIBS community, the double-pulse configuration emerged as a method for improving the sensitivity of LIBS in liquids. Specific details on the mechanisms involved in the double-pulse approach and its interpretation have been entirely described in the literature. The review of De Giacomo et al.291 provides a general description of the basic aspects of underwater LIBS and of the peculiarities of DP-LIBS as an invaluable analytical tool for the elemental analysis of bulk water and of submerged solids. DP-LIBS offers a greater sensitivity (1 or 2 orders of magnitude better), less broadening, and lower continuum when compared to conventional LIBS. Rai and co661

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appear in the bibliography. As an example, the analysis of trace elements in speleothems is of special interest in geology because of the significance of these elements as paleoclimatic proxies.182,183,295 Alvey and co-workers296 demonstrated the capability of the technique for the fast analysis and discrimination of minerals. The same authors have also developed LIBS for the recognition, classification, and provenance of different geomaterials (rocks, minerals, ...) using multivariate statistical classification techniques.78,297,298 Nowadays, the use of statistical and chemometric methods for sample classification is almost mandatory. Numerous authors have applied these methodologies in a broad variety of applications such as the identification of mineral ores,299 characterization of igneous rocks,300 and optimization of the ChemCam instrument.301 On the basis of this methodology, calibration of the ChemCam LIBS instrument for carbonate minerals on Mars was performed by Lanza et al.302 The relevance of this optimization lies in that the existence of carbonate may indicate the past presence of liquid water on Mars and the evolution of aqueous alteration processes that have occurred there. One of the main problems affecting LIBS signal is matrix effects. In order to overcome this drawback, calibration curves allow correction for chemical matrix effects when a proper training set of standards are used. Thus, calibration curves as well as multimatrix calibration curves have been applied in the LIBS analysis of rocks303 and zeolites.304 Dyar et al.305 and Fabre et al.306 recently discussed the potential of LIBS instrument for sulfur identification and igneous rocks characterization, respectively, under Martian conditions. It should be noted that, although LODs calculated in these reports were not extremely brilliant, results obtained by these methodologies were comparable and in good agreement with the requirements of ChemCam. Recently, Vaniman et al.307 developed the ceramic ChemCam calibration targets on the MSL. In addition, Lanza et al.308 demonstrated that the composition of rock varnish coatings and weathering rinds may be differentiated from that of fresh rock by LIBS. Depth profiling analysis was performed here with the objective to provide important information about the types of weathering processes. In a recent publication, Wiens et al.309 described the body unit of ChemCam, as well as the assembly, testing, and verification of the instrument prior to launch (Figure 6A). Curiosity was launched on November 2011 and successfully landed on Mars surface in August 2012. Figure 6B shows a pictorial interpretation of Curiosity analyzing at the Mars surface. At the time of writing of this paper, the Curiosity rover was 2 months in operation. The first LIBS spectrum has been published.310 A preliminarily analysis indicates the spectrum is consistent with basalt, which is known from previous missions to be abundant on Mars. Isotopic Analysis. Isotopic analysis is of crucial interest in many fields such as medicine, chemistry, materials science, radiochemistry, and archeology, among others. Nowadays, isotopic analysis is performed by isotope-ratio mass spectrometry (IRMS), thermal ionization mass spectrometry (TI-MS), secondary ion mass spectrometry (SI-MS), gas chromatography/mass spectroscopy (GC/MS), ICPMS, and MALDI-MS, with mostly satisfactory levels of sensitivity and selectivity. However, the miniaturization of these techniques significantly compromises the analytical figures of merit and does not eliminate the need for sample preparation. As discussed in this review, LIBS is a good candidate for fast analysis at atmospheric pressure thanks to its well-known versatility and capability for multielemental in situ analysis. Nevertheless, isotopic analysis is not a typical LIBS application due to the high spectral resolution needed. In addition, Stark

evaluated the effects of experimental parameters on the reproducibility and dynamics of laser-induced cavitation bubble in DP-LIBS. This report is very interesting since it combines laser pulses in the ns and ps range. Although future works must verify its suitability for LIBS measurements, it seems that infrared ps laser pulses are preferable for inducing stable and reproducible bubbles (first pulse) and also for plasma formation inside the bubble (second pulse). In 2012, Cabaliń et al.292 used a collinear DP-LIBS configuration to study the effect of deposition of water droplets on the ablation of metallic samples. The authors demonstrated that liquid coverage increases the material removal rate and also reduces the ablation threshold. This fact is in agreement with thermal and mechanical properties of the metals, thus suggesting that photothermal and mechanical effects play a significant role in water-assisted plasma formation. Furthermore, Lee et al.293 and Rifai et al.89 demonstrated the high sensitivity of DP-LIBS for the analysis of trace metals in aqueous solution. In conclusion, DP excitation increases the range of LIBS applications in submerged environments and makes this approach extremely attractive in many fields. As an example, LIBS has been demonstrated in oceanographic investigations.113 Several elements (Na, K, Ca, Mn, and Mg) were analyzed for understanding the chemistry of deep ocean hydrothermal vent fluids and seawater. In this case, DP-LIBS did not improve the LODs previously measured by SP-LIBS.294 More recently, Guirado et al.164 demonstrated the potential of a fiber-based instrument specially designed for the remote analysis of archeological material underwater (see Cultural Heritage). In conclusion, all the configurations examined here improve the sensitivity of LIBS analysis but the improvement is at the expense of system cost and experimental complexity. What is clear is that, depending on the matrix, the experimental conditions must be chosen case by case. Space Exploration and Isotopic Analysis. Space Exploration. The space exploration is an exotic LIBS application that highlights the versatility of the method. The application of LIBS to Mars exploration is closely related to its capability for stand-off analysis and mainly focused on the identification and discrimination of minerals. Hyphenation of LIBS with other spectroscopic methods such as Raman and LIF has increased the possibility of the implementation of LIBS in future space missions. It should be stressed that data obtained during the limited lifetime of a mission are of crucial interest as they could answer questions related to the solar system and the geological processes and weathering conditions for life in the earth. Since the 1990s, the potential and viability of LIBS for planetary exploration has been studied. In 2004, the efforts of the LIBS community in the field of planetary exploration were recognized when a new LIBS instrument was selected for the mobile NASA Mars Science Laboratory (MSL) rover. The ChemCam instrument, that is, the Chemistry and Camera instrument, is one of 11 science instruments onboard NASA’s 2011 MSL rover named Curiosity. The ChemCam package consists of two standoff sensing instruments: LIBS and a Remote Micro-Imager (RMI), mounted on a rover body, for elemental analysis at distances from 2 to 9 m. The LIBS provides elemental compositions, while the RMI places the LIBS analyses in their geomorphological context. The main objective of ChemCam is to evaluate whether the Martian environment is capable of supporting microbial life. For this reason, and on the basis of orbital data, the Curiosity landed in a region rich in minerals. In fact, geochemistry is a topic of great interest for the LIBS community. Thus, a huge number of papers concerning this task 662

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Figure 6. (A) Schematic diagram of ChemCam instrument (Reprinted with permission from ref 309. Copyright 2012 Springer Science & Business Media) and (B) self-portraits of NASA’s Curiosity rover on Mars surface (Courtesy of NASA).

and Doppler broadening of spectral lines makes the determination of isotopic shifts in the order of a few picometers difficult. To overcome this problem, several researchers proposed the use of a vacuum or reduced pressure environment since collisional, Stark, and Doppler broadening mechanisms are minimized at low pressure. In the past few years, most of LIBS applications require fieldable systems which are associated to low-performance instrumentation in terms of spectral resolution. Doucet et al.311 described the determination of U-235/U-238 and hydrogen/ deuterium isotope ratios from partially resolved spectra and applying PLS using a portable LIBS sensor. Authors obtained a suitable solution for this application and also demonstrated once more that the combination of LIBS with chemometrics is an excellent approach for the fast determination of isotopes ratio using a portable sensor. In the case of molecular spectra, isotopic shifts can be orders of magnitude larger than in atomic spectra. On the basis of this consideration, Russo et al.312 developed a new approach, called LAMIS (laser ablation molecular isotopic spectrometry), for performing optical isotope analysis using LIBS in air at atmospheric pressure. Authors demonstrated the new concept in hydrogen, boron, carbon, and oxygen, observing that for larger isotopic shift (light elements) the measurement requirements were less stringent. In a recent publication, the same authors described how boron isotope abundance can be quantitatively determined using LAMIS.313 Also, sensitivity was improved using a double-pulse configuration to re-excite the expanding plume created by the first laser ablation pulse. Most recently, Mao et al.314 discussed the capability of LAMIS for the quantification of 86Sr, 87Sr, and 88Sr isotopes and its possible application in radiogenic age determination. LAMIS can determine not only chemical composition but also isotopic ratios of elements in the sample, increasing the potential of LIBS in nuclear and forensic applications.

for preparation and excitation of all sample components. This property confers to LIBS an unprecedent performance in terms of sampling and analysis capability. The mainstreams of the recent advances in LIBS may be characterized by the following features: (1) Improved quantization has been achieved by a widened knowledge of fundamental interactions and physical parameters of laser-induced plasmas. Properly employed, LIBS may today be considered not only as outstanding in elemental screening and identification tasks but also as an essentially quantitative analytical technique. (2) Elemental detection limits have been improved in instrument designs using stable and reproducible laser systems, means of precise pulse energy delivery to the sample, and detectors with signal integration capabilities and accurate timing performance. Particularly, the use of intensified CCD detectors has considerably widened the range of elements routinely detectable in subppm concentrations. (3) Surface and in-depth description of materials has gained increased acceptance among scientists and engineers. Especially, inspection of valuable assets in art and archeology has benefited from controlled and confined laser beam interactions with solid surfaces and from a better understanding of ablation dynamics and thermal input effects. (4) Instruments using more rugged and light components have broadened the fieldability of LIBS. Distinctive opportunities offered by LIBS such as the elemental analysis of distant objects and the analysis of submerged solids provide unique tools for extreme applications including the exploration of planets and the deep ocean. A more careful look must be taken at the precision and accuracy. The critical factors in the analysis of solids are the atomization efficiency and the fraction of emitting particles resulting from laser ablation. Both factors depend ultimately on the beam interaction with the surface. Owing to the usually inherently fluctuating character of the later process, the laserinduced plasma is less stable, and even its multiple pulse average is less reproducible than the instantaneous sensitivity of other highly stable analytical plasmas. This is most disadvantageous when analyses of ultratrace amounts are to be carried out. On the other hand, representativeness of the laser-induced plasma is not always guaranteed. A significant effort is needed in analytical

■

CONCLUSIONS AND FUTURE OUTLOOK As a form of atomic emission spectroscopy, LIBS is typically a multielemental method of analysis. Different from other atomic emission techniques, LIBS requires only a single analytical operation 663

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practice to ensure that both the qualitative and quantitative composition of the plasma is consistent with the sample analyzed. From the chemical analysis standpoint, what is needed is the study of analytical processes in the light of the newly developing concepts, not just from the viewpoint of developing a new analytical method but for improving an existing one. Concepts such as local thermodynamical equilibrium are relatively well understood in laser-induced plasmas, and the effect of experimental factors, such as multipulse laser excitation and the use of lasers of different wavelength and pulse width in combination with refined measurement timing, are well established, but a systematic examination of analytical methods using these and other concepts remains to be done. One important occurrence during the recent years has been the spreading of LIBS technology throughout the developing world, and that suggests that there is an enormous amount of headroom for the LIBS community, including the instrument manufacturers. For the first time since the launch of this technology, a wide selection of commercial instruments is currently available. From custom-made systems to serial production instruments, the market offers many different alternatives in terms of performance and cost. As in the case of every “new” analytical technique, LIBS has been first looked upon as a universal remedy for all problems. This is not the case, of course, but especially in metals analysis, process monitoring, and field applications, LIBS is a very powerful, even indispensable, analytical tool.

■

advisement of Prof. Dr. J. Javier Laserna. The investigation was performed in the Laboratory of Materials Analysis by Laser of the Central Research Services at Málaga University. She is currently forming part of the research staff of the same laboratory. Her research lines are mainly based on the improvement of optical sensors for standoff and real-time diagnosis through the study of the LIBS standoff response as well as its laser−matter interaction, and the development of approaches to improve its analytical performance. Luisa M. Cabaliń is an associate Professor in the Department of Analytical Chemistry at University of Málaga. She received her B.S. degree from the Faculty of Science of University of Málaga (Spain) in 1989 and her Ph.D from the same University under the direction of Professor J. Laserna and A. Ruperez in 1994. She then spent two years as a postdoctoral researcher with Professor Jean M. Mermet at the University Claude Bernard Lyon-I. Laboratoire des Sciences Analytiques Lyon, France. Her research interest is in the area of lasers in chemical analysis, laser-induced plasma spectrometry; surface analysis using laser ablation with optical detection, imaging techniques; instrumental solutions for chemical analysis in industry; on-line analytical methodology; fieldable analytical instrumentation; development of spectroscopic instrumentation. J. Javier Laserna is Professor of Analytical Chemistry at the University of Málaga, Málaga, Spain. He graduated in Chemistry at University of Granada and received his PhD from the University of Málaga in 1980. He did postdoctoral work with Jim Winefordner at University of Florida for two years from 1986 to 1989. He has been a titular member of the IUPAC Commission V.4 on Spectrochemical and other Optical Procedures for Analysis, from 1996 to 2001, and head of the Office for Technology Transfer of the University of Málaga, 1994−1997. He has been president of the Spanish Society for Applied Spectroscopy (SEA), 2001−2004, and president of the Working Group in Spectrochemical Analysis of the Spanish Royal Society of Chemistry (RSEQ), 1998−2001. He is coinventor of 6 patents held by the University of Málaga and has published over 250 papers plus 5 books and book chapters. He was section editor for Raman Spectroscopy of the Encyclopedia of Analytical Chemistry, John Wiley & Sons, 2000. Prof. Laserna’s current research interests include the use of lasers in chemical analysis, laser-induced plasma spectroscopy; time-of-flight mass spectrometry; secondary ionization mass spectrometry; surface analysis using laser ablation with optical and ion detection, imaging techniques; laser remote chemical analysis; instrumental solutions for chemical analysis in industry; on-line analytical methodology; fieldable analytical instrumentation; development of spectroscopic instrumentation; analysis of energetic materials; development of sensors for CBNRE threats; lasers for Cultural Heritage; materials analysis. By the end of 1990’s, he succeeded in demonstrating largescale optics standoff laser induced breakdown spectroscopy for analysis of distant objects. Later, this technique has been used in the analysis of explosives and in space exploration. He has given numerous invited plenary and keynote talks and is member of the advisory board of several scientific journals. Prof. Laserna was awarded with the RSEQ National Award for Research in Analytical Chemistry in 2009 and the SEA National Award for his research career in Applied Spectroscopy in 2010.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Francisco J. Fortes graduated as a chemist at the University of Málaga, Spain, in 2003. The same year, he joined the Laser Laboratory to start his Ph.D. thesis under the supervision of Prof. Dr. Javier Laserna, focusing in the development and miniaturization of portable LIBS technology for Cultural Heritage applications. Nowadays, he is a postdoctoral researcher at Laser Laboratory in University of Málaga (UMA). Currently, his research interest is focused on the better understanding of the fundamental processes involved in laser−matter interaction, the application of laser spectroscopy in analytical chemistry, the development of portable LIBS instruments, and the optical trapping of nanoparticles generated by optical catapulting. Javier Moros received his B.Sc. in chemistry from the University of Valencia, Spain, in 2001. He carried out his thesis work at the same university under the supervision of Prof. Dr. Miguel de la Guardia and Prof. Salvador Garrigues, focusing on the application of different chemometric approaches to analytical data obtained by vibrational spectroscopy and Raman spectroscopy, thus receiving his Ph.D in 2007. At present, he is a postdoctoral researcher under the direction of Prof. Dr. Javier Laserna in the Laser Laboratory at the University of Málaga (UMA). His current research interests are focused on optical sensing techniques, specifically, LIBS and Raman spectroscopy, the analysis in stand-off mode, and the design and application of chemometrics approaches for improving the selectivity of LIBS, mainly in the analysis of compounds sharing similar chemical composition.

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ACKNOWLEDGMENTS Research supported by the Excellence Project P07-FQM-03308 of the Secretariá General de Universidades, Investigación y Tecnologia,́ Consejeriá de Innovación, Ciencia y Empresa de la Junta de Andalucia.́ The authors are also grateful for support for this work provided by Project CTQ2011-24433 of the Ministerio de Ciencia e Innovación, Madrid.

■

Patricia Lucena graduated in Chemistry and specialized in Analytical Chemistry at University of Granada, Spain, in 1994. She received her Ph.D. in Chemistry at University of Málaga, in the department of Analytical Chemistry of the Faculty of Sciences, in 2003. Her thesis work focused on the analysis of automobile catalysts by laser induced breakdown spectroscopy under the

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Analytical Chemistry

Review

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dx.doi.org/10.1021/ac303220r | Anal. Chem. 2013, 85, 640−669