Laser-Induced Breakdown Spectroscopy

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

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

■ CONTENTS

General Information: Books, Reviews, andConferences 640Fundamentals 641

Interaction of Laser Beam with Matter 641Factors Affecting Laser Ablation and Laser-Induced Plasma Formation 642

Influence of Target on the Laser-InducedPlasmas 642Influence of Laser Parameters on the Laser-Induced Plasmas 643

Laser Wavelength (λ) 643Laser Pulse Duration (τ) 643Laser Pulse Energy (E) 645

Influence of Ambient Gas on the Laser-InducedPlasmas 645

LIBS Methods 647Double Pulse LIBS 647Femtosecond LIBS 651Resonant LIBS 652

Ranging Approaches 652Applications 654

Surface Inspection, Depth Profiling, and LIBSImaging 654Cultural Heritage 654Industrial Analysis 655Environmental Monitoring 656Biomedical and Pharmaceutical Analysis 658Security and Forensics 659Analysis of Liquids and Submerged Solids 660Space Exploration and Isotopic Analysis 662

Space Exploration 662Isotopic Analysis 662

Conclusions and Future Outlook 663Author Information 664

Corresponding Author 664Notes 664Biographies 664

Acknowledgments 664References 664

Laser-induced breakdown spectroscopy (LIBS) has experi-enced a spectacular growth in the past decade. From the

time LIBS was studied only in a few laboratories around theworld to the present, nearly fifty years have elapsed during whicha striking progress has been accomplished. The first experimentson LIBS were reported early after the demonstration of laseraction. In 1962, Brech and Cross used a ruby laser to producevapors which were excited by an auxiliary spark source to analyzemetallic and nonmetalmaterials by atomic emission spectroscopy.1

Early in 1963, spectra produced solely by laser excitation wererecognized to result in fairly reproducible quantitative relation-ships among the various elemental constituents of the sample.2

Many additional advantages of LIBS as an analytical tool wererecognized in the pioneering works of the 1960s. By 1966, time-resolved LIBS was reported,3 and the electron temperature andnumber density were calculated on a hydrogen plasma. Otherdiagnostics studies included pressure and pulse width depend-ence of the breakdown threshold; absorption, scattering, andreflection characteristics of the plasma; Doppler-shift of scatteredlight from the luminous front; and expansion of the shock waveinitiated by the spark. This fundamental knowledge in com-bination with the appealing features of LIBS as an analyticaltool fostered an extraordinary interest among the scientificcommunity on a technique that nowadays is still in continuousexpansion. Today, LIBS is deployed and working at the surfaceof Mars, on an impressive demonstration of experimentalmaturity.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 literaturepublished in this period. Instead, an ample, critical selection ofthe most important contributions is presented. After introducingthe general information sources of LIBS, the paper discusses theadvancement in the understanding of fundamental principles ofLIBS and the excitation strategies based on dual and multipulseschemes, resonant LIBS, and ultrafast lasers. Approaches to theanalysis of distant objects follow. Applications are presentedon the basis of a broad selection of new areas of research andinnovative uses of LIBS. The Conclusions and Future Outlookbriefly summarize recent advances and provides a prospective ofshort-term developments in LIBS. No specific section has beendevoted to instrumentation as this issue is covered in each of thespecific sections. In the last 5 years, a vast amount of experimentaland theoretical work has been developed in the area of laserablation (LA). Since this process is extremely versatile, its applica-tions have proliferated over a broad front of disciplines, rangingfrom science to engineering. Although many features of laserablation are relevant to LIBS, there is no specific coverage for LA inthe present paper.

■ GENERAL INFORMATION: BOOKS, REVIEWS, ANDCONFERENCES

During the period covered by this article, a monograph on LIBShas been published.6 The book provides an extensive coverageof fundamental principles, experimental parameters, and plasmadynamics, with a chapter devoted to modeling of plasma emission.

Special Issue: Fundamental and Applied Reviews in AnalyticalChemistry 2013

Published: November 8, 2012

Review

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Several chapters provide a broad treatment of LIBS applicationsin analysis of metal alloys, nonconducting materials, surfaces andinterfaces, and a final chapter devoted to industrial applications.Although out of the period covered by this Review, it is worthmentioning a number of books published since 2000. The bookby Lee, Song, and Sneddon7 provides an excellent overview ofLIBS achievements during the past century. The instrumentationrequired and options available are covered in the first part. Thesecond part discusses fundamental studies of the laser plasma,and the third part deals with applications. The book edited byMiziolek, Palleschi, and Schechter8 provides an extensive accountof LIBS fundamentals and applications until approximately 2005.Another book, authored by Cremers and Radziemski, uses acombination of tutorial discussions ranging from basic principlesup to more advanced descriptions of equipment, methods,and techniques.9 The second edition of the handbook is to bepublished early in 2013.Singh and Thakur edited a monograph intended for analytical

chemists and spectroscopists and also for graduate students andresearchers engaged in the fields of combustion, environmentalscience, and planetary and space exploration.10 A number of usefulreview articles published since 2008 summarize the developmentsand applications of LIBS. In a series of two articles, Hahn andOmenetto provided a comprehensive overview of LIBS. In the firstpaper, basic diagnostics and plasma-particle interactions arecovered.11 The second paper focuses on instrumentation, method-ology for material analysis, and applications.12

Aragon and Aguilera focused on the progress achieved in thedetermination of the physical parameters characteristic of theplasma such as electron density, temperature, and densities ofatoms and ions.13 Optically thin spatially integrated measure-ments and local measurements characterized by nonopticallythin conditions are discussed. The review article by Gornushkinand Panne describes modeling of laser-induced plasmas and over-views plasma diagnostics carried out by pump−probe techniques.14The emphasis is given to models relevant to spectrochemical analy-sis, with special attention to collisional-radiative and collisional-dominated plasma models where radiative processes play animportant role. An overview of spectroscopic diagnostics tech-niques for low temperature plasmas has been presented with anemphasis to electron number density measurement.15 Theattention is drawn to techniques used for line intensity and lineprofile measurement, which are often overlooked in experimentalwork.The so-called calibration-free method for multielemental

quantitative analysis has been reviewed by the first proposersof this approach.16 An extensive list of applications and a dis-cussion on the weak points of the method are presented. Theauthors ask for systematic studies involving large numbers ofsamples and variable experimental settings in order to confirmand enlarge the knowledge gathered. Michel reviewed the appli-cations of single-shot laser-induced breakdown spectroscopy.17

A review focuses on what has been reported about the per-formance of LIBS in reduced pressure environments as well as invarious gases other than air.18

Cremers and Chinni presented an overview of LIBS as ananalytical method, discussing its many advantages and someimportant limitations and how these relate to potential appli-cations.19 Applications of LIBS in specific areas have also beenreviewed in some useful articles. A review assesses the appli-cations of LIBS for chemical analysis mainly centered in bio-materials and plants.20 Analysis of plant materials has been furtherreviewedby Santos and co-workers.21 Sample preparationprocedures

and calibration strategies are revisited. Burakov et al. reviewedthe analysis of soils.22 The efficiency of double-pulse LIBS hasbeen demonstrated in solving a number of environmental prob-lems such as the determination of heavy and toxic metals insoil and the detection of sulfur in coal. Detection of explosivesresidues has been reviewed.23 Recent advances in laboratoryinstrumentation, standoff systems, and data analysis techniquesare discussed.Gaudiuso and co-workers reviewed the uses of LIBS for

elemental analysis in environmental, cultural heritage, and spaceapplications.24 Two review articles discuss biomedical applica-tions of LIBS.25,26 Developments on fieldable LIBS instrumen-tation have been reviewed.27 New trends in the implementationof LIBS systems were presented, with particular emphasis onportable analyzers, remote instruments, and standoff systems.Several specialized conferences and symposia are completely

devoted to LIBS. The International Conference on LIBS wasrecently held in Luxor (Egypt). The meeting is held bienniallysince 2000, with the 2014 conference scheduled for China. Twosymposia on LIBS are held every other year in the US and Europealternating with the international conference. The next NorthAmerican Symposium on LIBS will be organized within theframework of SCIX 2013. The Euromediterranean Symposium onLIBS of 2013 is to be held in Bari (Italy). LIBS has recently alsoappeared prominently in China with the organization of theChinese Symposium on LIBS. The first event held in 2011 had anattendance of about 40 conferees, whereas the second sym-posium held in March 2012 had over 140 participants. In addi-tion to the above-mentioned books and review articles, themedissues of the journals Spectrochimica Acta, Part B AtomicSpectroscopy and Applied Optics are usually published with paperspresented in the specific LIBS conferences and symposia.

■ FUNDAMENTALS

Interaction of Laser Beam with Matter. Interaction of afocused laser beam with matter is a complex and not yet fullyunderstood phenomenon, which is still under intensive inves-tigation. When a high-power laser pulse impacts on the surface ofany material, the irradiation at the focal spot leads to somematerial removal (ablation phenomenon). The ablated materialcompresses the surrounding atmosphere and leads to the forma-tion of a shock wave. During this process, a wide variety ofphenomena including rapid local heating, melting, and intenseevaporation is involved. Then, the evaporated material expandsas a plume above the sample surface, and because of the hightemperature, 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 measuredby LIBS. However, what authors seek to face in this section is notthe analytical strength, which can be drawn from the plasma, butthe factors affecting its generation, its dynamics, and its signifi-cant parameters.Notwithstanding, the goal of this section is to convey to the

potential readers some highlights of physics of the plasma plumefrom a review of the most recent LIBS literature. It will not be thesubject of this section, a special attention to the complex expres-sions that are given to refer on the diagnostic characterization ofthe complex scenario of physical−chemical processes leading tothe formation and expansion of the plasma plume, in particularon the theoretical assumptions made and the approaches usedfor its modeling, and on physical parameters used within LIBSliterature regarding plasma diagnostics.

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Readers with no substantial prior knowledge on this subjectare invited to thoroughly read the review of Hahn and Omenetto12

as well as the references contained herein. Additionally, if expertreaders find that this section lacks detailed information on thosetopics, 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 andcharacteristics of laser-induced plasmas are strongly affected bythe laser operating conditions, i.e., laser wavelength (λ), pulseduration (τ), and energy (E). At the same time, it should berecalled that, while specific mechanisms governing laser energyabsorption in the target depend on the type of material, thesurrounding atmosphere, both in composition and pressure, playsan important role because it is the surrounding medium where theplasma evolves.The effects of each of those factors on the plasma plume in

itself, through objective parameters that characterize it, have beenextensively discussed in the literature. The review article13 pre-sented by Aragon and Aguilera focuses on the progress achievedfrom 1980 to 2007 on characterization of plasmas. A full andquite updated description of the experiments carried out and theprocedures developed to apply the different characterizationmethods in order to determine accurate values of the plumeparameters is documented there. Also, the review manuscriptpresented by Konjevic et al.15 may help the reader to learn moreon theoretical and experimental procedures used to determineplasma electron density and temperature.In contrast, the review article of Gornushkin and Panne14

provides important information on the diversity of plasma pro-cesses and means of how these processes can be modeled orderived from experiments. Besides general information on exist-ing plasma models, the emphasis is given to models relevant tospectrochemical analysis, i.e., models of radiating plasma.

Accordingly, considerations of this section will only refer tothe newest research. Moreover, for the sake of clarity and ease ofcomprehension, the section will be divided into subsections, eachfocusing on the particular effects that each input variable, namely,the targeted sample, laser parameters (wavelength and pulseduration, as well as fluence), and surrounding atmosphere, has onthe parameters of the resulting plume. Although authors will tryto provide a critical review of progress made, the reader shouldnot forget that the high dependence of the values of physicalparameters on the experimental conditions complicates the com-parison of results from different experiments.

Influence of Target on the Laser-Induced Plasmas. Aguileraet al.28 investigated (Nd:YAG, 1064 nm, 100 mJ, 4.5 ns) plumesfrom Fe−Ni, Cu−Ni, and Al−Ni binary matrixes in air at atmo-spheric pressure. After a complete description of plasma emis-sions detected in the 3−4 μs time window using a set of param-eters (electron temperature, Te; electron density, Ne; total numberdensity in the plasma, N; the length of the plasma along the line-of-sight, l; the perpendicular radiating area of the plasma, β), theirresults reveal the existence of a weak matrix effect that leads to avariation of these plasma parameters, as only metallic samples areconsidered. Nevertheless, it is expected that variation of plasmaparameters will be larger for materials having bigger differenceson 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 andAr gas background of FeO targets prepared under differingforms, namely, nanopowder, pressed powder pellets, and sinteredceramics. As demonstrated, spectroscopy of the different targetswas comparable to each other and qualitatively independent oftarget morphology. In contrast, the different formats featured bythe targets strongly influence the processes occurring at thesurface of the treated material. Hence, plume dynamics as well asparticles ejected from the irradiated material seem to significantly

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



depend on the degree of target compaction. Also, in this direc-tion, the relationship between sample hardness and the laser-induced (Nd:YAG, 532 nm, 30 mJ, 15 ns) plasma parameters hasbeen investigated for aluminum−lithium alloys (Al−Sc−Li, Al−Mg−Li, Al−Cu−Li) and lithium ferrites.30 Differences in plasmatemperature for ferrites are not related to changes in samplecomposition but are caused by their different physical propertiesand structure. For aluminum alloys, variation of their composi-tion induced changes in hardness and both factors influencedplasma properties. It has been also proved that the ablatedmass isin inverse proportion with hardness.Schmitz et al.31,32 have given new experimental insights into

the characteristics of atmospheric-pressure ablation of molecularsolids with respect to analytical MALDI (matrix-assisted laserdesorption/ionization) applications. The early processes, ma-terial release and formation and expansion of hemisphericalshock waves, in ablation (nitrogen, 337 nm, 235 μJ, 4 ns) ofdifferent common MALDI matrixes, namely 2,5-dihydroxyben-zoic acid, a-cyano-4-hydroxycinnamic acid, and sinapinic acid, aswell as anthracene, have been studied. For a given fluence, similarcrater shapes of the same dimension for MALDI matrixes havebeen observed as compared with the consistently smaller, and ofremarkable “clear−cut” form, crater of anthracene. Furthermore,for ablation at whatever energy regime, smallest expansionsat any given time were observed for anthracene as contrastedwith those from MALDI matrixes. The pronounced differencesbetween anthracene and the striking similarity of MALDImatrixes have been justified by authors through the release CO2from 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 mostimportant variable affecting the characteristics of the plasmasince the effects of its parameters are 2-fold: first, during itsinteraction with the targeted sample and, then, with the plasmaplume 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 particulartemperature depending on the mechanism of interaction of thelaser pulse with that, and the onset of ablation (either thermal orphotochemical) occurs if the fluence is above a particular thresh-old. Once the plasma plume is generated, its density may obstruct(‘‘plasma shielding’’) partially or entirely laser radiation, depend-ing on the laser wavelength and pulse length. Consequently, notthe full energy is transferred from the laser pulse to the originalmaterial.With all these key aspects affecting the whole chain of possible

events occurring during plasma formation, authors have no otheraim than to put in situation to the reader on how different laserparameters may affect the physics of the plasma plume. To ensurea more user-friendly and understandable framework, influences oflaser 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 Hanifet al.33 where the spatial evolution of Cu plumes produced by1064 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 thanthat 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 of532 nm (2.60× 1016 cm−3). Both works also reveal thatTe andNedecrease along the direction of plume propagation due to therapid conversion of thermal energy into kinetic energy, in such a

way that the plasma expands and thermalizes by transferringthe energy to its surroundings. Influence of λs (1064, 532, and266 nm) on emissions from Sn plasmas have been also inves-tigated, specifically focusing on the changes in debris generation,in an effort to develop efficient (and debris free) light sources at13.5 nm for the next-generation extreme ultraviolet (EUV) lithog-raphy.34 Deposition studies have shown 266 nm to generate alarger amount of atomic particles, consistent with mass-ablationestimates. In contrast, their kinetic energy profiles are found to bebroader with decreasing λ; a fact attributed by the authors to theenhanced three-body recombination in dense plasmas produced byshorter λ. Results lead to the conclusion that, from a perspective ofdevelopment of a EUV source, 1064 nm is the best option of the λsstudied, owing to its higher 13.5 nm conversion efficiency andlower atomic debris.Campos35 and Harilal36 have also explored the effects of λ on

plasmas produced from planar Sn targets using 1.06 μmNd:YAG(6 ns) and 10.6 μm CO2 (30 ns) lasers. Several striking dif-ferences in the features of the two plumes have been noticed. Themajor difference involves the spatial−temporal evolution of Ne.Some light has shed on the nature of the hydrodynamic expan-sion of plasmas, revealing that Nd:YAG plasma forms a forwardbiased jet whereas the CO2 plasma expands almost spherically, inclear agreement with the less amount of debris per pulse that itemits. The analysis of craters created also confirmed a larger massablation rate (3.6 times higher) for Nd:YAG plasmas comparedto that produced with CO2 lasers. A significant difference in Tebetween CO2 plasmas and Nd:YAG plumes has been alsoobserved. From all these findings, the use of 10.6 μm CO2 as aEUV 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 μmNd:YAG laser and reheated bya 10.6 μm CO2 laser. Their results conclude that the Nd:YAG-CO2 laser combination improves the sensitivity by the effectivereheating effect resulting from efficient inverse bremsstrahlungabsorption of the longer λ laser. It is thus clear that a properchoice of the λ allows one to produce a plume with better prop-erties for analytical applications. To push more in this direction,Ma and co-workers have investigated the ablation of Al targets inone-bar Ar background using nanosecond UV (355 nm) or IR(1064 nm) laser pulses.38 Differences in absorption rate betweenUV and IR radiations leading to different propagation behavior ofthe produced plasma have been demonstrated. While for UVablation the background gas is principally evacuated by theexpansion of the vapor plume, for IR ablation the background gasis effectively mixed to the ejected vapor during at least hundredsof nanoseconds after the initiation of the plasma. As a con-sequence, higher Te and Ne are observed for UV ablation thanfor IR ablation. These parameters confirm a hotter, confinedAl plasma for UV ablation, whereas for IR ablation, a largeraxially extended Al vapor plume with a better homogeneity isobserved. Their observations suggest descriptions by “laser-supported combustion wave” and by “laser-supported detonationwave” for the propagations of plasma produced by UV and IRlasers, respectively.

Laser Pulse Duration (τ). Irrespective of their duration, laserpulses usually reach the required conditions for ablation of tar-gets since the rate of energy deposition greatly exceeds the rate ofenergy redistribution and dissipation, thus resulting in extremelyhigh temperatures in those regions where energy absorptionoccurs. However, as a consequence of the different mechanismsof energy dissipation in the sample, differences in pulse duration



result in fundamental differences of the ablation process. Indeed,interaction of nanosecond (ns) pulses with materials are sub-stantially different from those of femtosecond (fs) pulses sincethe rate of energy deposition is significantly shorter in this lastinstance. Thus, for ns pulses, the material undergoes transientchanges in the thermodynamic states from solid, through liquid,into a plasma state. Furthermore, the leading edge of the laserpulse creates plasma, and the remaining part of the pulse heatsthe plasma instead of interacting with the target. In the case ofultrashort laser pulses, at the end of the laser pulse, only a veryhot 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 fused silica targetshave been compared.39 These investigations reveal a sole maincomponent (‘‘fast’’) in the plume from fused silica. Contrarily,two components in the case of Cu have been found: a ‘‘slow’’component of high intensity that evolves close to the targetsurface and a less intense, but ‘‘fast’’, component observed fartheraway. Also, the component of fused silica plumes is 3 times fasterthan the fastest component of Cu plasmas. Moreover, whilenanoparticles (NPs) represent a large fraction of matter ablatedfrom Cu, they are not observed during ablation of fused silica.Spatial−temporal maps of ionic, neutral, andmolecular species

generated from planar graphite targets irradiated by fs laser(Ti:sapphire, 800 nm, ∼87.5 J cm−2, 40 fs, 10 Hz) under varyingambient N2 gas pressures have been reported.40 Space-timecontours for those species within the plume have revealed that, inthe presence of an ambient gas, the distribution of each species isrelated to its next ionized level, whereas the molecular speciesspatial extension at the early stages of plasma life is directlycorrelated to the excited ions.Gacek et al.41 presented simulations on the fundamentals of

plume splitting at atomistic level under the influence of shockwave during the early stage of its propagation (up to 2 ns).According to their findings, at the very beginning of ablation, twodistinguishable density peaks at the plume emerge and quicklydisappear due to the spread-out of the slower moving part. Whilethe front peak (coming from the faster moving of atoms andsmaller particles) propagates out against time and experiencesstrong constraint from the ambient gas, the second peak (originatingfrom the larger clusters) moves slower and experiences very littleconstraint, eventually picking up their velocity during the earlyevolution. Regarding the ambient, the larger the pressure, theearlier is the plume splitting, that occurs at a distance closer to thetarget surface. In contrast, when the ambient pressure is reduced,the plume splitting becomes weak and barely visible. Further-more, under stronger laser fluence irradiation, the plume splittingalso happens earlier.At the other end of the temporal regime, Zhou et al.42,43 have

experimentally studied plasmas induced during ablation (SPIG3.0 laser, 1064 nm,∼0.2 to 0.4 mJ) of polished Ti and Al targetsin air at atmospheric pressure using relatively long (200 ns) laserpulses. A rapid growth of the plasma size with time during thelaser pulse (from 50 to 200 ns) has been observed. Subsequently,the radiation intensity of the plasma region increases, but it isnot uniform. Plume again shows two distinguishable regions: abright spot located just above the target surface and other high-radiation-intensity region behind the expanding plasma front.Later, the region just above the target surface disappears firstmoving upward and, then, through merging with the otherregion, thus forming a single high-intensity zone. The importantrole of laser−plasma interaction during the plasma evolution

when using a relatively long pulse is underscored. Additionally,an interesting physical phenomenon for 100 ns pulse (but not for200 ns pulse) LA has been observed: while the plasma bottom isdetached from the ablated target surface shortly after laser pulseends, this plasma bottom grows backward toward the target. Inshort, due to themuch longer ns laser pulse duration, distributionof plasma radiation intensity and propagation of its front havedifferent physical features from those produced by much shorterns laser pulses.A suggestive investigation has been made by examining some

aspects in the plasma plumes from Cu plates immersed in waterwhen using different pulse widths, namely, 19, 90, or 150 ns, of aNd:YAG laser.44 As compared to a short pulse, a minimizeddamage and efficient heating of the plume due to the directabsorption of the later part of the long pulse has been observed.According to these authors, long ns pulses are more favorable forthis LIBS application due to the relatively slow heating of theplasma, causing a larger and less-dense plume and, therefore,fairly intense and less broadened emission lines. Also, a weakercontinuum has been observed.Some comparative studies, even shuffling different experimental

variables, seem to be in clear concordance with the improvedanalytical performance of DP-excitation mode, due to the muchlower density in its plasmas and to the temperature growth aswell as to the increase of both plasma lifetime and dimensions ofthe plume along the observation line. A detailed comparativestudy of collinear SP and DP-LIBS (KrF excimer-dye laser, 248nm, 450 fs) on the basis of emission lifetime, Te, and Ne, for fsablation in ambient air of brass (Cu−Zn), has been carried out.45As expected, the DP arrangement yields hotter and longer-livedplasmas and, in consequence, a significant enhancement of theintensity 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-backgroundratio (although its broad background is still higher) and a con-siderable decrease of line broadening. As a result, an overallimprovement in spectral resolution is observed even using anongated data acquisition mode. Besides, the DP arrangementreduces the threshold fluence for plasma formation by about afactor of 2, allowing the acquisition of good analytical spectra atlower fluence values while minimizing damage of the originalsurface.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 delaytime [≠f(λ)] and pulse energy ratio [=f(λ)], the DP approachyielded an emission signal enhancement of over 300-fold ascompared to the single pulse instance. This signal enhancementis also underpinned by thermal reheating of the plasma plume.Higher values for Te and Ne have been also calculated. Theauthors suggest to keep the pulse energy of the second laserlarger than that of the first one for a better experimental realiza-tion of DP-LIBS.In another turn of the screw toward deeper knowledge, plasmas

from Fe generated by SP and cross-beam DP configurations usingNd: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 whenthe CO2 pulse interacts with the sample before the Nd:YAG pulsehas been observed. The CO2 pulse heats the target during the first700 ns before the arrival of the Nd:YAG pulse without melting thesample and without any noticeable deformation of the surface.Then, a few microseconds after the arrival of the CO2 pulse, heavyand slowmoving particles are extracted and ejected from the target



surface, thus providing fuel for subsequent plasma creation by theNd:YAG laser.To close this matter, a comparative study of effects of SP as

well as both collinear and orthogonal preablation DP config-urations on plasma emissions fromAg samples using twoNd:YAGlasers operating at 532 and 1064 nm has been presented.48 Asexpected, DP configurations yield up to 12 times (collinear) and6 times (orthogonal) signal enhancement as compared to the SPmethod. For these enhancements, the optimum value of theinterpulse delay time between both pulses is independent fromDP configuration, whereas the optimized value of pulses energyratio depends on the DP configuration. A shorter wavelengthpulse for generating plasma, and a delayed pulse of longerwavelength for plume reheating, is recommend for betterDP-LIBS.Laser Pulse Energy (E). Ablation and plasma formation are

largely affected by the laser pulse energy, E (closely connectedwith pulse duration). In fact, for LIBS, the energy per unit areathat can be delivered to the target is more important than theabsolute value of E. Thus, the primary energy-related parametersinfluencing the laser−matter interaction are usually termedeither as irradiance (energy per unit area and time, W cm−2) or asf luence (energy per unit area, J cm−2). Experimental determi-nation of irradiance or fluence requires a careful evaluation of thespot size over which the laser beam is focused. However, despitethat change on focusing distance introduces a degree of flexibilityin 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 energiesand 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 laserpulses (Nd:YAG, 1064 nm, 200 mJ, 7 ns, 1 Hz) of variableenergies (50, 70, and 95 mJ) have been reported by Sarkar andcollaborators.49 Vanadium oxides (VO, V2O3, VO2, and V2O5) inair at atmospheric pressure were studied. Their findings are ingood agreement with the predicted increase ofTe with E. Besides,its temporal profile also reveals faster decay of Te with increasingE. The rate of this decay, following power law, is assumed to beindependent of the nature of the target. The same decay holdstrue for Ne but faster with reduction in E; this is the reversebehavior of that observed for Te. Since the amount of ablatedmaterial 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 plasmaparameters has been studied by Abdelhamid and collaborators.50

Au thin films deposited onto Cu substrates were studied. Despiteslight fluctuations attributed to high reflectivity of metals in thelayered material, Ne values nearly independent of the workingdistance, and subsequently of the irradiance, have been observed.Similarly, no measurable change has been found in Te, so therange of irradiance values studied by these authors seem notenough for modifying the plume parameters.Both Ne and Te on transient and elongated plumes have been

also evaluated when focusing pulses at seven different incidentirradiances (3.7, 6.3, 9.7, 12.4, 15.9, 19.0, and 21.9 GW cm−2) ofthe 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 (rangingfrom 6085 to 7498 K) with the raise of the irradiance (from 3.7 to21.9 GW cm−2). Furthermore, spatial evolution of these plumeparameters from 0.1 to 4.0 mm axial heights above the target

surface was observed. Hence, while Ne is maximum near thetarget surface and decreases at larger distances from the targetsurface due to the recombination with ions, Te slightly decreasesat both the plasma edge, due to the higher radiative cooling at thisarea (larger emitting surface as the plasma expands) and therapidly conversion of thermal energy into kinetic energy, andclose to the target surface, because thermal conduction from theplasma toward the target considering the equipartition time forenergy 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 Fesamples (cleaned, but not polished, surface) has beeninvestigated.52 The influence of irradiance on plasma expansionhas been highlighted. At high irradiance, the plume, containingmore ablated matter and having higher internal energy, is capableof pushing the surrounding air far enough in front of itself inorder to expand into a hemispherical shape; whereas at lowerirradiances, the ablation plume has less energy, so its expansion inthe radial direction prevails over that in the longitudinal direc-tion, 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 shorttemporal existence, transient in nature and containing particles ofthe ablated matter, expands at supersonic velocity toward thesurrounding atmosphere in front of the target. The interaction ofthe plume with the ambient gas is a complex gas dynamic processdue to the rise of new physical processes, including deceleration,thermalization of the ablated species, interpenetration of gascomponents into the plasma, radiative recombination, formationof shock waves, and clustering. In this respect, not only is thecomposition of the surrounding atmosphere but also the pres-sure under which such plume is evolving, combined with thecrucial requirements of LA to be performed in background gaswith residual pressure typically for pulsed-laser deposition(PLD), perhaps, are the reasons which have led many groupsto 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 natureand pressure of various ambient environments on the spatialand 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 simula-tions from theoretical models play an important role in gettingmore detailed information about what could happen in the pro-cess. Despite its common goal, several theoretical studiesidentified in the literature are based on different assumptionsaccording to the several aspects of LA that the model intends todescribe. Thus, Gonzalez and collaborators53 have used theparticle-in-cell (PIC) computational method to study theCoulomb interaction between the particles of the initial statesof Al plasmas (Nd:YAG, 1064 nm, 500 mJ, 9 ms, 10 Hz) expan-sion in vacuum. An ideal model assuming that the plasma is in alocal thermal equilibrium as well as the ablated particles have afixed temperature, and a constant evaporation flux (J) from thetarget surface has been considered.In contrast, considering a wide range of ambient pressure con-

ditions and for a high vacuum ambience, Antony and co-workers54

used an approach based on hydrodynamic equations to modeldynamics of plume temperature and plume velocity from LIBSexperiments (Nd:YAG, 355 nm, 90mJ, 5−10 ns, 10Hz) on Al, Cu,and ZnO targets.



Using both adiabatic expansion and kinetic models, Kumar etal.55 have made an attempt to estimate the lengths of plumes andto investigate plasmas expansion rates across the laser beam axisfor 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. Alter-natively, a theoretical thermal model consisting of equations ofconservation of mass, momentum, and energy for studying thetarget heating, plasma formation, and plume expansion duringinteraction with ns laser pulses has been presented and experi-mentally corroborated on Al plumes (Nd:YAG, 1064 nm,450 mJ, 10 ns), by Moscicki and co-workers.56 The work fromBogaerts’ research group offers an overview of their modelingactivities in ns- and fs-LA.57

On the other hand, from a more experimental perspective,influence of the surrounding ambient is limited toward specificdiagnostics of physical parameters defining the state of theplasma as well as its morphology profiles with respect to thespatial and temporal evolution of the plume. In this connec-tion, in a thorough manner, temporal evolution of those plumesunder a wide range of pressures (from 103 to 10−4 mbar) hasbeen characterized.58 A general decrease in Te and Ne values isobserved with decreasing ambient pressure. However, the moststriking feature is the step change in the plasma behavior,between ambient pressures of ≥10 mbar and ≤1 mbar, despitethat in the early stages of plasma formation (110 ns) plasmasgenerated are comparable in terms of size and luminosity what-ever the ambient pressure. Then, as pressure decreases, a lower

confinement and a larger acceleration on the plasma disper-sion, and subsequently highly distinguishable plumes, have beenobserved. Also, the role of air in the dynamical evolution andthermodynamic state of polished Al plumes (Nd:YAG, 1.06 μm,70mJ, 20 ns) has been investigated,59 and a strong presence of airin the plasma core was detected. Thus, while a very limitedpresence of Al species is found in the region farther from thetarget, where mostly air emissions are observed, a prevalence ofair species in the plume composition, even in the region closerto the target, is noticed. Similar investigations on Al plasmas(Nd:YAG, 532 nm, 100mJ, 8 ns) have been carried out under theinfluence of both vacuum and air at atmospheric pressure.60 Inthis case, overall behaviors of spatial−temporal variations for Te

have been observed almost the same. Contrarily, forNe, a highestvalue has been observed close to the target for both differentambient conditions, as well as posterior decreasing at both longertimes 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 airpressures, 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 alsostudied.61,62 A decrease of intensity of the Al plasma with raisingpressure has been observed. Such drop is justified from the factthat, under low pressure, the plume interacts less strongly withambient 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

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



processes proceed faster since energy exchange between particlesinside 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, 4ns, 10 Hz), has been studied by Patel et al.63 As before, a largerconfinement of the plume near the target surface, with a decreas-ing on the velocity of the plume front, has been noticed as thepressure increases. This confinement also reported emissionintensity longer sustained in time because of enhanced collisionalprocesses. Decreasing of Te and Ne is larger when the pressuredecreases because expansion of the plasma becomes faster, thusresulting in faster cooling.The expansion dynamics of different plumes after LA, under

other different background atmospheres than air, have been alsoevaluated by several research groups. Hence, the influence of O2gas pressure on propagation of plumes from complex oxides(LaAlO3 and LaGaO3) has been investigated64 to elucidate anddiscuss the role of the surrounding background on characteristicsof ablation (KrF excimer, 2.0 J cm−2, 25 ns) plume and, in turn,address consequent effects on the growth of interfaces forproducing high quality oxides thin films. As with air atmosphere,also similar regimes of influence have been noticed: almostfree plume expansion at low pressures; plume splitting at middlepressures; and the braking effect of the gas on plume propagationalong the normal to the target surface at high pressures. Besides,pressure only slightly affects the plume at early delays after thelaser pulse. At larger delays, despite similar plume elongation,whatever the pressure, the interaction with the background gasstarts affecting the plume spatial characteristics. Investigations ondynamics of the plasma and its properties under the influence ofsome inert gases as Ar and He, how they change with pressure aswell as critical comparison with those evolutions observed underair ambient, have been also made.On considering Ar atmospheres, hydrodynamic expansion

features of Al plumes (Nd:YAG, 1064 nm, 6 ns) at atmosphericpressure have been investigated by Harilal et al.65 In theirexperiments, the main temporal regimes for plasma expansionunder Ar gas, namely, the initial-stage asymmetric expansion, theinternal shock-wave-like plume structure, and the vortical motion,have been observed. In contrast, dynamics of the Fe and graphiteplumes (Q-switchedNd:YAG, 532 nm,∼30 J cm−2,∼8 ns) duringits propagation under different pressures (from 2 × 10−4 to20 mbar) have been also reported.66 Identical dependence onpressure has been observed for plume expansion. However,different temporal expansion regimes have been noticed: aninitial expansion up to ∼100 ns, whatever the gas pressure is; anintermediate plume expansion where the temporal elongation ispressure dependent; and finally, a spherical expansion for alloperating pressures after several tens of μs. Besides, the increasedpressure reduces not only the kinetic energy of species within theplume but also the velocity of the expanding plume front. Addi-tionally, the plume splitting phenomenon for Fe and Al plasmas,as they expand through different pressures (0.5 and 1.0 mbar forFe, and 0.2 mbar for Al) of Ar gas, has been more deeplyinvestigated.67 Unlike for lower and higher gas pressures, atwhich plasma plume expansion does not show any visible frontaledge, at moderate gas pressures the plume splitting is observed.Furthermore, the pressure regime resulting in plume splitting istarget dependent. Also, a detailed comparison of the interplaybetween 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 beendescribed by Ma and co-workers.68 A soft confinement by Ar

over the Al plasma core, having quite uniform distributions ofNe and Te but with a large amount gas of mixed within it, hasbeen observed. In contrast, Mendys et al. applied the Thomsonscattering method to quantify Ne and Te at different instants ofthe evolution, at atmospheric pressure, of a plume core, firstinduced (Nd:YAG, 532 nm, 2.0 kJ cm−2, 4.5 ns, 10 Hz) and thenperturbed 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 Neup to about 1.5 μs has been found. Then, a significant decelera-tion on the decay rate for Te (cooling governed rather by gasdynamics and energy losses than by the equilibrium state of theplasma) as compared with that of Ne (directly related to the rateof the plume expansion) has been reflected.Regarding He ambient, spatial−temporal dependencies of

plasma emissions from ablation (266 nm, 10 ns) of Cu, undervarious both pressure (100, 500, and 760 Torr) and irradiance(0.5, 0.7, and 1 GW cm−2), have been investigated by Mehrabianet al.70 As expected, at specified irradiance, the increasing of Hepressure 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 to3 Torr) on dynamics of laser (Nd:YAG, 1064 nm, 1.6 J, 8 ns)blow-off plasma plume from multilayered LiF−C thin films havealso been evaluated.71 A deep dependence of intensity, size, andshape of the plume on the nature and composition of the ambientgas has been observed. Hence, while velocity of the plume hasbeen found to be higher in He ambient, intensity enhancement isgreater in an Ar environment. Finally, the effects of Ar, He, andair under different filling pressures (from 5 to 760 Torr) on Cu72

and Cd73 plasmas (Nd:YAG, 1064 nm, 200 mJ, 10 ns) have beeninvestigated. A strong influence of the pressure and nature of theambient gases, due to their differences in density, mass, ioniza-tion potential, and thermal characteristics, on optical emissionintensity, Te, and Ne has been revealed. Surface morphologicalchanges 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 reviewof Effenberger and Scott18 as well as the references containedherein. The article focuses on compiling an understanding ofLIBS phenomena that have been gained through the variouspressure 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 scientificliterature. However, the goal of authors here was simply to raiseawareness about some basic knowledge on this topic to thereader through some subtle accents. Moreover, this section seeksto bring the reader into a deep thought about the complexitywithin the complete sequence of events that take place during theLA, depending on the variables by which it is governed. Thus,despite the large number of scientific and practical applications ofLIBS that will be reflected below, the study of its basic mechan-isms is still a great challenge, since there is no universal model tocompletely describe this phenomenon.

■ LIBS METHODS

Double Pulse LIBS. The original purpose of DP-LIBS wasthe improvement of the observed signal in an attempt to increasethe analytical sensitivity. However, the advantages of thisapproach and their combined andmultiple optical configurationslead also to improvements in LIBS applications such as under-water analysis (see section on Analysis of Liquids and Submerged



Solids), which would not be feasible without the benefits ofdouble-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 havethe same propagation pathway, is the simplest but less versatileapproach. In the orthogonal approach, a pulse ablates the sample

(perpendicular to its surface) and the second pulse (parallel tothe sample surface) is sent either before to form a preablationspark or after in order to reheat the plasma generated by the firstpulse. Additional combinations have been proposed usingdifferent 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

Table 1. Collinear Double-Pulse LIBS Applications

sample collinear configurationa enhancement factorb/remarks ref.

arsenic (mine tailing soils) Nd:YAG @ 1064 nm, width 6 ns, combined 90 mJ, dt 1−180 μs intensity 13% 80SNR 165%

chromium (dyed wool fabric) Nd:YAG @ 1064 nm, width 10 ns, 90 mJ pulse−1, dt 7 μs LOD 5−10mg kg−1 81crop plants (K, P, Mg, Ca) Nd:YAG @ 1064 nm, width 12 ns, 65−78mJ pulse−1, dt 7 μs 82explosives residues Nd:YAG @ 1064 nm, width 5 ns, 335 mJ pulse−1 intensity 2−3× 74food (potatoes) Nd:YAG @ 1064 nm, width 5 ns, 10 mJ pulse−1, dt 100 ns intensity 2× 88geomaterials Nd:YAG @ 1064 nm, width 7 ns, 320 mJ pulse−1, dt 1 μs no advantages for sample classification except in

the case of soils78

carbonates (mineral and rock)natural fluorite (mineral), silicate(rocks and soils)

graphite Nd:YAG @ 532 nm, 20 mJ pulse−1, 36° incident angle intensity 5× 87metallic alloys (steel) Nd:YAG @ 1064 nm, 60 mJ pulse−1, dt 1 μs intensity 10× 84metallic alloys (Pb) Nd:YAG, width 8 ns, combined 40 mJ, dt 7.4 μs LOD 10× all wavelength combinations 92

first pulse @ 532 nm; second pulse @ 1064, 532, and 355 nm best LOD (0.0017%) for 532 nm/355 nmmetallic aqueous solutions (Cr) Nd:YAG @ 532 nm, width 5 ns, combined 300 mJ, dt 2−3 μs ablated mass ≥3.5× LOD 10× 76multielement aqueous solutions(Fe, Pb, and Au)

quasi-collinear LOD 10× 89first pulse: Nd:YAG @ 266 nm, width 7 ns, 32 mJ pulse−1

second pulse: Nd:YAG@1064 nm, width 7 ns, 200mJ pulse−1, dt 2−3 μs−1

multielement aqueous solutions(Ca, Ba, Sr,and Mg)

Nd:YAG @ 532 nm, width 3−5 ns, dt 50 ns intensity 1.5−2.5× (aerosol) 902 × 65 mJ (aerosol), 2 × 35 mJ (microdrop) intensity 4× (microdrop)

Ti solid Nd:YAG @ 532 nm, width 8 ns, 20 mJ pulse−1, dt 0.5 μs intensity 5× 79solid samples (brass, Fe, Si,BaSO4, Al)

excimer−dye @ 248 nm, width 450 fs, 15 mJ pulse−1 intensity 3−10× 97

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

Table 2. Orthogonal Double-Pulse LIBS Applications

sample orthogonal configurationa enhancement factorb/remarks ref.

Al (metal) ablation laser: Nd:YAG @ 532 nm, width 8 ns, 7 mJ pulse−1 decrease Al I signal and increase Al II signal 96prespark 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 improvement in the depth resolution 75reheating laser: Nd:YAG @ 532 nm, width 5 ns, 60 mJ pulse−1 irradiated area 4×

ash (coal fired power plant)(Ba, K, Mg, Ti, Fe, Ca, Al)

Nd:YAG @ 1064 nm, width 6 ns, 100 mJ pulse−1 unequal ablation could be partially surpassed 85Nd: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 comparable in analytical performance 83reheating 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 intensity 2× 93reheating 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 LOD (Cl) of 80 ppm 94prespark laser in He gas: Nd:YAG @ 1064 nm, width ns, 110 mJ

copper-based alloys ablation laser: Nd:glass @ 527 nm, width 250 fs R2 (0.998−0.999) 77reheating 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 LOD 20 μg g−1 91prespark 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 intensity 25× 95prespark or reheating: Nd:YAG @ 532 nm, 10 ns, 30 mJ pulse−1

fossil (snake) ablation laser: Nd:YAG, @ 266 nm, 10 mJ pulse−1 86reheating 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 nsreduction of the plasma threshold 100× 98

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



(532/1064, 532/532, and 532/355 nm) have been tested fordetermining Pb in metal alloys.92 In all cases, the DP approachimproves the limit of detection (LOD) by 1 order of magnitudeas compared to the single pulse method. Although a dependenceof Pb intensity with the matrix was found for all combinations, abetter correlation coefficient of the calibration curves is reportedfor the combination of 532/355 nm. An orthogonal DP-LIBSmethod for in-depth characterization of ceramic multilayersamples has been presented.93 In this study, a first defocusedpulse (1064 nm, 50 mJ) was used to ablate the material and asecond one (532 nm, 40 mJ) to excite and reheat the vaporizedsample. The results demonstrate that the signal-to-backgroundratio (SBR) was improved by almost 2-fold as compared to thesingle-pulse approach. In addition, a lower ablation rate and abetter 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 smallercrater sizes compared with SP. This means that the orthogonalarrangement is a useful tool for almost a nondestructive ana-lysis. In this sense, a crater diameter of 10 μm has been ob-served in the surface of zircalloy samples.91 A prespark inhelium at atmospheric pressure, sent 1 μs before a ps abla-tion laser at an output energy of 26 mJ, was used. A LOD ofdeuterium in zircalloy very low (20 μg g−1) was found. Asimilar arrangement has been reported for the analysis of Cl inconcrete.94 In this study, a prespark in He with a delay timebetween pulses of 10 μs allowed the use of ablation energies ofonly 2.5 mJ per pulse.Experimental approaches using various pulse widths in

orthogonal DP-LIBS have been tested. For the analysis of gado-linium oxide, ns or fs pulses were used for the sample ablationand ns pulse was employed as prepulse (prespark in air) orreheating pulse.95 For prepulse mode, no intensity enhancementwas observed compared with SP-LIBS mode. For reheatingmode, intensive emissions have been obtained for the fs−nscombination of pulse widths.In prespark orthogonal DP-LIBS, the signal enhancement has

been attributed to the increase in the plasma temperature.96 Inaddition, an increase in the intensity of ionic emissions and acorresponding decrease in the atomic emissions (which is relatedto the role of Saha equilibrium) have been observed. The selectiveprolongation of emission lifetime only for the enclosed part ofthe plasma in a rarefied region by preablation spark has been alsonoted.DP-LIBS methods describing several combinations of laser

beams have also been published.99−101 An UVNd:YAG laser wasused for standoff analysis at 55 m, whereas a second pulse from aCO2 laser at 10.6 μm was simultaneously delivered to thetarget.99 This study reports that the signal enhancement factor(100×) for the targets assayed (metals, ceramics, and plastics) iscaused by the higher temperature (several thousands of degrees)of the plasma as compared with that achieved using SP. The samelaser combination was used for the analysis of polystyrene film ona Si substrate and for trinitrotoluene (TNT) residues.100 Also, aNd:YAG laser at 1064 nm (8 ns, 120 mJ pulse−1) synchronizedwith a CO2 laser (200 ns, 1.5 J pulse−1), in He at atmosphericpressure, have been used for the determination of H in zircalloysamples, an analysis difficult to perform using a conventionalLIBS system.101 In this case, the CO2 laser was guided per-pendicular to the sample surface, and the ablation laser wasfocused at an incidence angle of 45°.

Recent advances in instrumentation have allowed incorpo-ration of the DP-LIBS configuration into portable sensors ofapplication, for instance, in cultural heritage (where the use ofmicroinvasive techniques is mandatory), and standoff analysis.For in situ LIBS applications, a portable laser system weighing aslittle as 3 kg has been designed and constructed.102 This low-costand compact laser even permits the use of nonintensified charge-coupled device (CCD) detection.Commercial and mobile DP laser instruments have been used

for numerous purposes.84,103−105 The setting of these sensors isusually collinear DP at 1064 nm with an energy per pulsein the range of 50−120 mJ at a maximum repetition rate of10 Hz. For LIBS measurements on bronze alloys, a mobile DPlaser instrument has been used.103 A set of reliable LIBSparameters for the quantitative analysis of Cu, Sn, and Zn hasbeen identified. The sensor caused minimal damage to thesample surface. Also, fast and reliable quantitative analysis ofcomplex metallic alloys (steel, in this case) has been per-formed with a cheap experimental setup.84 Mn, Fe, Ni, and Znoxides in molten glass were quantified.104 Intensity enhance-ment factor in the range of 1.5−3 and double reproducibilitywere obtained with a mobile dual-pulse system. LODs between7 and 194 ppm have been achieved for these metals in liquidglass. A portable DP-LIBS instrument for a rapid qualitativeanalysis of materials of interest in cultural heritage has beenused.105 A comparison with micro-X-ray fluorescence (XRF)analysis was performed.In DP-LIBS arrangement, the enhancement in signal inten-

sities leads one to expand the use of the CCDs. Hence, CCDsrequire the accumulation of a larger number of laser shots, but itcan be advantageous for trace element detection as in the case ofP in an FeO matrix where the estimated LOD for P(I) line at214.91 nm is 10 ppm.106 This fact is due to the low noise levelrecorded by the CCDs which produces an improvement in theLOD of minority elements. Otherwise, iCCDs are more versatileand 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), andaerosols111 have been reported. DP-LIBS has been applied for thedetermination of heavy and toxic metals (Pb) in soils and toimprove the detection of S in coal.107 In the analysis of polymers,such as polyamide, polyvinyl chloride, and polyethylene, theLIBS signal enhancement has been found to depend on thedelay time and the type of polymeric material investigated whencompared to single-pulse measurements.108 In a recent report,different DP-LIBS approaches have been evaluated for theanalysis of macro- and microelement concentrations in algalbiomass.109 The purpose of this research was to improve theanalytical performance (selectivity and sensitivity) for thedetection of toxic heavy metal and elements with biologicalsignificant (K, Mg, Ca, Na) for potential industrial biotechnologyapplications.The efficiency of the DP-LIBS method for the analysis of

liquids was optimized by studying the influence of the excitationenergy in the LIBS response.110 An enhancement in the LOD ofMg in water of about 1 order of magnitude was obtained bydecreasing the energy of both pulses. The authors suggested thatthe energy for the first pulse should be close to the plasma thresh-old where the gas bubble has its maximum lateral expansion andthe 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



particulate and gaseous forms of carbon species in an aerosol.The C response was improved in DP configuration, and the useof the ratio of the DP to SP responses has been proposed as ananalytical tool.An improvement in the analytical performance is not guaranteed

as a result of the double pulse effect in LIBS. A critical study of theexperimental parametersmust be done before any new application.For example, calcified tissues analysis by DP-LIBS was basedon the measurement of relative intensities of ionic and atomic linesfor Mg and Ca.112 Compared to SP, the intensity ratio of Mgwas similar and the intensity ratio of Ca was worse in the DParrangement. DP was also tested to classify geomaterials (carbo-nates as mineral and rock, natural fluorite as mineral, and silicateas rocks and soils) using chemometric tools.78 The DP-LIBSsystem did not provide any advantage for sample classificationover the SP-LIBS system, except in one of the cases tested (soilsample). Also, DP-LIBS has been employed to detect keyelements in aqueous solutions at high pressures to simulate thechemistry of the deep ocean.113 Each element (Na, Mg, Ca,Mn, K) was found to have a unique optimal set of parameters fordetection. The results suggest that the plasma lifetime is veryshort (around 500 ns) in high pressure aqueous solutions andLOD did not improve in the DP experiment compared with theSP configuration.Several models to achieve a better understanding of the

physical processes arising in DP-LIBS and their effectivenesshave been reported. The main cause of improvement in thesignal LIBS in the collinear configuration is an increase of themass ablated with the second pulse. A theoretical model hasbeen published by Rai et al.114 This work indicates that thesecond laser pulse increases the mass ablated more than 3.5times in comparison with single pulse mode. The enhance-ment factor produced by the DP configuration depends onthe square of plasma density, its volume, and the fraction ofsecond laser pulse absorbed by the plasma of the first laserpulse (inverse bremsstrahlung absorption due to electron−ioncollisions). The increase in plasma temperature causes a greatervolume, but this fact only has enhanced effect through enhancedablation.The role of the matrix composition on the emission enhance-

ment in collinear DP-LIBS has been studied for pure metals byCristoforetti et al.115 This study attempts to relate optical prop-erties of metals with the correlation between enhancement ofablated atomized mass and the increased plasma temperature.As a conclusion, among all the optical properties of the metalsstudied, authors have reported that only a relation with the metaldiffusivity exists.Nagli et al.116 have studied the first 500 ns of the plasma

lifetime in collinear DP configuration on Si and Al targets. Whenthe second pulse arrives, the increase of the plasma volume ishigher than the increase of ablated mass. Consequently, theelectron density decreases, the number of collisions with free-electron is reduced, and the plasma lifetime increases. Further-more, free electrons absorb the remaining energy of the secondpulse so that the energy of the free electron increase, favoring theconcentration of ionized species in the plume. Thus, for thedouble pulse arrangement, lower continuous radiation andnarrower lines implying a lower electron density should beexpected, the relative intensity of ionized species being higherthan for neutral species.The expansion dynamics of the plume in SP and DP excitation

regimes was studied on glass samples.117 In reference to thedouble pulse dynamics in collinear mode, local rarefaction of the

ambient gas due to the propagation of the semispherical shockwave generated by the first laser was demonstrated to be respon-sible for the enhanced material ablation and enlarged plasmadimension for the second laser pulse. An enhancement factor upto 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 hasbeen observed under a range of low pressures, and it seems to bestrongly dependent on the ambient pressure. The density andtemperature of the plasma have also been demonstrated that varycritically with plasma morphology. Three ambient pressureregimes have been identified where the plasma evolution hasbeen observed to differ markedly.The effect of the atmosphere surrounding the plasma in collin-

ear DP-LIBS has been also studied for brass samples.118 Anincrease in the spectral intensities of several lines has beenobserved in DP-LIBS in Ar compared to DP-LIBS in air. Theenhancement in spectral intensity dropped as the pressure wasreduced.A numerical model, describing laser−solid interaction, vapor

plume expansion, plasma formation, and laser−plasma inter-action, has been developed to describe the effects of DP-LIBS ofCu in He ambient gas at 1 atm.119 The results of DP-LIBS werecompared with the SP-LIBS measurements with the same totalenergy. The DP configuration might be more efficient becausethe target remains for a longer time in the molten state althoughthe temperature is a bit lower. The mass ablated rate was higherin DP configuration since the total laser absorption in the plasmawas 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 lasershielding effect and that the enhancement of intensity and massremoval were produced when the laser shielding was lower. Also,orthogonal DP laser ablation of Al-based alloy has been inves-tigated.121 In this study, the effect of the sample heating by thepreablation pulse and the mechanisms for the mass removalincreasing were discussed.De Giacomo et al.122 have experimentally and theoretically

compared SP and DP modes. The two laser beams were used incollinear configuration with an incidence angle of 45°. The authorsindicated that SP-LIBS has a marked recombination character andit is affected by chemical reactions with the surrounding air. TheDP-LIBS, expanding in the first pulse induced plasma, keeps itsenergy for longer times which turns in a higher ionization degree,better fulfillment of the local thermodynamic equilibrium condi-tion and in a more stable signal. The analysis of the results demon-strated that the most important feature of DP-LIBS, as far asanalytical applications are concerned, is the possibility to increasethe detection time window and the emission volume, thus obtain-ing 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 laserpulses; it was found that the first pulse produces the bubble (withoutproduces plasma), and the second pulse induces the mild plasmain the bubble. Recently, the problem of the poor reproducibility ofthe DP-LIBS analysis of samples underwater has been inves-tigated, and laser ablation process in water has been discussed (seesection on Analysis of liquids and submerged solids).124

In multipulse excitation or MP-LIBS (more than two sequentialpulses), only a few papers have been published. A homemade MP-Q-switchedNd:YAG laser system has been presented.125 The laser



basically consisted of a train of pulses (up to 6) and width in therange of 20−30 ns with average energy per pulse of 28 mJ (totalenergy about 170 mJ). MP-LIBS has been applied to the analysisof steel.126 In this case, a train of pulses (up to 11) of lower energyand separated by a few μs was obtained by reducing the delaybetween the Q-switch opening and the flash lamp. A crater depthof 90 μm in Zn suggests that MP-LIBS is a suitable arrangementfor the online analysis and quality control of layered materials.MP-LIBS has been also evaluated to the quantitative analysis ofgold alloys.127 Good results in terms of accuracy and precisionwere achieved. Twenty spectral lines from eleven elements wereinvestigated under MP-LIBS (up to 6 pulses, interpulse delay of20−40 μs). The improvement found in the LOD compared withDP has been discussed.128 In this context, the processes involvedin signal enhancement have been reviewed by Galbacs et al.129

The enhancement in MP-LIBS is caused by the increasedmaterial ablation but also because the reheated plume provideslong excitation for certain species. As a conclusion, MP-LIBScauses a lower breakdown threshold and consequently an improve-ment in the ablation efficiency.Femtosecond LIBS. This section reviews recent LIBS work

using ultrashort pulses. Under this approach, the beam does notinteract with the resulting plasma. The interaction of fs pulseswith the material provides special features to the ablation processas a lower threshold and a larger efficiency which are different forlonger pulses. Other advantages include the smaller heat affectedzone, a better depth resolution, a faster broadband-backgrounddecay, and matrix-independent sampling. Progress in fs tech-nology is occurring quickly and new applications of short-pulsedablation are expected to grow. However, for this to happen, theultrafast laser should be more affordable. The principles and usesof LIBS, including coherent Raman spectroscopy (CARS) andterahertz (THz) spectroscopy using ultrafast lasers, have beenreviewed.130

The influence of pulse duration for LIBS analysis using nspulses or fs pulses has been studied in the analysis of bronzes.131

By comparing single-shot LIBS with fs and ns laser pulses, the fsspectrumwas well-resolved and presented a very low backgroundemission, allowing signal accumulation for a large number ofpulses. For fs-laser pulses, plasma emission was found to vary muchmore rapidly with time than in the case of ns-laser-produced plasma.This plasma behavior using fs pulses results in a significant reductionof the continuum as mentioned. In this sense, plasmas produced byultrashort laser seem to reach Local thermodynamic Equilibrium(LTE) after its formation. Spectra of depleted U metal obtainedusing ns Nd:YAG (1064 nm) and fs Ti:sapphire (800 nm) laserpulses have been compared.132 Fs pulses generate cooler plasmas,and the lines are short-lived compared with longer pulses. Neutral Uatoms lines appear immediately after excitation. However, the lineintensities are generally higher in ns LIBS whereas the sensitivity ofns LIBS is better than that of fs LIBS.The detection power of fs LIBS however may change dras-

tically when different samples are analyzed. For instance, aTi:sapphire laser (40 fs) was used to generate plasmas on the surfaceof aqueous solutions. LOD values for Al, Cu, Fe, K, and Zn in waterwere 6.4−200 times lower compared with ns laser excitation.133A study on the influence of buffer gas (air, Ar, and He) and the

pressure on the LIBS spectrum has been reported for UV fspulses on solid samples (brass, Cu, Al, and Si).134 Even thoughthe maximum emission intensities were measured when Ar gaswas used, He led to a higher signal-to-noise ratio (SNR) and lessbroadening of the emission lines (attributed to the Stark effect)resulting in an enhancement of the spectral resolution. Air or Ar

atmosphere showed noticeable line broadening particularly athigh ambient pressure.Although typically performed using ns lasers, some recent

investigations used fs lasers for surface diagnostics in materialsanalysis: surface inspection, depth profiling, interface character-ization, and thin film analysis. Fs laser ablation was applied tothe chemical analysis of Si with the aim of determining the cratersize from which spectral emission could be measured.135 ATi:sapphire laser delivering 100 fs pulses was used. The beam inthe far-field was able to produce subμm craters and in the near-field resulted in the formation of sub-30-nm craters. Depthprofiling with an average ablation rate of 15 nm per pulse hasbeen achieved.136 The fs LIBS plasmas were generated using apulsed laser at 795 nm with pulse energy and duration of 200 μJand 130 fs, respectively. A study of depth-profiling and interfacecharacterization have been performed using fs LIBS.137 Theauthors used a Ti:sapphire laser working at 780 nm, with 150 fspulses and a maximum energy of 720 μJ. Small individual circularcraters with approximate diameter of 30 μm without crackingor melting were observed. However, ablation-induced surfaceroughening was noticed. In another work, spectrochemical analy-sis of microcracks and their propagation on the surface withlateral resolution of 2 μmhave been studied.138 Fs LIBS has beenused for the detection of heavy metal dopants in porous thinfilms.139 The benefit of selective ablation and better control ofmaterial removal may overcome the additional expense of ultra-fast lasers for thin film analysis.Other applications of fs LIBS include the analysis of euro

coins140 and of animal tissues141 and the monitoring of silvertransport through silicon layers of fuel particles for a hightemperature gas reactor.142 Also, fs LIBS has been evaluated forthe detection of explosive residues.143 A comparison of LIBSusing 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 withns pulses was dominated by elemental lines. However, De Luciaet al.144 suggested that several advantages attributed to fs pulsesare not realized at higher laser fluence for the analysis of explosivetraces.Ultrashort laser pulses have been also studied in DP arrange-

ments. Pinon et al.97 employed a collinear geometry and havestudied the effect of the interpulse delay time. Compared with fsSP-LIBS, the energy of one pulse divided into two pulses ledto improve the optical emission by 1 order of magnitude at adelay time comprised between 50 and 1000 ps. The fs DP modeincreases the plasma temperature and the electronic densityresulting in an increase of plasma lifetime, signal intensity, andsignal reproducibility.45 For lines with higher energy levels, theintensity enhancement has been more noticeable than for lowenergy levels. A reduction of the plasma threshold by a factor of 2minimizes the surface damage. Surface effects have also beenstudied in Ni-based superalloys.98 The use of fs DP-LIBS in thesesamples reduced the crater depth to less than 60 nm comparedwith the SP mode (depth of 200 nm). Crater depth on Ag foilby fs laser pulses has been measured in vacuum at differentfluences.145 The effect of the second collinear fs pulse and the delaytime has been studied. Results suggest that the laser ablation depthhas 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-basedalloys, excellent linear regression coefficients (0.998−0.999)were obtained by integrating all emission intensity data along thewhole interpulse delays used.77 For the analysis of gadolinium



oxide cited above, ns or fs lasers were used to the ablation anda ns laser was used as prepulse or reheating pulse.95 In thereheating mode, the signal intensity enhancement was 25-fold forthe fs−ns combination.In addition, applications of ultrashort LA, namely, fs pulse thin

films deposition and fs/ns pulses in DP-LIBS for elementalanalysis have been discussed.146 One of the most importantfeatures of the plasma generated with ultrashort laser pulses is thelarge 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 havebeen demonstrated using this method. A requirement for themethod 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 tunedto a resonance transition. The loss of energy in exothermic colli-sions of laser excited atoms was concluded to be the majorprocess for atomic vapor heating and subsequent formation ofLTE plasmas.RELIBS provides the capability to selectively enhance atom

generation by careful selection of the ablation laser wavelengthand incident energy. The application of RELIBS to quantitativeanalysis has also been evaluated.148 Resonant laser ablation(RLA) was assessed for steel analysis. The signal from an elementat a wavelength coinciding with the selected wavelength improvesover the signals of other elements. This fact supports the hypothe-sis that desorption induced by electronic transitions is involved inthe enhancement process.Detection of Pb at low concentration in contaminated water

was improved when a second resonant laser was delivered to thesample.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 laserpulse tuned to a specific transition of Pb. In this case, the LOD forPb in water was found to be approximately 60 ppb when 1000shots 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 lasertuned at a selected λ transition in lead was used. In this work, theconfiguration of DP-LIBS was named LIBS-LIF because theselected λ was used to excite the traces of the element. The DPsetup consisted of a Q-switched Nd:YAG laser and a ns opticalparametric oscillator (OPO) laser. The LODs for Pb in bothsolid and water samples were 2 orders of magnitude with respectto LIBS. For RELIBS analysis where the second pulse excites thematrix, 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 usedfor ablation and a OPO laser pulse (tuned at 396.15 nm, 7 ns)was used to resonantly excite the aluminum host atoms.151 TheLODs achieved using RELIBS were comparable to LIBS, but amuch lower amount of matter could be ablated from the sample.In other study, RELIBS was investigated for enhancement of theLOD of Pb in Cu alloys.152 An optical parametric oscillator laserwas used to ablate the sample, and it was tuned to the Pb(I)283.31 nm line. The Stokes direct-line fluorescence signal at405.78 nm was recorded. The LOD for Pb (8 ppm over 500 lasershots) 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 laserpulse.153 Cheung et al. discussed the enhancement in the signal-to-noise ratio (SNR) of Mg, Pb, Si, and Cu, when the laser beam

for the ablation step is Gaussian (2−3-fold) and when the heatingpulse 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 laserwavelength was tuned to the vibrational transition of the polymerwas studied by Khachatrian et al.154 The degree of enhancementdepends on the particular vibrational mode excited. Significantenhancements were found for the C−H stretch fundamentalvibrational transitions. Resonant LIBS in the mid-infrared (mid-IR) spectral region have been discussed for the detection ofresidues on surfaces. However, this approach seems to be moreappropriate to bulk samples.

■ RANGING APPROACHESField analytical instrumentation is an attractive option for fastchemical response (industrial, military, and security applica-tions), on-site measurement ability (geological exploration andenvironmental monitoring), and in those cases where the object/material cannot be transported to the lab (archeological andcultural heritage applications). In other words, in situ analysis isneeded in those applications where access to the sample isdifficult 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, spec-trographs, and detectors make possible the development ofcompact and rugged instrumentation. In especial, the use of fiberlasers, compact spectrometers, and fiber optics for guiding theplasma emission offers greater flexibility while reducing the riskof instrument failure.Fortes and Laserna reported a full revision of fieldable LIBS

instruments and applications.27 On the basis of this report, wecategorize the ranging approach of LIBS as portable, remote, andstand-off configurations. In a portable system conf iguration, that is,a man-portable sensor for field analysis, the operator and thesensor are close to the target. The number of portable instru-ments designed by different research laboratories has consid-erably increased in the past few years. Due to the provenreliability and ruggedness of solid-state lasers, the majority ofworks reported in the literature used a Nd:YAG laser working at1064 nm. The specific application (environmental, industrial,geological, cultural heritage, and security, among others) deter-mines the use of detectors with an analytical response in theentire spectral range. This fact requires a compromise betweenthe spectral resolution of the detector and the requirements for aminiature instrument. Most portable systems were based on asampling probe (containing the laser head) and a main unit(mainly a suitcase or an aluminum case containing the laserpower supply, the spectrograph, the detector, and the laptop).Cunat et al.155 designed a man-portable laser system in whichboth the spectrometer and the computer are enclosed in aspecially adapted backpack. This prototype was successfullyevaluated for geochemical analysis of karstic formations,155 fordetermination of Pb in road sediments,156 and for the inspectionof oil spill residues in the coast.157 The analyzer was a tech-nological evolution of the original instrument proposed by thesame authors. A similar configuration was also used by Munsonet al.158 for the detection of indoor biological hazards. A novelcompact and portable pulsed laser system was developed for insitu SP- and DP-LIBS applications102 in the IESL-FORTH. Inthis case, Goujon and co-workers demonstrated the versatility ofthe instrument for identification purposes within a broad variety



of materials, such as pigments in paintings and icons, metals,ceramics, etc. Most recently, Rakovsky et al.159 designed a com-pact portable instrument for geochemically recognizing tephralayers in lacustrine sediments and fossilization processes inammonites. It should be noted that, whether it be a commercialor homemade portable instrument, results obtained usingthis technology are in good agreement with those reported inlaboratory.160,161

Several options can be considered when LIBS measurementsat a distance are needed. In this context, some confusion existsregarding the use of the terms remote and stand-off when referr-ing to the analysis of distant objects. For the purposes ofthe present article, we distinguish both approaches in Figure 3. Ina remote system (Figure 3A), the laser and the signal are trans-mitted through a fiber-optic cable, whereas in a stand-of f system(Figure 3B) both the laser and the signal are transmitted along anopen path. In both cases, the operator is located far from thetarget. Laser technology and optical fibers play an increasinglyimportant role in the design and construction of LIBS sensorsand measuring systems for remote analysis. The integration offiber-optic cables is a solution for applications in which the targetis 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 arecapable of transmitting laser radiation in the range of MW cm−2

without any damage to the fiber. Furthermore, there exists thepossibility of collecting the plasma light using the same fiberemployed 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 forthe in situ analysis of polluted soils at 10 m. The authors usedindependent optical fibers for transmitting the laser radiation andfor collecting the plasma light. A single lens at the end of the fiberwas used to focus the laser beam. In contrast, Dumitrescu et al.163

tested a movable fiber probe for gas-phase LIBS. The authorsevaluated the associated effects of delivery fiber curvature onLIBS signal in order to improve the fiber to laser coupling. Mostrecently, Guirado et al.164 demonstrated the capability of LIBSfor the recognition and identification of archeological materialssubmerged in seawater at depth of 30 m in the Mediterranean

Sea. This demonstration opens a new front of applications to theLIBS 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 andthe returning light from the plasma are transmitted through theatmosphere. As demonstrated, the use of a laser system with agood beam quality factor is mandatory for laser-induced plasmaformation at long distances.165 One of the major problems asso-ciated with ST-LIBS is the attenuation of light by the atmo-sphere.166 However, nowadays, the range of ST-LIBS applica-tions has considerably increased with interesting demonstrationsin the area of forensic science and in analysis of explosives(see Security and Forensics).167 Although LIBS is essentially anelemental analysis technique, it has been successfully tested as apowerful method for detection and identification of residues andbulk explosives.23,74,168−173

Recently, the possibility of combining LIBS with other spec-troscopic techniques has been explored. Particularly well suitedtechnologies for the combination with LIBS are Raman spec-troscopy and molecular fluorescence spectroscopy as thesetechniques use essentially the same instrumentation and also thethree of them are stand-off technologies. In a series of threepapers, Moros et al. explored the Raman-LIBS sensor fusion as apowerful approach for detection of explosives and relatedmaterials.174−176 The information extracted from the same singlespot when both sensors work side-by-side was first demon-strated.174 Later, the strong and weak points of each techniquewere analyzed.175 Finally, the ability for Raman-LIBS detectionand discrimination of explosives residues at 20 m was demon-strated. Using the 2D image generated from preprocessedmolecular and atomic data, a wide range of compounds was easilydistinguished from each other by simple linear correlation.176

The potential of hyphenation of these spectroscopic methods hasbeen also demonstrated in the authentication of inks.177

On the other hand, the combination LIBS/LIF also yieldsvaluable information since it combines the atomic informationprovided by LIBSwith themolecular data provided byLIF. Further-more, LIF has been widely used in the analysis of vegetation,pollutants, and cultural heritage samples.

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.



■ APPLICATIONS

Surface Inspection, Depth Profiling, and LIBS Imaging.During the last decades, the potential of LIBS as a surfacecharacterization tool for spot analysis, line scan, depth-profiling,area analysis, and compositional mapping with a single instru-ment in air at atmospheric pressure has been demonstrated. Ascompared to other techniques of surface analysis, LIBS presentsthe advantages of sampling flexibility in terms of size and shape ofthe analyzed specimen in combination with a fairly good lateraland in-depth resolution and surface sensitivity. Some interestingexamples include trace elements in teeth,178 accumulation ofheavy metal in vegetal tissues,179 characteristic elements in anengine valve,180 dopants in transparent dielectric,181 and spatialdistribution 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 forthe analysis of explosive residues in human fingerprints depositedon glass.184 Results obtained by OC-LIBS corroborated theabsence of spectral interferences when compared with conven-tional LIBS and the freedom from spectral contribution of thesubstrate 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 differentsurface finishes. The authors concluded that differences on thesurface state yield changes on the LIBS signal that can thus leadto a misinterpretation of the results. However, this effect wasminimized when enough pulses were delivered on the sameposition, and the intensity detected was similar on all theanalyzed 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 thisapplication.186 For instance, Galmed et al.136 have evaluated theability of fs-LIBS to generate depth profiling for a Ti thin film ofthickness 213 nm deposited on a silicon (100) substrate beforeand after thermal annealing. An average ablation rate (AAR) of15 nm per pulse was achieved. The depth profiling was verifiedwith a theoretical simulation model, and a very good agreementwith the experimental results was obtained. In other signifi-cant studies, an orthogonal DP-LIBS arrangement was appliedfor first time to the depth characterization of ceramic multi-layered materials as an alternative to the SP approach that useddefocused beams to decrease the power density andconsequently the AAR.93 The depth resolution with the doublepulse configuration was improved by almost 2-fold as comparedto the regular single-pulse approach whereas the reproducibilityin the description of depth profiles was also twice better. Thepotential of MP laser excitation scheme to in-depth character-ization of electrolytically galvanized steel has been presented byCabalin et al.126 Here, bursts of 11 laser pulses with interpulsegaps of 7.4 μs were produced from a commercial electro-opticallyQ-switched Nd:YAG laser containing a KDP* crystal. WithMP excitation, the ablation efficiency was increased 10-fold oniron and 22.5-fold on zinc with respect to dual pulse or single-pulse excitation. The results demonstrated that it was possible toobtain the sample stratigraphy up to depths of 90 μm on zinc-coated steel sheets, using a single burst with a total energy of only60 mJ. Sundaram et al. have performed a systematic study tounderstand the size and shape of nanopatterns generated onselected semiconducting (Si and Ge) and metallic (Cr, Cu, andAg) targets under different laser pulse durations, laser energies,

and number of laser pulses.187 Compared with nanosecond laserpulses, fs excitation provided lower damage thresholds to thetargets, although higher damage thresholds to the near fieldscanning optical microscope (NSOM) probes were at the wave-length studied.In the conventional applications of LIBS discussed in this

Review, laser pulse energies range between 10 and 100 mJ. In thisenergy regime, typical laser focal spot sizes are on the order of afew micrometers. However, improved lateral resolution andsurface sensitivity can be obtained by decreasing the pulse energydown to hundreds of μJ. Godwal et al.188 have integrated a LIBSsystem with a microfluidic platform demonstrating the sensitivedetection of Na in a microdroplet. These same authors149 havegenerated 2D maps of a fingerprint on a Si wafer using 5 μJultraviolet laser pulses. Here, the detection sensitivity has thelimiting factor in the amount of material ablated at very low laserpowers. In these circumstances, increasing the excitationefficiency by resonant ablation is largely beneficial. Laser-inducedplasma formation followed by resonant excitation by a secondpulse tuned to a specific transition of Pb has resulted in a limit ofdetection of approximately 60 ppb for Pb in water. A setup hasbeen proposed for analyzing artworks using a microscope.189

Best working conditions in order to obtain the least amount ofmaterial 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-dimensionalechelle images has been developed.190 This method is based onthe comparison of two 2D images and the identification of pixelsgroups (particles) that can be considered as representative ofactual signals. In this methodology, the majority of noise peaks iseliminated, and identification of weak signals is possible withoutaltering the intensity ratio between emission lines. It has theadvantage 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 nearfield apertured laser processing has been demonstrated.135 NoLIBS signals have been reported from such a limiting amount ofablated material.

Cultural Heritage. In the past decade, LIBS has become avaluable analytical tool for cultural heritage. No sample prepara-tion, minimally destructive, fast analytical response, depth-profiling analysis, and the capability for in situ analysis are themain features that make LIBS a very attractive technique for thecharacterization and conservation of archeological samples,artworks, and other important materials.24,191 The chemicalcomposition extracted from a LIBS analysis gives archeologistsadditional information to better understand our history. Thus, aclose collaboration between the LIBS community and thearcheology 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 forproducing tools) using partial-least-squares discriminant analysis(PLS-DA) of LIBS data. Although information extracted fromthis analysis allows archeologists to better understand artifactproduction and past trade patterns, LIBS still requires furtherimprovements in data processing analysis for discriminationanalysis. Bronze materials are one the samples receiving moreattention in cultural heritage applications. Recently, Gaudiusoet al.193 proposed a new procedure for determination of plasmaexcitation temperature in copper alloys. This method is based onlocal thermodynamic equilibrium and consisted of inverting the



calibration-free (CF) approach, overcoming some of the disadvant-ages of CF-LIBS and calibration curves. Also, only one standard isrequired for the analysis (no matrix-matching is necessary). Theprotocol was performed in a set of archeological bronzes. Highprecision data comparable to those obtained with CF-LIBS andcalibration curves were reported.One of the most important problems regarding the analysis of

archeological materials arises from the fact that most of thesepieces are structured in different layers, either from the manu-facturing process or due to degradation phenomena. Whencompared to other analytical techniques, LIBS is capable ofdescribing the surface and subsurface composition of a materialwhile preserving the sample integrity. This capability is largelyadvantageous when attempting the discrimination betweengenuine archeological findings and modern counterfeits. Con-sequently, depth profiling analysis has been performed in a broadvariety of materials. Significant examples include Roman wallpaintings from the archeological area of Pompeii,194 character-ization of iron Age pottery from Turkey,195 analysis of frescoes,196

authentication studies of unglazed earthenware artifacts,197 andchemical analysis of multilayered painted surfaces198 and metallicartifacts.199

Recently, LIBS has emerged as a promising technique for theanalysis of bioarcheological materials such as calcified tissues,namely, teeth and bones. Harith et al.200 evaluated the influenceof biological degradation and environmental effects on theinterpretation of archeological bone from three different ancientEgyptian dynasties. Authors demonstrated that post-mortemeffects must be taken into consideration on studying dietaryhabits. Also, a clear correlation between the degradation of thetissues in the archeological bones with the absence of CN and C2molecular band in the LIBS spectra was found. Rusak and co-workers201 used calcium-to-fluorine ratios as indicators of bonepreservation quality. Thus, a value for this ratio of 5.70 could beused to distinguish well-preserved and poorly preserved bones,regardless of the species considered.LIBS has been extensively used in combination with multi-

variate statistical approaches such as linear discriminant analysis(LDA), principal component analysis (PCA), and soft indepen-dent modeling of class analogy (SIMCA), for establishing thegeographical origin of historical building materials.202 Classi-fication of silver Roman denarii203 and the characterizationof calcareous and refractory materials from the ancient Greek-Roman theater of Taormina105 have been also demonstrated. In

all these cases, XRF was used as a validation method. Resultswere in good agreement with LIBS data. In addition, the hyphena-tion of LIBS with Raman spectroscopy has broadened the range ofapplications of both techniques. As discussed above, sensor fusionyields complementary information about the sample underexamination (see Ranging Approaches). Sharma et al.204 integrateda combined Raman-LIBS system for the chemical characteriza-tion of minerals. Here, Raman spectroscopy yields informationconcerning the minerals and their polymorphs while LIBS issensitive to minor/trace elements in the elemental composition ofthe sample. In addition, Osticioli et al.205 also demonstrated theversatility of a Raman-LIBS instrument for giving information onthe chemical composition of different artworks such as fresco andterracotta samples and a bronze figure from the Ghibertis “Porta diParadiso”.At present, continuous advances in mobile technology have

resulted in the construction of portable LIBS instrumentscapable for on-site analysis in museums, art galleries, caves, orarcheological excavations. Hence, transport of the artwork to thelaboratory is eliminated, thus reducing the total analysis time andthe risk of irreversible damage to the object.Of particular interest in contemporary archeology is the in situ

characterization of archeological materials from the marine envi-ronment. Certainly, extraction of submarine assets is quite oftennot practical and/or not permitted, thus making the develop-ment of analytical technology for submersed material a parti-cularly appealing activity. Recently, Guirado et al.164 provided anew solution for this problem and demonstrated for the first timethe analysis of submarine archeological materials using a remoteLIBS instrument. Figure 4A shows a picture taken during the on-site trials on the Mediterranean Sea. The chemical signatures ofceramic materials, metallic samples, and precious metals wereacquired at depths of 30 m during a field campaign performed inthe Mediterranean Sea (Figure 4B).

Industrial Analysis. LIBS has been extensively investigatedfor industrial process control in the steelmaking industry. Signifi-cant applications include characterization of hot and moltenmetals, inspection of surface dust on steel sheets, online mea-surement of coating thickness and composition, classification ofmetals and alloys, sorting of steel parts, analysis of slag, andclassification of scrap.8,206,207 Robustness, stability, reliability, analy-sis speed, and operational availability are important performancecharacteristics that make LIBS an important tool in the steelfactory.208,209

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 bronzematerial acquired at 30 m depth.



Noll et al.210 have demonstrated versatile high-speed steelanalyzers that use a Paschen−Runge spectrometer, photomulti-plier tubes (PMTs), and multichannel integrator electronicscapable for the simultaneous detection of a wide variety ofelements with measuring frequencies of up to 1 kHz. On theopposite side, the University of Malaga has designed and con-structed a portable LIBS system based on high resolutionminiature spectrometers using integrated linear CCD arraysexhibiting a spectral resolution of 0.08 nm. This instrument wasdesigned for online production control of the thickness of Mg onelectrolytically galvanized steel.211 Figure 5 shows a photographof the LIBS demonstrator as mounted in the field trials carriedout at ThyssenKrupp Steel (TKS) pilot plant in Dortmund,Germany. For variableMg thicknesses (depending on strip speedof the pilot line), a satisfactory agreement between plant LIBSmeasurements and data from laboratory chemical analysis byICP-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 scrapmaterials. In this case, the greatest constraints arise from the irregularsample geometry, the presence of surface debris, matrix effects, andinterferences, all of which result in poor reproducibility andrepeatability. To overcome these difficulties, several approacheshave been developed. For instance, Werheit et al.212 proposed theuse of a 3D scanning LIBS setup, where the focal point can berapidly changed to improve analytical performance. Their systemsuccessfully classified and automatically separated Al postconsumerscrap 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 Theresults demonstrated that LIBS can be considered as a viablealternative to inductively coupled plasma optical emission spec-trometry (ICP-OES) and XRF for the determination ofrecovered precious metals.Recently, LIBS together with discriminant function analysis

(DFA) has been utilized for identification and classification of six

groups of themost used polymers in manufacturing and packagingof 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 asinput variables in DFA. Results show that LIBS/DFA was capableof 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 beenevaluated for this application.

Environmental Monitoring. Soils and slurries constitutethe environmental workspace most affected by heavy and toxicmetals from many anthropogenic sources. Numerous LIBS investi-gations have focused on the in situ semiquantitative and quantitativecharacterization of these metals.22,24 The most recent studies relatedto 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 plantnutrition and can affect crop yields when not present in appro-priate concentration levels in soils. LIBS seems a suitable in situand real-time technique to determine nutrient distribution insoils, although soil heterogeneity and matrix effects are twofactors that make this application difficult. Trevizan et al.228,229

have determined macro- and micronutrients in pellets of plantmaterials using an approach based on univariate calibration withcertified reference materials (CRMs). Some discrepant resultswere observed, which indicates that matrix effects and differencesin particle size distribution of CRMs and samples are relevant forthe analysis. Recently, LIBS possibilities and drawbacks in thequantification of the total elemental concentration of soils underlaboratory-controlled conditions have been studied.230 Otherinteresting applications of LIBS have focused on the investigationof the metal accumulation in vegetal tissues.231 Galiova et al.demonstrated the capability of LIBS for mapping Ag and Cudistribution directly in plant leaves of Helianthus Annuus L.232

Environmental challenges from different types of industries areassociated with wastewater generation contaminated with toxic

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.



Table3.Summaryof

LIBSApp

lications

toAnalysisof

Heavy

andToxicMetalsin

Soils

andSlud

ge

sample

issueanalyzed

byLIBS

elem

ents

limitof

detection

ref.

soil

quantificatio

nofCr,Cu,Pb

,V,and

Zn;determ

inationofanthropogenic

index(AI)forCr(AI C

r)andZn(AI Z

n)Cr,Cu,Pb

,V,and

Zn

Cr(17mgkg

−1 ),C

u(61mgkg

−1 ),P

b(20mgkg

−1 ),V

(29mgkg

−1 ),

Zn(55mgkg

−1 )

215

soil

screeningof

cadm

ium

contam

inationinsoils

Cd

CdII214.441nm

(1.3μg

g−1 )CdII226.502nm

(3.6μg

g−1 )

CdI228.802nm

(4.0

μgg−

1 )216

soil

insitu

semiquantitativeanalysisof

pollutedsoils

Cu,Pb

,Fe

Pb(200

ppm),Cu(80ppm)

217

soil

quantificatio

nof

PbinsoilusingDP-

PbPb

(20ppm)

107

soil

comparison

ofAsdetectionby

theSP

andDP

As

As(85mgkg

−1 )

80soilandsludge

quantitativeandqualitativedeterm

inationof

heavymetal

Al,Ca,Cr,Cu,Fe,M

g,Mn,Pb

,Si,Ti,

V,and

Zninsoil

218

soil

onlinemonito

ringtheremediatio

nprocessof

soilcontam

inated

with

chromium

metal

Cr

Cr(2

mgkg

−1 )

219

soil

developm

entof

amobile

system

andaclassificatio

nof

thesamples

byPC

AFe,C

a,Na,Si,and

Al

162

soil

quantitativemultielementalm

ethodforcontam

inantdeterm

inationin

soilundersewagesludge

application

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

Zn

Ba(8.01mgkg

−1 ),C

o(9.33mgkg

−1 ),C

u(9.94mgkg

−1 ),

Mn(114

mgkg

−1 ),N

i(7.86

mgkg

−1 ),V

(46.9mgkg

−1 ),Zn(30.7mgkg

−1 )

220

soil

multivariateandunivariateanalysisto

determ

inetotalC

concentration

totalC

221−

223

slurry

quantitativeanalysisof

slurry

samples

usingunivariatecalibratio

n,multip

lelinearregression,and

partialleast-squares

regression

Al,Ca,Fe,N

i,andSi

224

slurry

quantificatio

nof

metalapplying

univariateandmultivariatecalibratio

nMg,Si,and

Fe225

slurry

analysisof

simulantslurry

samples

used

inthevitrificatio

nprocessof

liquidradioactivewastes

Al,Fe,Si,Na,andLi

226

roadside

materials

quantitativedeterm

inationof

Pbusingaman

portableanalyzer

PbPb

(190

mgkg

−1 )

156

rock,sandstone,

beachsand

qualitativedeterm

inationofoilspillresiduesusingaman

portableLIBS

system

C,C

N,and

C2bandsH

,N,O

,Mg,Na,

Fe,and

VandCa,Si,and

Al

157

soil

toxicmetalsinGulfW

aroilspillcontam

inated

soil

Al,Ba,Ca,Cr,Fe,M

g,Na,S,Sr,T

i,V,

andK

Al(12

ppm),Ba(3

ppm),V(2

ppm),Ti(6ppm),Sr

(7ppm),S(7

ppm),

Fe(12ppm),Ca(14ppm),Mg(9

ppm),Cr(2

ppm),K(10ppm),

Na(7

ppm)

227



metals such as Ni, Cr, Pb, etc. In this field, LIBS is particularlyinteresting for on-site and remote monitoring of the concen-tration of these toxic metals in liquid environmental samples suchas wastewater from industrial plants and in seawater. As discussed inthe section devoted to the analysis of submerged solids, plasmaformation in liquid media is a complex phenomenon dominated bya continuum background and consequently a low emission signal.Generally, direct analysis of liquid samples by LIBS is implementedby focusing a laser pulse on the surface or in the bulk liquid or on thesurface of a liquid jet. Inherent drawbacks of such a method includesplashing and formation of surface ripples in the liquid produced bylaser-induced shock waves. However, several interesting results havebeen reported in this area.233,234 For example, Hussain and Gondalhave developed a LIBS system for the analysis of Ca, Mg, P, Si, Fe,Na, andK inwastewater from adairy product plant.235 Toovercomethe problem of splashing of water on optical components, a specialcell was designed. In addition, relatively high energy laser pulses(100 mJ) were used to enhance the emission intensity. Thisapproach provided accurate analytical results that were in goodagreement with those obtained with ICP-OES. Determination of Crin industrial wastewater from electroplating industries has been alsoconducted.236 In this case, a liquid jet was used. LIBS results were ingood agreement with atomic absorption spectrometry data. Rai etal.237 also explored the effect of additional elements like Cd and Coin the Cr contaminated water. Recently, a similar configuration wasevaluated for analysis of Pb and Cd.238

LIBS has been extensively used for continuous and in situ moni-toring of gas and particle emissions (heavy metals) originating fromexhaust stacks (incinerators, industry, foundries, etc.). Approachesreported in the literature include the focalization of the laser onparticles collected on a filter or direct analysis of the aerosolcloud.239−241 Gallou et al.242 have compared these approaches forquantification of Cu particles with sizes ranging from 1 μm to 7 μm.

Indirect analysis appears to be significantlymore efficient than directanalysis. For direct LIBS analysis, the minimum detectable concen-tration of CuSO4 particles in air was 15 μg m

−3 for an analysis timeof 1 min. The second approach consisted of analyzing quartz fiberfilters enriched with the same CuSO4 particles. The LIBS LODwas60 μg m−3 for a similar measurement time. LIBS has shown itspotential as a valuable technique for unburned carbon (UC) analy-sis in fly ash from furnaces of pulverized-coal-fired powerplants.85,243,244 Under optimized conditions, the UC content infly ash is in the range of 2−5 wt %, while this value may be up to20 wt % under nonoptimized operating conditions. Online moni-toring of UC by LIBS could provide a close-up monitoringoperation that could help in optimizing the combustion processthrough an adjustment of the air−coal ratio. Zhang et al.245 havedeveloped a LIBS system for online analysis of unburned carbon infly ash without being affected by the type of coal burned. A suctioncapacity of 1.4 m3·min−1 was suggested to enhance the stability andreliability of the quantitative analysis. For minimizing matrix effects,a second-order polynomial multivariate inverse regression methodwas considered. The accuracy observed using the C line at 247.86nm was 0.26%, whereas the average relative error was 3.81%.

Biomedical and Pharmaceutical Analysis. Several reviewpapers20,25,26 (cited in the General Information: Books, Reviews,and Conferences) have been published summarizing the effortrelated to the fundamentals, instrumentation, and applications ofLIBS for the analysis of biomaterials. The characterization of bio-materials is interesting for obtaining clinical diagnostic information,which may better guide treatment and may also be helpful inoptimizing the therapeutic technique. According to Rehse et al.,25

biomedical applications of LIBS can be broadly classified into twocategories: (1) analysis of human clinical specimens (including hardtissue such as teeth and bones and soft tissue such as human hair andskin or tissue samples, human blood, or other fluid samples) and (2)

Table 4. Biomedical Applications of LIBS

applications samples elements issue studied by LIBS ref

analysis of carious toothdecay

teeth Ca, Na, and Sr spectroscopic investigation of carious tooth decay 246, 247

Ca, Mg, Cu, Zn, Sr, Ti, C, P, H,O, Na, and K

rapid identification of caries and the roles of the presence ofmetal elements for caries formation

in dentistry teeth Ca, Mg, P, and Zn microleakage in dentistry; microleakage between infrastructureand veneer materials in dentures

248, 249Cr, Co, and I

analysis of nails nails Ca, Na, and K quantitative analysis of normal and pathological nails 250in gastroenterology gallbladder stone C, Ca, Cu, Fe, K, Mg, Mn, N, Na,

O, P, Sr, and Znqualitative and quantitative analysis of different kinds ofgallstones and variations in the elemental composition acrosstheir width

251, 252

C2 swan bands and CN violetbands

laser ablation of stones andapplication innephrology

kidney stones, urinarystones

Ca, Mg, Cu, Fe, Zn, Sr, Na, K, C,H, N, O, P, S, Cl, etc.

qualitative and quantitative analysis of different kinds of kidneystones, urinary stones and cross-sectional study

253, 254

C, H, N, O, Ca, Mg, Na, Sr, K,and Pb

analysis of body fluids glucose and NaCl analysis of organic matter such as glucose and metal elements 255in diagnosis malignant tissues Ca and Mg spectrochemical analysis to identify and characterize 256

some types of human malignanciestissue classification brain, lung, spleen, liver,

kidney, and skeletalmuscle

Ca, Al, Fe, Cu, Na, Zn, Cr, Mg, K,P, C, Li, Ni, Mo, Sn, Sc

characterization of different normal animal tissues 257

analysis of bones calcified bones Sr, Ba, Al, Pb, and CN and C2swan bands

biological degradation and environmental effects in bonesamples

200

analysis of blood blood samples O, H, C, N, Fe, Mg, Ca, K, andNa

qualitative analysis of blood and other liquid organic compounds 258

biodiagnostic pathogens and viruses onsubstrates

differentiation of pathogens and viruses on substrates 259

analysis of biologicalspecimens

salt and soil Ca, Mg, Na, K, Al, Si, Sr, Ti, Zn,Fe, S, C, H, N, and O, C, and N

analysis of common edible salts, soils and heterogeneousbiological samples

260



analysis and detection of pathogenic microorganisms (e.g., bacteria,pollen, virus, fungus, yeasts) that can infect human subjects andcause disease. Table 4200,246−260 summarizes the most recent LIBSbiomedical applications.LIBS has been successfully used for discrimination, classi-

fication, and identification of bacterial specimens on the basis oftheir atomic composition.261,262 For this purpose, advancedcomputerized chemometric methods in combination with high-resolution and broadband echelle spectrometers using sensitiveCCD and iCCD detectors have been employed. A method basedon LIBS and neural networks (NNs) for rapid identification anddiscrimination of specific bacteria strains (Pseudomonas aero-ginosa, Escherichia coli, and Salmonella typhimurium) has beenreported.263 The effect that adverse environmental and meta-bolic stresses have on LIBS identification of bacterial specimenshas been evaluated.264 All bacteria were correctly identifiedregardless of their metabolic state, and the LIBS-based diagnosticretained its selectivity and sensitivity.An interesting review focused on the analysis of plant materials

by LIBS has been recently published.21 Applications to fresh ordried surface of leaves, roots, fruits, vegetables, wood, and pollenare quoted.From the perspective of consumer safety and human health

hazardous contaminant, the potential of LIBS to determine theconcentration of toxic elements in four different lipstick brandssold at local markets in Saudi Arabia has been examined.265

Important findings of this study are that the concentration ofsome of the toxic species like Pb, Cr, and Cd was much higherthan the safe permissible limits for human use and could lead toserious health problems. Godoi et al. evaluated LIBS for theidentification of Ba, Cd, Cr, and Pb in plastic toys. The LIBSsignal correlated well with ICP-OES concentrations.266 LIBScould be then used as a simple and fast screening method capableof discriminating toys that offer potentially toxic effects from safeproducts.Several applications of LIBS for monitoring the distribution of

active pharmaceutical ingredients (API), drugs, lubricants, andother components used in the formulation of powder blends andtablets have been reported.267 Doucet et al.268 have reported that,using selective molecular bands such as CN, CH, and C2, theatomic lines of C, H, and Mg and two ionic lines of Ca, coupledwith chemometric tools, it is possible the complete and simul-taneous qualitative and quantitative prediction of all ingredientspresent in a complex matrix such as a pharmaceutical formula-tion. Similarly, two chemometric algorithms, namely, PCA andSIMCA, have been implemented for the classification and dis-crimination of pharmaceutical tablets of brufen, brufen (coated),glucosamine, glucosamine (coated), paracetamol, and vitaminC.269 While PCA was a valuable tool for recognizing similaritiesbetween sample types, SIMCA was employed to assign classgroups to the tested tablets. A set of 30 test samples were dis-criminated by SIMCA with an average rate of approximately 94%correct classification.LIBS has been evaluated for studying the distribution of macro-

and micronutrients in multielement tablets.270 The resultsobtained for Ca, Mg, P, Cu, Fe, Mn, and Zn were comparedwith the analysis of the corresponding acid digests by ICP-OES. Ingeneral, elemental concentrations measured by LIBS were in goodagreement with those obtained by ICP-OES, although differencesdue 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 theinterior to the surface. To evaluate drug migration, Yokoyama

et al.271 prepared model tablets containing nicardipine hydro-chloride as API (pale yellow) and excipients (all of white color).Visual inspection, FT-IR mapping and LIBS analysis werecompared. These authors demonstrated that LIBS analysis wasthe simplest and the fastest method for migration monitoring.Recently, aerosolized drug delivery methods have gained

popularity due to their improved efficiency in administration ofnano-sized drug particles to the specific target organs. In thiscontext, the viability of LIBS for chemical analysis of carbon-containing aerosolized drugs has been demonstrated.272 Relativeelemental ratios of carbon to various trace elements in differentdrugs were examined. Three different carbon-based powderedvitamin drugs with different carbon contents and trace elementssuch as Fe, Ca, and Mg were evaluated, and the elemental ratiosof [C]/[Mg], [C]/[Fe], and [C]/[Ca] were calculated from theLIBS spectral data analysis on each of the drugs. An excellentagreement with the expected stoichiometric ratios based on theirchemical formulas was found.

Security and Forensics. A considerable amount of literaturehas been published in this area. In the security field, an assess-ment of the application of LIBS to the detection of explosiveresidues has been published.23 Methods to improve the sensi-tivity and selectivity of LIBS for detection of explosives such asthe 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 LIBSapproach due to the often small sample size available for analysis.Two other important areas that could benefit from single-shotLIBS are explosive detection and military applications. Suchapplications require rapid detection thus using a minimumnumber of laser shots. Applications of single-shot LIBS have beenreviewed by Michel.17

A recent article reviews the information over the last 5 yearsrelated to the advances in explosive detection techniques: innova-tive detection approaches, improvement of existing techniques,instrumentation developments (miniaturization, portability, field-ruggedness, improvements in standoff distances), and selectivityand sensitivity enhancements.273 The authors summarize spec-troscopic approaches for explosive detection that could competewith LIBS, including: ion mobility spectroscopy; mass spectrom-etry; terahertz spectroscopy; Raman spectroscopy; and cavity ringdown spectroscopy. A critical review of laser-based methods forstandoff detection of explosives including LIBS has also beenpresented.171

Recently, forensic applications of LIBS have also been dis-cussed, and they enclose the analysis of glass, gunshot residues,papers and inks, paints, and soil samples.12 A significant effort hasbeen performed in developing explosive detection using LIBS incombination with chemometrics tools. The efficacy of chemo-metric techniques such as linear correlation, PCA, and PLS-DAfor the identification of explosive residues has been evaluated.74

The use of the full spectra or the intensities and ratios specific toexplosives in the chemometric model has been discussed. Thecombined use of the two models seems to find better results.172

In this regard, the importance of variable selection for maxi-mizing residue discrimination with PLS-DA has been high-lighted.274 Moreover, LIBS combined with PLS-DA has beentested to discriminate explosives from plastics.275 Dingari et al.have proposed the use of support vector machines (SVMs) fordiscrimination based on LIBS measurements applied to forensicarea.276



Other strategies have been designed to improve thediscrimination of explosives from nonexplosive substan-ces.277−279 Concerning the analysis of explosive residues,selective sampling and analysis of explosive residues on solidsurfaces has been successfully tested. As demonstrated, thesubstrate contribution could be minimized under certainexperimental conditions.279

Other attempts to improve the detection of explosive residueby LIBS include the use of ultrashort laser ablation and resonantLIBS, which have been cited above (see LIBS methods, fs-LIBSand RELIBS).The combination of LIBS with Raman spectroscopy has been

presented as an alternative for the detection of explosives. Thesuccessful integration of both sensing technologies in a singlehybrid sensor capable of simultaneously acquiring, in real time,both multielemental and molecular information from the samelaser pulses on the same cross section of the sample at standoffdistances, has been displayed.174 Additionally, strong and weakpoints 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 fusionof the responses of both sensing technologies.175 Furthermore,how to combine the spectral outputs of LIBS and Raman forgenerating a new pattern of identification (2D image) andachieve synergy for obtaining knowledge about the identity ofcompounds better than that achieved when each technique actsalone has also been proved.176

New strategies for the detection of explosive residues havebeen tested as the detection of explosive behind a barrier(polymethylmethacrylate and glass) placed between a target anda standoff LIBS sensor.170 Analysis of explosive residues inhuman fingerprints using a laboratory scale instrument has beendescribed by optical catapulting-LIBS (OC-LIBS).184 Finally, thepotential of LIBS to detect chemical and biological threats hasbeen studied.169,280 Simulant samples such as Bacillus atrophaeusspores, Escherichia coli, MS-2 bacteriophage, a-hemolysin fromStaphylococcus aureus, 2-chloroethyl ethyl sulfide, and dimethylmethylphosphonate were tested and correctly classified by LIBStogether 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) andarcheology. In recent years, a number of interesting ideas con-cerning the study in liquids and its applications have emerged.Among all the applications inside liquids, this Review focuses inliquid samples and submerged targets. Table 5 provides anoverview of the different configurations reported in the literaturefor 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 plasmagenerated is very short due, among other factors, to the increasedoccurrence of electron-ion recombination processes. Direct con-sequences are a relatively poor signal in the conventional singlepulse LIBS approach, and spectral emission is characterized by abroad spectral continuum. Under these conditions, the accu-mulation of several laser pulses turns into the unique alternativefor improving the signal-to-noise ratio. Nevertheless, the mostimportant mechanical effect when a laser pulse is focused on aliquid is the formation of a cavitation bubble since its char-acterization allows the understanding of the laser−matter interac-tion inside a liquid and provides an explanation of the phenomenaobserved in the DP-LIBS configuration.

A number of fundamental studies attempted to explain, from atheoretical point of view, the mechanisms and processes insideliquids, with particular emphasis on bubble dynamics. Peelet al.281 provided a comprehensive overview of the dynamics oflaser-induced cavitation. In fact, authors generated plasmas inbulk water in order to investigate the rate of bubble expansionand to estimate the maximum bubble diameter and bubblelifetime prior to the unavoidable final collapse. The total durationof the oscillating cavitation bubble was obtained using twotechniques, namely, pump−probe beam deflection and high-speed photography. The mean value measured was ∼800 μs.Most recently, Lazic et al.282 studied light transmission through alaser formed bubble during the ablation of a metallic target insidewater. Due to its importance in many applications, the authorsinvestigated optical properties of the vapor cavity formed underthese conditions, observing that inhomogeneous water vaporclustering inside the cool expanded bubble further perturbs thelight transmission and induces irregular ablation by thesuccessive 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 aCu target in water. Shadowgraphy images revealed that longpulses (150 ns) were more favorable for LIBS analysis. Underthese conditions, the relatively slow heating of the plume causes alarger and less dense plume, and consequently, the spectralemission shows less broadening and a weaker continuum. On thebasis of this report, the same authors performed a compositionalanalysis of a Cu/Zn alloy in water using a 150 ns pulse.283 Dif-ferent ablation efficiency for Cu and Zn was observed, suggestedby a significantly Zn-rich plume compared with the target Zncontent. Schechter et al.284 investigated the earlier stages ofoptical breakdown in water and alcohols using nanosecondpulses of 1064 nm. Authors observed the discrete structure of thelaser spark column, consisting of abundant micro plasma balls inthe tested liquid. Also, they found that the discrete nature of theplasma column lasts up to 100 ns. On the other hand, ultrashortlaser pulses have been also used to evaluate the spectral andtemporal characteristics of plasmas produced in seawater.132,285

The LODs for Al, Ba, Ca, Cu, Fe, K, Mg, Na, and Zn in waterwere 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 usedin the determination of Ca and Mg in aqueous solution,286 quan-titative determination of Pt in liquid samples287 and chemicalcharacterization of crude oil.157

Several attempts to improve the sensitivity in liquids usingdifferent instrumental developments have been presented. Asknown, sample introduction is a weak point in LIBS analysis ofliquids. In order to overcome this drawback, Yalcin et al.288

designed a continuous flow hydride generation LIBS system forthe determination of Sn in aqueous environment. Althoughresults were quite satisfactory, the method still requires furtheradvances 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 of2−3-fold) for toxic elements using an ultrasonic nebulization-sample introduction system for LIBS analysis.289 Rai et al.237

investigated the matrix effects for Cr in liquid jets and observedthat the introduction of additional elements such as Cd and Co inthe Cr decreases the detection sensitivity in binary and tertiarymatrixes. In addition, Feng and co-workers290 evaluated theinfluence of experimental parameters on the LIBS signal of Pbin a Pb(NO3)2 aqueous solution. The LOD was calculated in



60 ppm, still requiring a large effort to achieve trace levelperformance in liquid samples.Analysis using the single-pulse approach is limited by a number

of shortcomings (splashing, limited lifetime of the plasma, largebackground, poor excitation efficiency...) that affect thesensitivity and analytical precision of LIBS. Although nowadaysit is fully established in the LIBS community, the double-pulseconfiguration emerged as a method for improving the sensitivityof LIBS in liquids. Specific details on the mechanisms involved inthe double-pulse approach and its interpretation have beenentirely described in the literature. The review of De Giacomoet al.291 provides a general description of the basic aspects ofunderwater LIBS and of the peculiarities of DP-LIBS as aninvaluable analytical tool for the elemental analysis of bulk waterand of submerged solids. DP-LIBS offers a greater sensitivity(1 or 2 orders of magnitude better), less broadening, and lowercontinuum when compared to conventional LIBS. Rai and co-

workers evaluated the temporal dependence in the signal enhance-ment of Cr emission lines by DP-LIBS.76 Authors indicate that theincrease in ablated material and subsequent signal enhancementmay be due to the rarefied gas density inside the region enclosedby the shock wave produced by the first laser pulse. In addition,the LOD of Cr in aqueous solution was measured in 120 ppbwith DP-LIBS, 1 order of magnitude better than in conventionalLIBS. Recently, the synergetic effects of DP-LIBS for theformation of low-density plasma in water were investigated bySakka et al.123 Authors demonstrated that pulse interval andpulse energies of 15−30 μs and ∼1 mJ, respectively, were appro-priate for the acquisition of narrow emission spectral lines (low-density plasma) without time-gated detection. It must be notedthat the pulse energy employed here is much smaller than thosetypically used in DP-LIBS. They emphasized that this unpre-cedented finding suggests a new type of mechanism for laser-induced breakdown in water. Most recently, Cristoforetti et al.124

Table 5. Overview of the Different Configurations Performed in the Literature for the Analysis of Liquids andSubmerged Solids

laserλ

(nm) τ (ns) f (Hz) E (mJ)LIBS

configurationcollectiongeometrya samples LOD observations ref.

Nd:YAG 1064 10 140 SP- S.V. (90°) water bulk dynamic of laser-induced cavitation 281

Nd:YAG 1064 6.5 210 SP- Al light transmission through a laserformed bubble

282

Nd:YAG 19/90/150 SP- S.V. (90°) Cu effect of pulse duration on LA in liquid 44

Nd:YAG 1064 150 6.6 SP- S.V. (90°) Cu/Zn effect of fractionation 283

Nd:YAG 1064 6 20−150 SP- liquids time-dependent structure of opticalbreakdown using interferometrictechniques

284

Ti:sapphire 800 40 fs 100 1.1 SP- S.V. (90°) seawater determination of elements in seawater 132, 285

Nd:YAG 532 16 1−10 SP- S.V. (45°) Ca 1.9 μg mL−1 chemical analysis 286

Mg 3.2 μg mL−1

Nd:YAG 266 3 20 SP- C.V. Pt 100 mg kg−1 chemical analysis 287

Nd:YAG 1064 4 20 50 SP- S.V. (45°) crude oil in situ detection of oil spill residues 157

Nd:YAG 532 10 10 150 SP- S.V. (90°) Sn 0.3 mg L−1flow hydride generation LIBS system 288

Nd:YAG 532 10 10 45−150 SP- S.V. (90°) metals inliquids

289

Nd:YAG 532 4 10 425 SP- S.V. (45°) Cr 1.1−2 ppm LIBS for liquid jet analysis 224

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 signalenhancement of Cr by DP-LIBS

76

2 Nd:YAG 18/16 1 DP- S.V. (90°) Al, Cu synergetic effects of DP for theformation of mild plasma in water

123

2 Nd:YAG 1064 25 ps/20 ns 10 DP- Al, Au effects of experimental parameters onthe laser-induced cavitation bubble

124

532

2 Nd:YAG 1064 4−6 20 400 DP- S.V. (90°) metals LIBS of metals covered by waterdroplets

292

1064 13 20 35

2 Nd:YAG 266 7 32 DP- C.V. Fe 8 ppm chemical analysis 293

1064 7 250 Pb 6 ppm

Au 3.5 ppm

2 Nd:YAG 532 6 20 30−100 DP- S.V. (45°) B 0.8 ppm chemical analysis 89

Li 0.8 ppb

2 Nd:YAG 1064 5 200 DP- C.V. Na 50 ppm LIBS at oceanic pressure(2.76× 107Pa); not improved LODs

113

Mn 1000 ppm

Ca 500 ppm

Nd:YAG 1064 5 60 SP- C.V. Na 50 ppm LIBS at oceanic pressure(2.76 × 107Pa)

294

Mn 500 ppm

Ca 50 ppmaS.V. (side view); C.V. (collinear view to the incoming laser beam).



evaluated the effects of experimental parameters on thereproducibility and dynamics of laser-induced cavitation bubblein DP-LIBS. This report is very interesting since it combines laserpulses in the ns and ps range. Although future works must verifyits suitability for LIBS measurements, it seems that infrared pslaser pulses are preferable for inducing stable and reproduciblebubbles (first pulse) and also for plasma formation inside thebubble (second pulse). In 2012, Cabalin et al.292 used a collinearDP-LIBS configuration to study the effect of deposition of waterdroplets on the ablation of metallic samples. The authorsdemonstrated that liquid coverage increases the material removalrate and also reduces the ablation threshold. This fact is inagreement with thermal and mechanical properties of the metals,thus suggesting that photothermal and mechanical effects play asignificant role in water-assisted plasma formation. Furthermore,Lee et al.293 and Rifai et al.89 demonstrated the high sensitivity ofDP-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 thisapproach 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 forunderstanding the chemistry of deep ocean hydrothermal ventfluids and seawater. In this case, DP-LIBS did not improve theLODs previously measured by SP-LIBS.294 More recently,Guirado et al.164 demonstrated the potential of a fiber-based instru-ment specially designed for the remote analysis of archeologicalmaterial underwater (see Cultural Heritage).In conclusion, all the configurations examined here improve

the sensitivity of LIBS analysis but the improvement is at theexpense of system cost and experimental complexity. What isclear is that, depending on the matrix, the experimental condi-tions must be chosen case by case.Space Exploration and Isotopic Analysis. Space

Exploration. The space exploration is an exotic LIBS applicationthat highlights the versatility of the method. The application ofLIBS to Mars exploration is closely related to its capability forstand-off analysis and mainly focused on the identification anddiscrimination of minerals. Hyphenation of LIBS with otherspectroscopic methods such as Raman and LIF has increased thepossibility of the implementation of LIBS in future spacemissions. It should be stressed that data obtained during thelimited lifetime of a mission are of crucial interest as they couldanswer questions related to the solar system and the geologicalprocesses and weathering conditions for life in the earth.Since the 1990s, the potential and viability of LIBS for plane-

tary exploration has been studied. In 2004, the efforts of the LIBScommunity in the field of planetary exploration were recognizedwhen a new LIBS instrument was selected for the mobile NASAMars Science Laboratory (MSL) rover. The ChemCam instru-ment, that is, the Chemistry and Camera instrument, is one of 11science instruments onboard NASA’s 2011 MSL rover namedCuriosity. The ChemCam package consists of two standoff sens-ing instruments: LIBS and a Remote Micro-Imager (RMI),mounted on a rover body, for elemental analysis at distancesfrom 2 to 9 m. The LIBS provides elemental compositions, whilethe RMI places the LIBS analyses in their geomorphologicalcontext. The main objective of ChemCam is to evaluate whetherthe Martian environment is capable of supporting microbial life.For this reason, and on the basis of orbital data, the Curiositylanded 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

appear in the bibliography. As an example, the analysis of traceelements in speleothems is of special interest in geology becauseof the significance of these elements as paleoclimatic proxies.182,183,295

Alvey and co-workers296 demonstrated the capability of the techniquefor the fast analysis and discrimination of minerals. The sameauthors have also developed LIBS for the recognition, classi-fication, and provenance of different geomaterials (rocks,minerals, ...) using multivariate statistical classification techni-ques.78,297,298 Nowadays, the use of statistical and chemometricmethods for sample classification is almost mandatory. Numer-ous authors have applied these methodologies in a broad varietyof applications such as the identification of mineral ores,299 char-acterization of igneous rocks,300 and optimization of theChemCam instrument.301 On the basis of this methodology,calibration of the ChemCam LIBS instrument for carbonateminerals on Mars was performed by Lanza et al.302 The relevanceof this optimization lies in that the existence of carbonate mayindicate the past presence of liquid water on Mars and theevolution 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 curvesallow correction for chemical matrix effects when a proper train-ing set of standards are used. Thus, calibration curves as well asmultimatrix calibration curves have been applied in the LIBSanalysis of rocks303 and zeolites.304 Dyar et al.305 and Fabreet al.306 recently discussed the potential of LIBS instrument forsulfur identification and igneous rocks characterization,respectively, under Martian conditions. It should be noted that,although LODs calculated in these reports were not extremelybrilliant, results obtained by these methodologies were compar-able and in good agreement with the requirements of ChemCam.Recently, Vaniman et al.307 developed the ceramic ChemCamcalibration targets on the MSL. In addition, Lanza et al.308 dem-onstrated that the composition of rock varnish coatings andweathering rinds may be differentiated from that of fresh rock byLIBS. Depth profiling analysis was performed here with theobjective to provide important information about the types ofweathering processes.In a recent publication, Wiens et al.309 described the body unit

of ChemCam, as well as the assembly, testing, and verification ofthe instrument prior to launch (Figure 6A). Curiosity waslaunched on November 2011 and successfully landed on Marssurface in August 2012. Figure 6B shows a pictorial interpretationof Curiosity analyzing at the Mars surface. At the time of writingof this paper, the Curiosity rover was 2 months in operation. Thefirst LIBS spectrum has been published.310 A preliminarily analy-sis indicates the spectrum is consistent with basalt, which isknown from previous missions to be abundant on Mars.

Isotopic Analysis. Isotopic analysis is of crucial interest inmany fields such as medicine, chemistry, materials science, radio-chemistry, and archeology, among others. Nowadays, isotopicanalysis is performed by isotope-ratio mass spectrometry (IR-MS), thermal ionization mass spectrometry (TI-MS), secondaryionmass spectrometry (SI-MS), gas chromatography/mass spec-troscopy (GC/MS), ICPMS, andMALDI-MS, with mostly satis-factory levels of sensitivity and selectivity. However, theminiaturization of these techniques significantly compromisesthe analytical figures of merit and does not eliminate the need forsample preparation. As discussed in this review, LIBS is a goodcandidate for fast analysis at atmospheric pressure thanks to itswell-known versatility and capability for multielemental in situanalysis. Nevertheless, isotopic analysis is not a typical LIBS applica-tion due to the high spectral resolution needed. In addition, Stark



andDoppler broadening of spectral lines makes the determinationof isotopic shifts in the order of a few picometers difficult. Toovercome this problem, several researchers proposed the use of avacuum or reduced pressure environment since collisional, Stark,and Doppler broadening mechanisms are minimized at lowpressure.In the past few years, most of LIBS applications require field-

able systems which are associated to low-performance instru-mentation 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 andapplying PLS using a portable LIBS sensor. Authors obtained asuitable solution for this application and also demonstrated oncemore that the combination of LIBS with chemometrics is anexcellent approach for the fast determination of isotopes ratiousing 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 thisconsideration, Russo et al.312 developed a new approach, calledLAMIS (laser ablation molecular isotopic spectrometry), forperforming optical isotope analysis using LIBS in air at atmo-spheric pressure. Authors demonstrated the new concept inhydrogen, boron, carbon, and oxygen, observing that for largerisotopic shift (light elements) the measurement requirementswere less stringent. In a recent publication, the same authorsdescribed how boron isotope abundance can be quantitativelydetermined using LAMIS.313 Also, sensitivity was improvedusing a double-pulse configuration to re-excite the expandingplume created by the first laser ablation pulse. Most recently,Mao et al.314 discussed the capability of LAMIS for the quantifica-tion of 86Sr, 87Sr, and 88Sr isotopes and its possible application inradiogenic age determination. LAMIS can determine not onlychemical composition but also isotopic ratios of elements in thesample, increasing the potential of LIBS in nuclear and forensicapplications.

■ CONCLUSIONS AND FUTURE OUTLOOK

As a form of atomic emission spectroscopy, LIBS is typically amultielemental method of analysis. Different from other atomicemission techniques, LIBS requires only a single analytical operation

for preparation and excitation of all sample components. Thisproperty confers to LIBS an unprecedent performance in terms ofsampling and analysis capability.The mainstreams of the recent advances in LIBS may be

characterized by the following features: (1) Improved quantiza-tion has been achieved by a widened knowledge of fundamentalinteractions and physical parameters of laser-induced plasmas.Properly employed, LIBS may today be considered not only asoutstanding in elemental screening and identification tasks butalso as an essentially quantitative analytical technique. (2) Ele-mental detection limits have been improved in instrumentdesigns using stable and reproducible laser systems, means ofprecise pulse energy delivery to the sample, and detectors withsignal integration capabilities and accurate timing performance.Particularly, the use of intensified CCD detectors has con-siderably widened the range of elements routinely detectable insubppm concentrations. (3) Surface and in-depth description ofmaterials has gained increased acceptance among scientists andengineers. Especially, inspection of valuable assets in art andarcheology has benefited from controlled and confined laserbeam interactions with solid surfaces and from a better under-standing of ablation dynamics and thermal input effects. (4)Instruments using more rugged and light components havebroadened the fieldability of LIBS. Distinctive opportunitiesoffered by LIBS such as the elemental analysis of distant objectsand the analysis of submerged solids provide unique tools forextreme applications including the exploration of planets and thedeep ocean.A more careful look must be taken at the precision and

accuracy. The critical factors in the analysis of solids are theatomization efficiency and the fraction of emitting particlesresulting from laser ablation. Both factors depend ultimately onthe beam interaction with the surface. Owing to the usuallyinherently fluctuating character of the later process, the laser-induced plasma is less stable, and even its multiple pulse averageis less reproducible than the instantaneous sensitivity of otherhighly stable analytical plasmas. This is most disadvantageouswhen analyses of ultratrace amounts are to be carried out. On theother hand, representativeness of the laser-induced plasma is notalways guaranteed. A significant effort is needed in analytical

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



practice to ensure that both the qualitative and quantitativecomposition 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 developingconcepts, not just from the viewpoint of developing a newanalytical method but for improving an existing one. Conceptssuch as local thermodynamical equilibrium are relatively wellunderstood in laser-induced plasmas, and the effect ofexperimental factors, such as multipulse laser excitation and theuse of lasers of different wavelength and pulse width in combina-tion with refined measurement timing, are well established, but asystematic examination of analytical methods using these andother concepts remains to be done.One important occurrence during the recent years has been

the spreading of LIBS technology throughout the developingworld, and that suggests that there is an enormous amount ofheadroom for the LIBS community, including the instrumentmanufacturers. For the first time since the launch of this tech-nology, a wide selection of commercial instruments is currentlyavailable. From custom-made systems to serial production instru-ments, the market offers many different alternatives in terms ofperformance and cost. As in the case of every “new” analyticaltechnique, LIBS has been first looked upon as a universal remedyfor all problems. This is not the case, of course, but especially inmetals analysis, process monitoring, and field applications, LIBSis a very powerful, even indispensable, analytical tool.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.Biographies

Francisco J. Fortes graduated as a chemist at the University of Malaga,Spain, in 2003. The same year, he joined the Laser Laboratory to start hisPh.D. thesis under the supervision of Prof. Dr. Javier Laserna, focusing in thedevelopment and miniaturization of portable LIBS technology for CulturalHeritage applications. Nowadays, he is a postdoctoral researcher at LaserLaboratory in University ofMalaga (UMA). Currently, his research interestis focused on the better understanding of the fundamental processesinvolved in laser−matter interaction, the application of laser spectroscopy inanalytical chemistry, the development of portable LIBS instruments, andthe optical trapping of nanoparticles generated by optical catapulting.

Javier Moros received his B.Sc. in chemistry from the University ofValencia, Spain, in 2001. He carried out his thesis work at the sameuniversity under the supervision of Prof. Dr. Miguel de la Guardia andProf. Salvador Garrigues, focusing on the application of differentchemometric approaches to analytical data obtained by vibrationalspectroscopy 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 Malaga(UMA). His current research interests are focused on optical sensingtechniques, specifically, LIBS and Raman spectroscopy, the analysis instand-off mode, and the design and application of chemometricsapproaches for improving the selectivity of LIBS, mainly in the analysisof compounds sharing similar chemical composition.

Patricia Lucena graduated in Chemistry and specialized in AnalyticalChemistry atUniversity ofGranada, Spain, in 1994. She received her Ph.D. inChemistry atUniversity ofMalaga, in the department ofAnalyticalChemistryof the Faculty of Sciences, in 2003. Her thesis work focused on the analysis ofautomobile catalysts by laser induced breakdown spectroscopy under the

advisement of Prof. Dr. J. Javier Laserna. The investigation was performed inthe Laboratory of Materials Analysis by Laser of the Central ResearchServices at Malaga University. She is currently forming part of the researchstaff of the same laboratory. Her research lines are mainly based on theimprovement of optical sensors for standoff and real-time diagnosis throughthe study of the LIBS standoff response aswell as its laser−matter interaction,and the development of approaches to improve its analytical performance.

Luisa M. Cabalin is an associate Professor in the Department of AnalyticalChemistry at University of Malaga. She received her B.S. degree from theFaculty of Science of University of Malaga (Spain) in 1989 and her Ph.Dfrom the same University under the direction of Professor J. Laserna and A.Ruperez in 1994. She then spent two years as a postdoctoral researcher withProfessor Jean M. Mermet at the University Claude Bernard Lyon-I.Laboratoire des Sciences Analytiques Lyon, France. Her research interest isin the area of lasers in chemical analysis, laser-induced plasma spectrom-etry; surface analysis using laser ablation with optical detection, imagingtechniques; instrumental solutions for chemical analysis in industry; on-lineanalytical methodology; fieldable analytical instrumentation; developmentof spectroscopic instrumentation.

J. Javier Laserna is Professor of Analytical Chemistry at the University ofMalaga, Malaga, Spain. He graduated in Chemistry at University of Granadaand received his PhD from the University of Malaga in 1980. He didpostdoctoral work with Jim Winefordner at University of Florida for twoyears from 1986 to 1989. He has been a titular member of the IUPACCommission V.4 on Spectrochemical and other Optical Procedures forAnalysis, from 1996 to 2001, and head of theOffice for Technology Transferof the University of Malaga, 1994−1997. He has been president of theSpanish Society for Applied Spectroscopy (SEA), 2001−2004, and presidentof the Working Group in Spectrochemical Analysis of the Spanish RoyalSociety of Chemistry (RSEQ), 1998−2001. He is coinventor of 6 patentsheld by the University of Malaga and has published over 250 papers plus 5books and book chapters. He was section editor for Raman Spectroscopy ofthe Encyclopedia of Analytical Chemistry, John Wiley & Sons, 2000. Prof.Laserna’s current research interests include the use of lasers in chemicalanalysis, laser-induced plasma spectroscopy; time-of-flight mass spectrom-etry; secondary ionization mass spectrometry; surface analysis using laserablation with optical and ion detection, imaging techniques; laser remotechemical analysis; instrumental solutions for chemical analysis in industry;on-line analyticalmethodology; fieldable analytical instrumentation; develop-ment 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 large-scale optics standoff laser induced breakdown spectroscopy for analysis ofdistant objects. Later, this technique has been used in the analysis ofexplosives and in space exploration. He has given numerous invited plenaryand keynote talks and is member of the advisory board of several scientificjournals. Prof. Laserna was awarded with the RSEQ National Award forResearch in Analytical Chemistry in 2009 and the SEA National Award forhis research career in Applied Spectroscopy in 2010.

■ ACKNOWLEDGMENTSResearch supported by the Excellence Project P07-FQM-03308of the Secretaria General de Universidades, Investigacion yTecnologia, Consejeria de Innovacion, Ciencia y Empresa de laJunta de Andalucia. The authors are also grateful for support forthis work provided by Project CTQ2011-24433 of theMinisteriode Ciencia e Innovacion, Madrid.

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Laser-Induced Breakdown Spectroscopy

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