focal point review

DAVID W. HAHN AND NICOLO´OMENETTO DEPARTMENT OF MECHANICAL AND AEROSPACE ENGINEERING, DEPARTMENT OF CHEMISTRY, UNIVERSITY OF FLORIDA, GAINESVILLE,FLORIDA 32611 Laser-Induced Breakdown Spectroscopy (LIBS), Part II: Review of Instrumental and Methodological Approaches to Material Analysis and Applications to Different Fields

The first part of this two-part review focused on summarizing the current state-of-the-art of plications. An attempt is made to provide an the fundamental and diagnostics aspects of analytical LIBS, providing a contemporary updated view of the role played by LIBS in the laser-induced plasmas, only touching briefly snapshot of LIBS applications, and highlighting various fields, with emphasis on applications upon concepts such as sensitivity and detection new directions in laser-induced breakdown considered to be unique. We finally try to assess limits and largely omitting any discussion of the spectroscopy, such as novel approaches, instru- where LIBS is going as an analytical field, vast panorama of the practical applications of mental developments, and advanced use of where in our opinion it should go, and what the technique. Clearly a true LIBS community chemometric tools. More specifically, we dis- should still be done for consolidating the has emerged, which promises to quicken the cuss instrumental and analytical approaches technique as a mature method of chemical pace of LIBS developments, applications, and (e.g., double- and multi-pulse LIBS to improve analysis. implementations. With this second part, a more the sensitivity), calibration-free approaches, applied flavor is taken, and its intended goal is Index Headings: Laser-induced breakdown hyphenated approaches in which techniques spectroscopy; LIBS; Instrumentation; Analyti- such as Raman and fluorescence are coupled cal sensitivity; Limits of detection; Signal-to- Received 22 December 2011; accepted 10 with LIBS to increase sensitivity and informa- noise ratio; Calibration; Matrix effects; Dou- January 2012. tion power, resonantly enhanced LIBS ap- * Author to whom correspondence should be ble-pulse LIBS; Multi-pulse LIBS; Microanal- sent. E-mail: [email protected]fl.edu. proaches, signal processing and optimization ysis; Hyphenated methods; Chemometrics; DOI: 10.1366/11-06574 (e.g., signal-to-noise analysis), and finally ap- Remote detection; Applications.

APPLIED SPECTROSCOPY 347 focal point review

INTRODUCTION TABLE I. Ideal requirements of a spectroscopic method.a 1 s stated in our first article, Is the feature laser-induced breakdown spec- a LIBS A troscopy has become a very Characteristic feature characteristic? Comments attractive and popular technique in the No sample preparation YES Indeed, a laser-induced plasma can be directly field of chemical analysis. The first obtained on any sample in any location. Note, proof of this statement is that the LIBS however, that this may limit the accuracy of the community keeps increasing, as testified technique in quantitative analysis. Liquids, solids, gases YES This has been demonstrated, although we note the by the growing number of participants relative difficulties of both liquids and solids each year at the conferences dedicated to analysis. the various aspects of the technique (see 3D resolution in solids YES The ability for 2D spatial mapping, as well as Table I in Ref. 1). The other proof is that (i.e., depth profiling) depth profiling at a given location, provides unique 3D capabilities. the number of papers published is No spectral interferences NO This is strongly related to the available spectral increasing every year at a remarkable resolution and choice of analytical lines. rate, meaning that an attempt to give No matrix interferences NO In general, LIBS is subject to matrix effects, proper credit to all of them is impossible (perfect accuracy) although as discussed, steps may be taken to minimize these effects. in a single review. But even more Absolute (standardless) NOT YET Calibration-free offers an approach for absolute, important, such an increase makes it standardless analysis, although the CF approach difficult sometimes to distinguish be- is not perfect. See pertinent discussion in the tween applications in which LIBS is text. Single-atom and single- NO Laser-based method can reach this limit. However, taking a step forward, either in terms of molecule detection limits the overall detection efficiency of LIBS (as well simplicity or rapidity of analysis or as of other emission techniques) is too low (see better sensitivity, and others in which Table V and Ref. 17). Precision limited only by NO Emission techniques are limited by extrinsic noise, the approach described does not seem to fundamental counting i.e., by noise that can be minimized by proper offer decisive advantages over already choice of experimental parameters. Intrinsic established methods. This is particularly noise is not reached (see Ref. 17). important because, in general, a well- Stand-off (remote) YES A capability that is well-suited to LIBS, including single- and double-pulse configurations. established analytical standard proce- Useful in many spectral YES LIBS is amenable to the UV, visible, and NIR dure will hardly be replaced by a new regions regions. Moreover, spectra in the mid-IR region technique if the analytical figures of can also be used advantageously. merit of the latter are not all clearly Simultaneous all-channel YES The use of CCD and echelle spectrometers allows spectral acquisition simultaneous collection of a broad spectral superior to those of the already existing range. and accepted procedures. Compatible with other methods YES Both tandem and hyphenated methodologies The emphasis of the present review provide great diversity to the LIBS technique. will be on instrumental and quantitative Information-rich (multivariate) YES LIBS is well-suited to a broad range of multivariate chemometric data analysis schemes. aspects of the technique. Clearly, the Simple, inexpensive, compact, YES LIBS is relatively simple with regard to basic principles discussed in the first portable implementation and amenable to low-cost, review are needed here, and the reader compact and portable instrument configurations will often be referred to some of the and packages. Nondestructive NO Strictly speaking, LIBS may be considered a references listed in Part I. minimally destructive method in that a small amount of material is ablated. THE IDEAL ANALYTICAL a The various characteristic features reported here have been kindly provided by Gary M. Hieftje (Indiana METHOD University). The comments are ours. There are certain characteristic fea- tures of LIBS that undoubtedly make the surrounding environment using a single ments on all other characteristics. We technique unique and superior to other laser shot. note that isotope analysis can now be analytical methods. These characteristics The analytical capability of any added as a welcome feature of the have been repeatedly stressed (some of spectroscopic (and also non-spectro- technique. them, namely the ‘‘no need of sample scopic) method is perhaps best assessed preparation,’’ have become cliche´) in the if one refers to the requirements of an REVIEW OUTLINE literature. Suffice it to say that LIBS, in ideal method of analysis, as illustrated in principle, can directly address and Table I. Clearly, no analytical method Literature Citations. We would like simultaneously detect all neutral and possesses all the requisites listed. The to highlight some explanation of how ion spectral features of all atomic and two unique features of LIBS are the lack the literature cited at the end of the molecular species of all elements pres- of sample preparation and its stand-off article is arranged, since this will ent in any type of sample and its capability, together with some com- facilitate the task of finding a reference,

348 Volume 66, Number 4, 2012 associate it to the pertinent topic, and medicine,30 HandDanalysis,21 industri- operation in other gaseous environments cross-link it to that topic. We note that: al,9,28,37 and explosives.36 Finally, in at ambient pressure, and in particular at (i) some references omitted in this article addition to a comparison with the other different pressures, is receiving increas- have been cited in the first review,1 major analytical superstar methods,17 four ing attention in view of the envisaged while other references listed in the first reviews are referenced16,34,46,47 in view of LIBS application in space research and review can also be found here when the close link between LIBS and optical exploration. Several references67–82 are duplication was felt necessary; (ii) in emission and mass spectrometry tech- therefore listed addressing this topic. several instances, there are papers listed niques using an inductively coupled Liquid targets and submerged solid in one topic, which could have been plasma as ion source (laser ablation– targets are considered later. listed equally well in another topic. One inductively coupled plasma–atomic emis- A series of old and recent papers example would be an article describing sion spectroscopy, LA-ICP-AES, and describing the parameters relevant to the use of a double-pulse, stand-off laser ablation–inductively coupled plas- monochromator, collection optics, and apparatus to detect an explosive sub- ma–mass spectrometry, LA-ICP-MS), detector types follows.83–101 Here, as stance where extensive chemometrics is and the link with the laser ionization well as in other sections, whenever it needed for the interpretation of the data: mass spectrometry technique (LI-MS). was felt useful, reference to classic such an article can either be found in the After the various reviews, the ratio- papers is provided, in the same spirit double-pulse, stand-off, explosive detec- nale behind references48–140 follows a that permeated Part I of our review, i.e., tion or chemometric literature, respec- trend whose logic reflects the order in that of alerting the reader of the tively. which the essential components and usefulness of reading and learning how In general, all references within a characteristic features of a typical LIBS the same concepts were previously given topic are arranged in chronologi- experiment (mainly referring to a solid treated in depth and to take advantage cal order. The order in which the target) are assembled. These consist of a of the already existing knowledge. different topics are arranged is described laser, some focusing optics to direct the Concepts such as ‘‘Optical Conduc- as follows. beam into the target, an optical system to tance’’ and ‘‘Luminosity-Resolving Three books,2–4 and many reviews convey the plasma radiation into a Power’’ are introduced and their rele- and chapters are given first.5–20,21–47 monochromator, and a detector to con- vance to LIBS measurements illustrated. General reviews encompass all aspects vert the photons into an analytical signal Selected papers discussing imaging op- of the technique, and the information (e.g., counts). The above scheme fol- tics,85,86 echelle monochromators, and provided is obviously updated to the lows and expands on similar consider- photon detectors (photomultiplier tubes, period in which they were written: some ations given, for example, by Tognoni et PMT; charge-coupled devices, CCD; of the early ones are reported here al.12 and by Sirven et al.48 in their intensified charge-coupled devices, mainly to show the improvements made discussion of the essential steps taken in ICCD) can be found,87–90,93 together in all pertinent aspects of the technique the optimization of a LIBS experiment. with a comparison between gated versus (e.g., lasers, spectrometers, data process- Because the ablation process, together non-gated acquisition.94–101 Operation ing, miniaturization, to cite a few) and to with the shaping of the laser profile, as in the vacuum ultraviolet (UV) as well demonstrate how interest in LIBS re- well as the influence of the focusing as the infrared (IR) is discussed in Refs. search and applications has grown arrangement chosen to form the plasma 102–114. during the last years. Reviews focusing on the target (fluence and irradiance The two final sections focus on one on a specific field or application are an effects) were already described in Part I important aspect of LIBS experiments, indication of the increasing interest of (see Table II),1 just a few papers have namely the acquisition of space- and the analytical community in that topic. been selected, reporting on uncommon time-resolved information during the General reviews, some of which are optical arrangements based on the use of entire persistence of plasma emis- indeed very informative, can be found in ellipsoidal mirrors,49,50 the use of un- sion.115–129 As stressed in the present Refs. 6, 7, 11, 14, 16, 23, 32, and 33. common laser shapes51 and pulse dura- review, improvements in the experimen- Reviews on specific topics include a tion,52,53 and the potential and attractive tal data acquisition have progressed at a tutorial on the concept of calibration in features provided by novel compact, slower rate in comparison with the spectroscopy,43 normalization approach- high-power fiber laser sources.54 The advancements made in plasma model- es,40 semi-quantitative calibration-free implementation of an algorithm based ing, bringing new attention to the methods,29,39 modeling,41 surface anal- on best in-focus imaging search55 and reconciliation of modeling and experi- ysis15 and microanalysis,12,16,25 double- laser spatial structure effects56 are also mental data. The instrumental section pulse approaches,22,26 stand-off detec- discussed. Finally, even if this topic has regarding spectrally resolved optical tion,24,42 single-shot analysis,44 and a been treated in our first review,1 a few detection includes papers130–140 discuss- tutorial on the use of chemometrics in references are added57–66 addressing the ing a recent topic, namely the possibility analytical spectroscopy.31 effects of changing the ablation laser of observing polarized plasma emission Reviews on laser ablation and LIBS wavelength. The target is considered to in order to improve the signal-to-noise applications to specific topics include be a solid in ambient air, as this is the ratio. cultural heritage,8,13,19,20,45 geochemistry most commonly encountered situation in Modeling of laser-induced plasmas and environment,18,38,45 aerosols,35 bio- LIBS experiments. On the other hand, continues to be a favorite topic studied

APPLIED SPECTROSCOPY 349 focal point review in many laboratories.141–174 As stated to a variety of analytical problems in elemental information, as in the cou- above, the degree of sophistication different fields with varying degrees of pling of LIBS with Raman spectrosco- reached nowadays in predicting the success. CF approaches have been, are, py,394–403 or again to improve the evolution of the plasma parameters and and will be of significant relevance in sensitivity (laser ablation–laser-induced the excitation-ionization of the target LIBS for many reasons that will be fluorescence, LA-LIF).404–420 constituents, in addition to their chem- outlined in this review, and they there- A preliminary discussion of the con- ical interaction with the environment, fore deserve careful consideration. cept of sample homogeneity421–427 in- has reached a highly sophisticated level. It is a common consensus that LIBS is troduces the topic of Micro-LIBS, which In line with the applied analytical generally less sensitive than other spec- stands on its own in terms of number of flavor of the present review, the concept troscopic emission methods. Although papers devoted to it.428–451 Since this of calibration and linearity of the this statement needs to be better quali- instrumental approach can be accom- analytical curves of growth is addressed fied and will therefore be more closely plished either with low energy per pulse in detail.175–191 The basic expressions scrutinized, the search for improving the or with microchip lasers, the term Laser pertinent to this topic have been already sensitivity, and consequently the detec- Microprobe Analysis may be more given and discussed in Part I of our tion limits (assuming the signal-to-noise appropriate.218 Here, the capabilities of review (see Table IV).1 Here, emphasis improves as well), has been actively the technique as a surface analysis tool is reserved for the practical evaluation of pursued in the last 20 years. One of the are challenged, and therefore the con- the self-absorption factor and optical most studied approaches is the use of cept of sample homogeneity becomes thickness of the plasma. Because cali- double-pulse LIBS. A large body of essential. The ultimate capability of the bration curves, in particular the portion literature exists describing double-pulse technique in terms of minimal spatial obtained at low analyte concentrations, approaches,295–342 which indeed en- (lateral and depth) resolution is still are directly related to analytical param- hance signal levels and improve limits unknown, and near-field scanning mi- eters such as signal-to-background and of detection. Collinear and orthogonal croscopy approaches452–458 are currently signal-to-noise ratio, sensitivity,and configurations have been tested, together actively investigated. detection limits, both relative and abso- with a discussion of the reasons for the Molecular LIBS459–468 seems to be an lute, several references are devoted to observation of the enhancement. Multi- oxymoron, since no molecules should these topics. Tutorial articles and ‘‘old’’ pulse approaches343–354 have also been resist the dissociating character of the treatments are reported in Refs. 192– described and enhancements reported. high-temperature plasma. On the con- 207, while literature more specifically Other more recent approaches, albeit trary, by an appropriate selection of the referring to LIBS is reported in Refs. less popular, are also reported (micro- laser energy and measuring delay, 208–221. In these sections, the discus- wave enhancement and magnetic field molecular emission has been commonly sion will focus on data processing enhancement).355–368 observed in LIBS. A welcome conse- procedures to evaluate the dominant Resonant laser ablation (RLA) and quence of addressing molecular species sources of noise in a LIBS experiment resonantly enhanced LIBS (RE-LIBS) in the plasma is the possibility of and the type of noise involved. are experiencing a renewed interest and performing isotope analysis469–481 at Needless to say, in many practical many references are reported on these ambient pressure. cases, precision and accuracy are features topics.369–393 The main features of these Two essential features of LIBS are its of overwhelming importance in chemical approaches are minimal sample destruc- analytical versatility, in the sense that it analysis. In this respect, many laborato- tion and improved limits of detection. It can be applied to any sample type, and ries have devoted their efforts to studying will be shown that RLA and RE-LIBS its remote sensing (stand-off) capability. matrix effects and ways of normalizing are performed with much lower energy References 482–542 collect a number of the signal to some of the key parameters pulses from the ablation laser (which studies regarding the use of LIBS with involved in the vaporization-excitation- explains the minimal sample destruc- liquid targets and the analysis of detection process.222–261 tion) and that the excellent detection underwater objects. Much of the inter- These efforts have their natural fol- limits refer to the absolute rather than est in dual-pulse work has probably low-up in the attempt of making stan- relative detection limit. This trend is originated from the necessity of increas- dard reference materials and matrix- similar to that which will be addressed ing the sensitivity of the analysis of matched standards redundant by at- in the section regarding Micro-LIBS liquids. And the interest in the analysis tempting to perform an absolute analy- (see later). of liquids can be mostly driven by sis. This daunting task has been Following the double-pulse approach, medical applications of lasers such as challenged before by several atomic tandem (hyphenated LIBS) approaches, for ophthalmic microsurgery and stone spectroscopic methods, such as absorp- involving (usually) two lasers and the fragmentation. Recently, the possibility tion and fluorescence, mass spectrome- same or different optical detection of characterizing archeological objects try with plasma and glow discharge apparatus, are described in Refs. 394– under water has received considerable sources, and X-ray spectrometry.262–272 420. Under this category, the underlying attention. In LIBS, calibration-free (CF) proce- purpose is either to improve the infor- Because of the large amount of dures273–294 have become popular in mation power of the approach, i.e., by information provided by a LIBS spec- many laboratories and have been applied providing both molecular as well as trum, and in cases of environmental and

350 Volume 66, Number 4, 2012 forensic interest in which discrimination of substances becomes the final goal, the use of chemometric approaches is be- coming a standard tool for processing the data. A personal selection of papers is made in this topic, with an attempt to be inclusive.543–576 References 577–588 specifically ad- dress stand-off applications of the tech- nique and in particular space explora- tion.589–599 In this context, the ambient background is always a problematic issue, in particular when the target is an organic substance (e.g., an explosive material), and data treatment becomes essential in order to discriminate the signal due to the target from the background response. Stand-off ap- proaches have been particularly exploit- ed in the detection and discrimination of organic compounds and explo- sives.600–615 Moreover, remote tech- FIG.1. Overview of some typical LIBS methodologies and applications discussed in this review. The double arrow linking ‘‘methodology’’ and ‘‘application’’ is meant to niques are inherently suitable for stress their bi-univocal relation, in the sense that it is the development of a new applications in the field of cultural methodology that leads to a new application, but it is the need of solving an existing heritage.616–626 analytical problem that triggers the development of a new methodology. As stated before, the number of LIBS applications is impressive. In fact, as setups used in different LIBS applica- nal-to-noise ratios, linear dynamic range reported by Cremers and Chinni in their tions. of the calibration curve obtained (self- recent review:33 ‘‘It is not an exaggera- absorption), and most importantly the tion to state that LIBS is probably the A TYPICAL LIBS existence of matrix effects and their most versatile method of elemental EXPERIMENT: FROM SAMPLE relevance to the quality of the data analysis that is currently practiced.’’ In TO SIGNAL this review, several examples are given. provided. Our selection includes topics relevant to As noted above, the considerations The above issues, together with other the environment (soils and vegeta- have been derived and expanded from a selected topics treated in this review, tion627–649 and minerals650–655), aero- series of questions given, for example, have been collected in Table II and will sols and bioaerosols,656–670 combus- in the review by Tognoni et al.12 and in now be discussed below. tion,671–682 forensics,683–691 pharmaceu- the paper by Sirven et al.48 These Laser and Focusing Optics. The tical,692–697 medical,698–709 and nu- questions summarize the logical steps main parameters characterizing the laser clear.710–715 of a LIBS experiment, where one is beam and the focusing effects on the To conclude our review, a few papers faced with the choice of laser (wave- target were extensively treated in our 1 illustrating the use of spark-induced length, pulse duration, and energy) and first review (see Table II and III ); breakdown spectroscopy (SIBS) as an its focusing on the target (fluence versus hence, we will keep this section brief. alternative approach in selected applica- irradiance), the sample environment (air, Several reviews (see, e.g., Refs. 12, tions are also cited.716–718 mixed gases, vacuum), the choice of the 20, and 23 to cite a few) discuss the Despite the high number of references analytical line (atomic versus ionic and different types of lasers used in LIBS reported, many have been necessarily spectral interference), the efficient col- and in particular the ablation wavelength omitted and we apologize to the authors. lection of plasma radiation (optical selection criteria and the pulse duration. For more complete coverage, the reader conductance, spectral resolution, space- As pointed out by Fantoni et al.,20 the is referred to a number of recent special resolved measurement), the choice of choice of the wavelength will be dictat- issues, which are collecting LIBS papers monochromator (luminosity-resolving ed by the material (e.g., UV is best for devoted to specific themes highlighted power product, imaging), and the choice ceramics, stones, and metals) and by the in the various LIBS conferences (see of detector (gated versus non-gated, application. Sometimes, one needs to Table I of Part I).1 spectral efficiency, gain and linearity of compromise, as for example in the case An overview of the methodologies response). After choosing the setup, the of water analysis, where strong absorp- discussed in this review and some major experimentalist needs to critically eval- tion in the UV and IR would favor the applications are shown in Fig. 1. Figure uate the analytical data collected by choice of a visible wavelength. Regard- 2 illustrates some typical experimental assessing signal-to-background and sig- ing the pulse duration, ultrashort pulses

APPLIED SPECTROSCOPY 351 focal point review

an alternative to electro-optically switched operation. Focusing effects on the target have been addressed in Part I of our review. The effect of the beam numerical aperture was addressed by Sirven et al.48 and an autofocus algorithm based on searching the best in-focus image was described by Novotny et al.55 The complexity of the laser–sample interac- tion can be seen from the study of Lednev et al.,56 who investigated the influence of the spatial beam profile of the ablation laser on several analytical figures of merit such as limit of detection, sensitivity, linearity of cali- bration curves and analytical precision. The results are difficult to rationalize in a single general conclusion. In fact, while a multimode profile results in better sensitivity, detection limit, and linearity for single-spot sampling, it also results in worse analytical precision. Moreover, a single-mode profile should be the choice for single-shot analysis, but a multimode profile performs better for single-spot sampling. While, as expected, a single-mode Gaussian beam profile performs best in terms of lateral resolution, depth profile studies, and chemical mapping, a multimode profile should be chosen for the analysis of FIG.2. Experimental LIBS setups. (a) A ‘‘traditional’’ setup, consisting of a single laser 56 focused with a spherical lens close to the target position and axial collection of the plasma trace elements. radiation with a pierced mirror and a spherical lens to form an image of the plasma on the In concluding this section, we draw entrance of a fiber optic optically coupled with the monochromator. (b) Double-pulse LIBS the readers’ attention to a recent paper setup in an orthogonal configuration. The delay generator controls the delay between the by Gravel et al.54 evaluating the poten- two pulses. With this arrangement, both pre-spark and re-heating of the plasma can be chosen. (c) Photon collection can be accomplished in the vacuum UV as well as in the IR tial of a fast growing laser technology, regions. Purging of the optics with nitrogen is useful for measurement below ~200 nm and namely the high power, high repetition down to ~175 nm, while below ~175 nm, vacuum operation is necessary. (d) Stand-off rate fiber lasers. The authors have configuration. The plasma radiation is collected by the telescope and sent to the investigated systematically the charac- spectrometer via fiber optic. teristics of plasmas formed on aluminum (Al) and copper (Cu) matrices, reported produce cleaner craters, intense neutral optical system, derived from laser fusion limits of detection and ablation rates, atom emission, and close to stoichio- experiments,49 based on two ellipsoidal and concluded that these sources are 20 very promising for LIBS work, in metric sample evaporation. gold-coated mirrors to study spherical 54 A random selection of papers related shock waves in laser-induced plasmas in particular for industrial applications. Sample Environment. We give here to the use of different laser wavelengths gases, while Cabalı´n and Laserna51 a few considerations on ablation exper- on the ablation characteristics of differ- generated a ring-shaped micro-plasma iments performed at different pressures ent solid and gaseous targets is provid- by focusing the laser beam with an and with different gases, together with ed.57–66 Some refer to LA-ICP-AES59 axicon in combination with two spher- 62 65 52 their effect on the LIBS signal. The and LA-ICP-MS. In one case, the ical lenses. Rodolfa and Cremers used impetus behind understanding the be- influence of wavelength in the near-field cylindrical optics to create a linear spark havior of the plasma at different pres- ablation nanostructuring of Si was ~ 1 cm in length for spatial surface sures can be linked to attempts to described. mapping. The use of long laser pulses improve the critical performance and to A few papers specifically dealing with (150 ns) provided by an acousto-opti- space and planetary exploration, since laser types and focusing optics are also cally switched Nd : YAG laser was the pressure of Mars is a few torr of CO2 cited. Gatti et al.50 have described an investigated by Yamamoto et al.53 as and that of Venus is ~ 90 atm.

352 Volume 66, Number 4, 2012 The use of different gases (helium TABLE II. A LIBS experiment: from laser–sample interaction to signal detection and presentation of analytical result. (He), argon (Ar), air, CO2)hasa significant influence on the evolution, morphology, and emission characteris- Relevant parameters and Topics considered in this review concepts treated in this review tics of laser-induced plasmas on solid targets. Moreover, LIBS measurements Choice of lasers Wavelength, pulse duration, space and time profile, in pure gases and gaseous mixtures are fluence and irradiance. Transfer optics (from laser to sample) Focusing laser on (near) target surface. Microanalysis interesting on their own. considerations. Stand-off operation. Effenberg and Scott have reviewed Sample and sample environment Homogeneity considerations. Ambient pressure, low this field recently.72 The following pressure, different gases. considerations are taken from several Emission parameters. Choice of line. Transition probability and self-absorption. Linearity of papers.67–71,73–76,79 In general, the type calibration curves. Most suitable wavelength region. Optical collection of plasma emission. Orthogonal versus axial collection. Space resolved and magnitude of the effect found by Transfer optics (from plasma to detector) measurements. Plasma imaging. working at different pressures and with Monochromator and detector Optical conductance. Luminosity-resolving power product. different gases can be summarized as Spectrometer (grating, echelle) and detectors (PMT, follows: (i) with ns ablation pulses, and CCD, ICCD). Optimization of signal-to-background and Choice of delay and integration times. Evaluation and in a given range of reduced pressures, signal-to-noise ratios. Limit of detection. quantification of dominant noise sources. Evaluation of the spatial distribution of the emitting the types of different noises affecting a measurement. species increases in size, and the inten- Different approaches for evaluating LOD. sity of neutral lines first increases and Calibration and matrix effects Interferences related to mass removed and to plasma physico-chemical behavior. Normalization approaches. then decreases by reducing the working Choice of internal standard. pressure (see, e.g., Iida,68 Sdorra and Theoretical evaluation of detection Detection efficiency. Calculation of the number of atoms Niemax,69 Knight et al.73); (ii) similar sensitivity as an absolute figure of seen by the detector. Numerical evaluation of LOD. results, although in different pressure merit of LIBS Enhancing the sensitivity and detection Double-pulse and multi-pulse LIBS. Hyphenated methods intervals compared to ns pulses, have limits (LIF, Raman, LA-LIBS). been observed for femtosecond ablation Treatment of analytical data Single-shot analysis. Spectral fluctuation approach. on Al, silicon (Si), and Cu targets Chemometrics. (Yalc¸in et al.70). In this case, the Absolute analysis and calibration-free Standardless versus absolute analysis. emission enhancement observed at 4 approaches Torr was significantly dependent upon the measurement delay time; (iii) the change in intensity shows a similar core characterized by a fairly uniform the primary plasma is dominated by trend, although with different rates of distribution of temperature and species continuum radiation while that of the change, for the different gases tested number density. Moreover, a ‘‘soft’’ secondary plasma is predominantly (air, He, Ar), the intensity obtained with confinement of the vapor plume by the characterized by line radiation. If a Ar being higher than with the other gas is found, since a large amount of Ar lower temperature characterizes the sec- gases at pressures less than ~ 300 is mixed with the Al plume: this ondary plasma, absorption of the con- Torr;68,69 (iv) the observed change in indicates that the interplay between the tinuum radiation emitted by the primary emission intensity is clearly related to plume and the gas needs further study to plasma could be observed. In the images the change in mass removal and vapor- be completely understood. reported by Glumac and Elliot,71 a ization68,69 and to the change in plasma The fact that argon increases the size bimodal plasma structure is clearly parameters such as temperature and and temporal emission persistence of the observed at lower pressures and longer electron number density; (v) spatially plasma is accompanied by the beneficial delay times. resolved measurements taken by Agui- effect of reducing the self-absorption, as 74 Summarizing the above findings, it is lera et al. with iron (Fe) in an Ar specifically reported by Kuzuya and difficult to compare the results presented atmosphere indicate that Ar and Fe are Mikami.75 We note, however, that it is on a common ground, since different separated due to the existence of a the self-reversal of the lines that dimin- lasers, different energies, different time region of minimum Ar density resulting ishes, not the self-absorption, indicating from the displacement of the ambient a temperature gradient. Referring to the regimes, and different ranges of pressure gas by the propagating shock wave; (vi) finding of Ma et al.76 that a fairly were used. In order to rationalize the the background gas has a confinement uniform temperature is found in the effect of different gaseous environments effect on the plasma, as predicted by gas emitting plasma core, self-absorption on targets ablated by the laser, one needs dynamic modeling,79 and the relevance will still be present. As reported in other to take into account the atomic mass of of the effect depends upon the atomic studies cited by Yalc¸in et al.,70 and also the gas, the mass ablated and vaporized, mass of the gas; and (vii) time- and by Knight et al.,73 two plasmas exist, a the confinement effect, the plasma space-resolved measurements taken by primary plasma close to the target shielding effect, and the variation of Ma et al.76 on an Al plasma in Ar at surface and a secondary plasma at temperature and electron number densi- atmospheric pressure show a plasma reduced pressure. The emission from ty. All this information is needed for a

APPLIED SPECTROSCOPY 353 focal point review complete description of the interplay plasma morphology during its temporal experiment does indeed depend on between the gas and the plasma plume. evolution or to image the distribution of the spectral efficiency of the spec- LIBS characterization of a He plasma the emitting plasma species. In this trometer (see Eq. T3.6). induced by a high-power CO2 laser has second case, whatever the final goal is, (iii) For a given optical setup, simple been recently reported,77 as well as the knowledge of the optical and spectro- algebraic manipulations of the rela- detection of trace quantities of He and scopic parameters involved is needed. tion linking the fundamental param- Ar in binary and ternary gas mixtures A discussion of the figures of merit of eters of the optical elements (lenses, with nitrogen.78 Since hydrogen is an an optical train can be found in several mirrors, limiting apertures and lin- important constituent of atmospheric books dealing with optics and analytical ear magnification) with the spec- pressure plasmas, in particular for the spectroscopy, but it is not covered in trometer figures of merit (spectral determination of electron number densi- great detail in books and chapters band pass, reciprocal linear disper- ty based upon the Stark broadening of dedicated to LIBS instrumental aspects sion, resolving power) allows one its lines, several papers focus on its or in the short courses given at various to derive Eqs. T3.12 and T3.13, detection in LIBS spectra. Kurniawan et LIBS conferences, a laudable exception which represent the number of al.80 reported the quantitative analysis of being the course given by Gallou and atoms present in the emission 87 hydrogen in a Zircaloy sample (a Sirven at EMSLIBS 2007. Our choice volume that are ‘‘seen’’ by the material used in light water nuclear here is to refer to the book of Alkemade 83 detector and therefore contribute to power stations to contain the uranium et al., in addition to a selection of the observed signal.92 This expres- fuel) by LIBS at reduced pressure. Two papers taken from the literature. The sion will be used for calculating the recent studies81,82 addressed the ques- papers selected basically deal with the 83,84 analytical sensitivity of a typical tion of the origin of the hydrogen found laws of propagation of radiation, 85,86 LIBS experiment. in laser plasmas induced on metal targets imaging spectrometers, spectral effi- 91 at ambient pressure, i.e., whether its ciency, emission volume seen by the de- Referring to some of the cited literature, 92 88–90,93 source is the sample itself or the tector, echelle monochromators, we draw the readers’ attention to the 94–99 85 surrounding atmosphere. The question gated and non-gated detectors, and paper by Olesik and Hieftje, who is important, since hydrogen is used as a the description of a spectrometer allow- describe two optical imaging systems ing full spectral coverage and multi- based on the use of a monochromator, test indicator of the inner conditions of 101 the plasma. Both studies81,82 have channel operation. namely an electronic slitless spectro- shown that the major contribution to The pertinent terminology and the graph (EES) and a monochromatic the presence of hydrogen (H) atoms in fundamental relations between the vari- imaging spectrometer (MIS). In the last the plasma spectrum can be attributed to ous parameters are collected in Table III. case, an image of the source is formed the target and only to a minor extent to All the terms used in the table are on the grating, recollimated inside, and the surrounding air. defined in the description column as focused outside the monochromator on Optical Transfer of Plasma Radia- well as in the table captions. We the detector. This system was assembled tion, Spectrometer, and Detection. therefore make only a few additional by Iida et al.86 for LIBS work. The transfer of the radiation from the remarks here: Several papers deal specifically with a plasma to the detector, the so-called (i) As Eq. T3.7 in Table III clearly comparison between gated (ICCD) and optical train, plays an important role in indicates, the optical conductance non-gated (CCD) detectors. Normally, determining the signal collection effi- remains constant through the opti- the possibility of selecting the time ciency of the system utilized. There are cal system assembled. This funda- interval in which the signal is measured two considerations worth stressing here, mental relation is also known as and the time delay from the onset of oneinthecaseinwhichaLIBS ‘‘the invariance of radiance.’’ It also plasma formation is beneficial to maxi- experimentalist uses a commercially conveys the concept of the limiting mize the signal-to-noise ratio. This was 95 available system, and another in which aperture of the optical system, pointed out by Carranza et al. and by 97 the optical train is assembled in a which in most cases corresponds Sabsabi et al. On the other hand, 98 custom-made fashion. While in the first to that of the spectrometer. Mueller et al. did not find a substantial case one has no choice other than (ii) The luminosity-resolving power difference in the performances of the relying upon the optical quality and product (denoted as spectral effi- CCD and ICCD, their conclusion being spectral characteristics of the instrument, ciency of the spectrometer) is also a based on the similarity of signal-to-noise in the second case one can put together constant in most conventional spec- ratios and detection limits obtained for different optical components with a trometers, clearly stating that an several elements in steel samples: the specific task in mind, for example, that increase in the resolving power of findings were also confirmed by a of maximizing the transmission of the spectrometer does necessarily simulation exercise. As correctly point- radiation, favoring high spectral resolu- entail a decrease in the luminosity. ed out by the authors,98 one would need tion or devising a system capable of This was also pointed out in a to compare detectors alone and not providing space-resolved information. comparison of imaging detectors.91 spectroscopic systems, since different Last but not least, one can assemble an It can also be appreciated that the spectrometers and collection geometries imaging spectrometer to follow the signal-to-noise ratio in an emission may indeed result in different luminos-

354 Volume 66, Number 4, 2012 TABLE III. Definitions related to the spectral detection process and parameters involved.a

Eq. Expression Description number

L ¼ AX Definition of optical conductance (luminosity, throughput, etendue, light gathering power). T3.1

k0 Rs ¼ Resolving power defined at the central peak of the line (k0). T3.2 Dks k0 Ps [ LsRs ¼ AsXs Definition of luminosity-resolving power product. This term can be interpreted as the ‘‘spectral T3.3 Dks efficiency’’ of the optical collection apparatus. Z 0 S } Ls IsðkÞdk } LsIs The expression for the line emission signal, spectrally integrated over the line profile, is T3.4 Dk proportional to the optical conductance of the system. Z line k0 B } Ls IBðkÞdk}LsIBðk0Þ Dks}LsIBðk0Þ The expression for the continuum background emission signal is proportional to the optical T3.5 Rs conductance of the system and the spectral resolution of the spectrometer. Dks 0 ffiffiffiffiffiffiffiffiffi ffiffiffiffiffi S LsIs p p } rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi}K LsRs}K Ps This expression shows that in the shot-noise limit, the signal-to-noise due to plasma T3.6 NB LsIBðk0Þ k0 background, the signal-to-noise ratio increases proportionally to the luminosity-resolving Rs power product.

Ae Xe ¼ As Xs The optical conductance is a constant throughout the optical system. This is a fundamental T3.7 relation valid for any optical system.  ðDkshsÞ p ðdeLeÞ Xe ¼ 2 Eq. T3.7 rewritten in terms of plasma and spectrometer parameters, i.e., reciprocal linear T3.8 Rd 4F dispersion, spectral bandwidth, and F-number.

Dks ¼ Rdws Definition of the spectral bandwidth of the spectrometer. In most cases, this quantity defines T3.9 the spectral resolution, since it is much larger than the diffraction-limited spectral width. Dk h M ¼ s ¼ s The linear magnification of the collection optics, rewritten in terms of plasma and spectrometer T3.10 Rdde Le parameters, allows linking the diameter and length of the emission slab in the plasma with the spectral and geometrical parameters of the spectrometer. hsRdde Le ¼ T3.11 Dks

2 3 p de Le p de hsRd Vseen ¼ ¼ Expression defining the volume of the emitting plasma seen by the detector. T3.12 4 4 Dks

3 p de hsRd Nseen ¼ nTVseen ¼ nT Expression defining the number of emitting atoms seen by the detector, given a total atom T3.13 4 Dks number density in the plasma. a Definition of parameters in the equations of the table. 2 Equations T.3.1–T.3.6: L = Luminosity of the monochromator (cm sr); Xm = Solid angle of the monochromator (sr); Rm = Resolving power of the monochromator; k0 = Central wavelength of the spectral line (cm); Dkm = Spectral bandwidth of the monochromator (cm); Pm = Luminosity-resolving power 2 -2 -1 0 product (cm sr); S = Measured emission signal (counts); Dk‘ = Spectral line width of the transition (cm); Is(k) = Spectral line intensity (W cm nm ); Is = -2 -2 -1 Spectrally integrated line intensity (W cm ); B = Background signal (counts); IB(k) = Spectral background intensity (W cm nm ); NB = Background noise (counts). Note the proportionality sign in the equations, implying the presence of a factor which makes the units on either side of the equations consistent. 2 Equations T.3.7–T.3.13: Ae,s = Areas relative to the emission source and spectrometer (cm ); Xe,s = Solid angles relative to the emission and spectrometer (sr); ws = Spectrometer slit width (cm); hs = Spectrometer slit height (cm); M = Linear magnification of the collection optics; F = F-number of the spectrometer; de = Diameter of the emission slab (cm); Le = Length of the emission slab (cm); Rd = Reciprocal linear dispersion of the spectrometer (nm/mm); Dks = Spectrometer bandwidth (nm). ity, thus invalidating the comparison of ICCD detectors, making this combina- Chen et al.96 have described a spec- the detectors. tion useful in LIBS work (see, e.g., early trometer coupling a two-dimensional The advantage of covering a large instrumental developments and spec- CCD detector with an integrating grat- spectral interval with a single laser shot trometer characterization88–90). As a ing consisting of 10 sub-gratings. The is a welcome feature of many LIBS curiosity, it is interesting to note that instrument allows full spectral coverage applications, in particular stand-off anal- the World Space Observatory Ultravio- from 200 nm to 1000 nm without any ysis and characterization of samples let (WSO/UV) will comprise a high mechanical moving elements.96 producing complicated spectra where resolution double echelle spectrograph In concluding this section, we draw spectral interferences are likely to occur. (HIRDES) covering the near vacuum the readers’ attention to a paper by Echelle spectrometers are known to UV range (174–310 nm) and the Taschuk et al.100 comparing four com- cover a large wavelength interval and vacuum UV range (102–176 nm) with mon LIBS detection systems. The au- can be combined with both CCD and a resolving power greater than 55 000.93 thors considered two Czerney–Turner

APPLIED SPECTROSCOPY 355 focal point review spectrometers coupled with an intensi- increasing the emission intensity of ed information may result in better fied photodiode array and ICCD, an characteristic lines of Br, Cl, and I in analytical figures of merit, for example, Echelle spectrometer coupled with an the region 130–190 nm and also in in terms of linearity of calibration ICCD, and a field-portable broadband decreasing self-reversal (called self-ab- curves. Most important, the magnitude spectrometer composed of eight com- sorption by the authors) effects.108 of matrix effects may vary with the pact CCD spectrometers optimized for Using vacuum UV lines, Cl and Br position of observation within the plas- different spectral regions. This work is were detected in organic samples using ma. useful because it describes a radiometric carbon as an internal standard.110 The Space-resolved measurements are not approach aimed at characterizing the authors presented calibration curves suggested here to become a routine combined performance of the spectrom- showing a non-zero intercept.110 Finally, analytical protocol in quantitative LIBS eter and the detectors in absolute terms, arsenic (As), Br, Cl, P, and S were work; nevertheless, it is important to so that the data reflect the behavior of detected at 7 Torr CO2, therefore emphasize the high relevance of such the LIBS plasma itself, rather than a simulating the Martian atmosphere.111 measurements since they help clarify the combination of the LIBS plasma and the A few papers have discussed the use analytical performance of the technique experimental equipment.100 of near-infrared lines for the detection of for quantitative applications. Spectrometers based on a Paschen– S, fluorine (F), and Cl.112,113 Space-resolution in plasmas can be Runge optical layout were described by Detection limits and linearity of achieved with setups allowing axial Noll et al.28 and by Neuhauser et al.101 calibration curves similar to those ob- resolution along the direction of the The detectors in this case are either tained with vacuum UV lines were expanding plasma toward the ablation photomultiplier tubes or CCD linear reported. All major lines of F (centered laser, perpendicular to the target, and sensors. Simultaneous detection with around 837 nm) and Cl (centered around lateral resolution along the orthogonal PMTs allows independent optimization 687 nm) were detected quantitatively direction. Both lenses and mirrors are of signal-to-noise ratio for each element. using gated gallium arsenide (GaAs) used to accomplish this result. Aguilera As Noll et al. point out,28 the highest photocathode-based detectors without and Arago´n122 used two mirrors to fold measuring frequency with single-pulse the use of an internal standard.113 In a and focus the plasma emission at unity LIBS, Paschen–Runge spectrometers, He atmosphere at ~100 mbar, best magnification onto the entrance slit of PMTs, and multi-channel integrator results for S (869.46 nm) were obtained the spectrometer. The use of mirrors was electronics was 1 kHz. Further increases forming the plasma with a 355 nm YAG preferred to avoid chromatic aberration to 5 kHz will be the basis for future laser.113 In a survey, spectra of soils effects typical of focusing lenses. In this high-speed LIBS microanalysis for in- using a 532 nm YAG laser and a cooled respect, we note that chromatic aberra- dustrial applications.28 indium gallium arsenide (InGaAs) diode tion of refractive optics can be eliminat- Paschen–Runge spectrometers have array detector, strong near-infrared ed (and spherical aberration minimized) been commonly used for LIBS analysis (750–2000 nm) lines for calcium (Caþ), by using the system described by of steel samples with detection in the Al, C, potassium (K), Si, S, and uranium Padgett and Farnsworth,117 consisting vacuum UV. The elements of interest (U) were observed.114 In the case of U, of two plano-convex lenses mounted on are carbon (C), sulfur (S), phosporous results were given for a flow cell purged a translation stage that are driven in (P), nickel (Ni), chromium (Cr),102–112 with Ar at 0.84 3 105 Pa. Limits of opposite directions. The space between and the halogens bromine (Br), chlorine detection ranged from 260 ppm (Ca) to the lenses can be adjusted with wave- (Cl), and iodine (I).113,114 P, S, and C in 6400 ppm (Al).114 length so that the first lens always acts as steels using lines in the 178.28 to 193.09 Space-Resolved Information: In- a collimating lens and the second as a nm range have been determined with version Approaches. It was pointed focusing lens.117 The authors reported limits of detection (LODs) below 10 lg/ out in our first review that terms such as sub-millimeter spatial resolution of g.103 In another study using a micro- emissivity and intensity of spectral lines (small-size) emission sources with this channel plate (MCP) photodiode ar- have different meanings (see Table IV, system.117 ray,104 doubly ionized (45.96, 97.70, Eqs. T4.10, T4.19).1 This difference is A hybrid detection system has been and117.57nm)andsinglyionized essential when space-resolved diagnos- described123 in which a plasma interme- (68.73 and 90.41 nm) carbon lines tic measurements are performed in LIBS diate image is first formed and then is provided LODs in the 100 lg/g range, with the aim of retrieving the distribu- rotated by 90 degrees by means of an while the use of Ar, He, and air resulted tion of ions, neutrals, temperature, and optical periscope, ensuring that the in an unprecedented LOD for C2þ (97.70 electron number density (see, for exam- expansion direction of the plasma lies nm) of 1.2 lg/g.106 Bulk analysis of ple, Aguilera and Arago´n122). In analyt- along the orientation of the slit of the steels with an echelle vacuum UV ical practice, the emission signal from spectrometer. Bulatov et al.116 achieved spectrometer providing full spectral the plasma is integrated spatially and spatial resolution through the use of an coverage in the region 150–300 nm therefore only ‘‘average’’ values can be eight-furcated optical fiber bundle: sev- gave LODs for C (193.091 nm), P obtained for the above parameters. In en fiber-ends were placed along the axis (177.495 nm), and S (182.034 nm) of quantitative analysis, in particular when of the plasma plume, 1.5 mm apart, 7, 36, and 73 lg/g, respectively.109 The using calibration-free approaches (see resulting in better than 1 mm spatial use of argon was found beneficial in later), local rather than volume-integrat- resolution, while the last fiber collected

356 Volume 66, Number 4, 2012 the entire plasma plume, which is the reconstruction of the original object will the former case, during the observation usual arrangement in most LIBS work. never be obtained from the Abel trans- time, the lens views an almost stationary Whatever arrangement is chosen, form. This is valid for any source, plasma, while in the last case, because of space-resolved data will always be including the inductively coupled plas- the narrow acceptance cone, the moving ‘‘line-of-sight’’ integrated data, since ma, where the region of greatest analyt- plasma can partially or even completely radial information is not directly ame- ical interest is in the center of the disappear from the sight of the fiber. nable to observation. Therefore, if discharge and the Abel inversion suffers In order to overcome the drawbacks desired, axially and laterally resolved from the greatest uncertainty in the of the Abel transform method, Monning data must be converted into radially reconstructed data for points in the et al.118 discussed the application of a resolved data using mathematical tools, central region.118 Moreover, in the case more general computer-tomography the most popular being the Abel inver- of laser-induced plasmas, not only the (CT) algorithm, which requires no sion approach. A selection of papers is lack of symmetry, but also its non- assumption of symmetry and is less given here; however, the reader should stationary character and non-negligible sensitive to the presence of noise.118 The be aware that the number of original optical thickness cause problems. Gor- technique, which is a generalization of works dedicated to Abel inversion is nushkin et al.127 have tested the validity the Abel transform method, is called the significantly larger. It is therefore advis- of the Abel inversion method in the Radon transform method, or Radon able to consult not only the references reconstruction of radiative and thermo- tomography. This approach has been listed, but also the bibliographic cita- dynamic parameters of a transient, non- applied to several types of plasmas and tions contained in those references. homogeneous laser plasma and paid combustion flames, and very recently, The problem of computing radial special attention to its dynamic aspects Merk et al.128 have tested the Radon profiles has received considerable atten- and the errors associated with time transform to LIBS with synthetic emis- tion will all kinds of radiation sources integration. The authors127 have used sivity functions that mimic typical LIBS exhibiting cylindrical symmetry utilized an algorithm developed by Aguilera et plasma features. As the authors point in several emission spectroscopic meth- al.121 and subsequently modified by out, the approach can reveal asymmetric ods, the closest to LIBS being spark Cristoforetti et al.124 to account for plasma features that could not have been emission spectroscopy. Scheeline and plasma asymmetry, which was called observed with the Abel transform.128 Walters115 have thoroughly described an ‘‘onion peeling’’ algorithm, since it For more details on the characteristics the use of Abel inversion applied to was mathematically equivalent to peel- and applications of the Radon transform, plasma discharges (arcs, sparks) and the ing an onion. Among the drawbacks the reader should consult the original meaning of the radially resolved data identified were the small plasma size, its papers.118,128 derived when background, self-absorp- fast dynamics, and the noise.127 In order Polarization of Plasma Emission. tion, discharge wander, and noise affect to reduce the integration time, and The issues of polarization from plasma the measurements. Noise effects on therefore improve the accuracy of Abel radiation date back several decades in lateral emission profiles were specifical- inversion, Shabanov and Gornushkin129 the literature, although the early work is ly addressed by Chan and Hieftje:125 proposed the use of large acceptance largely focused on anisotropic plasmas seven test profiles commonly encoun- angle apertures (at the expense of and recombination radiation.130–134 tered in analytical plasma emission spectral resolution), allowing sufficient More contemporary studies have fo- sources were artificially modified by signal-to-noise ratio to be achieved with cused on polarization-resolved emission the addition of noise, and the authors much shorter integration times, in the in the context of LIBS. For example, the provided a method for estimating the range of several tens to several hundreds use of polarization-resolved plasma statistical confidence intervals for radi- of nanoseconds. A special numerical emission as a means to enhance the ally resolved emissivity data derived data processing technique compensated sensitivity of femtosecond LIBS analy- from Abel inversion.125 Alvarez et al.120 for the loss of spatial resolution due to sis was reported, with a focus on the applied the Hankel–Fourier method (one the large acceptance angle aperture.129 observed polarization of the continu- of several numerical algorithms used for Shabanov et al.126 have also calculated um.135 The extension of this approach to Abel inversion) to obtain the radial the emission spectrum of an asymmetric, more traditional nanosecond-laser LIBS dependence of temperature and electron ellipsoidal plasma plume with specific measurements was reported,136,137 with number density in a microwave He reference to the use of a lens or an the observation that the continuum plasma produced at atmospheric pres- optical fiber to collect the plasma emission displayed polarization behav- sure by the axial injection torch (TIA). radiation. The paper is mathematically ior preferentially over discrete atomic All papers dealing with Abel inver- oriented, since the model incorporates emission lines, offering an opportunity sion point out its limitations, namely that the solution of the radiative transfer for baseline continuum suppression. We the source must possess axial symmetry equation along the emission sight: note, however, that such studies did not and that the results are very sensitive to nevertheless, it has an important practi- explore the polarization phenomena the amount of noise present in the cal relevance, since it shows clearly the temporally. Other studies examined the system. Indeed, as pointed out by different effects produced by the moving use of polarization filtering to suppress Monnig et al.,118 if the basic assumption plasma on the (simulated) signal col- background elastic scattering rather than of circular symmetry is violated, a true lected with a lens or with a fiber optic. In continuum emission.138

APPLIED SPECTROSCOPY 357 focal point review

With a goal of better understanding realize that this is a nearly impossible all the physical phenomena one wishes the observed polarization with nanosec- task. This is true even if one considers a to understand.170 In view of this con- ond LIBS, Asgill et al.139 performed single analytical problem, i.e., the quan- sideration, the amount of useful model- temporally and spatially resolved mea- titative determination of one element in ing done and published in the LIBS surements for analysis of both solid one matrix with a well-defined experi- literature is rather impressive. It is not systems and gaseous systems, noting mental setup. The versatility of LIBS our goal to cover this topic exhaustively that the latter system eliminates any aggravates the situation in the sense that here: the interested reader can consult potential effects from time-resolved the number of experimental cases to be the selection of papers given here, as surface reflectivity. The authors139 considered increases excessively. Suf- well as the references provided in these found no evidence of inherent polariza- fice it to say that in order to describe just papers. Due to the large amount of tion of either the continuum or atomic the laser interaction with the target material available and the mathematical emission; however, they observed that material, one is faced with the following flavor of many contributions (e.g., some polarization effects are present at situations: (i) the target can be a solid, a detailed description of numerical simu- early times for analysis of solids at liquid, or a gas; (ii) the target can be lation codes), we note that important oblique angles of observation, which transparent or absorptive to the laser papers may have been left out, and for were attributed to Fresnel reflectivity. wavelength; (iii) if the target is a solid, it that we apologize to the authors. Since the Fresnel effects were limited to can be a metal, a semiconductor, or a Gornushkin and Panne41 have pub- early times in the plasma evolution, wide band gap dielectric; (iv) only one lished a very useful review on modeling 170 where continuum emission dominates, it laser pulse, or a succession of pulses efforts in LIBS, and Bogaerts et al. was concluded that such an effect could separated by the laser operating period have edited a cluster issue consisting of account for the previously reported (reciprocal of the operating repetition 20 invited contributions assessing the studies.139 In a follow-up study, Penczak rate), is used; (v) the laser operates at a state of the art in plasma modeling and et al.140 concluded that Fresnel effects specific wavelength; (vi) the duration of numerical simulation. The flavor of the 170 accounted for some but not all of the the laser pulse can be in the nanosecond, editorial review and the LIBS mod- 41 reported polarization effects. picosecond, or femtosecond range; (vii) eling review is different and addresses At the present time, the polarization the laser pulse is characterized by a different audiences: the last review has a dependency with nanosecond LIBS certain energy (proportional to the direct, specific link with LIBS work and analysis of solids remains an area of attainable laser fluence on the target) emphasizes models relevant to spectro- continued study, although the research and power (related to its irradiance), chemical analysis, while the cluster suggests that both pulse energies and depending upon the temporal shape of issue deals with major modeling theories surface orientation may play key roles. the pulse; (viii) the target environment and simulation strategies and their can be the vacuum, reduced pressure, or applications to contemporary plasma- MODELING LIBS the ambient atmosphere, including dif- based technologies, reviewing modeling work on different plasma sources (e.g., The importance of modeling the LIBS ferent gases; and finally (ix) the irradi- ation can be provided by two laser radiofrequency plasmas, magnetron dis- plasma has been repeatedly stressed in charges, and microwave plasmas). The the literature. As Gornushkin and Panne pulses (derived from the same laser or from different lasers), where the time editorial review explains clearly how the point out in their recent review of this theoretical foundations on which plasma 41 delay between the pulses of the first and field, further progress in the improve- science was built were laid down in the second laser can be varied at will. ment of the quantitative aspects of the 19th century.170 More recently, Bogaerts When the complex variety of physical technique can only be expected from et al.174 have given an overview of their and chemical plasma processes is added deeper understanding of plasma process- own work, carried out in the PLASM- to the picture, thus involving consider- es. A naive, ambitious interpretation of ANT research group of the University of ations on thermodynamic equilibrium, the word ‘‘modeling’’ would imply the Antwerp, focused on laser ablation, capability (and ability) to provide a statistical distribution, radiation theory, plume expansion, and plasma formation. description of the entire sequence of kinetics and matrix effects, and photon- Some considerations, resulting from processes occurring in a LIBS experi- to-signal conversion, one understands our personal selection of litera- ment. In other words, one would expect why the above statement is justified. ture,41,141–174 are summarized below. a full description of the laser–sample On the other hand, it is also true that interaction, plasma formation and ex- striving for completeness and striving (i) Several groups of models exist:41 pansion, excitation processes occurring for understanding need to be balanced. the continuous models based on in the plasma, collection of radiation, Understanding is most easily gained fluid dynamics, the discrete mod- generation of the analytical signal, and from modeling as few parameters as els based on Monte Carlo or conversion of this signal into concentra- possible. Moreover, these parameters molecular dynamics simulation, tion. The ideal outcome would then be a should be amenable to experimental and the hybrid models, where the simulation of the spectrum, plasma observation. To paraphrase Bogaerts et output of one model is used as parameters, spectral line features, sam- al.,170 all present plasma models are input for the other. ple composition, and associated analyt- compromised, judiciously reduced prob- (ii) Collisional-radiative (CR) and col- ical sensitivity. It does not take long to lems, which nevertheless try capturing lisional-dominated (CD) models

358 Volume 66, Number 4, 2012 are mostly used to describe radia- interaction and associated process- terial (RDX),164 and aerosol particles tive plasmas, the major difference es (thermal effects, phase transi- (magnesium (Mg), cadmium (Cd)).166 being that CR plasmas are non- tion, plasma–surface interaction) Particle formation and size distributions equilibrium plasmas and provide a with a ‘‘black box’’ to be ignored, are specifically considered in Refs. 156 more adequate description of as done in items (iv) and (v) and 158. LIBS plasmas compared to CD above, is of course questionable. Figure 3 shows the complete scheme models, which assume local ther- Nevertheless, this black box is of the direct-inverse problem outlined in modynamic equilibrium.41 considered to be less important in item (v) above. (iii) Laser–matter interactions, radia- LIBS than in laser ablation ICP tion transfer, and chemical kinetics methods. As pointed out by Gor- QUANTITATIVE LIBS nushkin and Panne,41 ‘‘the uncer- can be implemented with each General Considerations. As stated tainties in the evaluation of the group of models. In particular, in our first review,1 the quantitative mass input into the plasma are modeling of laser–solid interaction aspects of LIBS may be figuratively today equivalent to starting a and laser-ablated plume expansion considered its Achilles’ heel, first due to radiative model with an educated behavior is mostly relevant for the the complex nature of the laser–sample guess of initial conditions.’’ techniques using ICP emission or interaction processes, which depends (vii) Despite the last statement in (vi), mass spectrometry. This is proba- upon both the laser characteristics and there is a rising feeling that, as the bly the best evidence of the the sample material properties, and models continue to be refined and indissoluble link between laser second due to the plasma–particle inter- become more robust, the initial ablation and LIBS. action processes, which are space and distribution of parameters needed (iv) Pure-plasma modeling, or ‘‘post time dependent. Together, these may as input to the models should be breakdown’’ plasma modeling, cause undesirable matrix effects.1 This verified experimentally. This rep- which starts after the laser inter- apparently critical statement regarding resents a very challenging task, action with the sample is complet- the analytical capability of LIBS to since highly resolved data, both in ed and the supply of energy from perform quantitative analysis is justified time and space, would then be the laser is terminated, has been by the fact that in practically all papers needed at early times of plasma described extensively: the plasma reviewing LIBS for chemical analysis, evolution. As a consequence, is supposed to ‘‘forget’’ its laser the qualitative aspects of the technique model refinements must be con- origin and expands, relaxes, and take the lion’s share of all its positive, sidered in the context of experi- decays like any other type of unique features. The situation, however, mental limitations. plasmas. Model inputs are the is different when the quantitative aspects (viii) Double-pulse LIBS has also been initial temperature and total num- are considered, since in this case, there modeled:22,165,167 this will be dis- ber of plasma species, together is a near-unanimous consensus that cussed later in the section dealing with their distribution, the initial these represent a major limitation of with double- and multi-pulse ap- plasma radius, and the plasma the technique. This sentiment has been proaches. front speed. The model outputs variously expressed by stating that are the spatial and temporal distri- Referring more specifically to our refer- ‘‘calibration is the most difficult issue butions of atoms, ions, electrons, ence list, examples of application of in the development of LIBS, especially line profiles, and absolute spectral group models described under (i) can be for field measurement,’’ that LIBS is ‘‘a intensities. The practical analyti- found. We note a typical trend in the mix of great potential and severe cal outcome is the evolution of the development of laser ablation-LIBS limitation,’’ that ‘‘one of the major plasma spectrum in any desired models: first the model is applied to problems that precludes more quantita- spectral window. vacuum, then to air, then to different tive use (of LIBS) is a lack of reproduc- (v) A two-fold approach results from gases (e.g., He, Ar), and then it ibility of spectra at a given wavelength the above: first, plasma radiation addresses more specifically the problem on a shot to shot basis,’’ and that ‘‘the dynamics are computed with pre- of particle formation and size distribu- sensitivity remains modest, precision set initial conditions (the direct tion. In most cases, pulse durations are mediocre and matrix dependence problem) and then compared with in the range of several nanoseconds strong.’’ The necessity of using matrix- experimentally measured parame- (Nd : YAG lasers at 1064 nm, excimer matched standards has been repeatedly ters, thus obtaining information lasers such as XeCl at 308 nm or KrF at stressed. To summarize a common about the initial state of the post- 248 nm). Apart from the paper by sentiment, we refer to Mermet et al.,218 breakdown plasma (the inverse Vorob’ev141 discussing the interaction who pointed out that: ‘‘regardless of the problem). We will refer again to of laser radiation with different types of field of application, the acceptability of these models later in the section solid samples, the targets considered are LIBS is still related to the problem of discussing calibration-free ap- metals, e.g., Al,142,143,146,168,169 quantitation, involving accuracy, i.e., proaches to quantitative LIBS Cu,150,153,158,161,163 lead (Pb),148,155 ti- repeatability and trueness.’’ analysis. tanium (Ti),143,147,151 Si,156,158,159 In support of LIBS, we would like to (vi) The identification of laser–sample SiC,152,154 Ti oxides,144 explosive ma- point out that even superstar techniques

APPLIED SPECTROSCOPY 359 focal point review

FIG.3. Flowchart of the essential steps involved in the implementation of a model in the theoretical calculation of the concentration of the various species and a comparison with the experimentally obtained values (see text for discussion). have weaknesses (e.g., fractionation LA- level considered adequate for the spe- render the calibration issue more diffi- ICP-MS) when applied to trace analysis. cific analytical task investigated, and we cult to tackle and solve. However, this Moreover, the fact that either internal refer the reader to the Applications issue is of essential relevance for the standard calibration or external calibra- sections. In addition, as pointed out by further development and acceptance of tion methods are needed is not a Russo et al.16 in the discussion of laser LIBS as a quantitative method, and it drawback per se. All quantitative ana- ablation, the occurrence of fractionation will only be through understanding and lytical techniques rely on calibration and does not preclude the use of laser appreciation of these processes that standards. Ironically, the versatility, ablation sampling for accurate chemical LIBS analytical schemes will be opti- which is the best advantage of LIBS analysis.16 Therefore, the problem is not mized to realize their full quantitative due to its capability of directly address- that LIBS cannot perform quantitative analytical potential. ing any sample, is inherently also the analysis, but rather how to make LIBS In conclusion, if quantitation is indeed very reason that matrix-matched stan- become as accurate and reliable as the the only vulnerable aspect of LIBS (as dards are not available in many applica- other spectroscopic methods. In this Achilleshadhisheelastheonly tions. Moreover, the lack of sample respect, it is also worth cautioning that vulnerable part of his body), this is preparation, which is another unique comparing LIBS with bulk analysis where research should then be focused and attractive feature of LIBS, will be methods, e.g., ICP-AES, may sometime and vigorously continued. challenged if extensive chemical or be meaningless, since bulk methods will Calibration and Curves of Growth. physical (mechanical) sample manipula- determine the average concentration of The relation between the (spectrally tion prior to laser ablation were the only the analyte in a sample mass that is integrated) thermal radiance, Bthermal,of way to achieve good quantitative ana- much larger than that sampled by an emission line as a function of the lytical data. LIBS.23 number density of the atomic (ionic) LIBS has been shown to be capable of The complexity associated with laser– species in the plasma is exhaustively performing quantitative analysis at a sample and laser–plasma interactions treated in all books related to spectro-

360 Volume 66, Number 4, 2012 Z chemical analysis and plasma spectros- the plasma and not to the concen- Bthermal ¼ðBÞk ðTpÞ kkðkÞ‘ ð4Þ copy. These works are listed in our first 0 tration in the sample. article, together with the most important line (ii) iThe emitting atoms in the plasma theoretical expressions (see Table IV, As a consequence, in optically thin and the plasma temperature are Eqs. T4.10–T4.13).1 supposed to be uniform in the condition, Bthermal grows linearly with We repeat below only the relations the wavelength-integrated absorption observed plasma volume. pertinent to the shape of the analytical coefficient, which in turn is directly (iii) The link with the more convention- calibration curves, known as curves of related to the atom number density. This al expressions used in the LIBS growth. The fundamental expression is the linear part of the curve of growth. literature (see Table IV, Eqs. relating the emission ‘‘signal’’ and the T4.14–T4.19 in our previous re- As the number density continues to 1 analyte atomic number density in the increase, the Doppler core of the line view ) can be seen from the direct plasma is given by the classical equation:1 saturates, but the (Lorentzian) wings of relation between k(k) and r(k), the the line are still optically thin. Depend- relation between r(k)andthe B absorption oscillator strength, f, thermal Z ing upon the value of the a-parameter, ÈÉ the relation between f and the Planck the curve of growth may show a plateau ¼ðBÞ ðTpÞ 1-exp½-k ðkÞ‘ Einstein spontaneous transition k0 k for a given range of number density. probability (A) and the use of the line Ultimately, the pressure broadening Saha–Boltzmann equation describ- ð1aÞ contributes to the wings, and Bthermal ing the atomic distribution function grows with concentration at a slower in the plasma under the assumption rate. This is the square root part of the Bthermal of local thermodynamic equilibrium Z curve of growth. (LTE). Wien In the other extreme (optically thick) ¼ðBÞ ðTpÞ fg1-exp½-kkðkÞ‘ k0 conditions, i.e., when the optical thick- An interesting aspect of the above line ness is much larger than unity, the considerations is that the essential fea- ð1bÞ blackbody value at the plasma temper- tures of the emission calibration curves ature is obtained, i.e., are dictated by the absorption parameter (At), showing that, fundamentally, emis- kkðkÞ‘ ¼ skðkÞð2Þ B ¼ðBÞ ðT Þð5Þ thermal k0 p sion and absorption are always co- Z existing in the plasma. and the analytical dependence of the We note again here that line profiles fg1-exp½-kkðkÞ‘ [ A ð3Þ t emission on analyte number density is showing a dip in the center should be line lost. called self-reversed, not self-absorbed The spectral line profile plays an lines. Although the two effects have In the above equations, ðBÞ (Tp) k0 essential role in dictating the shape of been correctly distinguished (see, e.g., (Wcm-2sr-1nm-1) is the spectral radi- the curve of growth. For analytical Fig. 8 in Ref. 20), some confusion in this ance of the blackbody radiation given by purposes, the quantity relating the lin- terminology still exists13,229,434 (see, the Planck or by the Wien laws at the earity, and any deviation from it, is the e.g., Fig. 6 in Ref. 13). temperature of the plasma, and Tp, self-absorption parameter, defined by All the concepts outlined above, and kk (k), and kk(k) are the wavelength- in particular the effect of self-absorp- dependent absorption coefficients in- kkðkÞ‘ 27,175–191 Kk [ tion have been discussed, in cluding and neglecting stimulated emis- fg1-exp½-kkðkÞ‘ detail or to some extent only, in the sion, respectively. k(k) = r(k)n, where skðkÞ literature. With regard to the calibration r(k) is the wavelength-dependent ab- [ ð6Þ curves, the reader is referred to the paper 2 1-exp½-skðkÞ sorption cross section (cm )ofthe by Mermet43 and to the various exam- -3 transition and n (cm ) is the atom The above considerations result in the ples taken from the LIBS literature and number density. sk(k) is the optical theoretical shapes shown in the central reported in Fig. 4. thickness (dimensionless, despite its upper part of Fig. 4, which shows the Analytical Sensitivity and Detec- name), given by the product of the double logarithmic plot of several theo- tion Limits. The concept of limit of absorption coefficient and the homoge- retical curves of growth (see Alkemade detection and its statistical meaning has neous length, ‘, of the plasma in the et al.,83 page 170). been the subject of many theoretical direction of observation. The integral in The above equations contain all the treatments and standardization proce- Eq. 3 is called the total absorption analytical information necessary to in- dures by international bodies, with the factor, At, (with units of length, e.g., terpret the shape of the calibration plot clear aim of proposing harmonized nm). obtained in practice. Several consider- internationally accepted definitions In the case of negligible self-absorp- ations are in order here: (see, e.g., Refs. 195 and 199). tion, i.e., if the optical thickness over the Being an emission spectroscopic tech- whole wavelength range of the line (i) iiThe expressions refer to the number nique, LIBS should therefore benefit profile is 1, one obtains the relation: density of the emitting species in from the large literature existing on this

APPLIED SPECTROSCOPY 361 focal point review topic and described in detail, for exam- signal and background, and NS and NB detector response and analyte concen- ple, in the case of flames, inductively as the corresponding noises, the above tration, is given by the simple expres- coupled plasma, and gas discharges (see, definitions can be written as sions: e.g., Refs. 196, 197, 200, 201, and 203). ¯ ¯ ¯ ¯ Moreover, it should be kept in mind that XS ¼ XSþB-XB ð7aÞ XL ¼ XB þ ksB ð9aÞ a ‘‘complete analytical procedure’’ im- plies carrying out exactly the same steps NS ¼ XS-X¯ S ð7bÞ XL-X¯ B ¼ ksB ð9bÞ carried out for the sample also for the blank and performed with all the N ¼ X -X¯ ð7cÞ ksB 192 B B B c [ LOD ¼ ð9cÞ paraphernalia of the laboratory. L b Because of the fundamental analytical The S/N ratio is the quantity used to relevance of this topic in all methods of evaluate the quality of a physical In Eq. 9, XL is the smallest discernible chemical analysis, it is not surprising measurement. This ratio is defined as signal (e.g., counts), sB is the standard that an overwhelmingly abundant num- the ratio of the signal amplitude to the deviation of the background (counts), or ber of articles exist in the literature. The noise amplitude, the latter represented of the analytical blank, and b is the discussion of concepts like ‘‘decision by its root mean square (rms) value, i.e., magnitude of the slope associated with limit,’’ ‘‘detection limit,’’ ‘‘limit of the linear part of the calibration curve ¯ ¯ quantitation,’’ and the associated statis- S pXffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiX (e.g., counts/ppm). tical analysis is made at widely different ¼ ¼ ð8Þ N N¯ 2 ¯ 2 Both signal-to-background and sig- levels of sophistication and mathemati- ðX-XÞ nal-to-background noise can enter the cal elaboration. We will therefore try to n definition of the detection limit using an avoid ‘‘misplaced erudition’’192 and adroitly partitioned variant of the con- attempt to summarize the essential The identification of the definition of the cept205 introduced by Boumans200 for information from a practical LIBS root mean square noise and the defini- ICP emission. The formula resulting standpoint while referring the reader to tion of the standard deviation (s) of the from this approach is the following: the personal choice of the literature finite population (n replaced by n - 1)is  reported: as we said above, the choice evident in the above equation. It is sB X¯ B cL ¼ k ð10aÞ made should satisfy every palate. indeed customary to refer to the relative X¯ B b The most useful way to deal with standard deviation as the reciprocal of  detection limits in LIBS is to refer to the the signal-to-noise ratio. 1 signal-to-noise (S/N) and signal-to- As can be appreciated from the above cL ¼ kc0ðRSDÞ ð10bÞ B X¯ =X¯ background (S/B) ratios, since during definitions, both signal and background S B the process of optimization of a LIBS are affected by noise. From the series of  experiment these are the quantities most single-shot spectra obtained, one can sB cL ¼ k ðBECÞð10cÞ often measured and reported. The total derive individual values of XSþB , XS, XB X¯ B detector response observed at a given (together with their corresponding aver- chosen wavelength in a LIBS spectrum age values), NSþB, NS, NB, and the In the above equations, k = 3, (RSD)B is will be the sum of the emission signal of corresponding ratios as figures of merit the relative standard deviation of the the analyte, the emission signal due to of the measurements. One can therefore background, i.e., (sB/X¯ B), and BEC plasma continuum, and the detector- optimize the figures of merit in terms of (=c0X¯ B/X¯ S) is the background equiva- associated signal (dark current, stray signal-to-background or in terms of lent concentration. light, etc.). In order to simplify the signal-to-noise ratios. If (XSþB/NSþB)is The use of one of the forms of Eq. 10, discussion, we will neglect the last cause evaluated, it should be clear that one in particular Eq. 10b, is recommended of detector response. If multiple LIBS deals here with the measurement repeat- when reporting the LOD of a given spectra are taken as a result of multiple ability, and this ratio should not be used LIBS experiment. laser ablations of the target, the analyte for the evaluation of the limit of Before referring to some specific signal is defined as the average net detection (LOD) since this analytical LIBS literature on LODs, we would like response obtained after subtraction of figure of merit is related to the noise in to point out some general consider- the average response due to the back- the background (or in the blank, see ations: ground, which is measured in the below) rather than the noise in the (i) iiWhen discussing LODs, back- vicinity of the analyte peak (see below). signal. Its definition, based upon the ground and analytical blank are Since any signal is affected by noise, signal-to-noise approach (see, e.g., Ref. used as synonymous terms, as both defined as the random deviation from 196), identifies the detection limit as the refer to the detector response in the the average, both signal and the back- concentration (amount) that results in a absence of analyte. Ideally, the ground will be accompanied by noise. signal-to-background noise ratio of k, analytical blank is identical to the Defining XS and XB as the responses with k = 3. As discussed by Voigtman sample analyzed, with the excep- observed for each individual spectra, X¯ S in his part 4 of Ref. 205, probably the tion of the analyte, which is absent. and X¯ B as the average responses due to simplest definition of LOD, in terms of This blank would allow the back-

362 Volume 66, Number 4, 2012 FIG.4. Theoretical and experimental curves of growth. (a) Double-logarithmicpffiffiffiffiffiffiffiffi plot of theoretical curves of growth based on a Voigt profile with various values of the a-parameter. At = total absorption factor; b = (pdvD)/ ln 2, where dmD is the Doppler width; n = uniform atom number density in the absorbing state; f = absorption oscillator strength; l = flame depth measured along the line of observation. The a-parameter is defined in panel (a), where dmL is the Lorentzian width. [Reproduced from Ref. 83, with permission.] The other panels show selected examples of calibration curves reported in the literature. (b) Effect of choosing an appropriate reference line for calibration purposes.40 (c) Matrix effects caused by different sample matrices.226 (d) Different calibration sensitivities observed with different sample phases.246 (e) Effect of line choice on linearity.183 (f, g) Onset of self-absorption effects.182,412 For a more detailed discussion, see text. [All these figures have been reproduced from the original papers, as cited, with permission. Inset (d) Copyright 2005 American Chemical Society.]

ground to be measured exactly at uation of the true background may directly applied to liquid samples, the spectral position of the line. If not be easy, especially with low including, for example, liquid jets such blank is not available, the spectral resolution spectrometers and droplets, water is not a common background is taken with a sample and complex spectra, where spec- reference matrix for LIBS. As a containing the analyte at very low tral interferences may easily occur. result, LOD charts, typical of ICP- concentrations. Note that the eval- (ii) iDespite the fact that LIBS can be OES and ICP-MS, collecting sin-

APPLIED SPECTROSCOPY 363 focal point review

gle-element numerical values di- (i) iiMany papers and chapters express the profile can be used since it will rectly inserted in the periodic table the LOD by Eq. 9c. In some cases, be proportional to the integrated for most of the elements present in it is not specified whether the noise line intensity (see the discussion on an ‘‘ideal’’ sample matrix, i.e., an refers to the signal or to the line profiles and instrumental width aqueous solution, are not available. background. Also, some papers presented in Ref. 200 of our first Extensive compilation of LODs can choose k = 2 instead of 3. A clear review1). be found in Cremers and Radziem- discussion on the evaluation and (ii) iThe use of the standard deviation ski book,3 and several works report optimization of LOD in the micro- calculated from the sample at low LODs, together with their defini- analysis of minor constituents in analyte concentration can also be tions, for a limited number of aluminum using a KrF laser with found in other works (see, e.g., elements in specific matrices and 50–300 lJ pulse energy was given Refs. 3 and 26). Also, sometimes for specific applications (see., e.g., by Rieger et al.212 The authors the slope b multiplies the standard Mu¨ller and Stege,13 Salle´ et al.,24 compare the procedure of construct- deviation rather than appearing in De Giacomo et al.,26 Cremers and ing a calibration curve and then the denominator of Eq. 9c. This Chinni33). determining the LOD using the results from the definition of the (iii) The above definitions are also valid slope of the plot (see Eq. 9c) with slope b being (DC/DX) (see e.g., in the case of a spectrum obtained that of using a single sample of Ref. 13). Finally, the importance of with a single ablation shot and a known concentration. With refer- considering absolute rather than CCD or ICCD. All the information ence to our discussion above, relative LODs in LIBS was pointed is indeed there, since the detector Rieger et al.212 have extrapolated out (see, e.g., Ref. 211). allows the simultaneous acquisition the (X/sB) value at a given concen- (iii) The importance of considering the of the spectral peaks and the tration to an (X/sB) value of 3. The signal-to-noise ratio when optimiz- background continuum. This allows authors also emphasize that the rms ing the analytical result is demon- the calculation of the net signal and noise of the background signal can strated in the comparison between of the various noises (peak to peak be obtained by either using multiple the enhancements obtained using and root mean square, which is ~1/ laser shots and corresponding single double-pulse (DP) versus single- 5th of the former, if the noise is pixel (wavelength) readings for pulse (SP) LIBS. In fact, using the normally distributed). Note that this each shot, or a single laser shot same intensity enhancement factor cannot be accomplished with PMT and multiple adjacent pixel (wave- as the improvement in the detection 212 detection, since at least two mea- length) readings. The authors limit assumes that the noise in the surements are needed in this case to finally optimize the LOD values as DP approach is the same as that of evaluate the signal and the back- a function of the measurement the SP. The results shown in the ground. delay time. Elhassan et al. discussed case of steel and aluminum targets (iv) Since LIBS is sampling only minute in detail the calculation of LOD in indicate that the LOD improvement amounts of sample, relative detec- the quantitative LIBS analysis of is in fact lower than the intensity 215 tion limits are not inherently rele- bronzes. Although some of the enhancement, in particular in the 214 vant as the absolute counterpart.202 terms used may be confusing, the case of Al. In fact, the common knowledge that authors define a reliable blank as a (iv) The observation of nonlinear cali- LIBS is not as sensitive as other sample with zero analyte concen- bration curves does not affect a analytical methods is somewhat tration, thus identifying the blank correct calculation of the LOD, misleading, in the sense that an with the background response be- since it is the linear part of the ~1 ppm relative detection limit low the peak of the line. A curve that is used for this purpose. (which admittedly does not sound calculation of LOD then involves On the other hand, a non-zero very spectacular nowadays) often the standard deviation of the inter- intercept will directly affect the translates into an absolute detection cept and of the slope of the accuracy of the calculation. -15 calibration curve, as proposed by (v) As a final remark, when reporting limit in the femtogram range (10 198 g). The advent of micro-sampling at Long and Winefordner. The LODs obtained with a given setup the sub-micrometer level is expect- authors also found that better results and operating conditions, it is ed to even lower such a limit into were obtained by integrating over a essential to ascertain that all rele- the attogram range (10-18 g). spectral interval around the peak of vant parameters (such as laser Moreover, this is also relevant when the line. In another paper, however, parameters, fluence and irradiance LIBS is applied to liquid samples no difference in the LOD was on the target, number of repetitions, (see later). reported using the peak signal or integration time, delay time, back- integrating over the line profile. In ground evaluation and subtraction, Some additional considerations spe- this respect, we would like to point just to cite a few) are given. For cifically referring to the way LODs are out that, if the spectral resolution of example, if one compares two LOD reported in the LIBS literature are the the spectrometer is not sufficient to values obtained with different num- following: resolve the line profile, any point in bers of shots, one could assume that

364 Volume 66, Number 4, 2012 TABLE IV. Some pertinent expressions relative to the use of the spectral fluctuations approach in LIBS.a

Eq. Expression Description number no 1=2 r ¼½rðBÞ 2 þ½rðSÞ 2 þ 2h ½rðBÞ ½rðSÞ This expression refers to a single isolated spectral line (signal S) T4.1 sum p;koff-peak net;k0 p;koff-peak net;k0 superimposed on a flat background (signal B). The standard deviation resulting from the sum of the two signals contains the correlation nocoefficient h. 1=2 r ¼½rðBÞ 2 þ½r 2-2h ½rðBÞ ½r This expression relates the standard deviation of the difference of the two T4.2 diff p;koff-peak sum p;koff-peak sum signals to the standard deviation of the sum and the correlation coefficient. 2 2 2 ðrsumÞ þðrBÞ -ðrdiff Þ h ¼ exp exp exp The correlation coefficient cannot be calculated directly from the standard T4.3 2 ðrsumÞexpðrBÞexp deviation of the sum of the two signals, but can be evaluated from the standard deviations of the sum of the difference and of the background. 2 2 1=2 rsum ðrB þ rS þ 2hrBrSÞ ðRSDÞsum ¼ ¼ This expression shows how the relative standard deviation of the sum of T4.4 ðS þ BÞ ðS þ BÞ the analyte and background signals is related to the individual standard deviations and the correlation coefficient.  2 r2 1=2 rP Pis 1 ðRSDÞratio ¼ 2 þ 2 -2h rPrPis This expression is relevant when an internal standard is used to normalize T4.5 P P PPis is the signal fluctuations. P and Pis refer to the signal peaks measured for the analytical line and that of the internal standard chosen. a The equations reported in this table have been taken from H-Y. Moon, PhD Dissertation (University of Florida, 2009), and will be submitted for publication.

LODs scale with the inverse square sponding standard deviation and relative this, the approach has merits and it is root of the number of shots used: standard deviation values (assuming a easy to implement in the practical LIBS this, however, needs to be demon- Gaussian probability distribution func- routine. For example, it indicates how strated experimentally. tion of the data). Note that the back- the measurement precision evolves dur- ground and signal plus background are ing the plasma evolution while at the The Standard Deviation–Relative acquired simultaneously and may there- same time evaluating the degree of Standard Deviation Approach. We fore show some correlation. Moreover, correlation between the signal and the have recently become interested in the only noises considered are back- background, thus helping to assess the applying to LIBS spectra an approach ground noise and signal noise. Since the feasibility of single-shot normalization that was first described in the case of a background cannot be measured under approaches based upon the spectral highly heterogeneous system like atmo- the peak, it is assumed that the mea- continuum. An interesting outcome is 220 spheric aerosols. We call this ap- surement ‘‘off-peak’’ is representative of the possibility of directly spotting the proach the standard deviation–relative the ‘‘true’’ background under the line. presence of self-absorption in some of standard deviation (SD-RSD) approach The total noise expressions resulting the peaks observed (see the brass (see Fig. 5). It is based upon single-shot from the application of error propagation spectrum in Fig. 5). acquisition and processing of individual statistics are collected in Table IV. A final merit of the approach resides 44 spectra. As pointed out by Michel, an The ‘‘spectra’’ obtained by plotting in its capability of evaluating the investigation into current research draws the standard deviation and relative usefulness of using an internal standard attention to a range of applications for standard deviation versus wavelength for normalization purposes. From Eq. single-shot LIBS, yet exposes the infan- (pixels) can now be compared with the T4.5, it is easy to predict the effect cy of quantitative single-shot LIBS. We conventional spectrum obtained by plot- resulting from ratioing the analyte signal therefore hope that the following con- ting the average signal versus wave- by an internal standard (see later). If we siderations may trigger some additional length (pixels). Based upon the ex- assume that the two signals have interest in the application of this mea- pressions given in Table IV, one can comparable magnitudes and noises (a surement approach. predict the shape of the spectrum that reasonable assumption in most practical The principle is the following. With a would be obtained for different limiting cases), the relative standard deviation of given target and under given operating cases, in which the dominant noise may the ratio will improve only in the case conditions, a certain number of single- be due to the background or to the where the noises in the signal and in the pulse spectra are obtained (see Fig. 5). signal. As shown in Fig. 5, even under internal standard are correlated, i.e., Within the spectral region selected, for the simplifying assumptions made, mul- when h = 1. If the noises affecting the each pixel (or equivalently for each tiple outcomes are possible, since the analyte signal and the internal standard spectral resolution element) the average observed behavior will depend upon the are anti-correlated, i.e., if h =-1, the signal levels for the background, signal source of limiting noise, the relative RSD will worsen by a factor of 2 plus background, and net signal are magnitudes of the signal and back- compared with the RSD of the analyte computed, together with their corre- ground, and their correlation. Despite signal alone. Finally, if the two noises

APPLIED SPECTROSCOPY 365 focal point review

FIG.5. Illustration of the SD-RSD approach applied to single-shot measurements. The application of the statistical expressions given in Table IV result in the simulated spectral plots of the RSD shown below the 55 single-shot spectra of Cu in a brass standard. The simulations (a–d) show that, at the wavelengths corresponding to the two copper lines, the RSD can remain the same, increase, or decrease depending upon the limiting noise of the measurement, the relative values of the signal and background and their degree of correlation. The two spectra on the right, in panel (e), were obtained with Al-alloy standard R14 and show the SD (black trace) and the RSD (gray trace), n the vicinity of the Mg 285.213 nm line. Note how the RSD can increase or decrease, depending upon the line chosen. Finally, the FWHM of the signal at the Cu 324.754 nm spectrum (f) is smaller than that observed for the SD spectrum (g), indicating the presence of self- absorption. Note here that the wavelengths listed correspond to the instrumental readings. [Reproduced with permission from Heh-Young Moon, PhD Thesis, University of Florida, 2010.]

366 Volume 66, Number 4, 2012 TABLE V. Calculation of the expected sensitivity (counts per pulse per atom) assuming a typical LIBS setup involving nanosecond pulse irradiation and ICCD detection.a

Eq. Expression Description number ed ¼ 1-expð-/stobsÞ »/stobs The detection efficiency is considered in the case of a repetitive transient behavior like that T5.1 ed ¼ 1-expð-/stresÞ»/stres encountered in LIBS. One single shot is considered. The two temporal parameters refer to T5.2 the residence time of the atoms in the observation volume and the actual observation time of the emission. Their relative values define whether a measurement is stationary or non- stationary.  E gu - u wshsXeT0 -1 /s ¼ðAu‘Þ e kT gD This expression relates the mean flux of signal counts per atom in the probe volume (s )to T5.3 ZðTÞ 4pAe the parameters of the transition and to the parameters of the optical detection system.

S ¼ð/stresÞ Nprobedð/stDÞ Nprobed The product /s tres becomes /s tD by taking into account the integration time used in the T5.4 single-pulse acquisition.

Nprobed ¼ Nsampleeablationevaporization The total number of atoms probed will depend upon the efficiency of ablation and T5.5 vaporization. Note that the excitation efficiency is included in the Boltzmann factor. This NA Nsample ¼ mswa number is related to the total number in the solid by the sample mass, the Avogadro number T5.6 Ma and the atomic mass and weight fraction of the analyte.  gu -Eu wshsXeT0 S ¼ðAu‘Þ e kT ðgDÞðtDÞ This final expression gives the number of counts per pulse predicted for a given transition, a T5.7 ZðTÞ 4pAe given mass ablated, a given ablation and vaporization efficiency, and a given electro-optical  collection/detection efficiency. By using the numerical values collected in the caption below, NA -7 -1 5 3 mswa eablationevaporization we obtain ed = 3.1 3 10 counts atom S = 2.91 3 10 wa. Ma a The equations reported in this table have been derived from the analytical considerations given in Winefordner et al.17 The reader is referred to this paper for more details as well as for the citation of the original literature from which the expressions were first derived [see, e.g., C.Th.J. Alkemade, Appl. Spectrosc., 35, 1 (1981)]. Definition of the parameters used in Eqs. T5.1–T5.7: ed = Detection efficiency; tobs = Observation time (s); tres = Residence time of the atoms in the observation -1 -1 (probe) volume (s); /s = Mean flux of signal counts per atom in the probe volume (s ); Au‘ = Einstein coefficient of spontaneous emission (s ); gu = Statistical weight of the upper level of the transition; Z(T) = Partition function at the temperature T of the plasma; Eu = Energy of the upper level of the transition (eV); k = Boltzmann constant (eV/K); T = Plasma excitation temperature (K); ws = Monochromator entrance slit width (cm); hs = Monochromator entrance slit height (cm); Xe 2 = Emission solid angle (sr); T0 = Overall transmission of the optics and the environment; Ae = Plasma emission area (cm ); gD = Detector efficiency (counts/photon); tD = Detector integration time (s); ms = Sample mass (g); wa = Weight fraction of analyte in the sample; Ma = Analyte atomic mass (g/mol); NA = Avogadro number (atoms/mol); eablation = Ablation efficiency; evaporization = Vaporization efficiency. 8 -1 -3 Numerical values of the parameters used in Eq. T5.7: Au‘ = 10 s ; gu/Z(T) = 1; exp(-Eu/kT) = 3.3 3 10 , with Eu = 4 eV and T = 0.7 eV (8123 K); ws 3 hs 2 2 -6 -8 = 0.005 cm ; Xe = 0.047 sr (F/4.1); T0 = 0.1; Ae = 0.1 cm ; gD = 0.05; tD = 10 s; ms = 10 ng (10 g); wa = Weight fraction of analyte in the sample; Ma = (Cu) 23 = 64 (g/mol); NA = 6 3 10 (atoms/mol); eablation = 0.1; evaporization = 0.1.

are independent, i.e., if h = 0, the RSD terms of counts per atom per pulse, of 10% (i.e., S = 100) is 0.034%,or340 pofffiffiffi the ratio will worsen by a factor of a typical LIBS experiment. In this case, ppm. Although it is tempting to play 2. This represent the classical case in a conventional laboratory setup is con- with the parameters to improve such which the measurement is signal-noise sidered. The numerical values of the limit, one has to realize that the single- limited, and both signals (analyte and optical and detector parameters are shot sensitivity as calculated is well internal standard) are affected by shot typical of other calculations performed within the order of magnitude of the noise. These results are of course well by Winefordner et al.17 for other spec- typical experimentally reported values in known, and similar considerations in the troscopic methods. As explained in the literature. case of ICP-AES measurements have Table V, the usefulness of the final been pointed out recently.259 expression (Eq. T5.7) is that it can be MATRIX EFFECTS AND We finally note that this approach numerically evaluated for any analyte NORMALIZATION gives information about the limiting and other similar experimental setups, APPROACHES noise of the measurement but does not provided that the spectroscopic parame- identify the type of noise (e.g., shot, ters are known and the electro-optical Definitions and Methodology. Ma- flicker) present. This can be identified by detection parameters can be evaluated. trix effects are of critical importance in the slope of the plot of the signal-to- Assuming the values given in the several analytical spectroscopy tech- noise ratio versus signal. table caption, and taking copper as a test niques, and laser-induced breakdown Numerical Evaluation of the Abso- element, the signal counts are given by spectroscopy (LIBS) is not an exception. lute Sensitivity of a LIBS Measure- Eq. T5.7. Assuming that the limiting In fact, it is not uncommon that a ment. Table V contains a selection of noise of the measurement is signal shot significantly different signal response is expressions that are needed in order to noise, the weight fraction of Cu calcu- observed at a given spectroscopic tran- calculate the absolute sensitivity,in lated for a relative standard deviation of sition of an element present at the same

APPLIED SPECTROSCOPY 367 focal point review concentration level in two different the generally accepted feature that it reference to uncertainty, strategies for samples. Such different response can requires ‘‘little or no sample prepara- reducing matrix effects are collected in be attributed to differences in the laser– tion.’’ three groups: (i) matrix matching; (ii) sample interaction, resulting from The use of a spectrochemical buffer, internal calibration (e.g., standard addi- changes in the ablation mechanism and intended as ‘‘a substance that is added to tion and isotope dilution); and (iii) efficiency, or it can be due to a variation a sample in order to minimize the correction of the effect either by directly in the physical plasma parameters, influence of sample composition and measuring the effect or some property of namely to changes in the electron characteristics on spectral line intensi- the matrix and establishing a correction temperature and number density, or a ties’’ can be considered as an extrapola- function or possibly joint analyte/inter- combination of both. In other words, tion of the ‘‘internal standard’’ approach ferent calibration. like in other methods, the analytical (see below) in the sense that the buffer Considerations on the Concentra- response can be influenced by physical, element is now added to the sample tion Units. Before starting this section, chemical, spectral, and instrumental rather than being selected from among one consideration regarding the units causes. the sample constituents. used in the x-axis (concentration axis) of The following considerations are a Finally, with regard to the measure- the analytical calibration curves is worth concise summary of the treatment given ment of spectral intensities, two opera- pointing out. We note that what follows by Boumans in his book (see Ref. 14 of tional procedures used in dc arc is strictly relevant to the matrix effect 1 Part I). emission work may be, at least concep- related to the entry of the material into As in all spectroscopic emission tually, relevant to LIBS, namely the the plasma and not to the matrix effects techniques, the analytical signal in LIBS total energy procedure and steady-state originating in the plasma. It is therefore is the result of a series of intermediate methods. The former procedure refers to assumed that what follows is valid for and complex steps. The ‘‘spectrochem- the complete volatilization of the sample identical plasma conditions (i.e., same ical physicist’’ needs to unravel these and to the integration of the spectral line temperature and electron number densi- intermediate physical processes in order intensity over the entire evaporation ty, no chemical reaction, etc.) and to reach the final goal of spectrochem- time, while the last procedure, as the identical detection parameters (spectral ical analysis. Not surprisingly, a simple name implies, refers to a constant bandwidth, gain, integration time, etc.): feeding of the sample into the plasma. relationship between the intensity of a in other words, physical, chemical, and spectral line and the number of atoms in In the conventional LIBS analytical instrumental matrix effects are absent. the sample indeed exists if one succeeds practice, the total energy procedure The concentration of element X in the in establishing constant excitation con- would not be applicable in the majority matrix can be expressed in weight ditions. The problem is that the nature of of cases involving bulk solid analysis. fraction (percent), w ,orinmole the sample has a powerful influence on On the other hand, it would apply to X fraction (percent), v . The weight frac- the excitation conditions, leading to single-shot, micro-LIBS experiments on X what is generally known as matrix grains, inclusions, micro-aggregates, tion conveys direct information about effects. Paraphrasing Boumans (Refer- and single particles, with the assumption the mass of an analyte present in the ence 14 of Part I),1 matrix effects can be that the entire sample is evaporated in sample, while the mole fraction is not as divided in two broad categories, namely the plasma, and using an integration gate informative as wX, since the sample those associated with the entry of the large enough to encompass the entire composition must be known. On the material into the plasma and those temporal evolution (persistence) of the other hand, the emission signal obtained taking place in the plasma. The former emission. in the laser plasma is fundamentally process is the major topic of study in the The steady-state method is inherently related to the number density of atoms laser ablation literature while the last inapplicable to LIBS, in view of the of the analyte present in the plasma, and one resides more in the field of plasma transient supply of sample to the plasma therefore to vX (see Ref. 1, Table IV). modeling and diagnostics. and the continuous change of plasma The large majority (practically all) of Another interesting point worth men- parameters during the measurement LIBS papers report wX in the concen- tioning is that spectrochemical analysis time, unless very short integration times tration axis. This choice would not be an has always benefitted from the use of the are used, which are detrimental to the issue per se; however, in some cases, the so called ‘‘spectrochemical buffer,’’ achievement of adequate signal-to-noise conclusions regarding matrix effects which was intended to dominate the ratios. drawn from the analytical curves ob- physical characteristic of the plasma Interferences have been defined for tained in this way may be questionable. (flame, d.c. arc discharges). The use of both qualitative and quantitative analy- To keep our discussion simple, let us a buffer implies either adding an element sis. In the IUPAC definition,224 an consider a binary alloy in which an to a sample solution (in the case of ‘‘interfering substance is one that causes element X (the analyte) is present in liquids) or diluting the sample with an a systematic error in the analytical matrix A at a given concentration, admixture (in the case of powders). In result’’ and further classification distin- expressed as weight fraction, wX. From both cases, sample preparation is need- guished the kind of mechanism involved the basic definitions of weight fraction ed: this would obviously undermine one in the interference. In another paper247 and mole fraction, it is simple to derive mainstay of the LIBS technique, namely discussing matrix effects with special the following relation:

368 Volume 66, Number 4, 2012 wXRAX atoms is present in the observation present in the sample, or to the spectral vX ¼ ð11Þ 1 þ wXðRAX-1Þ volume. This, however, is equivalent to continuum. saying that, all other conditions being Before sharing our considerations on In the above equation, RAX is the ratio of constant, the total mass vaporized in the normalization approaches in LIBS, we the corresponding atomic masses of MA plasma from matrix B must be different would like to refer to two classic papers and MX [i.e., RAX = (MA/MX )]. It is from that vaporized from matrix A. In on this topic.222,223 In analytical emis- clear that vX and wX will differ from each this case, the conclusion that no matrix sion spectroscopy, measuring the ratio other unless R = 1. The corresponding AX effects are present as a result of observ- of the intensity of the analytical line to calibration plots will then reflect this ing the same signal level for the same theintensityofalineofasecond difference. In the case of a single matrix, weight fraction in the two matrices A constituent present in the sample dates the calibration plot made using wX gives back to almost a century ago. Barnett et the same information as the plot obtained and B will not be correct. Figure 6 illustrates the above consid- al.222 have identified nine criteria for the using mole fraction: both plots will show selection of line pairs, five related to the the presence of self-absorption. A more erations. The top figure shows the plot of Eq. 11 for a single matrix composed choice of the internal standard element, interesting situation occurs when the and four to the choice of the internal element X is present at the same of elements A and B, at different values standard line. Apart from a few, which concentration (weight fraction) in two of RBA. The vertical dotted line high- involve the addition of the internal different matrices A and B. As shown in lights the effect on vA at a given fixed standard element to the sample prior to several cases in the LIBS literature, the value of wA. The three calibration curves analysis, or are typical of photographic emission signal of X can be significantly in the top inset illustrate the difference methods of recording, all the others are different when the two matrices are resulting from the choice of either vA or relevant to LIBS. These criteria state that analyzed, and the calibration plots ob- wA in the concentration axis. The case the analyte and the internal standard tained using w , even if perfectly linear, considered is that of Cu (MCu = 64 g/ X should have: (i) similar volatilization show significantly different slopes, mol) in Ag (MAg = 108 g/mol) and Au rates; (ii) comparable ionization ener- which are attributed to a matrix effect. (MAu = 196 g/mol) matrices. The left However, if M and M are different, plot in the bottom part of the figure gies; and (iii) similar atomic weights. A B The spectral lines chosen for the ratio one should be cautious in attributing this shows that the same weight fraction of should: (i) have the same excitation outcome to a matrix effect. Indeed, from Cu in Ag and Au results in different energies; (ii) be free from self-absorp- the above considerations, if the total signal intensities and different calibra- tion; and (iii) have similar intensity. A mass ablated entering the plasma is the tion sensitivities. This weight, however, thorough discussion of the relative same for both matrices and if stoichio- corresponds to different values of mole importance of each of the above criteria, metric ablation (no fractionation effects) fractions, whose plot shows a single including the additional behavior of the occurs, i.e., (wX,A = wX,B)solid = (wX,A = calibration sensitivity. Correspondingly, partition function of the two elements, wX,B)plasma, the only case in which one the right plot shows that the same mole can be found in the work by Barnett et can expect the same signal is when MA = fraction of Cu must give the same signal al.,222 who considered a model plasma, MB, since only in this case the mole intensity, but now this will only happen and Myers and Tracy,223 who studied fraction of X in matrix A (vX,A) will be at two different values of weight frac- the noise behavior of twenty elements the same as that in matrix B (vX,B), both tion. compared to that of an ion line of Mn, in the solid and in the plasma. If MA This problem has been alluded to used as internal standard, in ICP emis- differs from MB, the signal originating rarely in the LIBS literature238,253,560 from matrix A must be different from sion spectroscopy. and in Chapter 3 of one book,2 but so far that originating from B and will be larger The topic has received considerable its relevance does not seem to have been or smaller depending upon MB being attention in LIBS work. A recent review specifically and systematically ad- 40 larger or smaller than MA. On the other by Zorov et al. discusses several hand, the same plot repeated using mole dressed. different normalization approaches used fractions rather than weight fractions for Normalization Approaches. As was in atomic spectrometry methods with X will give a single straight line and a stated earlier, matrix effects can be laser sampling, including LIBS. In this single slope. Therefore, under the as- related to the entry of the material into review, the ‘‘reference’’ signals consid- sumed conditions, reporting the exis- the plasma and to the fluctuation of ered are the acoustic wave, the plasma tence of a matrix effect as a result of the plasma parameters. It is therefore under- emission itself, the electrical current, different slopes obtained using weight standable that the three most important and Mie scattering.40 Single-variate and fractions would not be correct. normalization parameters for the inten- multi-variate corrections are also dis- An opposite case can now be hypoth- sity of a given spectral transition are the cussed.40 esized, in which the emission signal mass ablated, the plasma temperature, The selected examples reported obtained when (wX,A = wX,B)solid is and the electron number density. The here involve the use of the acoustic experimentally observed to be the same, most general approach to normalization, signals resulting from the ablation 229,233,242,257 even if MA differs from MB. Using the however, remains that of ratioing the process, the tempera- same logic as above, this is only intensity of the spectral line of the ture,230–232,235,243,253,258,260 the elec- possible if the same number of emitting analyte to that of a suitable element tron number density,231,232,243,258,260 the

APPLIED SPECTROSCOPY 369 focal point review

FIG.6. Illustration of the different outcomes resulting from choosing the concentration units in weight fraction or mole fraction. The top part of the figure shows the relation between the two units, while the inset highlights the different slopes obtained for one element in two different matrices and the single slope resulting from the use of mole fractions in the x-axis. The two simulated plots in the lower part of the figure are meant to highlight the different conclusions reached regarding the presence of matrix effects. See text for discussion.

370 Volume 66, Number 4, 2012 continuum background,228,234,243,254 the for calibration.261 The last two papers many different matrices in order to fully electrical current,244,245 the total plasma deal with ICP-emission250 and ICP-mass assess its usefulness. emission,239 and the surface density of spectrometry225 coupled simultaneously the material.241 Apromisingwayof with LIBS; however, the aim was not ABSOLUTE ANALYSIS AND reducing matrix effects by decoupling merely comparing the two techniques CALIBRATION-FREE LIBS the ablation laser and the LIBS but rather to correlate the ICP signal 255 Standardless versus Absolute Anal- plasma (LA-LIBS), will be dis- with the LIBS signal in view of using ysis. The following considerations have cussed later. such correlation as an internal standard- been taken from Winefordner et al.17 We offer here the following consid- ization factor. In all analytical methods, there is a erations: (i) the use of the acoustic signal As a final remark, we refer to an functional dependence of the concentra- 253 for normalization purposes was also important paper by Aguilera et al. on tion (variable) upon the physical param- advantageously used in resonant laser the characterization of matrix effects by eter (the signal). If the theoretical ionization work (see, e.g., Ref. 371); (ii) studying LIBS emission calibration expression describing this dependence the importance of time correlation be- curves. In addition to T and ne, the is reliable enough to allow a direct tween the signal and the background due volume of the plasma region seen by the calculation of the concentration from a to the simultaneous measurement of detector and another parameter propor- single measurement, in absolute units, of signal and background was repeatedly tional to the total number of atoms the physical parameter, then one can stressed by Mermet et al. in ICP present in that volume were also con- 227,259 speak of an absolute method of analy- emission work. Is it indeed upon sidered. The authors concluded that, if sis.17,262 This is the only acceptable this correlation that the fluctuation 228 only line intensities are used for com- definition of absolute analysis, i.e., the approach of Xu et al., as well as our parison, matrix effects may be hidden, capability of providing quantitative re- recent approach (see below), are based; as the variation of plasma parameters sults without the use of standard refer- (iii) the important issue of stoichiometric compensate each other.253 ence materials. Some methods of ablation has been addressed in several chemical analysis can be made stable 236,248,258 Figure 7 shows two of the normali- papers. In particular, in the zation approaches described above. with time and reproducible to the point case of bronze samples, Pershin et An Approach to Evaluate Matrix that standard reference materials need al.248 have considered the possibility of Effects. The true cause of the matrix to be used only infrequently: in this case, selective evaporation of sample compo- 17 effect is difficult to identify without one speaks of standardless analysis. nents at the heating-melting-evaporation resorting to complicated theoretical In the past, these concepts have been stage. The authors have therefore incor- modeling and accurate experimental repeatedly discussed, in particular with porated the concept of selective evapo- measurements. This work illustrates a regard to the atomic absorption rather ration in a model of plasma formation novel procedure, based on the observed than emission or fluorescence proce- and derived a correction coefficient for dures. The use of a temporally isother- the intensity of the spectral transi- behavior of the signals obtained for selected elements, present in the same mal graphite furnace atomizer ensuring tions.248 Moreover, a criterion of choice complete sample atomization and of analytical lines was formulated;248 concentration (or in a narrow range of concentrations) in different matrices, as known and constant residence times of (iv) the normalization method using the the atoms in the analytical zone, irre- a function of the energy of the excited parameters T and ne was extended by spective of the matrix composition of state of the transitions investigated. This taking into account the degree of the material, have brought atomic ab- 260 approach is similar to that used in ionization of the analyte. sorption close to achieving absolute double-pulse LIBS to interpret the Other papers worthy of mention refer analysis. Indeed, the characteristic mass, specifically to LA-ICP emission or mass enhancements observed (see later). defined as the amount of sample injected spectrometry.237,261,225,250 The first pa- Two matrices are used to construct in the furnace resulting in a time- per237 describes a normalization proce- independent calibration curves for each integrated absorbance of 0.0044 was dure, tested on a homogeneous stainless element investigated. For any given found to be reasonably constant not only steel sample, based upon adding the net element, the plot of the log of the in inter-laboratory comparison but for a signals of all elements in the sample to intensity ratio in the two matrices versus variety of matrices as well.263,264 Atom- produce a normalization factor. Such the excitation energy will not only ic absorption was not alone in this approach could also be tested in LIBS reveal the presence of matrix effects search for absolute analysis procedures, work. The second paper261 discusses but will provide a direct hint for the and atomic fluorescence in a graphite calibration strategies for the analysis of identification of the physical parameter furnace was discussed as well.265 In pure copper metal using external matrix- (temperature, ablated mass) responsible addition, other methods were discussed matched calibration standards and pow- for the effect when the matrix is as candidates for standardless analysis, der pellets doped with either multi- changed. including X-ray bulk- and micro-analy- element solutions or with a powder The above-described approach has sis,266,267 various mass spectrometric mixture containing the analyte: the not yet been published: it has been methods of analysis,269,271 and more micro-homogeneity of the samples was tested in a few cases but needs to be recently laser ionization orthogonal investigated in view of their suitability more systematically evaluated with time-of-flight mass spectrometry

APPLIED SPECTROSCOPY 371 focal point review

FIG.7. Signal normalization approaches in LIBS. Two normalization parameters have been chosen, i.e., the plasma temperature and the ablated mass, which is proportional to the acoustic signal resulting from the ablation process. [The temperature plot has been taken from our own data, while the two figures on the left were reproduced, with permission, from Ref. 229, and the two on the right from Ref. 40, with permission.]

(TOFMS), where the results obtained Whatever the definition of absolute different laboratories and different in- were barely affected by the sample versus standardless analysis may be, the strumentation, have made the analytical matrix.272 Notably, chemometric ap- complexity of the problems associated spectroscopic community look at these proaches were discussed as powerful with the functional dependence between approaches with skepticism. This is well means of analyzing spectroscopic data signal and concentration, and difficulty summarized in the following (anony- without using reference spectra.270 of reproducing the same results in mous) statement: ‘‘For many decades,

372 Volume 66, Number 4, 2012 the concept of absolute analysis has left-hand side of the top equation and Ek many ameliorations and variations reared its head in the field of atomic identify the coordinates of the Boltz- on the theme have appeared, as spectroscopy, only to have the practi- mann plane, which then contains several described in Refs. 39 and 41. calities of the world imposed.’’ On the parallel straight lines whose slope is Modeling papers have been al- other hand, Hulanicky,268 in the IUPAC dictated by T and whose intercepts are ready mentioned; in addition, we Technical Report, also expressed well equal to qs. The normalization to unity mention here approaches that take the opinion that: ‘‘Progress in this field of the concentrations of all species into account selective evapora- can result from a better knowledge and allows F to be calculated: this is the tion,248 include and/or correct for more precise description of the theoret- key idea of the approach.273 Since Cs self-absorption,274,279 provide in- ical fundamental phenomena responsible refers to one species, the total concen- ternal normalization by the plasma for generation of the analytical signal, tration of a given element results from continuum,283 calculate relative and from the development of instrumen- the sum of neutral and ion species. number densities of elements tation with perfectly reproducible and/or The working hypothesis was clearly from the intercept of Saha–Boltz- theoretically described characteristics.’’ pointed out by the authors,273 i.e., LTE, mann plots,286 and use an internal Clearly, ‘‘an intermediate step is fulfilled stoichiometric ablation, no self-absorp- standard at known concentra- by any approach aimed at decreasing the tion, and homogeneous plasma should tion.289,290 We note here that the need of exactly matrix-matched stan- all be valid assumptions.273 In retro- use of an internal standard has dards.’’268 spect, LTE may be warranted by the been discussed earlier; here, the Calibration-Free LIBS. Calibration- judicious selection of time delay and approach seems better suited for free LIBS (CF-LIBS) refers to a proce- integration gate width, while moderate the analysis of liquids. We also dure capable of providing analytical amounts of self-absorption can be ac- note that the correction of self- results without using calibration stan- counted for; however, the requirements absorption reported in Ref. 289 dards. This topic is treated here rather of stoichiometric ablation and plasma contains an inaccurate definition than immediately after the modeling spatial and temporal uniformity are of the self-absorption correction section for two reasons. First, in its beyond control in the conventional LIBS factor, since, in conditions of high original conception, the approach does practice. self-absorption, it predicts zero not model the plasma but uses the The idea put forward by the CF emission intensity rather than the approach has aroused lively interest in spectroscopic relations between emis- 273–292 plateau value given by blackbody sion intensity and concentration of the LIBS community. All general emission. emitting species in the plasma, while reviews on LIBS (see, for example, (iii) From various results obtained in circumventing the necessity of using Refs. 12, 17, 20, 23, 33, and 46) devote different laboratories, with differ- standard samples. If one would place the asectiontoCF-LIBS.Thereader, ent instrumentation and on differ- approach in the modeling arena, CF however, is referred to the specific ent sample types, one may reviews by Tognoni et al.39 and by could be considered as a post-break- 41 conclude that CF-LIBS can be down, quantitative analytical modeling Gornushkin and Panne, where the considered as quantitative, semi- approach. The second reason, as clearly current status of the CF methodology quantitative, or qualitative, de- is thoroughly and critically assessed. explained by Tognoni et al.,39 is that the pending on the relative percentage The last review41 does emphasize the CF approach analyzes the matrix togeth- errors calculated for the major, modeling aspects while the former39 er with the analyte, rather than consid- minor, and trace element compo- focuses on analytical considerations ering it as an external interference. nents (see, e.g., Ref. 287). The and figures of merit. A useful table The left part of Fig. 8 reports the fact that the approach can only collecting all the applications (updated essence of the idea and follows strictly provide a qualitative estimation of at the date of the review) of CF methods the first paper of Ciucci et al.273 In the trace components is a conse- to quantitative analysis of solid samples ¯ quence of the closure equation expressions given, Iki is the integrated was provided in Ref. 39. intensity (number of transitions per unit (see Fig. 8), since small uncer- -3 -1 In line with what was done for other volume per unit time, cm s ) be- topics, we offer here several summariz- tainties affecting a major compo- tween excited level k and lower level i, ing considerations: nent translate into large gk is the statistical weight of level k, Aki uncertainties affecting minor com- is the Einstein coefficient of spontane- (i) viiAssuming that the pertinent fun- ponents.39,281 -1 ous emission (s ), kB is the Boltzmann damental spectroscopic data are (iv) iiA study of the expected accuracy constant, T is the plasma temperature, Ek available, CF-LIBS can in princi- and precision in the application of is the energy of level k, (note that the ple be applied to all elements and CF-LIBS was carried out: this can product kBT and Ek have the same units) all kind of samples. It is also be considered a first step torward Us (T) is the partition function and Cs is essential in single-shot analysis the identification of the most the concentration of species s. F is the (with due care to the type of serious source of error of the experimental unknown factor that in- statistics involved) and stand-off method.281 The conclusions re- cludes instrumental and plasma geomet- applications. garding the importance of obtain- ric parameters. For each species s, the (ii) viSince the first seminal paper,273 ing reproducible intensities of

APPLIED SPECTROSCOPY 373 focal point review

FIG.8. The left part of the figure depicts a flowchart showing the essential steps of the calibration-free approach first proposed by Ciucci et al.273 (see text for discussion). The right part of the figure shows a comparison between the results obtained for several standard Al alloy samples with CF-LIBS and Monte Carlo-LIBS (measurements obtained at ~0.1 mbar). The results are shown for three spatially resolved positions (A, D, and G) taken at different points in the axial direction of the plasma. [Reproduced from Ref. 288, with permission.]

spectral lines and accurate detector and sample compositions need still were observed at very early delay spectral efficiency are not surpris- further scrutiny.281 times (150–200 ns) and should not ing: more important is the finding (v) iiiOnly singly ionized species are affect the accuracy of CF-LIBS. that poor knowledge of partition considered in the closure equation. (vi) iiThe technique has been applied to functions and electron number Doubly ionized species have been double-pulse LIBS,280 although a density seem to play less impor- predicted and indeed observed for systematic study is still lacking tant roles. Moreover, the most boron (B) and Fe in LIBS work by here. relevant aspects of plasma ideality Gaft et al.292 However, these ions (vii) iDespite its limitations, it is a fact

374 Volume 66, Number 4, 2012 that the results provided by the section (an absolute quantity). es have become the most popular ways application of CF-LIBS for ele- The theory developed in the paper of accomplishing this task. Other ap- ments at high concentration are allows one to calculate such cross proaches are magnetic field enhance- rather good in terms of distance (a sections.294 Apart from the funda- ment, resonant laser ablation, and measure of the overall accuracy, mental interest of the approach, resonantly enhanced laser ablation. see Ref. 39). Note that most resting on the validity of the These approaches will now be discussed results are obtained with standard, assumptions made, its attractive in order. conventional LIBS setups, i.e., analytical outcome is that it is Double-Pulse LIBS. Much of the without striving for space resolu- amenable to calibration-free anal- interest in double-pulse work has prob- tion or using integration gate ysis, since the analyte concentra- ably originated from the necessity of widths short enough to ensure tion can be derived from the increasing the sensitivity of the analysis LTE conditions. Two conclusions measured peak intensity of the of liquids (see later). In general, howev- may then be drawn here: the first selected transition.294 er, the rationale behind using double- is that the concern stressed in pulse LIBS is the fact that with one In conclusion, in view of the continuous many diagnostics studies against single step, the processes of ablation and efforts and developments in more re- the abuse of assumptions neglect- excitation of the resulting plume cannot fined modeling of the overall LIBS ing plasma inhomogeneity, ab- be controlled independently. This was process, including laser ablation and sence of thermodynamic equi- already pointed out in the early 1970s particle–plasma effects, it is expected librium, and the necessity of (see, for example, Ref. 345), and it is the that CF-LIBS will become more and performing time- and space-re- reason behind the use of ‘‘tandem’’ more mature. In the mind of the original solved measurements is over-esti- approaches, where laser ablation is proposers, this approach is standardless, mated. The second conclusion is combined with inductively coupled plas- a definition that does not conform to that that the good analytical results are ma emission/mass spectrometry, or with given above. Neglecting semantics, we just due to a fortuitous coinci- other emission sources, including LIBS note that even an infrequent use of dence, explainable by the simulta- (see later), or directly with mass spec- standards would be a welcome outcome neous occurrence of several errors trometry (laser ionization-MS).331 An of the various approaches described, acting in opposition and therefore additional consideration stems from the since it would imply that the reproduci- mutually compensating, thus can- observation that, since a large proportion bilty of LIBS data has reached the point celing their global effect. of atoms in the neutral state is present where matrix effects have been con- (viii) Two recent approaches are worth even 40 ls after the plasma formation, a tained to an acceptable level. In any discussing in this section. The first second laser fired at this time could re- case, the importance of the approach and is a paper by Ribie`re and Che´r- heat the plasma, thus causing further its relevant, and in some cases essential, on293 where an absorption tech- atom excitation and emission, as pro- role in many applications are a guarantee nique called LIPAS (laser-induced posed by Uebbings et al.295 The addi- that the method will continue to be used plasma absorption spectroscopy) tional attractive analytical feature of the and improved. was developed. Two point-like approach would be the possibility of laser-induced plasmas were gener- LIBS SIGNAL ENHANCEMENT matrix-independent analysis by selecting ated on solid targets very close to time delays at which the intensity ratio of each other: one laser provides a In consideration of the often repeated neutral lines with comparable excitation continuum source to backlight the statement that LIBS sensitivity is per- energies are independent of time.295 other absorbing plasma. Absolute haps lacking compared to other spectro- For these reasons, the number of atomic ground state number den- scopic techniques, it is not surprising research papers and applications papers sities of several elements were that a substantial amount of effort was on double-pulse LIBS on solid samples obtained by solving the radiative devoted to increase its sensitivity and has been steadily increasing in the past transfer equation (RTE) and fitting detection power (note that the detection decade. The interests of the research the results of the numerical calcu- power is the reciprocal of the limit of community have been equally devoted lation to the observed absorption detection, see, e.g., Ref. 192: this way to the analytical applications (since profiles. Note that, contrary to the we avoid the potential confusion result- enhancement factors of up to 200 times case of atomic emission, neither ing from the statement that a decrease in have been observed) and to the diag- LTE nor calibration of the system the value of LOD is actually an increase nostic and fundamental aspects of the is required.293 The second paper in the method’s performance). One approach, in this last case with the aim by De Giacomo294 discusses a direction is that of keeping the noise of better understanding the reasons for theoretical approach based on the constant while increasing the analyte the different enhancements and the idea of connecting the emission signal, as a larger signal-to-noise ratio mechanisms involved. Such an explo- intensity of optically allowed tran- will result in lower detection limits. sion of works was prompted on one sitions (which strongly depend on There are several ways of improving hand by the significant enhancements experimental conditions) to the the signal level in LIBS, noting that reported, and on the other hand by the impact electron excitation cross double-pulse and multi-pulse approach- very complicated, and therefore chal-

APPLIED SPECTROSCOPY 375 focal point review

dismissed with a few statements: the reader is therefore referred to the individual papers and reviews cited. Some of the conclusions can be sum- marized as follows: (i) the first laser pulse creates a plasma and the second pulse propagates at high velocity in the rarified medium created by the first pulse. In fact, the intensity of the lines due to air elements such as oxygen and nitrogen are weak in the double-pulse spectra obtained at these delay times. This situation therefore resembles that of laser ablation in vacuum, where hydro- dynamic expansion of the plasma is considerably high; (ii) the plume size is wider in the double-pulse case when compared to that observed with a single pulse. The region of the plume charac- terized by a high temperature is also FIG.9. Schematic illustration of the arrangements used in collinear and orthogonal wider in the dual pulse case; (iii) the double-pulse LIBS. In the orthogonal case, the pulse at k2 can precede or follow the ablation pulse at k1. Note that two wavelengths are shown here for the two lasers. enhancement in the intensity of the spectral lines is associated with a larger mass removal from the ablation pulse, lenging, mechanisms underlying such (iii) two lasers, incident on the sample at attributed to the lower shielding effect of enhancements. As a consequence, the a given angle, operated at a single the laser beam from the buffer gas, as interest in this field has produced a very wavelength or at different wavelengths. shown by the increase in the crater large number of publications and the A bird’s eye view of the literature volume ablated on the sample surfaces. presentation of a large amount of allows identifying the following param- This enhancing effect allows improving spectroscopic information.296–342 Fortu- eters which have been studied, experi- the detection limits and consequently nately, several very informative reviews mental arrangements tested, techniques extending the analysis also to elements and key papers have been written by used, and data provided: (i) beam present as traces in the sample; (iv) a leading groups in this field, in addition configuration (collinear or orthogonal); higher proportion of ionic line emission to a few modeling papers (see, for (ii) laser energy (fluence) of both pulses; is observed in some double-pulse con- example, Babushok et al.,22 Noll et (iii) inter-pulse separation, measurement figurations, the behavior of the intensity al.,307 Scaffidi et al.,325 Mao et al.,317 delay and gate width; (iv) dynamics of of ionic lines changes as a function of Colao et al.,301 Corsi et al.,306 Pinon˜ and plasma evolution, shadowgraphy, and inter-pulse separation and observation Anglos,335 De Giacomo et al.,26,333 morphology of craters formed; (v) delay times, while the enhancement is Bogaerts et al.,165 and Rai et al.167). characteristics of the spectrum obtained, different for ions and neutral atom Figure 9 illustrates the most typical e.g., ionic versus neutral lines, spectral transitions; (v) re-heating seems to be arrangements investigated so far. The line widths, persistence of plasma emis- effective at short inter-pulse separation collinear approach is less versatile than sion; (vi) temperature and electron used and transitions with much higher the orthogonal, because with the latter number density; (vii) signal enhance- excitation energy are observed with two- one can either pre-spark above the ment (dual pulse versus single pulse), pulse operation; and (vi) there is a sample and then ablate, or ablate first detection limits, and signal-to-noise correlation between the excitation ener- and then re-heat the plasma. On the ratios. gy of the transition investigated and the other hand, the former approach is more Referring more specifically to our enhancement observed in double-pulse amenable to practical applications, in literature selection, we note that exper- operation. particular to standoff detection. iments have been performed with dif- The reader is cautioned against gen- Basically, the experiments involve the ferent pulse durations (e.g., fs/fs eralization of the above conclusions, following operational characteristics: (i) pulses,335 fs/ps pulses,311 fs/ns puls- since the relevance of each will depend one laser operated at a single wave- es304,308,327,329), and different wave- on the experimental parameters of each length, two or more pulses, normal or length combinations (e.g., 532/1064 experiment. A useful critical assessment oblique incidence on the sample; (ii) two nm,310,312 266/1064,303 mixed-wave- of the above conclusions can be found in lasers, parallel or orthogonal with each length 266/1064 nm,303 266/10 600 the paper by De Giacomo et al.333 in the other, operated at a single wavelength or nm328,336). case of metallic targets. at different wavelengths, and with the The resulting overall picture is com- We conclude this section by sharing same or largely different pulse widths; plex and cannot be elucidated and several observations:

376 Volume 66, Number 4, 2012 creating changes in the materi- slightly more elaborated approach (i) viiAs stressed by De Giacomo et al.314 is explained in Table VI, where all 333 al., the double plasma expands (v) iiiDouble-pulse LA-ICP-MS has the terms and equations are illus- in an environment that is hotter been attempted by Gonza´lez et trated. The essence of the ap- than that of a single plasma and al.,318 who found a five-fold proach is to create a sort of therefore keeps its energy for a improvement in the precision. Boltzmann plot of line enhance- longer time. The plasma spatial Apparently, to our knowledge, ment, called ‘‘enhancement plot,’’ region observed and the observa- this paper was not followed by by using the log of the enhance- tion delay play a major role in other publications. ment in the vertical axis and the defining the enhancement ob- (vi) iiStandoff experiments (up to 55 m log of the excitation energy in the tained, since the single-pulse plas- ranges) were described by Kill- horizontal axis. LTE and optically 328 ma is located closer to the target: inger et al. using a CO2 laser thin conditions are assumed. In as a result the enhancements combined with a UV laser at 266 this plot, the slope of the curve is a obtained at the same location and nm. Targets included metals, ce- measure of the change in plasma delay time are inconclusive. Apart ramics, and plastics. In another temperature and the intercept is a from this conclusion, based on experiment, Weidman et al.339 measure of the change in ablated scientific grounds, from an analyt- used a combination of 1064 nm mass when double pulse and ical point of view, it seems only with 10.6 lmtodetectTNT single pulse are compared. Both fair to compare the best double- residues. The authors concluded neutral (see Eq. T6.4) and ionic pulse signal with the best single- that the small improvement in (see Eq. T6.7) transitions are used: pulse signal, which are obtained in signal-to-noise ratio was exceeded in the last case, the intercept different experimental conditions. by the increased complexity of the contains information about the (ii) viWhen a mixture of gas-phase and approach.339 electron number density and tem- solid-phase analytes are present in (vii) iThe fact that double-pulse LIBS perature. the plasma, as in the case of an emission lasts longer than single- Multi-Pulse LIBS. This section is a aerosol, the different interaction of pulse emission has been described corollary of the previous one, since the the expanding shock wave with by the terms decay time or lifetime considerations are indeed similar to gaseous and particulate analytes of the plasma (see, for example, those already described. Several publi- could be exploited in differentiat- Refs. 22, 148, 302, 327, and 335). cations are listed and should be consult- ing between the two phases of the Indeed, the emission temporal 343–354 323,338 ed for more details. same element species. waveform (observed with a photo- All papers give credit to the early (iii) vThe enhancement factor observed multiplier) shows a maximum work of Piepmeier and Malmstadt343 experimentally could be directly followed by a near-exponential and Scott and Strasheim.344 We also add translated to improved detection decay of the emission signal. The a chapter written by Laqua.345 As stated limits, the improvement being the choice of this terminology, i.e., by Sattman et al.,347 and stated earlier enhancement factor. This has been decay time and lifetime, may be here, the use of a single pulse (single reported by Scaffidi et al. (see confusing, especially when it is step) does not allow independent control Table 1 in Ref. 325). As the extended to the emission of spe- of the ablation and excitation processes. authors point out, this can be done cific atomic/ionic levels, for ex- The double-pulse approaches in the re- in the assumption that LIBS is a ample, when modeling chemical heating scheme configuration described detector-noise limited technique. kinetics of plasma evolution for in the previous section, in addition to This may well not be always the Pb.148,155 Since lifetime and decay using two lasers, is not always suited for case, and therefore this extrapola- times are intensive spectroscopic practical applications. The solution to tion must be taken cautiously. parameters, related to Einstein independent optimization of the ablation (iv) iiWharton et al.314 described an coefficients of spontaneous emis- and plasma excitation, obtained with a interesting sub-picosecond experi- sion and collisional relaxation single flash lamp pumped Q-switched ment to study the effect of the pre- rates, and are independent of the Nd : YAG laser, is to modify the laser pulse, i.e., light arriving at the concentration of the emitters, these operation in order to obtain a sequence solid target prior to the high terms may be avoided. As an of laser emission bursts at a given (low) intensity laser pulse. In this work, alternative, one may refer to a repetition rate, each burst consisting of the effect of the universal, long- term like ‘‘persistence time’’ to several (up to 6) Q-switched short laser duration pre-pulse, i.e., that caused describe the phenomenon. pulses emitted during a single flash lamp by ASE (amplified spontaneous (viii) Gautier et al.313,319 found a corre- pulse.347,348 The energy as well as the emission) was studied by creating lation between the enhancement inter-pulse separations of the short an artificial ASE pre-pulse from a observed in both collinear and pulses can be actively controlled. As high-power Ti-sapphire laser. The orthogonal configurations and the an example,348 one can have a sequence authors found a significant de- excitation energy of the transitions of repetitive laser bursts separated by a crease of the threshold fluence investigated. An extended and period of 100 ms (10 Hz repetition rate),

APPLIED SPECTROSCOPY 377 focal point review

TABLE VI. Theoretical expressions for the dependence of the signal enhancement in double-pulse LIBS on the physical parameters of the plasma and fundamental atomic parameters.a

Eq. Expression Description number

-Eu=kBT -Eu=kB T A;n plasma gue plasma plasma A A;n gue I ¼ n V WAu‘ A;n ¼ n V Wv a Au‘ A;n This expression is a simple modification of that presented in our first T6.1 U ðTÞ U ðTÞ review (see Table IV), with the introduction of some new symbols to emphasize its application to neutrals and ions, and the mole fraction of À Á the analyte. plasma plasma A;n E 1 1 I n V vA a UA;n T - u - DP DP DP DP DP ð SPÞ kB T T Rn ¼ ¼ e DP SP The ratio of the intensities measured for neutral species in a single pulse T6.2 I plasma plasma A A;n UA;nðT Þ SP nSP VSP vSP aSP DP and in a double pulse experiment depends upon the total mass ablated, the excitation energy of the line chosen, and the plasma temperature À Á obtained in the two operational schemes. plasma plasma A;i Ej 1 1 I n V vA a UA;i T - - DP DP DP DP DP ð SPÞ kB T T Ri ¼ ¼ e DP SP The corresponding expression obtained for ionic transitions is identical to T6.3 I plasma plasma A A;i UA;iðT Þ SP nSP VSP vSP aSP DP that for the neutrals, except that the proper superscript is now used and the excitation energy refers to the ion excited state. !"# plasma plasma A A;n A;n nDP VDP vDP aDP U ðTSPÞ Eu 1 1 lnðRnÞ¼ln þ ln - - The usual transformation in a logarithmic plot shows the dependence of T6.4 plasma plasma A A;n UA;nðT Þ k T T nSP VSP vsP aSP DP B DP SP the slope on the difference between SP- and DP-plasma temperatures, while the intercept indicates the difference in the ablated mass. !"# plasma plasma A A;i A;i nDP VDP vDP aDP U ðTSPÞ Ej 1 1 lnðRiÞ¼ln þ ln - - A similar result is obtained when the logarithmic plot is made using T6.5 plasma plasma A A;i UA;iðT Þ k T T nSP VSP vsP aSP DP B DP SP ionic transitions.

kA ðTÞ aA;i ¼ aA;n Saha These two relations show the definition of the ionization fraction as a T6.6 ne 3=2 A;i function of the electron number density and the Saha constant (see ð2pmekBTÞ 2U ðTÞ A 1 kA ðTÞ¼ e-Eion=kBT also Part 1, Table IV ). Saha h3 UA;nðTÞ A;n 1 aDP;SP ¼ A e These two expressions represent the neutral fraction of the analyte atoms T6.7 1 þ kSaha=n in the plasma (top) and the singly ionized fraction of the analyte atoms in the plasma. A;i 1 aDP;SP ¼ e A T6.8 1 þ n =kSaha !"# plasma plasma A A;n A;n nDP VDP vDP aDP U ðTSPÞ lnðRiÞ¼ln þ ln Using Eq. T6.6 results in this final expression of the log of the T6.9 plasma plasma A A;n UA;nðT Þ nSP VSP vsP aSP DP enhancement for ionic transitions. The plot is made using in the x-axis   A e the sum of the ionization energy of the atom and excitation energy of Ej þ Eion 1 1 3 TDP nSP - - þ ln þ ln e the ion. The intercept in this case is seen to be a function of the kB TDP TSP 2 TSP nDP temperature and electron number density, in addition to the ablated mass. a The equations reported in this table are the result of a collaborative effort between the University of Florida (N. Omenetto) and the CNR Pisa, Italy (E. Tognoni, G. Cristoforetti, and V. Palleschi). The experimental data obtained using the above equations have not yet been submitted for publication. -1 Definition of the parameters used in Eqs. T6.1–T6.7: I DP,SP = Spectrally integrated (over the line profile) net intensity (counts s ); gu = Statistical weight of the A -3 A;s upper level; nDP;SP = Number density of atoms of the analyte in the plasma (cm ); nDP;SP = Number density of neutral (s = n), singly ionized atoms (s = i) of the -3 plasma -3 A A plasma analyte in the plasma (cm ); nDP;SP = Total number density of atoms and ions in the plasma (cm ); = vDP;SP = (n /n )DP,SP = Molar fraction of the analyte in e -3 -1 the plasma; TDP,SP = Plasma excitation temperature (K); nDP;SP = Electron number density (cm ); kB = Boltzmann constant (eV K ); h = Planck constant (erg s); A A,s plasma me = Electron mass (g); Eion = ionization energy of the analyte (eV); U (T) = Partition function of the analyte in ionization stage s; VDP;SP = Plasma volume 3 (cm ); WDP,SP = Fraction of plasma volume seen by the detector; Eu (eV) = Excitation energy of upper level of the neutral line; Ej (eV) = Excitation energy of upper level of the ionic line; Ei (eV) = Excitation energy of lower level of the ionic line; Rn = (IDP/ISP)neutral = Intensity ratio for neutral lines; Ri = (IDP/ISP)ionic = Intensity ratio for ionic lines. each burst consisting of six individual performance by taking advantage of rial ablation are again discussed as pulses of 50–80 ns duration, separated multi-pulse operation, while still main- possible causes of signal enhancement from each other by 15 ls.348 Galba´cs et taining the advantage of being a com- in multi-pulse LIBS.353 Applications al.350 developed a micro-LIBS system pact and portable system.350 have been reported in the analysis of based on a Nd-GGG (gadolinium galli- We note here that one could adopt the steel samples using a vacuum UV um garnet) laser and produced from 1 to nomenclature used to describe the char- Paschen–Runge spectrometer, with de- 11 laser pulses of 8 to 10 ns duration in a acteristic output of a free-electron laser, tection limits below 10 ppm for the burst (called shot by the authors) of which consists of macropulses of 5–10 elements C, P, S, Al, Cr, Cu, manganese 100–200 ls: the inter-pulse separation ls duration, formed by a train of micro- (Mn), and molybdenum (Mo).347,349 was 25–50 ls. As the authors point out, pulses of a few picoseconds.346 Other applications include the quantita- their system improves the analytical Plume re-heating and increased mate- tive analysis of gold alloys,352,354 me-

378 Volume 66, Number 4, 2012 tallic targets (Al, Cu, Si, Zn) in air,350 Kexue et al.368 have demonstrated with ‘‘microwave-enhanced spark-in- commercially available soldering tin laser-induced plasma re-heating by cou- duced breakdown spectroscopy (SIBS)’’ sample, pyrolithic graphite, and polytet- pling a fast electrical spark discharge and concluded that, for both systems, the rafluoroethylene samples.354 Six-pulse taking place between two pure cerium most typical features are a prolonged limits of detection and limits of quanti- and tungsten alloy electrodes, whose tips persistence of emission, the ability of tation were found to improve, compared were positioned ~ 2 mm above the observing molecular spectra, and the to those obtained with double pulses, by sample surface. When the laser-induced suitability for volume measurements and a factor of 4.2 (Sn) to 16.7 (Cr).354 plasma forms, the additional electrical gas analysis.366 Other Signal Enhancement Ap- discharge deposits more energy and re- The final approach again takes ad- proaches. This section will review some heats the plume. An enhancement factor vantage of plasma confinement, this different approaches that have been of 38.5 was found for neutral Pb (283.31 time realized by the application of a described in the literature with the nm). The enhancement for As could not magnetic field. The approach is de- specific goal of enhancing the LIBS be quantified, since its emission at scribed by Rai and Thakur (Chapter 4 signal. Some approaches take advantage 286.04 nm could not be observed in in Ref. 4, page 106), as well as in several of plasma confinement effects, either absence of electrical spark.368 papers.357,359,363,367 The key parameter using wells and pipes or magnetic fields Perhaps the simplest approach to here, indicated by the symbol b, is the to spatially confine the plasma. Other LIBS signal enhancement takes advan- ratio between particles pressure and approaches, which could belong to the tage of the beneficial effect exercised by magnetic field pressure, or equivalently, category of hyphenated LIBS (see later), argon buffer gas (see also our earlier the ratio between the plasma kinetic are included here since the laser-induced discussion on the effect of argon envi- energy and the magnetic energy.357,359 It plasma and the additional plasma, e.g., a ronment on plasma characteristics). Son is shown that the plasma expansion microwave-induced plasma, an electrical et al.365 synchronized a pulsed gas jet depends on b: plasma confinement only discharge, or a glow discharge are with the laser pulses and obtained occurs if b is low, meaning that the arranged to form an almost spatially emission enhancements up to a factor plasma decelerates. When b = 1, the co-existing source. We note, however, of 10 for an Al sample. The authors plasma would be stopped by the mag- that this distinction is not important. evaluated the signal-to-noise ratios for netic field.359 Targets investigated in- 359 363 The final enhancement arrangements two strong Al neutral (396.1 nm) and clude Cu, Al, Cu, and cobalt (Co), 357 reported are the use of a pulse argon jet ionic (281.6 nm) lines and found that the Al-alloys, and aqueous solutions. In injected on the sample synchronously noise practically remained the same as this last case, enhancement factors were with the laser pulse and the application that without the jet, thus making the rather modest (~2 for solids and 1.5 for 363 of metallic colloidal particles on the improvement in signal-to-noise ratio liquids). In other cases, enhancement surface of a leaf sample: the observed equal to the enhancement factor. The factors up to 6 to 8 were reported for Cu. 367 enhancement in this case was attributed effect of laser energy, and the evaluation Joshi et al. carried out a diagnostic to localized surface plasmon reso- of parameters such as plasma tempera- study on a solid Li target using charac- nance.364 We now briefly summarize ture, electron number density, and teristic neutral and ionic lines. By each of the above approaches. ablated mass, together with the existence observing the behavior of the enhance- The space confinement effect pro- of LTE were also investigated.365 ment as a function of field strength, the duced by craters on plasmas formed at The enhancement of sensitivity re- authors were able to assess the relevance the bottom of the craters was shown to sulting from the coupling of the laser of mechanisms such as electron impact 358 excitation, field-induced ionization, and enhance the signal from the plasma. plasma with microwaves has also re- 367 Shen et al.360 exploited this finding by ceived attention, and the approach has recombination. constraining the plasma in a pipe of 20 been presented several times at LIBS mm depth with a curved end that was conferences. We refer here to a work RESONANT LASER secured tightly on the surface of a presented by Ikeda and Kuneko in ABLATION–RESONANTLY cylindrical Al alloy target. Observed 2008.362 The characteristics of laser ENHANCED LIBS enhancement factors up to 9 were ablation–microwave induced plasma Originally, the term resonant laser attributed to reflection and compression (MIP) emission, where the laser-induced ablation (RLA) was used to describe the of the shock wave by the pipe wall.360 plasma plume in a low-pressure Ar approach in which low-power tunable Tereszchuk et al.361 implemented atmosphere expands directly into the lasers were used to enhance the ioniza- laser ablation directly into a pulsed glow MIP, was discussed in the early 1990s tion yield for analysis by ion trap mass discharge by designing a discharge by Niemax et al.355,356 The appealing spectrometry.370–376 As described by chamber in which the cathode was the feature reported is the potential of the Gill et al.,375 the approach relies on target for laser ablation, with the laser method to perform matrix-independent irradiation of a sample in a mass beam entering through a ring-shaped multi-element analysis. In the recent spectrometer with modest intensity laser anode. A clear intensity enhancement LIBS-related work,362 the system is pulses tuned to one or two-photon was observed when laser ablation was believed to be compact and economical. resonant transitions in the analyte of assisted by the glow discharge excita- Ikeda et al.366 have subsequently com- interest. The key role played by the tion.361 bined ‘‘microwave-enhanced LIBS’’ single laser pulse here is that the leading

APPLIED SPECTROSCOPY 379 focal point review edge of the pulse ablates the material for the overall process to take place. As transition of the plasma components while the trailing edge of the pulse a result, unlike many experiments in- (matrix, minor or trace elements). excites and ionizes the resonantly inter- volving single-pulse LIBS with nano- This would be the case of double- acting atoms.375 Enhanced atomic emis- second lasers, absorption of the trailing pulse LIBS, as in case (i) above, the sion will also result from the resonant edge of the ablation laser by inverse difference being that the excitation ablation process, and the effect has been bremsstrahlung has to be avoided. This laser normally follows in time the exploited in hyphenated approaches explains why the fluence of the ablation ablation laser. Note that, in one such as laser ablation-microwave plasma laser needs to be kept low to avoid the particular case, using an ArF at 193 emission and ICP-MS,381 or as a variant above effect and also explains why the nm as the excitation laser, this of LIBS in which enhanced plasma interaction is minimally destructive. configuration was called LIBS-LIF emission is observed after resonant Another reason why the fluence of the by Cheung,382,383,385 who postulated ablation with a tunable laser.390–392 This laser needs to be low is that the effect is a direct population of a continuum of process can indeed be called resonance purely resonant only at low fluence atomic states by the 193 nm photons fluorescence. In addition, two lasers can since non-resonance effects start to be and subsequent collisional relaxation be used in an arrangement similar to important at high fluence. Finally, it is to discrete excited levels from which the collinear or orthogonal dual pulse worth noting that RLA should in radiation occurs. experiments (see previous section): in principle be observed when tuning the (iv) Same as in case (iii), except that this case, one laser is tunable and the laser to a resonant transition of any here the excitation laser is tuned to term resonantly enhanced LIBS is element in the sample, including the a strong resonance transition of the used.25,379,382–386 matrix. matrix element. Since this resonant It is interesting to note that the Dual Laser Resonantly Enhanced interaction leads to an enhancement resonant approach has been extended LIBS. Here two lasers are used, an of the emission intensity of several to molecular transitions, as described in ablation laser and an excitation laser. elements (other than the matrix) in two recent papers dealing with poly- The fact that two independent lasers are the plasma, this case would be mers.387,388 Here, a significant enhance- used makes it necessary to clarify from called resonance-enhanced LIBS ment was observed when the laser was the beginning whether the excitation (RE-LIBS). Note that the resonant resonant with a C–H stretch fundamen- laser interacts with the sample surface. character of the process here is due tal band.387,388 In fact, such laser may or may not be to the excitation laser, not to the The reader is referred to the cited directed toward the target. ablation laser. literature for a detailed treatment of the In order to elaborate on this point, we can then distinguish several cases (in all In the literature, the experiments per- experimental details and possible mech- formed under the configuration de- anism governing the process. Since the the cases listed the ablation laser is always present but not tuned into scribed in (iv) have been called RE- field seems to be very active nowadays, 25 resonance with any transition of the LIBS. It is important to stress that, it was considered useful to present here since the configuration allows the exci- some simple summarizing consider- plasma components and the excitation laser is fired at a given delay after the tation laser to reach the target (unless it ations on both approaches. At the same ablation laser): is totally absorbed by the plume created time, we would like to stress that the by the ablation pulse), the trailing edge above considerations are not meant to (i) iiThe excitation laser interrogates the of this pulse should not be absorbed by refute, or assent to, the interpretation of plume without directly touching the inverse bremsstrahlung, a requirement the results described in the literature, but surface and is not tuned to any that is analogous to that of single laser rather represent a series of questions and resonance transition of the plasma RLA. Moreover, the best enhancement possible logical outcomes, whose indi- components (matrix, minor, or trace is observed when the fluence of the vidual relevance in the description of elements). This would be the typi- ablation laser is kept low. Note that the overall process can only be assessed by cal case of orthogonal double-pulse enhancement here refers to the compar- devising more experiments, specifically LIBS in the pre-spark or re-heating ison with single-pulse LIBS: as in the targeted to answer that question. configuration (see specific section cases of all double-pulse approaches, the Single Laser Resonant Ablation. In on double-pulse approaches). enhancement needs to be clearly defined single laser resonant ablation, the laser is (ii) iSame as case (i), but with the in order to allow a fair comparison directly focused close to the sample excitation laser tuned to a strong between the two approaches. surface, as in a conventional LIBS allowed transition of a trace ele- As a typical example, consider here experiment. As said above, the process ment. This would be the typical the case of an Al target (Al being can be called resonant laser ablation– configuration of a LIBS-LIF exper- therefore the matrix element) and Mg laser-induced fluorescence (RLA-LIF) iment (see specific section on and Si as two elements present at levels since the leading edge of the ablation hyphenated approaches). of a few percent.391 The ablation laser is pulse ablates the material while the (iii) The excitation laser is directed into a Nd : YAG laser and the excitation laser trailing edge of the pulse excites the the sample surface, either collinearly is a tunable optical parametric oscillator fluorescence. It is then obvious here that or at an angle with the ablation laser, (OPO) system. This second laser is the trailing edge of the pulse is needed and is not tuned to any resonance tuned to two strong Al transitions, i.e.,

380 Volume 66, Number 4, 2012 394.401 nm and 396.152 nm. As a collisions would create ~3 eV electrons, tion of each source in the tandem result, the Mg line at 285.213 nm and which can subsequently excite (and even combination.394 Following this sugges- the Si line at 288.158 nm are both ionize) heavy particles by further colli- tion, approaches such as LIBS-MIP, enhanced at an optimal inter-pulse sions. This mechanism was called LI- LIBS-GD, and LIBS-SIBS can be con- delay.391 BORS (laser ionization based on sidered tandem approaches. The same It is not simple to propose a unique, resonance saturation) by Measures.369 A will hold for the last approach described unequivocal explanation of the enhance- direct energy transfer between ground- in this section, i.e., laser ablation (LA)- 1 2 ment effect. Some considerations are state ( S0)MgatomsandexcitedAl(S1/2) LIBS. 2 0 collected below: atoms, resulting in Mg excited ( P1 )and On the other hand, the combined use 2 0 ground-state Al ( P3/2 ) atoms does not of LIBS with Raman and with laser- (i) iiThe experiment resembles a collinear seem a favorable route because: (i) the induced fluorescence (LIF) is different. (or small angle) double-pulse LIBS process is endoergic by 1.2 eV, and In fact, in the analysis of solid samples, where the second pulse finds a more therefore, if it occurs, it would require the three techniques are used indepen- favorable environment, created by an extra energy gap supplied by electron dently (with two different instruments or the first pulse, to reach the target and collisions; and (ii) the spin conservation with the same instrument) and the ablate more material. Therefore, one rule, before and after collision, is not complementary information provided is should observe enhanced emission obeyed, rendering the process less prob- used for analytical purposes. The term intensity of the Al lines, as well as of able.393 ‘‘hyphenated’’ is therefore more appro- lines of other elements even when Another mechanism that has been priate. When LIF measurements are the excitation laser is out of reso- postulated, mainly based upon the performed inside the plasma formed by nance with any transition of any differences in the shape of the ablation laser ablation of a target, the acronym element in the sample. craters observed in resonance and out of LA-LIF should be preferred to LIBS- (ii) iAs in RLA, when the excitation resonance, relies on radiation trapping LIF. laser reaches the target surface and effects,381,390 which extend the persis- It is fair to conclude, however, by is tuned to the Al resonant transi- tence of the plasma emission. saying that this discussion about the tions, the leading edge of the pulse Finally, we would like to end this terminology to be used is not as relevant ablates while its trailing edge ex- section with one practical remark, which as other cases pointed out in this review cites the Al atoms in the plasma was already pointed out in early (e.g., self-absorption and self-reversal), 375 (being resonant with its transitions). work, stressing the importance of since the type and aim of the approach Fluorescence from the Al atoms normalizing the results with a suitable described in each paper is clear. would then result. internal standard. In fact, working at low The three approaches discussed below (iii) The peculiar feature of the experi- energies, close to the ablation threshold, have been developed with different ment is that other lines (Mg, Si) are makes the results very sensitive to goals. Raman spectroscopy will enhance also enhanced in intensity when the variation in the laser fluence. the information power of LIBS, since excitation laser is resonant with Al. As stated at the beginning of this molecular information will be added to This means that some kind of section, the field is active and more the elemental information provided by energy transfer process is occurring. papers are expected in the future. LIBS. Fluorescence will enhance the If the emission enhancement of Mg sensitivity and detection power for is due to direct energy transfer HYPHENATED (TANDEM) selected analytes, in particular in those between the Al and the Mg atoms, APPROACHES cases where the LIBS limits of detection then one should observe an en- are not sufficient to meet certain regula- hancement also when the excitation The hyphenated approaches are gen- erally those in which LIBS is combined tory limits (e.g., in environmental appli- laser, tuned at the Al transitions, cations). Finally, the independent traverses the plasma plume orthog- with another technique, for example, optimization of the laser ablation and onally without interacting with the Raman or laser-induced fluorescence. laser-excitation/ionization steps achieved target surface, as in a typical LIBS- Some of these have already been with LA-LIBS may improve the freedom LIF experiment. If enhancement is mentioned when discussing signal en- from matrix effects. observed also in this case, then one hancement approaches. We note here Enhance Information Power: LIBS- would call this process sensitized that in earlier analytical spectroscopy Raman. Twomainfieldsinwhich fluorescence.393 literature, combining one ‘‘source’’ with another, where the term source has a elemental as well as molecular informa- A plausible explanation of the enhance- specific task associated to it (for exam- tion are needed are remote sensing of ment of the emission intensity of Mg and ple, sampling, atomization, vaporization, minerals and analysis of art objects.395– Si when resonant excitation of Al is excitation, ionization) was referred to as 403 Both fields are covered in detail later accomplished by the excitation laser is a tandem sources approach. As pointed in our review. Here we give a short that more energetic electrons are formed out by Kantor and Hieftje,394 under this overview of the combined use of LIBS as a result of super-elastic (quenching) broad category one can include all and Raman. collisions following resonance saturation sources combined in sequence, with a The instrumental development fol- of the Al lines.369–372,389 Quenching descriptive qualifier indicating the func- lowed a logical trend: first, separate

APPLIED SPECTROSCOPY 381 focal point review

FIG. 10. Complementary use of LIBS and Raman in the identification of pigments in art works. [Reproduced courtesy of Anastasia Giakoumaki and Demetrios Anglos (IESL-FORTH and University of Crete, Heraklion, Crete, Greece).] instruments were used, then hybrid units LIBS was much smaller than that for ficial for both techniques. Raman can were developed consisting of single Raman.398 Giakoumaki et al.400 assem- identify the presence of quartz in a laser and a single detector, and more bled a hybrid unit designed to investi- mineral sample, but this is not sufficient recently new laser types and spectrom- gate samples and objects of cultural to differentiate between different miner- eters have been implemented in the heritage. Only one laser at 532 nm was als. Using LIBS on the same sample combination. used and the spectra were taken with a spot as the Raman spectra reveals Marquardt et al.395 described a fiber- nonintensified CCD. Pigment identifica- significant differences in the elemental optic probe system for LIBS, Raman tion in paintings represents a typical composition of the minerals, making point analysis, and Raman imaging application of the combined techniques their differentiation easy. On the other analysis of particles on soil substrate: (see also Refs. 396 and 397 and Fig. 10). hand, the LIBS spectrum of a copper ore 401 different lasers were used to generate the More recently, Hoehse et al. have reveals the presence of TiO2 from the Ti plasma and the Raman signal. Sharma et described a two-laser LIBS-Raman mi- spectral lines in the spectrum, but does al.398 and Wiens et al.399 described the cro-analysis system with a dual-arm not give information about its polymor- analysis of biogenic and inorganic high-resolution Echelle spectrometer phic forms (anatase, rutile, brookite), minerals in a standoff configura- and a single nonintensified CCD. The whose differentiation is provided by the tion.398,399 One laser system was used, system compares well with state-of-the- Raman spectra.401 the Raman excitation being provided by art Raman scanning microscopes, while Enhance Analytical Sensitivity and a frequency-doubling crystal in front of LIBS provides parts per million (ppm) Detection Power: LIBS-LIF and LA- the LIBS laser and a super-notch filter in detection limits and large spectral cov- LIF. The diagnostic value of the fluo- the path of the detector.399 Both spectra erage (290–945 nm).401 The authors rescence technique has been discussed were taken at the same location (at 10 m illustrate clearly how the combination in detail in our first review and will distance): the sampling diameter for of LIBS and Raman is mutually bene- therefore not be repeated here. The

382 Volume 66, Number 4, 2012 emphasis is on its quantitative aspects the element itself.413 The determination curve is therefore (much) larger and improvement in sensitivity and of iron and lead in aqueous solutions than that of conventional emission. detection limits. The fluorescence ex- was described by Loudyi et al.,417 while (iii) If one wants to achieve high spatial periments reported below have been Laville et al.418 characterized and opti- resolution, pre-filter and post-filter performed with pulsed frequency-dou- mized the laser and instrumental param- effects will modify the shape of the bled tunable dye lasers,406–408,412–414 eters for lead detection in brass samples. calibration curve. In the former frequency-doubled and -tripled Ti-sap- The popularity of lead as analyte is due case, the laser beam may be atten- phire lasers,411,416 and OPO tunable to the large spectral separation between uated before reaching the plasma lasers.418,419 the excitation (283.305 nm) and fluores- volume seen by the detector, and in Kwong and Measures405 in 1979 cence (405.781 nm) wavelengths, which the last case the fluorescence radi- described the hyphenated approach in- allows efficient rejection of spurious ation may be reabsorbed on its way volving laser ablation and laser-induced laser scattering. to the detector. Either way, the fluorescence under the acronym TA- The determination of trace phosphorus optical thickness of the plasma will BLASER (trace analyzer based on laser in steel samples was described indepen- influence the saturation behavior ablation and selectively excited radia- dently in two papers by Shen et al.419 and and the corresponding linear dy- tion). Among the various features of the by Kondo et al.416 In the former namic range of the analytical cali- approach, freedom from matrix effect, experiment,419 the fluorescence wave- bration curves.420 together with isotope analysis capability, lengths were in the low UV (213.547, (iv) Despite the fact that the resonance were the most attractive. Apart from 213.618, and 214.915 nm) after excita- fluorescence, i.e., identical excita- some excessively optimistic predictions, tion at 253.398 and 253.560 nm. Kondo tion and fluorescence wavelengths, 416 the authors405 described all the essential et al. used 255.49 nm as excitation has been used (e.g., for Cr under instrumental and analytical features of wavelength and fluorescence wave- vacuum conditions405 and for Cd in the approach. Other papers followed this lengths at 213.618 and 214.915 nm. soils412), this scheme should be seminal paper, emphasizing slightly From the practical analytical point of avoided whenever possible. Non- different characteristics of the fluores- view, some general observations and resonance fluorescence, i.e., differ- cence technique. Niemax et al.406,407 comments are worth stressing: ent excitation and fluorescence pointed out a very important feature, wavelengths, helps in eliminating (i) iiThe maximum fluorescence signal laser scattering: however, when the namely that the fluorescence ratios of will be obtained when the transition different elements were independent of excitation and fluorescence lines are is optically saturated. Saturation of close to each other, spurious signals the matrix investigated. Pesklak and the fluorescence signal will occur Piepmeier408 published a thorough dis- may still affect the result. The when the rate of stimulated absorp- angular dependence of the polariza- cussion of the use of nonresonance tion and emission dominate over fluorescence in laser micro-plumes tion of the scatter has been exploit- spontaneous and collisional rates. 412 formed in different atmospheres (He, ed to reduce the effect: however, Therefore, the relevant parameter it cannot be entirely eliminated. neon (Ne), and Ar) at reduced pressures here is the laser power (or irradi- on stainless steels and niobium alloy (v) iMolecular fluorescence from radi- ance) rather than energy (or flu- cals present in the plasma (e.g., CH, targets. Filtering effects were also con- ence). The saturation curve should 408 CN, C , OH, NH) can also in sidered. A series of papers describe then report in the horizontal axis the 2 the laser-ablation fluorescence approach principle be detected by fluores- laser spectral irradiance and not the cence. to detect lead in ultrafine (10–300 nm) laser fluence.415 The two parameters aerosols409 and in metallic matrices in a are of course correlated: however, Decrease Matrix Effects: LA-LIBS. low-pressure argon atmosphere,410 and the temporal shape of the pulse As we have detailed in this article, heavy metals in soils with a simultaneous should strictly remain constant matrix effects remain an important Paschen–Runge spectrometer.412 Telle et when the pulse energy is varied. aspect of LIBS as an analytical method. al.411 described a remote LIBS-LIF (ii) iIf the laser beam is made spatially Toward this topic, a fundamental point arrangement working at atmospheric homogeneous, and its size is larger is the concept of LIBS as both the pressure in air rather than in a low- than that of the laser-induced plas- sampling source and the analytical pressure buffer gas environment and ma, no self-absorption effects will plasma source. As noted above, the detected trace levels of Al, Cr, Fe, and Si. influence the calibration curve ability of LIBS for direct, in situ analysis Godwal et al.414 determined lead in a (fluorescence signal versus concen- is widely cited as a positive attribute of water jet sample ablated with a laser at tration) under saturated conditions. the method. However, the coupling of 266 nm, and Koch et al.413 detected This is a consequence of the fact the sampling step (i.e., ablation step) indium fluorescence in the cavitation that the net atomic absorption with the analytical processes (i.e., exci- bubble induced in liquids by the primary coefficient in the saturated volume tation and emission) can also exacerbate ablating laser: the choice of the element is equal to zero and the atomic the presence of matrix effects, as the (In) was dictated more by the conve- system becomes optically transpar- highly nonlinear plasma formation, ma- nience of the excitation/detection ent to the radiation. The linear terial sampling, and plasma growth scheme rather than the importance of dynamic range of the calibration processes depend strongly on the nature

APPLIED SPECTROSCOPY 383 focal point review

emission (PIXE), Rutherford backscat- tering spectrometry (RBS), spark source mass spectrometry (SSMS), ICP-AES and MS, glow discharge MS, laser microprobe analysis (LAMMA), sec- ondary ion MS, and transmission elec- tron microscopy with energy dispersive X-ray analysis (EPXMA).421 The typical sample masses involved in these tech- nique span a range from several grams down to femtograms. Spark emission spectroscopy is the most natural analogue of LIBS as a solid-sampling method: it was therefore felt appropriate to draw the readers’ attention to a paper by Danzer422 discussing sampling procedures to in- vestigate the chemical homogeneity of the magnesium distribution in a cast iron sample containing an average of 0.07% Mg. According to Danzer,422 ‘‘solids are defined to be analytically homogeneous if fluctuations in the chemical composi- FIG. 11. Schematic setup of LA-LIBS. With this approach, the sampling of the analyte tion over the whole sample volume (ablation) and the excitation-ionization processes occur in two different plasmas, which can then be independently optimized, leading to improved freedom from matrix effects. determined in different areas of the sample are not significantly larger than the error of the analytical procedure.’’ of the analyte matrix. Taking this to the second plasma is fully uncoupled from This concept was lucidly discussed by natural limit, one might conclude that the laser ablation event (i.e., analogous Kempenaers et al.423,425,426 using syn- direct LIBS analysis of solids is inher- to the ICP in LA-ICP-OES), no matrix chrotron micro-XRF for characterizing ently prone to matrix effects, thereby effects as related to laser–material cou- the micro-heterogeneity of heavy metals necessitating matrix-matched standards, pling as manifest by differing plasma in low-Z reference materials. As pointed advanced chemometric calibration ap- conditions are introduced. Figure 11 out in their paper, homogeneity is not an proaches, calibration-free schemes, or illustrates the operational scheme. absolute material property, but it de- some combination thereof. This stands As reported by Windom and Hahn,255 pends on the characteristics of the in contrast to the widely used plasma- linear relative calibration curves were analytical method used to characterize based analytical methods such as LA- realized for a rather disparate collection such material.423 Moreover, the total ICP-OES or LA-ICP-MS. With these of reference materials, thereby showing variance of a measurement is due to the schemes, the laser-ablation step used for the promise of this method as an method and to sample heterogeneity materials sampling is fully uncoupled additional step toward quantitative (sampling error) and depends upon the from the analytical plasma in the former LIBS-based analysis. value of the analyzed mass. As shown in and uncoupled from the vaporization Fig. 12, a critical mass exists below plasma and MS analytical scheme in the MICRO-LIBS (LIBS which the variance due to sample latter. Transferring such an approach to MICROPROBE) heterogeneity becomes increasingly im- the LIBS community, the recently portant, while above that value, the proposed LA-LIBS methodology255 Consideration on Homogeneity. material can be considered homoge- seeks to exploit the advantages of LIBS LIBS fits naturally the list of solid- neous and the total variance can be while uncoupling the potential for ma- sampling techniques that consume only identified with that of the method.423 In trix effects. The LA-LIBS scheme limited sample sizes. Zeisler421 has a subsequent work on glass reference separates the laser ablation step from discussed the problems associated with materials, the authors have concluded the analytical plasma step by using two standard reference materials for small- that the standards could be considered separated beam paths, namely, one for sample analysis, homogeneity being the homogeneous for 17 trace element laser-ablation and a second for tradition- most vital prerequisite for such materi- distributions provided that at least 60 al aerosol LIBS analysis of the resulting als. The solid-sampling panorama in- or 350 ng could be employed, corre- ablation plume. Using a carrier gas, the cludes techniques such as neutron sponding to sample volumes in the range ablation plume is transported from the activation analysis (INAA), X-ray fluo- 30 3 30 3 30 lm3 or 50 3 50 3 50 ablation cell to a free-standing aerosol rescence (XRF), and micro-synchrotron lm3.426 LIBS plasma for analysis. Because the induced XRF, particle-induced X-ray Similar considerations were presented

384 Volume 66, Number 4, 2012 by Efstathiou424 in relation to the use of very small volume test-portions of ultra- dilute solutions, thus containing a small number of analyte molecules. More recently, Sturm427 has described the use of an optical micro-lens array producing multiple closely spaced plas- mas on the sample surface, thus allow- ing extended areas to be analyzed without time-consuming sequential scanning of the single laser spot over the sample surface. The improvement in repeatability of spectra obtained from a slag sample was demonstrated (see Fig. 13).427 LIBS Micro-Probe. The advanta- geous features of laser microanalysis have been long known and discussed in the literature, and the reader is referred to the comprehensive review on the subject by Moenke-Blankenburg.428 La- ser microanalysis allows the following: (i) the qualitative and quantitative deter- mination of micro-amounts in micro- FIG. 12. Definition of homogeneity of a given material. The definition is based upon the regions: in other words, the direct contribution of the variance due to sample heterogeneity, sometimes denoted as the detection of local inhomogeneities in sampling error, to the total variance of the measurement result. As shown in the figure, mcrit solids; (ii) the change in composition as is the minimal mass above which a material appears to be homogeneous for a given method and manner of investigation. [Reproduced from Ref. 425, with permission, copyright 2002 a function of spatial coordinates, i.e., a American Chemical Society.] direct distribution analysis; (iii) a line analysis, by moving the sample after each laser spot; (iv) a layer analysis,by ablation efficiency achieved with low- emphasized when discussing micro- a vertical sequence of spot analysis; an energy pulses: lateral resolution of probe methods, in particular synchrotron area analysis, by a successive sequence several micrometers and picosecond X-ray fluorescence and PIXE. Electron of spot analysis; and (v) a volume ablation efficiencies ranging from 0.4 microprobe falls even below the funda- analysis, by a summation of microanal- to 200 lm3/lJ-1 were reported.446 mental absolute limit of detection, ysis.428 Two characteristics of micro-plasmas which is given by the square root of The use of ultra-short (ps, ns) laser are the short persistence of the radiation particle counting, taken as the lower ablation for surface elemental analysis (several tens of nanoseconds or less, limit for the sampling error. This error of solid materials (metals, alloys, dielec- instead of tens of microseconds), and the will be 1% or less if more than 10 000 trics) has been discussed by Semerok extreme sensitivity of plasma formation atoms are counted, corresponding (for and Mauchien,442 while micro-LIBS has on the distance between the sample and Cu) to a minimum mass of about 1 ag been reviewed a few years ago by the focused laser radiation: even small (10-18 g).429 Taschuk et al.446 As the authors point irregularities in the surface cause strong An important conclusion was reached out, one of the driving forces behind its fluctuations of the plasma emission: in by Mermet et al.218 after thorough success has been the development of extreme cases, no plasma is formed. On processing of shot-to-shot data to im- high-repetition-rate, compact microchip the other hand, this argument may be prove precision in LIBS microprobe, lasers,432 whose excellent characteristics reversed to the advantage of micro- namely that there was no penalty in terms of beam profile allow the LIBS, in the sense that careful position- associated to the use of a series of single formation of micro-plasma at very ing of the focused beam at the lm size shots with spot diameters below 10 lm, modest pulse energies (~ several lJ) scale can analyze the composition of at least for elements in the percentage since the short pulse duration (~0.5 ns) individual precipitate crystals: it is concentration range.218 results in high irradiances on the sample. therefore essential that for any applica- Scanning through the selected litera- The utility of microchip lasers has been tion at these size scales the analysis is ture reported here, many instrumental shown in several publications.436–441 done with a single laser shot.446 and analytical advances have been made Clearly, one of the best advantages of Micro-LIBS is clearly associated with in the past years. Menut et al.434 have high repetition rates is the resulting the concept of absolute detection limit. described a micro-LIBS instrument speed of analysis.435,451 Lateral and The difference between relative and based on a Nd : YAG laser operated at depthresolutionarelinkedtothe absolute LOD has always been clearly 266 nm, a modified optical microscope,

APPLIED SPECTROSCOPY 385 focal point review

FIG. 13. Left: (a) Microscope image of a hexagonal packed micro-lens array with circular micro-lenses of diameter ~0.9 mm. (b) Side-view and (c) 458 view of a laser-induced breakdown in ambient air generated by a 200 mJ/pulse laser beam focused with a micro-lens array. The laser beam direction is from the right to the left side of the photographs. Right: Overlay of three consecutive emission spectra of the laser plasma produced with focusing by (d) a single lens and (e) a micro-lens array in air at a rotating slag sample. The improvement in reproducibility obtained with the micro-lens is evident. [Reproduced from Ref. 427, with permission.] and a multichannel spectrometer cou- Several applications using microchip Zn at 213.9 nm).436 Other applications pled with an ICCD. The laser was and low-energy high-repetition-rate la- include 2D images of latent fingerprints characterized by an energy per pulse of sers were reported on low alloy steels using 400 nm pulses of 120 fs dura- 4 mJ, which could be decreased with a and aluminum targets.436–441,444 It is tion,443 elemental distribution of paper diaphragm to a few lJ, giving a spatial interesting to note that self-reversal was coatings,430 topographical and chemical resolution adjustable from 5 to 15 lm. observed in strong resonance lines (e.g., analysis of a meteorite, a basaltic rock,

386 Volume 66, Number 4, 2012 and individual aerosol particles,433 and the radius of curvature of the needle. chemical reactions with the ambient air. localized analysis of corals or spong- With such an approach, nanolocal anal- Examples are spectra given by OH, CN, 445 þ es. Micro-LIBS elemental analysis ysis by LA-ICP-MS was demonstrat- CH, NH, C2,C3, and N2 . These spectra was integrated into a Raman spectrom- ed.454 have diagnostic value and can be used, eter for space exploration.449 In order to A recent review of near-field laser for example, to evaluate the plasma improve the detection of weak signals ablation and its application to high- temperature and assess the presence of resulting from the ablation of small resolution surface analysis was written LTE. Molecular spectroscopy with LIBS masses, an improved method of signal by Cleveland and Michel.456 Near-field could be considered an oxymoron, since processing of 2D echelle images has LIBS with a ns laser directed through a the high temperatures provided by the been recently described.450 Finally, mi- bare dielectric fiber probe, fabricated by plasma would dissociate completely any cro-LIBS was also integrated with a the pulling method, was shown to molecular species present. Nevertheless, micro-fluidic platform, where sensitive generate enough emission to be detected since the plasma evolves in time and detection of Na in a micro-droplet was from ~2 lm diameter craters.453 Zorba cools down, an appropriate selection of demonstrated,447 and coupled with laser- et al.457 studied the ablation of Si for the experimental parameters (laser ener- induced fluorescence for improving the nanoscale chemical analysis in both far- gy, delay time, and buffer gas environ- detection sensitivity of lead in water.448 field and near-field configurations. A ment) allows the observation of In conclusion, micro-LIBS combines Ti : sapphire laser at 800 and 400 nm molecular emission. Molecular detec- successfully some of the unique advan- delivered 100 fs pulses at 1–10 Hz tion, on the other hand, implies the use tages of LIBS, namely the capability of repetition rates, with pulse energies in of atomic emission of selected elements elemental profiling in any material, the nJ range. With the laser operated at in the plasma to identify the chemical irrespective of its conductive status, no 400 nm, far-field data (obtained using a structure of the target. A typical example sample preparation, single-shot analysis, microscope objective lens and a fiber would be the chemical characterization and high speed. As Vadillo and Laser- bundle close to the sample surface for of an organic sample by linking to it the na15 pointed out, LIBS is ‘‘truly a collecting the plasma emission) were atomic emission of O, C, H, and N. surface analytical tool.’’ characterized by crater sizes as low as In practice, by changing the experi- Beating the Diffraction Limit: 1.13 lm with 15.2 nJ and less than 700 mental conditions, a LIBS spectrum may 457 Scanning Near-Field Optical Micros- nm with 4.2 nJ. In addition, a LIBS contain both elemental information copy (NFSOM). From the foregoing spectrum showing atomic lines of Si in (from atomic emission) as well as discussion, one wonders what could be the region 200–300 nm was clearly molecular information (from atomic the minimum size of the crater formed observed. In the near-field experiment, and molecular emissions) of the ablated from the ablation laser that would give the tip–sample distance was kept at 8–10 target. the best possible spatial resolution, while nm. With a laser energy of 0.18 nJ, both A detailed description of molecular still allowing enough photons to be craters and protrusions were observed, emission spectra observed in LIBS collected by the detection apparatus depending upon the number of pulses plasmas is out of the scope of the and generate an acceptable signal-to- delivered and the energy of the pulse. present review. We refer to a few papers Crater sizes and depths as low as 27 nm 460 noise ratio. by Acquaviva et al. (CN, C2), Bur- and 1.2 nm, respectively, were report- akov et al.459 (TiO), and Parigger et Spatial resolution is limited by dif- 457 ed. However, in this case, no plasma 463–465 fraction, which is approximately given al. (OH, C2,C3). The spectra were by (k/2) (see Part 1, Table II, Eq. T2.13, emission could be observed. obtained by experiments performed in in our first review, when the numerical Systematic studies will be needed to air or on targets like titanium metal or aperture is ~ 1.2).1 This limit can be understand not only the size and shape graphite. of the nanopatterns formed on the target 461 overcome by taking advantage of ‘‘near- 458 Ferioli et al. have used LIBS to field’’ effects occurring when the radia- in near-field experiments, but also to measure the equivalence ratio of a spark- tion is forced to pass through tiny identify the instrumental parameters in ignited engine using atomic C, O, and N apertures whose sizes are smaller than the optical collection system that should lines in the near-infrared region (700– the wavelength of the light. Such effects be optimized in the future in order to be 750 nm). Successful metrics were the can be controlled by adjusting the size able to observe an emission spectrum. peak ratios C/N and C/O together with and geometry of the aperture (see, e.g., CN/air ratios.461 Boyain-Goitia et al.462 Refs. 452 and 455). With this arrange- MOLECULAR LIBS have used molecular bands of CN and ment, surface resolution at the nanoscale The title of this section may need C2 in an internal signal calibration level can be performed. We also note some clarification: in fact, ‘‘molecular procedure to normalize strong fluctua- here that an alternative approach, pro- LIBS’’ may imply either molecular tions in single-pulse LIBS spectra of posed by Becker et al.,454 consists of spectroscopy using LIBS or molecular single bioaerosol (pollen) particles. In positioning close to the sample surface a detection using LIBS. The former term this work LIBS data were complement- silver needle immersed in the light refers to the spectra emitted by small ed by Raman data.462 radiation field. This produces a consid- molecules and diatomic radicals present Yang et al.466 have explored the mid- erable enhancement of the radiation in the plasma and resulting from the IR region (3.6–5.2 lm) by observing the intensity in a region comparable with ablation of the target or from subsequent LIBS plasma of carbon-containing tar-

APPLIED SPECTROSCOPY 387 focal point review gets at long delay times (50–200 ls). ISOTOPE ANALYSIS features of portability and rapidity of The authors466 used a liquid nitrogen analysis, which are associated to low- Laser-based atomic absorption and cooled InSb detector. The emission performance instrumentation in terms of fluorescence spectroscopy have been spectral resolution, Doucet et al.478 features, attributed to CO and CO2, used for isotope abundance determina- reported an accurate determination of were only observed in the spectra of tion: we cite here only a few papers by isotope ratio from partially resolved carbon-containing samples (cellulose Vera et al.469 (atomic and ionic fluores- paper fiber, polyethylene film, tetra- cence of U solutions nebulized in an spectra by applying chemometrics tools. fluoropolyethylene (TFPE)) while no argon ICP); Smith et al.472 (lithium Both U-235/U-238 and hydrogen/deute- features were observed in the case of a isotope ratio by laser ablation of a rium isotope ratios were considered, and YAG crystal, a quartz window, a lithium oxalate sample followed by dye the proof of principle of their approach microscope slide, and an aluminum foil. laser excited atomic fluorescence); was demonstrated. The authors conclud- Linking these features to the target was Smith et al.473 (U isotope ratios by laser ed that the performance of the combi- nation chemometrics-LIBS could be complicated by the observation that the ablation of UO2/graphite pellets fol- spectral intensity decreased when the lowed by diode laser excited atomic considered excellent for the rapid deter- sample was flushed with argon, indicat- fluorescence); and Bushaw and Anheier mination of isotope ratios using a low- ing the influence of atmospheric oxygen 476 152 160 weight, portable LIBS sensor to detect Jr. ( Gd : Gd isotope ratio—Gd is 478 in the formation of the emitting mole- biological and radiological threats. a suitable U surrogate—in individual 475 cules.466 It is interesting to note that micrometer-sized particles by laser ab- D’Ulivo et al. have studied the ambient CO did not produce a detectable lation, diode laser absorption ratio potential of LIBS in the determination of emission signal, while it was clearly spectrometry. The particles were diluted the D/H ratio in the gas evolved during visible in absorption against the spectral in sediment powder to simulate environ- hydrolysis of tetrahydroborates labeled background continuum. In a follow-up mental samples). with deuterium (TDB). The aim of the paper, Yang et al.467 reported atomic As pointed out by Smith et al.,474 measurement was the investigation of line emission features in the mid-IR observation of isotope shifts via optical the H/D exchange process in the borane emission spectroscopy is not a common reagents before the hydrogen transfer LIBS spectra of alkali metal halides 475 tablets (LiCl, NaCl, KF, KCl, KBr, and application of LIBS, mainly due to the from boron to analyte atoms. The authors have used the same optical setup RbCl). The emission was attributed to high spectral resolution needed. Indeed, previously described in this review.49,50 the alkali metal since no halide signature with overall Stark and Doppler broad- In order to obtain a sufficient separation was observed. Moreover, the emission ening of spectral lines in laser-induced between the peak wavelengths of H-a was not influenced by the nature of the plasmas of the order of 100 pm, it is lines for both isotopes, a long delay time halide. clear why LIBS is not suitable for the (~30 ls) and a two-peak Lorentzian Doucet et al.468 coupled LIBS in an direct determination of isotope shifts that can be of the order of a few deconvolution of the spectra were argon atmosphere with chemometrics used.475 for the qualitative and quantitative picometers. Viable solutions to this problem can be found by working under It is known that isotope shifts can be analysis of molecular compounds found orders of magnitude larger in molecular in a model pharmaceutical formulation. vacuum or at reduced pressures. Pietsch et al.471 determined the U-238/U-235 spectra than atomic spectra (see, for The authors took advantage of the 470 ratio using an ionic line at 424.437 nm example, the work of Niki et al. on molecular emission of selected diatomic characterized by a shift of 0.025 nm. boron isotope ratio using LIBS). Based fragments (CN, CH, C ), mathematical- 479–481 2 The operating pressure was 2.67 Pa. At on this knowledge, Russo et al. ly processed with chemometrics, to the delay time of the measurement, the developed a novel approach of perform- study the parent compound such as narrowest possible line width was of the ing optical isotope analysis using LIBS active pharmaceutical ingredients (API) 474 in ambient air and at ambient pressure: 468 order of 10 pm. Smith et al. used a and excipients. As said at the begin- high-resolution spectrometer and an the authors called the approach ning of this section, the basic hypothesis ICCD to evaluate Pu-239/Pu-240 iso- LAMIS.479 As pointed out in their first here is that the fragments are directly tope ratio: the wavelength chosen was paper, larger isotope shifts mean simpler linked to the parent molecule fragment an atomic line at 594.522 nm and the measurement requirements, in particular in the laser interaction process and are data were taken in a He atmosphere at for the light elements, where the shifts not the result of chemical reactions with 100 Torr with a delay of 1 ls and a gate are larger than for heavy elements.479 the ambient air (this could be the case, of 5 ls. It was found that the plasma Hydrogen, boron, carbon, and oxygen for example, for CN in ambient air). environment greatly affected the analyte were discussed in the first demonstration In concluding, we note that one of the intrinsic line width and thus the ability of the approach. A detailed investigation most interesting recent exploitations of to adequately resolve the isotopes.474 on quantitative boron analysis fol- molecular LIBS has been the approach The advantage of using He was demon- lowed,480 the choice of boron being named LAMIS (laser ablation molecular strated by Kurniawan and Kagawa21 in dictated by its importance in nuclear, isotopic spectrometry) described by their work on hydrogen/deuterium iso- medical, and material chemistry appli- Russo et al.;479 this will be discussed topes. cations. In this work, the authors used in the next section. In order to maintain the typical LIBS double-pulse ablation-excitation on bo-

388 Volume 66, Number 4, 2012 ron nitride pressed-powder disks with spark pairs’’ operation: the problems portable LIBS instrument); natural isotopic abundance as well as encountered were clearly outlined but Watcher and Cremers483 (U in isotope-enriched B2O3 pellets. Large the analytical potential of the approach solution; surface spark); Cheung isotope shifts were measured (from was also evident. This paper paved the and Yeung489 (liquid vaporization 0.74 nm up to 5–8 nm in the emission way to further developments in liquid- at fluences below breakdown; spectra of 10BO and 11BO molecular LIBS analysis. absolute Na detection of 0.4 pg); radicals. A third paper just appeared on In order to facilitate the linking of the Yaroshchyk et al.518 (optimized the strontium isotope analysis using SrO text with the references, we identify measurement conditions of plasma and SrF molecular radical emission.481 below a few broad categories: formation in liquid jets by the As a final consideration, LAMIS uses moving breakdown model). all the advantageous features of LIBS (i) viiBasic and diagnostic studies. Ex- (iii) vInstrumental developments. Exam- amples are the work of Cremers et with the added bonus of isotope analy- 482,498 ples are the work of Beddows et sis. Moreover, the method is not limited al. (spectroscopic spark pa- al.510 (single fiber plastic tube of to optical emission spectroscopy: ab- rameters; noise analysis; spark 20 m length to deliver ablation volume considerations; repetitive sorption and fluorescence measurements laser and buffer gas flow close to a spark characterization); Sacchi487 can be performed as well, extending the stainless steel sample under wa- 479 (water breakdown; electron ava- sensitivity to minor isotopes. ter); Yaroshchyk et al.517 (dual- lanche ionization); Golovlyov and beam spectrometer with one laser Letokhov488 (acoustical laser ab- LIQUID SAMPLES, COLLOIDS, split in two beams focused on two lation model); Ng et al.495 (plasma AND SUBMERGED TARGETS identical liquid jets forming the characterization at 532 and 193 analytical and reference channel); As often repeated in the literature, nm; ‘‘cold’’ 193 nm plasma results U¨ nal and Yalc¸in539 (continuous from an analytical point of view, liquids in three orders of magnitude larger do not seem to be the best sample for the signal-to-background); Escarguel flow hydride generation for tin exploitation of the LIBS technique. et al.501 (space and time resolu- (Sn) detection in aqueous environ- Nevertheless, especially after the reali- tion; plasma spectroscopic param- ments). zation of double-pulse approaches, a (iv) iiColloids and suspensions. Exam- eters; Abel inversion); De Gia- 491,493 large number of papers have been como et al.519 (metallic target ples are the work of Ito et al. published and many analytically useful under seawater; double-pulse plas- (FeO(OH) in water turbid solution results produced. The subject of this ma characterization); Casavola et with one and two sequential laser section has been treated in LIBS books al.522 (modeling double-pulse pulses; concentrations down to a (see, for example, Chapter 10 by Rai et few ppm; 16 ppb with double LIBS under water); De Giacomo 494 al. in Singh and Thakur’s book4) and in et al.528 (laser interaction with pulse); Knopp et al. (dye laser several reviews, for example.14,26,45 water and heavy water studies by at 500 nm, delivering 20 mJ in a LIBS offers the possibility of applying Stimulated Raman Scattering; 28 ns pulse focused in a cuvette; a rapid method, capable of standoff photolysis and breakdown pro- ions and suspensions monitored); 497 operation, to hostile environments (e.g., cesses); and Lazic et al.531 (diag- Haisch et al. (heavy metal nuclear waste) and to the characteriza- nostics of bubble and plasma colloids in the range 0.1–1 lm tion of objects submerged in water (e.g., formation; multi-pulse effects). diameter; miniaturized ultra-filtra- 503 for archaeometry). Moreover, the inter- (ii) General analytical studies per- tion system); Bundschuh et al. est in this field has been mostly driven formed with single lasers. Exam- (laser-induced breakdown detec- by medical applications of lasers such as ples are the work of Arca et al.496 tion (LIBD) of lanthanide oxide for ophthalmic microsurgery and stone (Cr, 100 ppb LOD), Samek et particle in the presence of lantha- fragmentation.487 As discussed by Sac- al.500 (Cr, Pb, Tc, U; few ppm nide aquo-ion); and Pu et al.514 chi,487 studies performed both in vitro, LODs; remote detection with a (lead carbonate colloids; double- on eye models, and in vivo have shown fiber assembly, hydrogen lines as pulse operation; ArF þ Nd : YAG that breakdown under laser irradiation is well as spiked species used as lasers). a probabilistic event and that the role of internal standards); Fichet et al.505 (v) iiiDroplets. Examples are the work beam irradiance is essential.487 Report- (12 elements in water and oil of Crouch and Arkontachi484 ing on the many papers appearing on samples, LODs varying from 0.3 (seminal work on LIBS character- this medical topic is clearly outside the to 120 ppm); Charfi and Harith507 ization of nanoliter droplets; iso- scope of the present review. (panoramic detection with an lated droplet generator); Chang et We have collected a number of echelle-ICCD system); De Giaco- al.485 (droplet breakdown theory; references dealing with various aspects mo et al.520 (single-pulse LIBS in droplet heating via stimulated of liquid-LIBS studies and applica- water); Rai et al.534 (Cr in several Raman scattering; location of tions.482–542 Cremers et al.482 first char- types of waters); Fichet et al.525 plasma formation with respect to acterized the detection of several (comparison of LIBS and ICP- the droplet); Huang et al.506 (elec- elements in aqueous solutions with OES); Fortes et al.537 (oil spill trospray ionization needle to form ‘‘repetitive single spark’’ and ‘‘repetitive residues and crude oils with a liquid droplets; Na, K, Al relative

APPLIED SPECTROSCOPY 389 focal point review

and absolute LODs; correlation Dı´az-Pace et al.526 (liquid con- detailed in specific papers. For the sake between plasma current and sin- verted into a solid pellet of of completeness, we report a succinct gle-shot integrated signal intensi- calcium hydroxide by mixing with summary of the main processes occur- ty); Poulain and Alexander492 CaO); Schmidt and Goode509 ring in the case of nanosecond single- (seawater aerosol droplets); Ikeza- (pre-concentration and immobili- and double-pulse LIBS of metallic wa et al.529 (micro-droplet ejection zation on a membrane; membrane targets under water. The following has using an ink jet system); and Groh is the LIBS target); Sarkar et al.533 been taken from Casavola et al.522 et al.538 (mono-disperse piezoelec- (filter membrane base collection When a laser pulse is focused on a tric micro-droplet dispenser; 0.1 and LIBS); Jijo´n and Costa541 liquid, the plasma is much different than nL volume droplets; LIBS and (steel surfaces scratched with an that obtained in a gas or in a solid. The ICP-OES). HB pencil; 1.0 lL salt solution plasma formed by a single pulse in water (vi) iiLaser breakdown-acoustic detec- deposited; high hydrophobicity has a short duration, as a result of the tion. Examples are the work of produces homogeneous spot col- plasma cooling due to processes such as Golovlylov and Letokhov488 (laser lecting the dried material); and ion-electron radiative recombination and ablation of absorbing liquids, Vander Wal et al.211 (1.0 mL chemical reactions. The most important acoustical micro-fragmentation solution deposited onto a carbon mechanical effect resulting from the 486 model); Kitamori et al. (CaCO3 planchet and evaporated). interaction is the formation of a cavita- micro-particles in water; Ca lines (x) iiiDouble-pulse operation. This top- tion bubble. The characterization of the observed only in presence of ic collects most entries. Examples bubble dynamics is important not only particles). are the work of Nyga and Neu490 for the understanding of the interaction (vii) iNanoparticle formation by laser (two XeCl lasers; calcite ablation of the liquid-submerged target with a ablation in liquids. Examples are in water); Pichahchy et al.498 single laser pulse but also for explaining the work of Tarasenko et al.524 (repetitive spark pair; Cr, Cu, the behavior observed in the case of (silver colloids prepared by abla- Mn, Si in steel samples under double-pulse excitation. The laser-in- 499 tion of a silver plate in acetone or water); Hosoda et al. (dual duced bubble dynamics have been distilled water; changes in particle pulse ps excitation; one laser with modeled522 by starting from the instant morphology; different laser wave- a double-pulse generator with when the bubble is mainly filled with 511 lengths); and De Giacomo et al.540 variable delay); Pearman et al. vapor. The vapor pressure inside the (single- and double-pulse laser (bulk aqueous solution; orthogo- bubble is much larger than that outside. ablation of graphite in water under nal arrangement; 250-fold ob- As a result of such difference, its radius pressure ranges 1–146 bar; Ti served enhancement); Kuwako et grows at supersonic speed. During the 512 target used to interpret the spectra al. (Na in water; 0.1 ppb LOD); growth phase, the pressure inside the 513 obtained). Pu and Cheung (liquid jet; bubble decreases, and the corresponding ArFþNd : YAG; Pb aqueous solu- volume increases, until a maximum (viii) High pressure bulk liquids. Exam- 515 ples are the work of Lawrence- tion); Rai et al. (Mg ion; six- expansion is reached. At this point, the 530 fold enhancement; 69 ppb LOD); Snyder et al. (orthogonal dou- 516 pressure outside the bubble is again ble-pulse LIBS; 138 bar; little or Kumar et al. (liquid jets; Mein- higher than that inside and a shrinking hard nebulizer); De Giacomo et no enhancement observed above 519 process begins. It can be shown that this 100 bar); Michel et al.532 (oceanic al. (basic aspects; metallic tar- inversion time is of the order of 150 ls. pressure; key operational parame- get in sea water; Ti, Cu, Pb, Sn, Inside the bubble, the decrease in and Zn in submerged objects); De ters optimization; Na, Mn, Ca, K, 521 volume and increase in pressure contin- and Li); and Michel and Giacomo et al. (submerged ues until the rate of condensation cannot 535,536 bronze sample in sea water); Lazic Chave (single- and double- 523 offset the volumetric reduction and the pulse LIBS; Na, Mn, Ca, Mg, and et al. (archeological materials; bubble collapses.522 The cycle may start K are key elements in understand- Fe, Cu-based alloys, precious al- over since the collapse produces a rapid ing the chemistry of deep-ocean loys, marble, and wood); De increase of temperature and pressure of Giacomo et al.527 (submerged hydrothermal vent fluids and sea- 542 the gas, thus giving rise to the formation water; double-pulse operation did targets ); and Lazic et al. of a second compression pulse and re- (underwater sediments; broad wa- not improve LODs). expansion of the bubble. ter sonoluminescence; automatic (ix) iiLiquid-to-solid transformation. When a second laser pulse, delayed selection of similar spectra used Examples are the work of Ca´ceres from the first, is used in view of for calibration curves). et al.502 (simple quick-freeze improving the sensitivity of the emission method to detect trace metal ions The reader is referred to the original technique, it is clear that the choice of in ice); Pardede et al.504 (electro- literature for more details, in particular the delay time between the two pulses chemical deposition as thin film for the interpretation of the single- and determines the conditions of pressure on a nickel plate electrode surface; double-pulse data obtained in water. The and temperature found by the second electrode placed in a vacuum basic interaction behavior and the char- pulse during the temporal evolution of chamber is the LIBS target); acteristics of the plasma formed are the bubble created by the first pulse. By

390 Volume 66, Number 4, 2012 changing the delay between the laser tional univariate approach. As noted by overfitting and noting the extreme im- pulses, it is possible to find the most Pretsch and Wilkins544 in a 2006 portance of model validation,546 and appropriate conditions for the second editorial introducing a feature issue on another paper focusing on the underly- pulse. One can note here that the bubble chemometrics, the field of chemometrics ing statistical issues associated with persistence time (expansion-collapse) is was intended to provide maximum partial least squares (PLS) and principal at least 100 ls, depending on the laser relevant chemical information by ana- component analysis (PCA) for discrim- characteristics, while the plasma induced lyzing chemical information, but one of ination, noting that PLS was not origi- by the second pulse persists for approx- the problems with today’s use of chemo- nally designed as a tool for statistical imately 5 ls.522 metrics is the use of sophisticated discrimination.549 With these comments Finally, we make only two observa- chemometric techniques by ‘‘uninitiated in mind, we now turn our attention to the tions: (i) in their work on double-pulse scientists’’ who lack the fundamental LIBS chemometric literature. LIBS of sodium (Na) in water, Kuwako knowledge of the capabilities and limi- We first discuss LIBS papers with a et al.512 consistently call the emission tations of the methods.544 While this is a primary focus on the chemometric signal ‘‘fluorescence,’’ and ‘‘fluorescence relatively strong statement regarding techniques themselves, such as method lifetime’’ the persistence of this signal chemometrics, the point is well taken comparisons and algorithm improve- with time. Some spectrally broad lumi- in that the use of sophisticated chemo- ments, rather than on a specific applica- nescence may be observed for a short metric algorithms may lead to apparent- tion. The use of linear and rank time (up to nanoseconds) during the ly successful discrimination among correlation have been compared for bubble collapsing time (sonolumines- samples in which the key identifiers classification,557 including the use of cence), as reported by Lazic et al.:542 are not related to the nature of the spectral filtering and masking,561 and however, this is not the case for the samples but rather to contaminant and/or the use of stored spectral libraries.547 sodium signal. As we said earlier, the background features. The processing of LIBS spectral data fluorescence term for describing LIBS Our goal here is to provide a discus- using techniques inspired by text re- emission should be avoided; (ii) Fichet sion of selected LIBS literature concern- trieval was explored with a goal of et al.525 compared the analytical perfor- ing various aspects and applications of unassisted elemental identification.567 mance of ICP-OES and LIBS on 19 chemometrics, illuminating what we feel The ultimate goal of such efforts is the elements with two different instruments, are important issues. For more compre- development of fully automated LIBS using different lines and instrumental hensive treatment of chemometrics in algorithms. A proposed extension of the parameters, as imposed by the instru- analytical chemistry, readers are referred PLS method using a nonlinearized PLS 2 mentation. A good agreement (R = to the 2004 text by Adams.543 In based on the multivariate dominant 0.9951) between the quantitative results particular, we note that in this review factor has been proposed as a means to for major and minor elements was we make no attempt to define or discuss treat nonlinear self-absorption in LIBS found. However, the LODs achieved the statistical methods of the various data,574 with the relevant merits of such with ICP-OES were about three orders chemometric techniques. an approach discussed in two related of magnitude lower than those obtained Our discussion is divided between papers.575,576 The issues of nonlinearity with LIBS. As pointed out earlier in this two key targets of chemometrics, as with LIBS spectral data as addressed review, this outcome deserves further nicely summarized by Sirven545 in a through chemometric methods clearly consideration, since the limits compared recent LIBS short course, namely, the remains an important point for further are concentration (relative) limits. One use of chemometrics for calibration and discussion in the LIBS community. should then also compare the absolute the use of chemometrics for classifica- The use of chemometrics for LIBS- LODs by taking into account the sample tion (i.e., discrimination or sorting). This based analysis of metals and ores has volumes consumed by each method. drawn considerable attention over the 482 is a very important distinction. As noted Cremers et al., for example, calculat- in a tutorial on multivariate calibration years, which is not surprising given the ed that the volume of water vaporized in atomic spectrometry, calibration is richness of such spectral data. Nearly all per laser spark in their particular exper- defined as the operation that determines of these studies fall into the calibration imental conditions was at most 10-5 3 10 the functional relationship between mea- category of chemometric studies. A cm , corresponding to about 10 Li sured values and analytical quantities.31 multivariate LIBS analysis was used to atoms in the volume. Both relative and It was further noted that calibration is an analyze slag samples in a steel plant as a absolute LODs have been indeed report- empirical process, and that in general, means to overcome matrix effects.551 ed in the above liquid-LIBS literature. the existence of universally good solu- Multivariate calibration was found to tions are precluded. Classification, on reduce the RSD from 3.4% to 1.8% as CHEMOMETRIC the other hand, is a means to distinguish compared to univariate analysis and to APPROACHES among a finite group of defined sample improve the coefficient of determination The use of chemometric methods for types and for the most part may be used to 95% as compared to 65%. Several the analysis of LIBS spectral data has synonymously with sorting. Several studies have reported on the chemo- proliferated in recent years, opening a studies have addressed the use of metric-based analysis of metal alloys, new avenue for semi-quantitative and chemometrics for classification, with including the use of linear correlation for quantitative analysis beyond the tradi- one paper focusing on the issue of brass and aluminum,548 univariate ver-

APPLIED SPECTROSCOPY 391 focal point review sus PLS calibration for brass,565 PLS for cation, the LIBS literature contains a dures and quantitative analysis, and gold,550 and multivariate calibration for wide range of applications. Representa- opening the door for sophisticated aluminum.555 LIBS has been success- tive studies include the identification of classification routines. These advances fully used for analysis of iron ores using Mars rocks using PCA, soft independent also come with several pitfalls; hence, both PLS regression566 and PCA regres- modeling of class analogy (SIMCA), one must use chemometrics with a firm sion.556 and PLS,554 the classification of archeo- knowledge of the fundamentals of LIBS An important point of discussion in logical pieces using PCA, SIMCA, and as an atomic emission spectroscopy these two studies is the use of spectral linear discriminant analysis (LDA),563 technique and not simply as a black- processing in conjunction with chemo- comparative analysis of automobile box methodology. metrics, such as background stripping, paints using a nonparametric permuta- Clearly the future of LIBS will be normalization, and spectral cleaning. In tion test and a parametric Wald test,564 coupled to the field of chemometrics, fact, the importance of spectral normal- quarry identification of historical build- and in fact, the use of advanced ization, as discussed earlier in this ing materials using LDA, PCA, and processing schemes in conjunction with review, is a common theme with chemo- SIMCA,569 discrimination of float glass LIBS seems to be growing rapidly. We metrics and is therefore emphasized here using a nonparametric permutation offer here a few summary comments on as a point that requires attention. Both test,568 identification of polymer materi- the use of chemometrics for processing PCR and PLS were used and compared als for recycling using artificial neural of LIBS spectral data. to univariate analysis for LIBS-based networks,571 and for classification of quantification of plutonium oxide surro- toys based on the presence of toxic (i) iiWe emphasize the point made gate residues.559 The calibration results elements, including Cd, Cr, and Pb.573 above with regard to the importance showed improvements in the coefficient In recent years, the use of LIBS and of fundamental knowledge of both of determination from 0.87 to 0.97 for chemometrics for the classification of the capabilities and limitations of cerium (Ce), with similar trends ob- explosives and chemical agents has chemometric methods.544 Chemo- served for Cr, Fe, Mo, and Ni. Multi- received attention. LIBS analysis of metric methods are perhaps the element calibration was examined using bacteria, pollens, and nerve agent sim- ultimate ‘‘black box’’ in analytical PCA for a wide variety of mineral ores, ulants were analyzed using linear corre- chemistry, with the ability to extract yielding average relative errors ranging lation, PCA, and SIMCA.552 The nerve an ‘‘answer’’ from spectral data sets from 1.4% for K to 5.5% for P at trace agent simulants all exhibited similar that is derived from spectral fea- levels.558 Other calibration studies using elemental composition (C, H, O, and tures wholly unrelated to the actual chemometrics include the analysis of P), making the accurate classification of (i.e., physical) differentiators in the basalts for the Mars Science Laboratory, the agents not always possible. Moving analytical specimens. In other comparing the performance of PCA and from single-shot analysis to a five-shot words, advanced chemometric al- PLS regression schemes.570 Multivariate average was found to improve the gorithms must be used with knowl- analysis was also used as a means to discrimination capability; however, edge of what emission features mitigate matrix effects and promote overlap of the clustered regions was still (e.g., atomic or molecular emission quantitative analysis for igneous and observed. The role of shot-to-shot var- peaks) are providing the associated highly metamorphosed rock samples.560 iability in spectral data was concluded to discrimination. The successful use of linear correlation play a significant role and the need for (ii) iThe use of chemometrics may be routines for depth-profiling with LIBS data pretreatments, echoing comments nicely divided into the use for was reported for a wide range of above, was stated. The influence of calibration and the use for classifi- materials, including archeological ce- variable selection for PLS discriminant cation (i.e., discrimination or sort- ramics and polymer coatings on steel.553 analysis for explosive residue classifica- ing).545 The use for calibration has A comparison of univariate and multi- tion was examined in a recent paper.572 the potential to address spectral variate calibration for analysis of plant It was found that the whole spectra interferences, matrix effects, and materials was reported, with the conclu- (200–970 nm) as the data input provided analyte signal nonlinearity due, for sion that PLS regression was the most the best results when classifying thirteen example, to the well-known curve of robust due to a lower occurrence of organic materials. However, variables growth and is an area where chemo- outliers.562 Finally, the extension of due to the surrounding atmosphere and metric schemes can bring significant LIBS from elemental analysis toward the substrate (e.g., Al and Ar) also benefit as compared to traditional molecular analysis in the context of contributed to the discrimination, and univariate analysis. Classification chemometrics was reported for analysis the authors added a note of caution affords the opportunity to sort be- of organic compounds in a complex regarding the robustness of such a model tween classes of materials. We note matrix.555 Using this approach, LIBS for future unknown samples. here, however, that classification was successful in predicting formulation In summary, the use of chemometrics implies that the unknowns are es- excipients and active pharmaceutical for LIBS brings an entire new dimen- sentially identical to the known ingredient in a complex pharmaceutical sion to spectral analysis as compared to samples used in the spectral training formulation. a traditional univariate analysis, with data. Extrapolation beyond the Turning from calibration to classifi- potential benefits to calibration proce- known samples and training sets to

392 Volume 66, Number 4, 2012 a wide range of sample conditions liquid steel at distances up to 1.5 m579 ploration instrument include a large (i.e., presence of interferents) opens and the quantification of major elements number of analyses per mission lifetime the door to misinterpretation of in an industrial mineral melt between and expanded analytical capabilities.73 chemometric algorithms, as the 1400 and 1600 8C.580 Heavy metals in Cremers et al.589–593 have carefully ‘‘best match’’ may have little phys- soils were analyzed using a mobile LIBS examined many of the issues related to ical similarity to the actual un- platform, with the authors reporting LIBS-based Mars exploration, including knowns. As noted in a recent semi-quantitative results using a data the analysis of water ice in the polar paper, spectral contributions not analysis algorithm to address spectral regions,591 a combined LIBS and reflec- originating from the analyte of interference and matrix effects.578 In tance approach for remote mineralogical interest (e.g., substrate lines or several studies, Laserna et al.581,586 have and elemental identification,589 stand-off atmospheric lines) may contribute focused on the instrument design for a detection of chlorine and sulfur ele- to the chemometric discrimination; coaxial system, including a critical ments,592 and the influence of ambient hence, model robustness is to be discussion of the relevant parameters581 pressure, which are typically modeled as 572 586 considered with a note of caution. and the application to moving targets. a few torr of CO2, on calibration curves (iii) The issue of training data vs. A fs-LIBS system was used to examine for soil and clay samples.593 The validation data remains very impor- the feasibility of stand-off LIBS over Chemistry Camera (ChemCam) instru- tant in the context of chemometric distances of 30 m or more for various ment suite package on the NASA Mars methods. The potential for over- targets of defense and homeland securi- Science Laboratory, launched in No- training is significant with LIBS ty.585 vember 2011, contains a LIBS-based spectral data, resulting in calibra- An alternative approach to stand-off instrument. The LIBS instrument, which tion or classification models that are LIBS analysis involves the use of will represent the first use of LIBS for less robust than even univariate ultrafast laser pulses and the resulting planetary science, has been detailed in a schemes. This is a contemporary phenomenon known as filamentation. series of recent papers that address topic within the LIBS community With traditional ns-LIBS in remote calibration and analysis using multivar- that still lacks well-formulated and configurations, the ability to focus the iate analysis for carbonate minerals,597 disseminated guidelines. laser beam on the target becomes limited the onboard calibration for igneous at distances approaching many tens of targets,596 the analysis of sulfur in Figure 14 shows some selected exam- meters. The optical process of filamen- geological samples,598 and the develop- ples of the application of chemometrics tation realized with ultrafast lasers in ment of data reduction schemes and to LIBS data. ambient air has been well documented, their use with a spectral database including the use with spectroscopic targeted for the Martian environment.599 REMOTE SENSING, STAND- techniques as summarized in the recent The use of the vacuum UV spectral OFF LIBS review paper by Leahy-Hoppa et al.588 region (100–200 nm) for analysis of Br, We touched on the use of remote or LIBS was successfully demonstrated at C, Cl, P, and S was examined, although stand-off LIBS for the analysis of distances of 25 m on copper and in the context of in situ close-up analysis explosives, although the emphasis was aluminum targets583 and at up to 90 m as a complement to stand-off detection on the specific application, while here via filamentation in open air using a 250 (see earlier section). In addition to the we focus on the methodology. In a mJ pulse from the Teramobile.582,584 A ChemCam instrument, a miniaturized review of LIBS for analysis of distant theoretical study of the atmospheric LIBS instrument for Mars exploration objects, Salle et al.24 summarize much propagation of both the laser beam and was developed with a total instrument of the literature up to 2007. Considering the return light was reported, with a goal package payload of about 1 kg.595 only open-path systems, the authors of understanding the effects of changing Other space applications of LIBS review a range of applications and atmospheric transmission in the context include the classification of meteorites, experimental configurations, noting that of stand-off LIBS.587 The authors em- including the goals of on-flight tech- the majority of studies reporting quan- phasize that stand-off LIBS is a bidirec- niques applicable to asteroid investiga- titative results have done so using the tional process in which the laser must tion,594 and the feasibility of LIBS- usual methods of calibration curves travel to the target and the plasma based analysis of rocks at high pressures through reference materials. In one of emission must travel back to the receiv- in the context of a Venus mission.590 For the first studies that focused on remote er. such an application, the LIBS measure- LIBS, measurements up to 100 m were Space Exploration. LIBS remains an ments would be performed at pressures reported using an optical fiber to both attractive analytical technique for space on the order of 90 atm and temperatures deliver the laser pulse and collect the exploration given the relative instrumen- in excess of 700 K, presenting consid- plasma emission.577 Metallic species tation simplicity combined with the erable challenges to the instrumentation. were targeted and successful detection spectrally rich output. In particular, the Organics: Explosives. The use of was reported, with the application fo- application of LIBS for Mars explora- LIBS for analysis of explosives has been cusing on hostile environments of nu- tion, including mineralogy and planetary explored in a number of papers, with a clear reactor buildings. Other industrial geology, has been the focus of many recent review paper summarizing much applications include the analysis of studies. Characteristics of a space ex- of the literature.36 Stated challenges

APPLIED SPECTROSCOPY 393 focal point review

FIG. 14. Selected examples of applications of chemometrics to LIBS experimental data. Top right: PCA scores of chemical agent simulants constructed using spectral regions from (A) single-shot and (B) average spectra.552 [Reproduced with permission from Ref. 552.] Bottom right: Three-dimensional scatter plot of a biological data set in the principal component coordinates. Each data point represents a single spectrum acquired with a single laser shot.662 [Reproduced with permission from Ref. 662.] Top left: Principal component analysis (PCA) plot of PC1 (47%) versus PC2 (26%) scores for carbonate samples (closed symbols) and geological reference standard materials (open circle).597 [Mineral images: Courtesy of Geology.com; Bacteria image by Bruce Arey/2010 EMSL. Plot reproduced with permission from Ref. 597.] Bottom left: X-loadings for PC1 (47%) and PC2 (26%) by wavelength. The most influential peaks in PC1 belong to Ca(þ) and Mg(-), while the most influential peaks in PC2 belong to Mg (þ) and Fe (-).597 [Reproduced with permission from Ref. 597.]

394 Volume 66, Number 4, 2012 include minimizing the effects of ambi- waysformolecularemissioninthe part due to the ability for in situ and ent air, which may contribute to the context of explosives discrimination,611 rapid minimally destructive analysis, observed N, C, and O signals, and as well as examining the use of multi- multi-element analysis, depth-profiling, controlling for signals related to the spectral schemes such as Raman- and compatibility with other techniques substrate materials on which the explo- LIBS.612,613 A fiber-optic-based LIBS such as Raman. Several reviews8,13,19,45 sives rest. Improvements for explosives sensor was reported for detection of have focused specifically on LIBS in art, discrimination were discussed, including explosives and landmines using a mi- archeology, and cultural heritage. The the use of double-pulse configurations, crochip laser for analysis of surface applications are many, and here we fs-lasers, resonance-enhanced LIBS, materials.603 review a cross-section of the relevant chemometrics, and multi-sensor tech- It is not surprising that a number of literature to demonstrate the breadth of niques. Overall, the potential for real- studies have focused on the ability of LIBS for this important application. time, standoff detection was seen as the LIBS to perform stand-off analysis for Anglos et al.618 provides a survey of biggest advantage of LIBS for explo- the specific application of explosives archeological applications, including sives, although the authors noted that analysis. LIBS was used at distances up painted works, icons, pottery, sculp- much improvement is still needed to to 45 m for characterization of energetic tures, and metal, glass, and stone make LIBS a viable sensor in a field materials, with the C2 Swan bands, in artifacts. The advantages of both early environment. conjunction with H, O, and N emission feedback in the field, as well as LIBS Some of the first work to address ratios, to discrimination between organic analysis in the conservation laboratory, explosives and non-explosives.602 In a discrimination among organic com- and the importance of close collabora- blind test, three explosive and non- pounds (i.e., other than elemental anal- tion between the archeological and LIBS explosive compounds were correctly ysis) was reported in two 2001 communities, is noted. LIBS was used 600,601 identified as such using a multi-step papers. In the first study, successfully for analysis of white lead C : H : O : N ratios were used to prepare data processing algorithm. A double- pulse LIBS system was used for stand- pigments in historic wall paintings, calibration curves for a range of solid noting that some transient discoloration, organic powders, with precision and off analysis of explosives and hazardous materials at a distance of 20 m.604–606 In attributed to PbO formation, was ob- accuracy in the range of 2 to 3%,with served near LIBS craters following either helium or argon used as a cover all cases, PLS data analysis was gener- analysis.619 Analysis of fresco by LIBS gas to displace ambient air.600 In the ally successful in discrimination among was reported, with elemental emission second study, F, Cl, and Br were various samples, including RDX, bio- lines giving insight into the use of analyzed in organic compounds, using logical compounds (e.g., B. subtilis), certain pigments.623 Other papers have both air and helium atmospheres, with and chemical warfare agent simulants. 609–614 focused on the use of LIBS for moni- the latter providing increased sensitivi- Laserna et al. have developed toring the cleaning of stones and mar- ty.601 several mobile platforms for explosives bles.621 Given the importance of explosives analysis, including a double-pulse sys- 610 detection for military and security ap- tem for detection up to 30 m, and an The analysis of ancient metals with 614 plications, a number of LIBS studies optical catapulting-LIBS system. LIBS would appear as a natural appli- have addressed this topic. Gottfried et Overall, the use of LIBS for explo- cation, giving the relatively strong al.607 have addressed the detection and sives analysis remains an important sensitivity of LIBS to many metals, as discrimination of explosive residues, application that continues to draw inter- well as the lack of concern due to applying PLS routines using the full est in the LIBS community. Advanced atmospheric background. LIBS has been spectral range from 200 to 980 nm. data analysis schemes are seen as used for the study of Porticello Bronzes, 620 Using both selected atomic emission essential for this application, given some in conjunction with XRF, helping to ratios in combination with plasma fea- of the challenges discussed above, as assign fragments to different statues. tures attributable to laser–material inter- well as the necessity to function in The investigation of LIBS for analysis actions improved the discrimination potentially widely varying background of corrosion processes in ancient bronz- power. Plasma characteristics were also environments. As noted in the chemo- es was successful, with depth profiling studied in the context of explosives metrics section, discrimination and clas- revealing information about the strati- analysis, with both atomic and molecu- sification generally work well among graphic layers of corrosion products.626 lar emission showing correlation with well-defined sample sets, but care must Depth-profiling with LIBS was also the amount of ablated material and the be taken to understand the analyte used to analyze coated metallic artifacts, plasma temperature.608 In a follow-on signals that provide discriminant power, including silver and gold coatings.622 study, sources for nonstoichiometric line including the use of emission lines that Other applications of LIBS in cultural intensities were presented, and a data do not actually arise from the target heritage include the analysis of stained analyte compound. processing procedure was developed for glass on St. Anthony’s Basilica,624 assessment of the preservation quality explosives classification in ambient CULTURAL HERITAGE air.615 Laserna et al.611–613 have dis- of archeological bones using the calcium cussed the challenges of explosives The use of LIBS in cultural heritage to fluorine ratios,625 and the use of UV detection in air, focusing on the path- has attracted attention for many years, in filaments for remote LIBS analysis for

APPLIED SPECTROSCOPY 395 focal point review distinguishing between metals and in a model for spectral correction, with tions.639 The use of LIBS for analysis stone.617 the express goal of minimizing matrix of nitrogen in soils presents a challenge effects.235 Another means to reduce due to the significant atmospheric nitro- APPLICATIONS matrix effects for analysis of polluted gen content and therefore the possibility soils is the double-pulse LIBS method- for spectral interference. The potential of Environmental Analysis. We dis- ology in combination with a calibration- LIBS for nitrogen analysis in soils was cuss here three environmental applica- free approach.280 The double-pulse investigated using both atmospheric and tions of LIBS, namely, for analysis of 632 soils, vegetative matter, and geological scheme improved the sensitivity, allow- reduced pressures. The authors noted samples. Given the recent interest in ing the detection of Cr, Ni, and Pb, the importance of the laser energy and resource conservation and sustainable which were not seen with a single-pulse the lens-to-sample distance, as well as agriculture, there is a significant moti- LIBS measurement, while providing cautioning about the spectral interfer- vation for LIBS-based analysis and quantitative analysis in excellent agree- ence due to titanium when working in associated promise of low-cost, rapid, ment with certified reference values. the 498 to 503 nm spectral region. Using and in situ analysis. Two reviews by The analysis of carbon in soils the 746.8 nm N(I) line, successful Bublitz10 and Burakov38 are available remains an attractive target for LIBS, calibration curves were realized at a that focus on the use of LIBS specifi- with a number of papers reporting reduced pressure of 0.04 Torr, giving a cally for soils analysis. various outcomes with regard to analyt- detection limit of 0.08%. In an early study, LIBS was used for ical performance. Using the 247.8-nm LIBS has been used rather broadly for analysis of environmental solids, includ- carbon line, LIBS was found to produce analysis of vegetative materials, includ- ing soils, sands, and sewerage sludge.627 a detection limit of 300 mg/kg, a ing for nutrient analysis as well as the A considerable focus was on optimiza- precision of 4–5%, and an accuracy of bioaccumulation of pollutants, for ex- 629 tion of the LIBS technique to address 3–14%. The authors noted the need ample, with phytoremediation. The use crater formation and aerosol production, for further analysis of the effect of soil of femtosecond LIBS for detection of size effects, timing, and reproducibility. texture, moisture content, and mineral- heavy metal accumulation in leaf sam- Good calibration results were obtained ogical composition on the LIBS analyt- ples was assessed and compared directly 642 for determination of heavy metal con- ical performance. The assessment of to X-ray microradiography. Single- centrations with detection limits in the total carbon (organic and inorganic) pulse LIBS was assessed for mapping 10 mg/g range. Pollution monitoring and using LIBS was examined using both silver and copper distribution in plants analysis of soils has been the focus of unwashed and acid-washed soils, with and compared directly to LA-ICP-MS 643 many additional studies, including for LIBS proving useful for determination analysis. The capability of both the the analysis of a wide range of toxic and of both organic and inorganic carbon in LIBS and LA-ICP-MS techniques was 630 heavy metals (Al, Cr, Cu, Fe, Mg, Mn, soil. Other LIBS-based studies for demonstrated for heavy metal mapping Pb, Ti, vanadium (V), and Zn) using carbon analysis have examined the use directly in plant leaves. LIBS in combination with an established of normalization, benchmarked with Both macronutrient and micronutrient laboratory-based analytical technique established analytical methods, and ex- analysis with LIBS was investigated in a such as AAS, ICP-OES, GF-AAS, and plored the relative merits of the 247.8 pair of studies,644,645 with the former ICP-MS.636,637,640,641 Overall, the LIBS nm lines versus the 193.0 nm carbon focusing on P, K, Ca, and Mg, and the analysis was generally consistent and emission lines, generally concluding that latter focusing on B, Cu, Fe, Mn, and reasonably correlated with the other LIBS is well suited for this applica- Zn. For the macronutrients, reasonable analytic methods, with the authors tion.631,633–635 A novel multivariate agreement of the LIBS results was found concluding that LIBS is useful for rapid analysis was used recently for carbon with ICP-OES analysis following wet screening and estimation of heavy metal measurements, using the 247.8 nm acid decomposition. For the micronutri- contamination. As noted in one study,640 carbon emission line in combination ent analysis, the precision varied from 4 LIBS is not described as an alternative with several other emission lines.638 to 30%, with limits of detection ranging quantitative analytical method to the The results showed that a robust cali- from 1.2 to 3.6 mg/kg. As a means to usual laboratory methods but simply as bration curve could be developed for enhance the performance of LIBS for an efficient time-saving tool to optimize carbon analysis using the multivariate analysis of plant materials, a simulta- sampling and to drastically reduce the analysis, although the need for addition- neous optimization strategy based on a number of soil samples analyzed. From al measurements for a broad range of neuro-genetic approach was investigat- an analytical perspective, several ap- soils was noted. ed.646 A Bayesian regularized artificial proaches have been used to increase the In addition to carbon, there is interest neural network was employed, followed accuracy and precision of the LIBS in the analysis of fertilizers and nitrogen by a generic algorithm in a approach method, in particular as a means to in soils with the LIBS technique. An called neuro-genetic. The technique was overcome the often significant matrix online LIBS instrument was successfully successful in arriving at an optimal set effects realized with soils analysis. The used for analysis of K, Na, and Mg in of experimental conditions for analysis influence of plasma temperature on soils, showing promise of LIBS as a of plant materials. The simultaneous quantitative analysis of heavy metals in perspective tool for control of these analysis of macro- and micronutrients soils was investigated and then included important nutrients in field condi- was investigated in sugar cane leaves

396 Volume 66, Number 4, 2012 using LIBS.647 Univariate and chemo- Aerosols and Bioaerosols. The ap- atomic emission and molecular bands) metric methods such as PLSR and iPLS plication of LIBS for aerosol analysis and data reduction algorithms, including were employed for analysis, and it was dates back nearly 30 years, as noted in a principal components analysis, all four found that the LIBS analysis did not recent review focusing on the path studies reported various degrees of differ significantly from the ICP-OES toward quantitative aerosol analysis.35 success in distinguishing between the results for a 95% confidence interval, LIBS-based aerosol analysis is a partic- various bioparticles, with considerably with the precision ranging from 0.7 to ularly useful microcosm to demonstrate more success in discriminating between 15% for the multivariate approach and the parallel advancement of a LIBS species (e.g., spores versus bacteria), from 1.3 to 25% for the univariate application that has benefited from and with less success in discriminating calibration. LIBS has also been used in challenging and important applications among like groups (B. globigii vs. B. several studies to determine the concen- such as bioaerosol analysis in combina- thurengensis). Overall, these four pio- tration of trace minerals in the skins of tion with prolonged attention to the very neering studies reached very similar potatoes.648,649 fundamentals of plasma–analyte interac- conclusions, which we summarize to We end this section with a discussion tions. The discrete nature of solid the general message that the LIBS of LIBS for analysis of inorganic, aerosol particles and their interaction technique can successfully discriminate mineralogical, and geological samples. with the laser-induced plasma present between broad classes of materials in A 2005 review of LIBS for in situ important issues unique to this applica- rather well controlled conditions, but geochemical and environmental analysis tion, including the importance of the provided consistent overlap when dis- highlights many of the important issues sample rate.657 A critical understanding tinguishing between biologically similar and results for this application.18 The of the sampling probability and the groups. One group noted that nearly all analysis of the minerals such as pyrite, selection of appropriate processing al- of the signature elements used for PbS, and zinc blende (ZnS), along with gorithms (e.g., ensemble averaging ver- classification are present in dirt and several others, was reported using LIBS sus single-shot conditional analysis) are other natural background contaminants. at the wavelength of 370 nm.650 Atomic necessary for successful LIBS analysis. The feasibility of detection and iden- emissions from a wide range of constit- Early work focused largely on environ- tification of individual bioaerosol parti- uent elements were observed, although mental and industrial monitoring of cles using LIBS was investigated in considerable matrix effects were report- particulate emissions, with studies ex- another study, noting the necessity of ed between the various minerals. The amining LIBS for heavy metal-contain- using single-shot LIBS analysis for this development of a method for the ing aerosol particles659 and sulfuric acid application.664 While calcium was de- automated quantitative analysis of ores aerosols.658 tected at the single femtogram level, using LIBS was reported based on a A series of fundamental studies have among the best LIBS detection limits commercial LIBS instrument.651 Target- addressed related matrix effects, calibra- reported for single-shot aerosol analysis, ed elements included P, Ca, Mg, Si, and tion issues, and upper size limits, as the authors were unable to detect other Al, yielding calibration curves with detailed in the above-mentioned re- trace elements or molecular emission linear regression coefficients ranging view.35 As an example, the temporal bands, leading to the conclusion that from 0.980 to 0.993. A key step was evolution of the particle-derived analyte real-time single spore analysis in ambi- the careful selection of analyte lines to signal was explored in the context of ent conditions was unfeasible. Spectral minimize interferences because of the plasma–particle interactions, leading to fingerprints of bacterial strains recorded observed matrix dependence of the the conclusion that an upper size limit directly in bacterial colonies was report- technique. LIBS was successfully used for quantitative analysis (now reported ed with a goal of classification using a for the analysis of carbonates and in the range of ~2–7 lm) is largely due set of atomic emission lines in combi- silicates using a broadband spectral to rate limitations of heat and mass nation with spectral bands attributed to region and regression analysis,652 with transfer rather than simply due to organic species.663 Discrimination be- the authors concluding that LIBS has available plasma energy.667 Our focus tween five bacterial strains was achieved promise for mineral identification and in here is on the specific topic of bioaerosol using a two-dimensional spread of select situ analysis. When minerals were analysis with LIBS, given the impor- emission ratios, and the potential of misidentified, it was generally identified tance of biosafety in homeland security, LIBS was stated for this application. as a mineral with a similar elemental healthcare, and food science. In one of the first papers to discuss composition. Principal components The first four studies using LIBS complementary analytical schemes for (PCs) and partial least squares discrim- specifically for analysis of bacteria and bioaerosol analysis, a comprehensive inant analysis (PLS-DA) were used to bioaerosols appeared in 2003.462,660–662 analysis of single-shot LIBS data was identify distinguishing characteristics of In this collection of papers, a rather wide reported for both spores and pollens.665 a wide range of geological samples.653 range of bacteria, including several Supporting measurements were also Both double-pulse and single-pulse Bacillus species, fungal and mold recorded using a single-particle aerosol LIBS systems were used, with no spores, and pollens were examined on mass spectrometer. In a critical assess- significant advantage realized with dou- substrates, in direct aerosol form, and as ment of their data, the authors concluded ble-pulse except for the analysis of isolated single pollens. Using a wide that the application of a LIBS-only solids. range of spectral signatures (e.g., both approach for bioaerosol detection may

APPLIED SPECTROSCOPY 397 focal point review be limited in the information provided, an autoclaved specimen, and a UV- laser-induced plasma was the ignition notably due to the low analyte signals irradiated specimen) were all correctly source.673 Taking laser ignition a step associated with the small dimensions of classified as E. coli, demonstrating that further, Phuoc674 examined laser-in- bioaerosol particles, and that more than LIBS can provide accurate analysis on duced breakdown as the simultaneous one technique (e.g., LIBS and Raman specimens rendered innocuous via com- ignition source and for direct analysis of and/or laser-induced fluorescence) monly available anti-microbial proce- the fuel-to-air ratio. Using ratios of the should or would have to be used. dures. Furthermore, 100% classification H and O atomic emission lines, a least- The interest in LIBS continues in the was achieved with LIBS when testing squares fit was used to produce an literature, with additional schemes and for the effects of nutrition via the growth acceptable calibration curve for the approaches being investigated in recent medium and bacterial starvation. fuel-equivalence ratio. The laser-in- studies. A femtosecond LIBS system Combustion. The use of LIBS for duced ignition of biogas–air mixtures was used to analyze homogeneous solid analysis of combustion systems is an was also examined successfully with samples (i.e., a pastille) of five different excellent match of the real-time capa- LIBS for a range of fuel-rich and fuel- species of bacterium.666 Using atomic bilities of LIBS with a potentially well- lean mixtures.672 emission from six trace minerals (Na, suited experimental matrix, although the Moving beyond analysis of gas-phase Mg, P, K, Ca, and Fe), quantitative possibility of complexity remains. Be- species, LIBS has been examined for differentiation was realized in a six- cause exhaust and in-cylinder applica- analysis of particulate combustion spe- dimension hyperspace with each axis tions alike involve gaseous systems, the cies in a range of industrial and flame representing a detected trace element, sample is perfectly homogeneous for the systems. In a comprehensive study at with the authors concluding that the most part, and since the gas composition three industrial facilities, including a spectral hyperspace enables LIBS to is largely nitrogen and oxygen, with power generation boiler, a glass-melting provide biologically significant analysis additional hydrocarbons, the matrix is furnace, and two black liquor recovery of biological materials. As discussed in considered constant with regard to boilers in a paper mill, LIBS was detail in the chemometrics section, one breakdown and plasma temperature. investigated for real-time analysis.671 must be careful in the distinction The use of LIBS for quantification of The probe configurations were impor- between LIBS-based absolute identifica- flame stoichiometry and local mixture tant to overcome rather harsh industrial tion and LIBS-based classification. With fractions has been examined in several considerations, including preservation of appropriate spectral data processing studies, including analysis of propane, optical surfaces. A range of elements, 675 schemes as available with advanced methane, and CO2 in air mixtures, including Ca, Fe, K, Mg, Si, Al, Ti, Na, chemometrics, LIBS can successfully spatially resolved measurements in and Cl, were successfully measured in classify among homogeneous samples methane–air diffusion flames,682 and real time, giving insight into the relevant in nearly all cases. The use of a local equivalence ratios in methane, processes. Several studies have exam- sequential dual-cover gas approach for ethylene, and propane air mixtures,677 ined LIBS for fast and quantitative LIBS analysis of bacterial strains was with relative standard deviations for analysis of fly ash, focusing primarily investigated using sequential analysis in single-shot measurements in the range on the elements Si, Al, Fe, Ca, and a cover gas of argon and then helium.668 of 2 to 3%. On-line and in-cylinder Mg.679–681 Given the wide variation in When analyzing with a discriminant analysis is obtainable with the LIBS fly ash, this application generally re- function analysis, 100% categorization method, as demonstrated using an opti- quires extensive calibration over a wide was realized when comparing a gram- cal-access spark plug for direct in- range of samples, as noted in all three negative and a gram-positive bacteria. In cylinder measurements of fuel equiva- studies. a new approach to sample introduction, lence ratios,678 showing a high linear Forensics. The use of LIBS in the LIBS was used for analysis of bioaer- correlation of the atomic emission ratios field of forensics provides opportunities osol particles following particle ejection with the equivalence ratios, and in the and presents significant challenges. by optical catapulting.669 The study exhaust stream of an engine,461 showing Competing with LIBS are established focused on the optimization of the laser agreement between LIBS measurements analytical methods such as X-ray fluo- parameters (e.g., interpulse delay, sam- and an on-line gas analyzer. A fiber- rescence, ICP-OES, and LA-ICP-MS. pling distance, and laser fluence) for optically delivered laser spark was used LIBS offers the advantages of rapid successful optical ejection of surface for fuel-to-air ratio measurements in analysis with little sample preparation particles to create an aerosol for subse- ultra-lean methane air mixtures.676 and therefore significant cost savings. quent LIBS analysis. In a recent study, The laser-induced plasma, in addition However, the forensic arena brings the effect of bacterial environment and to serving as an analytical plasma, also along high standards with regard to metabolic stress were examined in the serves as an ignition for combustible accuracy and precision, as the criminal context of LIBS-based analysis of E. gases given the high-temperature, high- justice system demands such perfor- coli and S. viridians.670 One key finding radical plasma volume, where interest mance. Applications include analysis of was that based on analysis of thirteen lies in advanced combustion schemes glass, which is promising given the normalized emission intensities using a for alternative fuels. LIBS experiments relatively consistent matrix of many discriminant function analysis, three were conducted in a high-pressure, glasses, and which has attracted the specimens of E. coli (a live specimen, constant-volume chamber where the most attention in the LIBS community.

398 Volume 66, Number 4, 2012 Other applications include detection of comparing 266 nm and 532 nm laser ing the chance for significant matrix gunshot residue, which was investigated wavelengths for various glass stan- effects. In addition, given the fact that in a 2003 study.683 Using adhesive tape dards.688 Given the large variance in many pharmaceutical formulations are to directly sample the hands of potential optical opacity of glass samples, the well-blended, sample non-homogeneity suspects, LIBS was found to successful- comparison of relatively deep UV and is not a significant concern. In aggre- ly identify persons that fired multiple visible radiation is important for this gate, such an application appears to be gunshots; however, reliable identifica- application. The 266 nm laser was found an ideal opportunity for the successful tion was not realized for the firing of a to remove more mass per shot, although implementation of the LIBS technique, single shot. the 532 nm laser produced stronger allowing a system to operate within a As discussed above, glass is a desir- emission signals. However, the 266 nm rather well defined sample space and able target for LIBS analysis for major, LIBS system produced better precision under well-controlled and repeatable minor, and trace elements, with the and was recommended for analysis of conditions, making possible precise common goal of matching crime scene glass, notably transparent glass samples. and accurate analytical measurements. samples to libraries or to individual In a similar study, 266 nm radiation was Here we review a few representative suspects. Automobile float glass, per- compared to 1064 nm radiation for LIBS studies for LIBS analysis of tablets and haps the most common glass for foren- analysis of glass samples, corroborating coatings. sics analysis, was studied with LIBS in a the earlier study that the 266 nm laser LIBS was used for real-time analysis direct comparison with LA-ICP-MS.684 provides improved laser–sample cou- of multi-component tablets, using atom- Elemental emission ratios were used pling and better analytical perfor- ic emission lines from unique elements with LIBS, allowing discrimination of mance.690 (e.g., phosphorus from the drug formu- 83% of the glass samples with 99% Other forensic applications of LIBS lation and magnesium from the lubricant confidence using LIBS alone, increasing include papers and inks, paints, and soil compound) for quantitative analysis.692 to 96–99% discrimination using a com- samples. The micro-analysis of docu- Significant improvement in the analyti- bination of LIBS and refractive index ment paper and gel inks by LIBS was cal performance was found through (RI) analysis. The LA-ICP-MS provided directly compared to LA-ICP-MS for signal normalization, using for example 100% discrimination without the use of discrimination of paper and ink sourc- the carbon line or the plasma continuum RI analysis. The authors conclude that a es.689 Both LIBS and LA-ICP-MS emission. For analysis of halogen spe- LIBS/RI combination is a useful analy- yielded excellent discrimination for pa- cies such as F or Cl, the use of a helium sis tool for those without access to LA- pers (.98%) and for inks (96–99%), atmosphere produced nearly an order of ICP-MS. In a follow-up study of four noting that the latter would otherwise be magnitude enhancement in the signal-to- glass types,685 the LIBS/RI combination inseparable by conventional methods. background ratio of the targeted analyte yielded 87.2% discrimination of 1122 TheanalysisofpaintsbyLIBSis lines. In a similar study, this group pairwise comparisons, while LA-ICP- perhaps a natural application for chemo- reported an accuracy of 0.5% for MS alone provided 98.8% discrimina- metrics, given that paints form a com- magnesium stearate analysis, with in- tion of 666 pairwise comparisons. Sam- plex heterogeneous system and that tra-tablet, intra-batch, and inter-batch ples of side-mirror glass were less forensic comparison remains a signifi- relative standard deviations of 13.8%, discriminated by LIBS due to a larger cant analytical challenge (see earlier 5.4%, and 7.4%, respectively.693 As an variance in emission intensities. The use section). Finally, we consider the use analytical benchmark, near-infrared of linear and rank correlation analysis of LIBS for forensic analysis of soil (NIR) spectroscopy was used to com- with LIBS spectral data provided effec- samples.691 Optimization of the LIBS pare the analytical performance of LIBS tive identification of glass fragments technique produced favorable compari- for analysis of magnesium stearate in with a 95% confidence level,686 provid- son with LA-ICP-MS for discrimination both powders and solid tablets.694 It was ing insight into an alternative data between regional soil samples. Using found that NIR provided better accuracy analysis routine. The Almirall group principal components analysis (PCA), and precision than LIBs; however, the has extensively examined LIBS in the the LIBS provided a 99.4% correct LIBS method showed superior selectiv- context of forensic glass analysis.687–691 classification rate, which compared fa- ity and robustness with regard to sample LIBS, LA-ICP-MS, and X-ray fluores- vorably to the LA-ICP-MS results. The matrix perturbations. As mentioned cence (XRF) were compared for dis- authors noted the importance of careful above, the use of LIBS within a crimination power using a set of 41 glass sampling for geographic site character- relatively controlled sample space is a fragments.687 Using 266 nm laser exci- ization. strength of the technique. In a recent tation, LIBS provided excellent preci- Pharmaceutical. The LIBS tech- study, LIBS was examined for analysis sion (,10% RSD for elements and nique has enjoyed success in the area of both macro- and micronutrients in ,10% RSD for atomic emission ratios), of pharmaceutical analysis, notably for multi-element tablets.696 Overall, the with the conclusion that LIBS compares analysis of powders and tablets. For this LIBS results were in agreement with favorably to the other analytical methods application, LIBS is well-suited for those obtained by ICP-OES, with the for a comprehensive data analysis pro- quality control and process monitoring, LIBS coefficients of variation ranging tocol. The wavelength dependency of given that the sample space is rather from 2 to 16%. The analytical figures of LIBS for glass analysis was examined, well-controlled overall, greatly mitigat- merit for the LIBS method were con-

APPLIED SPECTROSCOPY 399 focal point review cluded as consistent with the intended and LA-ICP-MS for trace metal imag- ICP-OES analysis. However, it is diffi- purpose, and the authors recommended ing.698 cult to extrapolate the results based on LIBS for analysis of tablets with ‘‘sim- Perhaps the most natural application trace elemental analysis to the larger ilar matrices’’ for the elements Ca, Cu, of LIBS in medicine is the analysis of applicability of LIBS for cancer analy- Fe, Mg, Mn, P, and Zn. hard tissues such as bone and teeth, sis. In a more recent study by this group, Several studies have examined LIBS which provide a more consistent matrix LIBS was used to differentiate between for analysis of tablet coatings in the and are high in elements giving rela- chicken brain, lung, spleen, liver, kid- pharmaceutical arena. One study as- tively strong atomic emission. Toward ney, and skeletal tissues. Using 21 sessed LIBS for rapid characterization this end, a range of calcified tissues selected analyte lines, LIBS was able of the coating thickness, uniformity, and (human teeth and eggshell) were exam- to successfully distinguish among the the photodegradation-predictive poten- ined with LIBS using the calcium and range of tissue samples. As discussed in tial.695 It was reported that batch-to- magnesium ion-to-neutral emission ra- more detail in the chemometrics section, batch variability in coating thickness tios.701 Both the Mg and Ca ion-to- discrimination among sample types is was easily measured with LIBS, and the neutral ratios were found to correlate generally successful with LIBS; hence, LIBS coating thickness measurements well with material hardness, the Mg the full potential of LIBS for true provided a good correlation (R2 . 0.99) ratio being preferred, although the ratios pathological diagnosis remains to be with photodegradation as measured in- also showed a dependency on the demonstrated. The use of LIBS for dependently with high-performance liq- specific matrices. Singh and Rai exam- identification of bacterial colonies in uid chromatography (HPLC). LIBS for ined the potential of LIBS for the rapid blood and bile samples was investigated identification of carious teeth, focusing for Pseudomonas aeruginosa and two analysis of tablet coatings was assessed 702 in a second study, specifically seeking to on the minerals Ca, Mg, Cu, Zn, Ti, P, strains of E. coli. Using nineteen 697 Na, and K, as well as H, C, and O atomic and ionic emission lines, LIBS quantify coating thickness. The LIBS 707 results were found to directly correlate emission. Significant differences in was able to distinguish differences in the with tablet weight gain, thereby validat- the elemental composition of the healthy P. aeruginosa and E. coli bacteria in ing the analytical results. The exact tooth and caries-affected tooth regions several background mediums, including TSA and blood agar. position of the LIBS sample point on were noted, including the observation that H and O emission was less in the The medical applications of LIBS the target surface was found to have a healthy portions. LIBS has been used for also extend to issues of safety and significant effect on the analytical re- the quantitative analysis of gallstones toxicity, in which the utility of LIBS as sults, which was attributed to sample through analysis of both major and a rapid analysis tool can play a role. curvature. minor species.703 A calibration-free Asbestos poses a significant health risk Biomedical. LIBS has drawn interest LIBS approach over the 200–900 nm to humans, with the chemical and for the application to biomedical fields, wavelength range was used and com- morphological properties of high impor- notably for pathological use, as well as pared with ICP-AES measurements. tance. LIBS was used for the analysis of for toxicology assessment. The use of Overall, LIBS was demonstrated as a asbestos in different samples in the LIBS for analysis of tissues, notably means for categorization of different context of field operations.700 A good hard tissues, is attractive for assessment types of gallstones. In another hard- correlation between the Mg/Si and Fe/Si of minerals, while the use for soft tissues tissue application of LIBS, the technique atomic emission ratios with dispersive can be challenging given the potential was used for analysis of pathological X-ray spectroscopy and SEM analysis for significant matrix effects. A recent nails.708 In addition to the analysis of the for discrimination between asbestos and 30 review by Liu and Zhang examines the Ca, Na, and K atomic emission lines, the cements was found. LIBS was used for application of LIBS for the biomedical CN emission line was used in consider- quantitative analysis of common Indian field. They note the major group of ation of thermodynamic equilibrium. edible salts in the context of kidney elements in the human body (H, C, N, The authors concluded that LIBS anal- disease.704 LIBS was successful in and O) account for about 96.6% of the ysis provides a distinct difference be- determining salt constituents and identi- mass, with the trace elements Na, Mg, P, tween normal and pathological nails fying the more beneficial salts for S, Cl, K, Ca, Fe, Mn, Co, Zn, and Ni (e.g., fungal infections). patients suffering from chronic kidney accounting for less than 5%, and the Soft tissues present more challenges, disease. From the perspective of con- minor elements V, Mo, Li, F, Si, As, Br, although the motivation remains high to sumer safety, LIBS was assessed for Sn, I, and Ba accounting for less than establish analytical tools suited to tissue rapid analysis of toxic substances (Ba, 0.001%. Given the high concentrations pathology such as determination of Cd, Cr, and Pb) in children’s toys.705 of the ubiquitous elements H, C, N, and malignancy. One such study examined Following optimization of experimental O, one can readily conclude that the the use of LIBS for discrimination parameters, LIBS was found to correlate human body presents an analytical between normal and malignant canine well with ICP-OES analysis, with the challenge for LIBS. Another recent hemangiosarcoma tumor cells.699 The conclusion that LIBS is feasible for review speaks to the importance of concentrations of trace elements were rapid analysis of potentially toxic ele- analytical techniques for biomedical found to be different in the normal and ments. In another potential application applications, addressing SXRF, SIMS, malignant cells, which was verified with for health risk assessment, LIBS was

400 Volume 66, Number 4, 2012 used in the analysis of carbon nanotubes (CNT) in a recent study.709 CNTs present a challenge for LIBS from several perspectives, including their small size, on the order of tens of nanometers, and to the ubiquitous nature of carbon in the environment. A cou- pling of LIBS and TEM grid analysis was found, although the results were not quantitative. Nuclear. LIBS has a rather wide range of applications in the nuclear area, including in situ materials analysis for inspection, as well as safety for contam- ination detection. In an early industrial application of LIBS, a fiber-optic-based LIBS system was used to successfully inspect the copper content of critical metal components in the reactor housing of a nuclear power station.711 Copper content over the range from 0.04 to 0.6% was successfully recorded with measurement times less than 3 minutes, noting the importance of rapid analysis for this application. LIBS has two potential roles in Tokamak facilities, namely for end-point detection to pre- serve the bulk materials and in chemical analysis.712 LIBS has drawn interest for rapid and real-time analysis of nuclear materials, where the potential for in situ and non- contact analysis is attractive. Direct FIG. 15. The miniaturization of LIBS systems has been due in part to the availability of detection of uranium in aerosol particles compact laser systems and compact spectrometers, as well as systems engineering and using LIBS was investigated in a integration. We show four typical compact laser systems, including those with an integrated laboratory environment.710 Using a fiber optic delivery, in the upper left and depict typical compact spectrometer configura- tions in the upper right. At the bottom we depict three miniaturized LIBS systems that have pulsed CO2 laser for plasma generation, been produced. Our choice of representative images for lasers, spectrometers, and uranium atomic emission was success- integrated LIBS systems is simply illustrative, and we fully recognize that there are many fully detected using ion lines at 409.0 such quality components and systems available to the LIBS community. [Fiber laser image and 411.6 nm for both aerosol particles courtesy of SPI lasers. All images have been reproduced by permission of the original and droplets, although no analytical sources. See Acknowledgments.] figures of merit are reported. Femtosec- ond LIBS was used to assess the feasibility of diagnosing and profiling study, time- and space-resolved atomic MISCELLANEOUS silver transport through the silicon emission was used to investigate the carbide layer of fuel particles for the We include here a brief discussion of 713 segregation of hydrogen and deuterium Pebble Bed Modular Reactor. The 714 an analytical technique that is rather femto-LIBS approach was concluded to in laser-induced plasma plumes. closely related to LIBS, namely, spark- provide the necessary spatial and depth Industrial. Industrial applications of induced breakdown spectroscopy resolution for the intended application. LIBS include, for example, process (SIBS).716–718 SIBS is a plasma-based Leak detection in nuclear facilities was monitoring, recycling and sorting, and atomic emission scheme; however, the addressed in a LIBS study that focused quality control during manufacturing. plasma is created by a high-energy, on boron detection for the precipitates of We refer the reader to two informative electrically produced spark discharge. boric acid on a range of low alloy chapters by Noll et al. dealing specifi- While the resulting plasma volume is steels,715 which is applicable to the cally with industrial applications (Chap- generally much larger than the typical detection of cooling water leaks follow- ter 11 in Ref. 2), and as related, to high- LIBS plasma, the overall plasma dy- ing the addition of H3BO3, a common speed LIBS applications (Chapter 14 in namics are analogous, and the atomic neutron poison. In a more fundamental Ref. 2). emission signal is subsequently opti-

APPLIED SPECTROSCOPY 401 focal point review

opportunity for LIBS applications, as we depict in Fig. 16. In view of the significant potential of LIBS in many different fields, for example, biomedical and geochemistry, one must consider the use of LIBS instrumentation by non-experienced us- ers. This brings an additional obligation to the LIBS community, namely, the design of robust instrumentation and user-friendly methodologies. In further regard to the growth and acceptance of LIBS as a spectroscopic method, we make an observation that the vast majority of literature cited in this review stems from the analytical spectroscopy field rather than from the fields corresponding to the specific application. It is felt, however, that acceptance of LIBS to the larger user community will be aided by the publi- cation of successful applications within the field of use. From the analytical perspective, LIBS may be considered sufficiently mature for the development of organized inter- laboratory comparison (i.e., round-robin tests). Toward this effort, a uniform set FIG. 16. The future of LIBS is strong, due in part to the broad versatility of LIBS as an of instrumental and operational parame- analytical technique. We show here a wide range of LIBS applications and implementations. ters are needed, which should then be Our choice of representative images is simply illustrative and is intended to show the followed rigorously for the comparison diversity of LIBS applications. [Image credit for the Remote sensing: NASA/JPL-Caltech/ LANL/J.-L. Lacour, CEA. All other images have been reproduced by permission from the of quantitative data obtained with dif- original sources. See Acknowledgments.] ferent instruments and different labora- tories. We note, moreover, that these mized for quantitative analysis. SIBS induced plasmas, as covered in the first exercises should focus primarily on has been used for continuous emission part.1 Our current focus has been a minor and major elements rather than monitoring of heavy metals at concen- critical discussion of calibration, matrix on trace elements (i.e., sub-ppm). In- interferences, detection limits, advances deed, at the ng/g level, as stated by To¨lg trations in the range of 15 to 75 lg per 429 cubic meter.716,717 The method was in instrumentation and methodology, and Klockenka¨mper, ‘‘the bitter real- successfully demonstrated for detection data processing, and related applica- ity of interlaboratory comparisons teach- of Cd, mercury (Hg), selenium (Se), tions. Throughout this review, we have es us that systematic errors increase antimony(Sb),As,Pb,andCr.In attempted to summarize the current state rapidly as the concentration becomes particular, the SIBS hardware was of the literature, pointing out the impor- lower and lower.’’ operated without failure for more than tant underlying analytical concepts rele- A common thread throughout this 100 hours. SIBS has also been investi- vant to all facets of LIBS. review concerns the issues of matrix- gated for rapid field screening of heavy As detailed throughout this review related effects in the context of quanti- metals in soils.718 Detection limits in the article and summarized in Table I, LIBS tative analysis, primarily accuracy. For range of 25 mg/kg were reported for Pb, has a number of positive attributes that example, much of the research related to Cr, barium (Ba), Hg, and Cd. make it appealing for a large number of double-pulse methods, calibration-free applications. A recent trend, as depicted schemes, and chemometrics are moti- CONCLUSIONS in Fig. 15, is toward miniaturization and vated by the goals of improved analyt- portability, opening the door for further ical performance. Along these lines, it is In this article, we have highlighted the fieldable LIBS instrumentation, as noted interesting to note an observation re- various steps and approaches of LIBS by Fortes and Laserna.42 Combining the ported by Ohls719 in 1996, namely, that from the sample to analytical signal, by versatility of LIBS as an analytical over a 30 year span of microsampling of focusing on the practical quantitative scheme along with the recent advance- solids by arc, spark, and laser ablation, aspects and relevant applications, rather ments in hardware, both configurations significant improvements have occurred, than on the diagnostic aspects of laser- and methodology, provides a timely as expected, in terms of speed of

402 Volume 66, Number 4, 2012 analysis and amount of precise data light, Ocean Optics, and SPI Lasers. We thank the 14. W.–B. Lee, J.Wu, Y.-I. and Lee, J. Sneddon, obtained, but surprisingly, little has following individuals for contributions to Fig. 16: Eds., Recent Applications of Laser-Induced Microanalysis: Richard Russo/LBNL; Industry- Breakdown Spectrometry: A Review of improved in terms of accuracy. This Recycling & Geochemistry: Reinhard Noll/ Material Approaches, Appl. Spectrosc. Rev. observation implies that the physics of Fraunhofer ILT, Aachen, Germany; Benchtop 39, 27 (2004). analyte sampling, vaporization, and Instrumentation: Steven Buckley/Photon Ma- 15. J. M. Vadillo and J. J. Laserna, Laser- excitation remain at the core of LIBS chines; Conservation: Anastasia Giakoumaki and induced plasma spectrometry: Truly a surface Demetrios Anglos, IESL-FORTH and University analytical tool, Spectrochim. Acta, Part B 59, as an analytical technique, and that of Crete, Heraklion, Crete, Greece; Industry - 147 (2004). improvements can only be expected Process Monitoring: Mohamad Sabsabi/NRCC, 16. R. E. Russo, X. L. Mao, C. Liu, and J. through a fundamental knowledge of Que´bec, Canada. Gonzalez, Laser assisted plasma spectro- these physical processes. Accordingly, chemistry: Laser ablation, J. Anal. At. we believe that the reconciliation of new Spectrom. 19, 1084 (2004). 1. D. W. Hahn and N. Omenetto, Laser-induced 17. J. D. Winefordner, I. B. Gornushkin, T. applications and fundamental research breakdown spectroscopy (LIBS), Part I: Correll, E. Gibb, B. W. Smith, and N. remains essential within the LIBS com- Review of basic diagnostics and plasma- Omenetto, Comparing several atomic spec- munity. particle interactions: Still-challenging issues trometric methods to the super stars: Special within the analytical plasma community, emphasis on laser induced breakdown spec- Appl. Spectrosc. 64, 335A (2010). trometry, LIBS, a future super star, J. Anal. ERRATA 2. A. W. Miziolek, V. Palleschi, and I. At. Spectrom. 19, 1061 (2004). We would like to draw the readers’ Schechter, Eds. Laser-Induced Breakdown 18. R. S. Harmon, F. C. De Lucia, A. W. Spectroscopy (LIBS), Fundamentals and Miziolek, K. L. McNesby, R. A. Walters, and attention to the following errors that Applications (Cambridge University Press, P. D. French, Laser-induced breakdown were unfortunately left in print in Part I Cambridge, UK, 2006). spectroscopy (LIBS)–An emerging field-por- (Appl. Spectrosc., 64(12), 335A, 2010). 3. D. A. Cremers and L. J. Radziemski, table sensor technology for real-time, in-situ At page 343A, the units of the electron Handbook of Laser Induced Breakdown geochemical and environmental analysis, number density in Eq. 1 should read Spectroscopy (Wiley, New York, 2006). Geochemistry: Exploration, Environment, -3 4. J. P. Singh and S. N. Thakur, Eds., Laser Analysis 5, 21 (2005). (cm ), as it is obvious from the Induced Breakdown Spectroscopy (Elsevier, 19. C. Fotakis, W. Kautek, and M. Castillejo, discussion following Eq. 1 In Table II. Amsterdam 2007). Eds., Lasers in the Preservation of Cultural Equations T2.11 and T2.12 contain the 5. E. H. Piepmeier, Analytical Applications of Heritage, Laser Chemistry (Special Issue) parameter D , defined as the lens lasers, E.H. Piepmeier, Ed., Chemical Anal- (Hindawi Publ. Corp., New York, 2006). ‘ ysis Series, P. J. Elving and J. D. Wine- 20. R. Fantoni, L. Caneve, F. Colao, L. Fornar- diameter. Note that this is valid under fordner, Eds. (Wiley and Sons, New York ini, V. Lazic, and V. Spizzichino, Laser the assumption that the laser beam 1982), Chap. 19. Induced Breadown Spectroscopy (LIBS). diameter uniformly covers the lens 6. D. A. Rusak, B. C. Castle, B. W. Smith, and The process, applications to artwork and J. D. Winefordner, Fundamentals and Appli- environment, B. Di Bartolo and O. Forte, diameter. If the contrary holds, D‘ cations of Laser-Induced Breakdown Spec- Eds., Advances in Spectroscopy for Lasers should be referred to the diameter of troscopy, Crit. Rev. Anal. Chem. 27, 257 and Sensing (Springer, New York, 2006), pp. the laser on the lens surface. In Table (1997). 229-254. IV, Eq. T4.11, the numerator in the last 7. F.-Y. Yueh, J. P. Singh, and H. Zhang, 21. K. H. Kurniawan and K. Kagawa, Hydrogen factor of the right-hand side of the Laser-Induced Breakdown Spectroscopy El- and Deuterium Analysis Using Laser-In- expression for the line spectral radiance emental Analysis, in Encyclopedia of Ana- duced Plasma Spectroscopy, Appl. Spec- lytical Chemistry, R. A. Meyers, Ed. 2066- trosc. Rev. 41, 99 (2006). should be replaced by the symbol ‘‘‘’’. 2087 (John Wiley and Sons, New York, 22. V. I. Babushok, F.C. De Lucia, Jr., J. L. Finally, in Eq. T4.15, two expressions 2000). Gottfried,C.A.Munson,andA.W. are reported, and therefore a space 8. D. Anglos, Laser-Induced Breakdown Spec- Miziolek, Eds., Double pulse laser ablation should be inserted between the second troscopy in Art and Archaeology, Appl. and plasma: Laser induced breakdown spec- Spectrosc. 55, 186A (2001). troscopy signal enhancement, Spectrochim. and the third ratios shown. 9. R. Noll, H. Bette, A. Brysch, M. Kraushaar, Acta, Part B 61, 999 (2006). I. Mo¨nch, L. Peter, and V. Sturm, Laser- 23. C. Pasquini, J. Cortez, L. M. C. Silva, and F. ACKNOWLEDGMENTS induced breakdown spectrometry applica- B. Gonzaga, Laser Induced Breakdown This work was supported by the National tions for production control and quality Spectroscopy, J. Braz. Chem. Soc. 18, 463 Science Foundation through grant CHE-0822469, assurance in the steel industry, Spectrochim. (2007). as part of the Plasma-Analyte Interaction Working Acta, Part B 56, 637 (2001). 24. B. Salle´, P. Mauchien, and S. Maurice, Laser Group (PAIWG), a collaborative effort of the 10. J. Bublitz, C. Doe`lle, W. Schade, A. induced breakdown spectr oscopy in open- University of Florida, Federal Institute of Materials Hartmann, and R. Horn, Laser induced path configuration for the analysis of distant Research and Testing (BAM) in Berlin, and the breakdown spectroscopy for soil diagnostics, objects, Spectrochim. Acta, Part B 62, 739 Institute for Analytical Sciences (ISAS) in Dort- European J. Soil Sci. 52, 305 (2001). (2007). mund, jointly funded by the NSF and DFG. We 11. L. J. Radziemski, From LASER to LIBS, the 25. N. H. Cheung, Spectroscopy of Laser Plumes would like to acknowledge first Dr. Ben Smith and path of technology development, Spectro- for Atto-Mole and ng/g Elemental Analysis, our graduate students for the continuous useful chim. Acta, Part B 57, 1109 (2002). Appl. Spectrosc. Rev. 42, 235 (2007). discussions and help provided during the prepara- 12. E. Tognoni, V. Palleschi, M. Corsi, and G. 26. A. De Giacomo, M. Dell’Aglio, O. De tion of this review. Moreover, we would like to Cristoforetti, Quantitative micro-analysis by Pascale, and M. Capitelli, Review: From acknowledge many colleagues in our LIBS laser-induced breakdown spectroscopy: A single pulse to double pulse ns-Laser Induced communitywhohavesharedwithustheir review of the experimental approaches, Breakdown Spectroscopy under water: Ele- expertise and knowledge through many presenta- Spectrochim. Acta, Part B 57, 1115 (2002). mental analysis of aqueous solutions and tions of their research, and ensuing personal 13. K. Mu¨ller and H. Stege, Evaluation of the submerged solid samples, Spectrochim. Acta, discussions, at the various LIBS conferences. analytical potential of Laser-induced break- Part B 62, 721 (2007). We thank the following companies for contri- down spectrometry (LIBS) for the analysis of 27. C. Arago´n and J. A. 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APPLIED SPECTROSCOPY 403 focal point review

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404 Volume 66, Number 4, 2012 Cremers, and M. J. Ferris, Characterization cal Optimization, EMSLIBS 2007 Short 102. T. L. Thiem, R. H. Salter, J. A. Gardner, Y. I. of Laser-Induced Breakdown Spectroscopy Course 2–3, Paris (2007). Lee, and J. Sneddon, Quantitative simulta- (LIBS) for Application to Space Exploration, 88. C. Haisch, U. Panne, and R. Niessner, neous elemental determinations in alloys Appl. Spectrosc. 54, 331 (2000). Combination of an ICCD wih an echelle using LIBS in an ultra-high vacuum, Appl. 74. J. A. Aguilera, J. Bengoechea, and C. spectrograph for analysis of colloidal mate- Spectrosc. 48, 58 (1994). Arago´n, Spatial characterization of laser rial by LIPS, Spectrochim. Acta, Part B 53, 103. V. Sturm, L. Peter, and R. Noll, Steel induced plasmas obtained in air and argon 1657 (1998). Analysis with Laser-Induced Breakdown with different laser focusing distances, Spec- 89. H. Becker-Ross, S. Florek, H. Franken, B. Spectrometry in the Vacuum Ultraviolet, trochim. 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APPLIED SPECTROSCOPY 411 focal point review

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