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International Scholarly Research NetworkISRN SpectroscopyVolume 2012, Article ID 285240, 12 pagesdoi:10.5402/2012/285240Review ArticleLaser-Induced Breakdown Spectroscopy:Fundamentals, Applications, and ChallengesF. Anabitarte, A. Cobo, and J. M. Lopez-HigueraPhotonic Engineering Group, Department of TEISA, Universidad de Cantabria, Edificio I D i Telecomunicacion,39005 Santander, SpainCorrespondence should be addressed to F. Anabitarte, anabitartef@unican.esReceived 12 September 2012; Accepted 1 October 2012Academic Editors: H. J. Byrne and G. LouarnCopyright 2012 F. Anabitarte et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Laser-induced breakdown spectroscopy (LIBS) is a technique that provides an accurate in situ quantitative chemical analysis and,thanks to the developments in new spectral processing algorithms in the last decade, has achieved a promising performance asa quantitative chemical analyzer at the atomic level. These possibilities along with the fact that little or no sample preparationis necessary have expanded the application fields of LIBS. In this paper, we review the state of the art of this technique, itsfundamentals, algorithms for quantitative analysis or sample classification, future challenges, and new application fields whereLIBS can solve real problems.1. IntroductionLIBS is an atomic emission spectroscopy technique whichuses highly energetic laser pulses to provoke optical sampleexcitation [1]. The interaction between focused laser pulsesand the sample creates plasma composed of ionized matter[2]. Plasma light emissions can provide “spectral signatures”of chemical composition of many different kinds of materialsin solid, liquid, or gas state [3]. LIBS can provide an easy,fast, and in situ chemical analysis with a reasonable precision,detection limits, and cost. Additionally, as there is no need forsample preparation, it could be considered as a “put & play”technique suitable for a wide range of applications [1]Considerable progress has been made during the lastfew years on very different and versatile applications ofLIBS, including remote material assessment in nuclear powerstations, geological analysis in space exploration, diagnosticsof archaeological objects, metal diffusion in solar cells, andso forth [4]. Today, LIBS is considered as an attractive andeffective technique when a fast and whole chemical analysisat the atomic level is required.Some of the established techniques for analytical atomicspectrometry are inductively coupled plasma-atomic emission spectrometry (ICP-AES), electrothermal atomizationatomic absorption spectrometry (ETA-AAS), and inductivelycoupled plasma-mass spectrometry (ICP-MS) [5], however,development on LIBS during recent years has reduced itsgap in performance with respect to these other well-knownapproaches [5].This paper begins with a brief explanation of the physicsinvolved in plasma induction and the features of this plasmain LIBS and is then followed by a description of the basicdevices which compose a LIBS set-up. These devices will bedescribed associating their features with the properties ofthe induced plasma. Moreover, different kinds of analysisalgorithms will be introduced in order to go beyond the“spectral signatures” obtained with the technique. Finally,some key LIBS applications will be described, and the mainresearch challenges that this approach faces at the momentwill be discussed.2. Fundamentals of Plasma Physics andIts SpectraUnderstanding the plasma physics of LIBS is essential toprovide an optimized setting for LIBS measurements. Alarge number of environmental factors affect the plasma lifetime and features, changing the spectral emission and the
2performance of this technique for chemical analysis at theatomic level.2.1. Laser Ablation and Plasma Physics of LIBS. Lasermatter interactions are governed by quantum mechanicslaws describing how photons area absorbed or emitted byatoms. If an electron absorbs a photon, the electron reachesa higher energy quantum mechanical state. Electrons tendto the lower possible energy levels, and in the decay processthe electron emits a photon (deexcitation of the atom).The different energy levels of each kind of atom inducesdifferent and concrete photon energies for each kind of atom,with narrowband emissions due to the quantization, withan uncertainty defined by Heisenberg uncertainty principle.These emissions are the spectral emission lines [6] foundin LIBS spectra and its features and their associated energylevels are well known for each atom [7].If the energy applied to the atom is high enough(overcoming the ionization potential), electrons can bedetached by the atom inducing free electrons and positiveions (cations). Initially, the detached electron is the mostexternal one (the furthest with respect to the nucleus)because it has the lowest ionization potential, but with higherenergy supply it is possible to detached more electronsovercoming the second ionization potential, the third, andso on. These ions can emit photons in the recombinationprocess (cations absorbs a free electron in a process calledfree-bound transition) or in the deexcitation process (thecations and the electrons lose energy due to kinetic processin a process called free-free transition). These emissionscan be continuum due to the different energies of theions and the different energy transitions, however cationsdeexcitation has discrete (or quantized) set of energy levelswith characteristic emission lines for each kind of element,allowing its identification together with the atomic emissionlines [7].The plasma, induced by the interaction pulsed lasersample, emits light which consists of discrete lines, bands,and an overlying continuum. These discrete lines, whichcharacterize the material, have three main features; wavelength, intensity, and shape. These parameters depend onboth the structure of the emitting atoms and their environment. Each kind of atom has some different energy levelswhich determine the wavelength of the line. Besides theidentification of the elements in the sample, the calculationof the amount of each element in the sample from the lineintensities is possible taking in account different necessaryconditions fixed by local thermodynamic equilibrium (LTEcondition) or problems related with matrix effects which canreduce the accuracy of quantitative analysis [8, 9].On the other hand, the intensity and shape of thelines depend strongly on the environment of the emittingatom. For not too high plasma densities, both the naturalbroadening (due to Heisenberg’s uncertain principle) andthe Doppler broadening (Doppler Broadening is due tothe thermal motion of the emitters, the light emitted byeach particle can be slightly red- or blue-shifted, and thefinal effect is a broadening of the line) dominate the linearISRN Spectroscopyshape [10]. For high plasma densities, atoms in the plasmaare affected by electric fields due to fast moving electronsand slow moving ions, and these electric fields split andshift the atomic energy levels. As a consequence of theseperturbations of the levels, the emission lines are broadenedand they change their intensity and shape. This effect isknown as the Stark effect [10] and it dominates the lineshape for dense plasmas. This broadening together withthe different parameters of spectral lines (intensities andshapes) and even the continuum radiation features can beuseful to determine plasma parameters, such as electrontemperature, pressure, and electron density [2, 7]. Theseparameters are very important to characterize the plasma,giving information about the physical state of it. Moreoverthe calculation of these parameters is necessary because theset-up has to be tuned to ensure LTE, key condition for anaccurate quantitative analysis [11].Basically, there are three stages in the plasma life time(Figure 1). The first one is the ignition process. This processincludes bond breaking and plasma shielding during thelaser pulse, depending on laser type, irradiance, and pulseduration.If the selected laser is a femtosecond one, nonthermalprocesses will dominate the ionization. The pulse is tooshort to induce thermal effects; hence other effects shouldionize the atoms, depending of the kind of sample. Thepulse has a huge amount of energy and effects like multiphoton absorption and ionization, tunneling, and avalancheionization excite the sample. With this amount of energy,the electron-hole created will induce emission of X-rays, hotelectrons, and photoemission. This will create highly chargedions through a process called Coulomb explosion [7]. Theabsence of thermal effects creates a crater with highly definededges without melted or deposited materials.In contrast, nanosecond lasers induce other effects. Theelectron-lattice heating time is around 10 12 s, much shorterthan the pulse time. This causes thermal effects to dominatethe ionization process. Briefly, the laser energy melts andvaporizes the sample, and the temperature increase ionizesthe atoms. If the irradiance is high enough, nonthermaleffects will be induced too and both will ionize the sample.Between 10 9 s and 10 8 s, plasma becomes opaque for laserradiation, thus the last part of the laser pulse interacts withplasma surface and will be absorbed or reflected, hence it willnot ionize much more material. This effect is called plasmashielding and is strongly dependent on environmentalconditions (surrounding gases or vacuum) and experimentalconditions (laser irradiance and wavelength) [12, 13]. Thisshielding reduces the ablation rate because the radiation doesnot reach the sample surface. This induces a crater withmelted and deposited material around it but at the same timethe plasma is reheated and the lifetime and size of plasma ishigher [13, 14].The next step in plasma life is critical for optimization ofLIBS spectral acquisition because the plasma causes atomicemission during the cooling process. After ignition, theplasma will continue expanding and cooling. At the sametime, the electron temperature and density will change. Thisprocess depends on ablated mass, spot size, energy coupled
ISRN Spectroscopyto the sample, and environmental conditions (state of thesample, pressure, etc.).If the plasma is induced in a vacuum, the plasmaplume expands adiabatically and the expansion of theablated material can be described by the Euler equationsof hydrodynamics [15]. In contrast, if the surroundingmedium is a gas or a liquid, the plume will compressthe surrounding medium and produce shockwaves. In thissituation the plasma plume is a mixture of atoms andions from both vaporized material and ambient gas. Theshockwave expansion can be described by Sedov’s theory[7, 16].Plasma temporal evolution changes with pulse duration.For pulses longer than about 5ps, the laser-plasma interactions result in plasma heating and the plasma temperatureincreases with pulse duration [17]. For short times (below30 ns) a fs-induced plasma emission intensity decays whilens-induced plasma becomes hotter [18]. The last part of thenanosecond laser pulse is absorbed by the plasma, reheatingit, elongating lifetime, and increasing line emission, but atthe same time, the background is higher and this decreasesthe sensitivity for nanosecond set-ups.A good way to improve the performance is to use afemtosecond laser to ablate the sample and nanosecond orpicosecond pulses to reheat the plasma [19].The last stage of the plasma life is not interesting for LIBSmeasurements. A quantity of ablated mass is not excited asvapor or plasma, hence this amount of material is ablated asparticles and these particles create condensed vapor, liquidsample ejection, and solid sample exfoliation, which do notemit radiation. Moreover, ablated atoms become cold andcreate nanoparticles in the recombination process of plasma.2.1.1. Plasma Emission Spectra. The emission lines from theatomic species can be hidden by continuum radiation thatis caused by two processes. The first one is due to radiativerecombination. Both continuum and line photons can beproduced in such a recombination event as the electronpasses from the free state into the upper bound levels ofthe ion and then cascades down to form a ground state ion.The other effect involved in continuum radiation is calledBremsstrahlung. This effect is related to free-free transitionscorresponding to the loss of kinetic energy by an electronin the field of an ion. The electron loses energy in thedeceleration process when it travels into the field of theion, emitting photons in different wavelengths, dependingof its initial energy and the loss of this energy. Continuumemission can hide atomic emission peaks, thus this effectshould be avoided.The continuum emission depends on both temperatureand density of plasma. These parameters are too highin the initial stages of plasma, especially in the ignition.For this reason, the time control unit has to delay theacquisition window in order to avoid this continuum. Forfemtosecond lasers, the continuum emission is observedwithin one nanosecond after the laser ablation; hence thedelay with these lasers should be greater than this time.For nanosecond lasers, atomic emission occurs after 13(a)(b)(c)Figure 1: Plasma life stages: (a) plasma ignition, (b) plasmaexpansion and cooling, and (c) particle ejection and condensation[7].microsecond and molecular emission occurs at later timesfrom recombination of species in the plasma [2, 7, 14]. Insidethis acquisition window, the initial stages of plasma life arecharacterized by a higher temperature and electron density.These parameters provide a better emission of ionic lines, forthat reason these initial stages are better to acquire ionic linesdespite the continuum emission. These optimal acquisitionwindows depend strongly on the sample material and bothenvironmental and experimental conditions, but the valuesgiven above can be interesting as a starting point for eachkind of laser [20].Lifetime for femtosecond-induced plasmas is shorter andwith less background. For that reason, the acquisition window size should be shorter for short pulses and femtosecondlasers are better for nongated measurements because thebackground is weak and the LIBS sensitivity improves [14].Generally, femtosecond lasers are better to obtain highlyaccurate craters and for nongated measurements, and usingdelays of a few nanoseconds and small acquisition windowscan improve LIBS sensitivity. Nanosecond lasers create amelted crater and need delays of a few microseconds andlarger windows with the advantages of low complexity inthe laser system. There are other kinds of lasers, such aspicosecond lasers, for which the time pulse is betweena femtosecond and a nanosecond, hence the features arebetween the two [7].
4ISRN SpectroscopyLaserbeamNd: YAG laser 1064 nmTime controlunitSpectrometerMirrorLensCCDOpticalfiberTo PCwith a combination of these parameters. Nanosecond-pulsedlasers are the most common for LIBS. Therefore, the most ofthis section is related to this kind of laser.PlasmaSampleFigure 2: Typical LIBS set-up.2.2. Environment. The plasma size, propagation speed,energy, and emission properties are related to the ambientgas into which the plasma expands. The ambient gas can helpor prevents the plasma shielding. For example, the gas canshield the sample from the laser beam if a gas breakdownoccurs before sample vaporization [21]. These undesirableeffects are less important for gases and aerosols, but they canbe important for solid samples.Gas pressure will influence plasma expansion. Lowpressures increase energy losses and uniformity of the plasmaenergy distribution. In addition, different gases have differentbehaviors at different pressures [21].The laser can be directed anywhere, allowing theresearcher to work near to the sample or far from it, forinstance with dangerous samples like explosives [22] inStand-off configurations. A Stand-Off LIBS (ST-LIBS) isdefined as a LIBS set-up which allows a remote, noncontactmaterial detection-characterization over a distance of at leastseveral meters [23]. This feature of LIBS enables flexible setups adapted to dangerous environments or samples.3. LIBS Set-Up ComponentsThe main devices involved in a LIBS analysis are shownin Figure 2. A high-energy pulsed laser (usually in thenanosecond range) is directed at the sample [2]. This lightenergy vaporizes the sample and induces the plasma.The spectrometer is in charge of diffracting the lightcollected, with a more or less complex optical system, inorder to obtain the spectral signature. Then, the light isdetected by using devices such as a photomultiplier tube(PMT), a photodiode array (PDA), or a charge-coupleddevice (CCD) [2, 7]. Finally, the acquired spectrum is processed by a computer for further analysis. LIBS set-ups needan accurate time control to avoid some plasma life stages andto improve the spectral signature [2, 3, 7]. The choice of thelaser combined with the set spectrometer-detector and timecontrol, adapted to environmental conditions, can determinethe success or failure of the experiment.3.1. Laser Configurations for LIBS. The main device of LIBSis the laser. It generates the energy to induce the plasma andmainly determines the plasma features. The main parametersrelated to the laser are the pulse time (explained above), theenergy per pulse, the wavelength, and the number of pulsesper burst [24]. Obviously, each application works better3.1.1. Laser Wavelength. The wavelength influence on LIBScan be explained from two points of view; the laser-materialinteraction (energy absorption) and the plasma developmentand properties (plasma-material interaction).When photon energy is higher than bond energy,photon ionization occurs and nonthermal effects are moreimportant. For this reason, the plasma behavior depends onwavelength in nanosecond LIBS set-ups. In the same way,the optical penetration is shorter for UV lasers, providinghigher laser energy per volume unit of material. In general,the shorter the laser wavelength, the higher the ablation rateand the lower the elemental fractionation [25].The plasma ignition and its properties depend ofwavelength. The plasma initiation with nanosecond lasersis provoked by two processes; the first one is inverseBremsstrahlung by which free electrons gain energy from thelaser during collisions among atoms and ions. The secondone is photoionization of excited species and excitation ofground atoms with high energies. Laser coupling is betterwith shorter wavelengths, but at the same time the thresholdfor plasma formation is higher. This is because inverseBremsstrahlung is more favorable for IR wavelengths [26].In contrast, for short wavelengths (between 266 and157 nm) the photoionization mechanism is more important. For this reason, the shorter the wavelength in thisrange, the lower the fluence necessary (energy per unitarea) to initiate ablation [27]. In addition, when inverseBremsstrahlung occurs, part of the nanosecond laser beamreheats the plasma. This increases the plasma lifetime andintensity but also increases the background at the same time.Longer wavelengths increase inverse Bremsstrahlung plasmashielding, but reduce the ablation rate and increase elementalfractionation (elemental fractionation is the redistributionof elements between solid and liquid phases which modifiesplasma emission) [28].The most common laser used in LIBS is pulsed Nd:YAG[29]. This kind of laser provides a compact, reliable, andeasy way to produce plasmas in LIBS experiments. Thefundamental mode of this laser is at 1064 nm and thepulse width is between 6 and 15 ns. This laser can provideharmonics at 532, 355, and 266 nm, which are less powerfuland have shorter time pulses (between 4 and 8 nm) [2,7]. The fundamental and the first harmonic are the mostcommon wa
Plasma temporal evolution changes with pulse duration. For pulses longer than about 5ps, the laser-plasma interac-tions result in plasma heating and the plasma temperature increases with pulse duration [17]. For short times (below 30ns) a fs-induced plasma emission intensity decays while ns-induced
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