1148 Volume 52, Number 9, 1998 APPLIED SPECTROSCOPY0003-7028 / 98 / 5209-1148$2.00 / 0

q 1998 Society for Applied Spectroscopy

Novel Probe for Laser-Induced Breakdown Spectroscopyand Raman Measurements Using an Imaging Optical Fiber

BRIAN J. MARQUARDT, DIMITRA N. STRATIS, DAVID A. CREMERS, andS. MICHAEL ANGEL *Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 (B.J.M., D.N.S.,

S.M.A.); and Group CST-1, MS J565, Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos,

New Mexico 87545 (D.A.C.)

A ® ber-optic probe designed for remote laser-induced breakdown

spectroscopy (LIBS), Raman spectroscopy, and Raman imaging hasbeen developed for the microanalysis of solid samples. The probe

incorporates both single-strand optical ® bers and an image guide

and allows atomic emission and Raman analysis of any spot on asolid sample within a 5 mm diameter ® eld of view. The real-time

sample imaging aspects of the probe are demonstrated by measur-

ing LIBS spectra from different regions of a granite sample and bymeasuring the Raman spectra of individual TiO2 and Sr(NO 3)2 par-

ticles on a soil substrate. The ability to obtain remote Raman images

of the TiO2 and Sr(NO3)2 particles on the soil substrate is also dem-onstrated. In this paper we discuss the design and implementation

of the ® ber-optic probe for obtaining LIBS spectra, Raman spectra,

and Raman images.

Index Headings: LIBS; Optical ® bers; Imaging; Remote sensing; Ra-man.

INTRODUCTION

Laser-induced breakdown spectroscopy (LIBS) is auseful method for determining the elemental compositionof various solids, liquids, and gases. This method hasbeen recently reviewed, and a number of applicationshave been described.1±6 In the LIBS technique, a high-power laser pulse is focused onto a sample to create aplasma or laser spark. Emission from atoms and ions inthe plasma is collected by a lens or optical ® ber andanalyzed by using a spectrograph equipped with a gateddetector. The use of a gated detector is necessary to sep-arate the delayed atomic and ionic emissions from theprompt thermal emission in the plasma. Atomic spectrallines can be used to identify the elemental compositionof the sample or to determine element concentrations.

LIBS is quite amenable to ® ber-optic remote sensingapplications because it requires no sample preparation,and only optical access to the sample is required. How-ever, optical ® bers have only recently been used for LIBSbecause of problems caused by high-powered laser pulsesdamaging the launch end of the optical ® ber. In recentarticles we described a simple optical setup for minimiz-ing or eliminating damage to optical ® bers during LIBSmeasurements of Pb in paint.7,8 This problem has alsobeen addressed by others, and several new ® ber-opticLIBS probes have been described for LIBS analysis.9±11

Optical ® bers are routinely used for remote Ramanspectroscopy, and recently m any ® ber-optic Ramanprobes and applications have been described. 12±15 In fact,

Received 15 October 1997; accepted 3 June 1998.* Author to whom correspondence should be sent.

several commercial ® ber-optic Raman probes are nowavailable. In most Raman probes, the optical ® bers areused as simple ``lightpipes’ ’ for the transmission of laserand Raman scattered radiation to and from the sample,respectively. Thus, while these systems are suitable forremote Raman measurements, they are limited to single-point measurements. Recently it was shown that small-diameter image guides (e.g., 350±1000 m m diameter)could be used for remote Raman and ¯ uorescence im-aging.16±18 The use of an image guide makes it possibleto do spectral imaging remotely or in situ in traditionallyhard-to-reach areas or hostile environments. The use ofan image guide in a LIBS probe would also make it pos-sible to perform elemental analyses of speci® c regions ona sample in remote locations and hostile environments.Since the designs of ® ber-optic LIBS and Raman probesare very similar, it should be possible to combine bothmeasurements into one probe and include a coherent col-lection ® ber for imaging capability. This possibility offersadvantages in applications where remote imaging is de-sired, such as cone penetrometry and hot-cell operations.

In this paper we describe a ® ber-optic imaging probefor remote LIBS, Raman point analysis and Raman im-aging and demonstrate its use for measuring atomic andmolecular spectra of solid samples remotely. The LIBSperformance of the probe was demonstrated by obtainingspectra of visually different areas on a granite sample.The Raman performance of the probe, both spectral andimaging, was demonstrated by using soil samples``spiked’ ’ with particles of TiO2 and Sr(NO3)2.

EXPERIMENTAL

System Design. The overall system design is shownin Fig. 1. The LIBS system consisted of a Q-switchedNd:YAG laser (Spectra Physics GCR-11 or ContinuumSurelite I) operating at 1064 nm, a 0.5 m f/7 (Spex Model1870), or 0.25 m f/4 (Chromex Model 250IS/RF) spec-trograph and an intensi ® ed charge-coupled device(ICCD) detector (Princeton Instruments, Model ITEA/CCD-576-S/RB-E) running WinSpec version 1.6.2 soft-ware. A Princeton Instruments PG-200 programmablepulse generator provided timing delays and synchronizedgating for the detector. The delay generator was used togate the detector for a 10 m s exposure, 700 ns after thelaser pulse, allowing the background continuum emissionto decay. For most LIBS spectra the laser repetition ratewas set at 2 Hz to eliminate breakdown at the ® ber face.The same detection system was used, with the 0.25 mimaging spectrograph, for Raman measurements. A 5 W

APPLIED SPECTROSCOPY 1149

FIG. 1. Schematic diagram of microprobe apparatus used to measure LIBS and Raman spectra and Raman images. The dotted lines designate® bers that are interchanged or moved for the Raman and LIBS measurements. A pellicle beamsplitter was used to launch the Nd:YAG and He:Nelasers into the same ® ber.

FIG. 2. Detail of the LIBS and Raman spectral components of themicroprobe. The Raman excitation and collection ® bers in the spectralcomponent were both equipped with in-line ® lters to reduce silica Ra-man interference. The coherent image guide with attached GRIN lenswas used to image a 5 mm diameter region of interest from the samplesurface and transfer the image to a video camera.

argon-ion laser (Coherent Model Innova 90) operating at514.5 nm was used for Raman excitation. Raman imageswere collected with the ICCD with the use of Winview(Princeton Instruments version 1.6.2) software. Ramanspectral data were collected at a nominal resolution of0.25 nm (i.e., ; 9 cm 2 1). For Raman imaging the spec-trograph slits were open to 1 mm to match the diameterof the collection ® ber.

Microprobe. The ® ber-optic microprobe consisted of® ve separate optical ® bers, four fused-silica single-strandmultimode ® bers for excitation and collection and oneimage guide, all housed in a 3.5 cm long 3 1.25 cmdiameter acrylic assembly. Although the probe contains® ve separate optical ® bers, there are two basic functionalcomponents, a spectral component and an imaging com-ponent. However, only three ® bers are used together forany particular measurement. Three of the ® ve ® bers (twolaser delivery and one collection) in the probe were usedfor spectral LIBS and Raman measurements of selectedregions within the ® eld of view of the image guide (seeFig. 2). For Raman imaging the same image guide wasused, but laser delivery was provided by a separate mul-timode optical ® ber with an attached GRIN lens (see Fig.3). Although Figs. 2 and 3 are shown as separate units,they were in fact incorporated into the same probe.

The spectral component of the probe (Fig. 2) consistedof a 1 mm core diameter silica/silica-clad LIBS delivery® ber (3M Specialty Optical Fibers, Model FG-1.0-UAT),a 1 mm core diameter hard-clad ® ber (3M Specialty Op-tical Fibers, Model FT-1.0-URT), used for collection ofboth LIBS and Raman spectra, and a 600 m m core di-ameter hard-clad Raman excitation ® ber (3M SpecialtyOptical Fibers, Model FT-600-URT). Both the Raman ex-citation and collection ® bers were equipped with in-line® lters to reduce the amount of silica Raman signal reach-ing the detector. The in-line ® lter was removed from thecollection ® ber during LIBS measurements. In the acrylicprobe, a 6.35 mm diameter, 12 mm focal length lens nearthe ® ber and a 6.35 mm diameter, 6.4 mm focal lengthlens near the sample were used to transfer excitation lightto the sample and collect the emitted and scattered light(see Fig. 2). The LIBS delivery ® ber was also used to

1150 Volume 52, Number 9, 1998

FIG. 3. Detail of the Raman imaging component of the microprobe. A1 mm core diameter excitation ® ber with attached GRIN lens illumi-nated a 5 mm diameter region on the sample. A coherent image guidewith attached GRIN lens imaged an overlapping region and transferredthe image to an image-corrected spectrograph with an ICCD detector.

deliver a 1 mW He:Ne beam to indicate the region ofinterest on the sample. The diameter, 1 mm, and positionof the He:Ne spot on the sample, visible in the imageguide ® eld of view, were nearly identical to the size andposition of the focused Nd:YAG spot, allowing very pre-cise positioning of the probe. The probe is positioned andaligned for analyzing speci® c sites on the sample by plac-ing it ¯ at on the sample surface and moving the probeor sample laterally while viewing a video monitor untilthe He:Ne beam illuminates a region of interest. The dis-tal end of the probe housing was machined so that, whenheld ¯ at against the sample, the focusing lens is at theoptimal focus for both the LIBS and Raman measure-ments. Optical alignment of the various ® bers and lensesin the probe housing is accomplished by using a set screwfor the image guide, while the other ® bers were prealignedand epoxied permanently into position. The lenses in theprobe are held in place by simple pressure ® ttings.

A 1 mm core diameter, 10 foot long coherent imageguide (Sumitomo Electric, Model IGN-1.0/13) with 13 000individual 8±9 m m elements was used both for samplevisualization and for Raman imaging. Attached to thesample end of the image guide was a 1 mm diameterSELFOC imaging GRIN lens from NSG America (ModelILH-100-W05-055-NCO), which was used to image a re-gion of the sample onto the face of the image guide (seeFigs. 2 and 3). Currently, the maximum length of imageguide available that maintains good image contrast is ap-proximately 30 ft. For real-time visual imaging of thesample, light was collected and transferred to a black andwhite CCD video camera (Genesys, Model GCB1324)using a 10 3 microscope objective (see Fig. 1). Real-timeimages of the sample were displayed on a video monitor,and still images were captured by using a commercialframe grabber (Snappy Model 2.0). A minilight ¯ ashlightbulb machined into the acrylic probe housing suppliedlight to the sample for real-time visual imaging.

For Raman imaging (see Fig. 3), excitation light was

provided by a 1 mm core diameter hard-clad ® ber (3MSpecialty Optical Fiber, Model FT-1.0-URT) with an at-tached GRIN lens. This arrangement resulted in an illu-minated spot on the sample approximately 5 mm in di-ameter. The same 1 mm diameter image guide used forsample visualization, shown in Figs. 2 and 3, was alsoused to collect the Raman images. The ® eld of view ofthe image guide was adjusted to overlap with the areailluminated by the excitation ® ber at the sample surface.

To illustrate the imaging ability of the probe, we se-lected samples that were visually heterogeneous and thatdisplayed unique Raman and LIBS spectra. A small pieceof granite was found to be ideal for LIBS because itconsisted of small (less than 2 mm diameter) light anddark mineral phases that displayed clearly different LIBSspectra. TiO2 and Sr(NO3)2 particles (Aldrich ChemicalCompany) were well suited for LIBS and Raman spectralmeasurements, as well as Raman imaging. Samples wereprepared by pressing a clay-based soil into 2.5 cm di-ameter pellets and adding small amounts of TiO2 and Sr(NO3)2 powder to the sample surface.

RESULTS AND DISCUSSION

Dark® eld images and LIBS spectra of the granite sam-ple are shown in Fig. 4. In the dark® eld images, Figs. 4aand 4c, the region of interest for the LIBS measurementsis shown by the He:Ne laser spot on the left side of theimages. The total imaged area was approximately 5 mmin diameter, the He:Ne spot and the Nd:YAG samplingareas were approximately 1 mm in diameter. The granitesample used in this study was typical, consisting of; 50% plagioclase feldspar (CaAl2Si2O8) with traces ofTi, Ba, and Sr (a light-colored mineral phase), ; 25±30%quartz (SiO2) (a light-colored mineral phase), and ; 20%hornblende (Ca2(MgFe2 1 )3Al4Si6O22(OH2) (a dark-col-ored mineral phase). In Fig. 4a, a dark region of the rockis selected, whereas in Fig. 4c a light region is selected.The LIBS spectrum of the dark region, Fig. 4b, is dom-inated by Fe lines, whereas the LIBS spectrum of thelight region, Fig. 4d, is dominated by Sr and Ca lines.The emission lines observed in Figs. 4b and 4d are con-sistent with the known mineral phases observed. Otherelements that could potentially be detected by LIBS inthis sample include Al, Si, and O in the light regions, aswell as Mg in the dark region. Emission lines for theseelements were not investigated. The light and darkregions selected were only 2 mm apart, yet differencesin composition of these regions are quite apparent, andthere is little or no interference between the two spectra.

Since the granite sample did not exhibit any strongRaman bands, as expected, the ability of the probe toperform both Raman and LIBS measurements on thesame sample was demonstrated by using a soil substratespiked with TiO2 and Sr(NO3)2 particles. Figure 5a showsa dark® eld image, taken with the probe, of three particles(700±1000 m m in diameter) on the surface of the soilsubstrate. On the left side of the image a particle ofSr(NO3)2 is seen, and two particles of TiO 2 are seen onthe right side of the image. A major problem with ® ber-optic Raman measurements is interference by strong sil-ica Raman bands found between 200 and 700 cm 2 1. By® ltering both the excitation and collection ® bers using in-

APPLIED SPECTROSCOPY 1151

FIG. 4. (a) Dark® eld image of a 5 mm region of interest on a granite sample and (b) the corresponding LIBS spectrum dominated mainly by Feand some Ca emission. Note the He:Ne laser spot ( ; 1 mm diameter) at the bottom left illuminating the dark spot on the sample. (c) Dark® eldimage of the rock sample with the He:Ne spot illuminating a light area as the region of interest, and (d) the corresponding LIBS spectrum dominatedmainly by Sr and Ca emissions.

FIG. 5. (a) Dark ® eld image of one Sr(NO3)2 and two TiO2 particles on a soil sample. (b) Raman spectrum of the top TiO2 particle obtained byusing the ® ltered Raman microprobe. (c) Raman spectrum of the Sr(NO3)2 particle obtained by using the ® ltered Raman microprobe.

line ® lters, we suf® ciently reduced the silica Raman tobe able to perform measurements in this region. A 514nm bandpass ® lter was used in-line near the sample endof the excitation ® ber, and a 520 nm longpass ® lter wasused in-line near the sample end of the collection ® ber.Without these ® lters, the spectra obtained in Figs. 5b and5c could not have been obtained. The Raman spectrumof the top TiO2 particle, shown in Fig. 5b, was obtainedby summing 100 ® ve-second exposures using ; 300 mW

of laser power at the sample. The Raman peaks at 146,411, 522, and 641 cm 2 1 are characteristic of TiO2. TheRaman peak at 146 cm 2 1 is actually the strongest of thefour peaks, but its intensity was reduced by the ® lters inthe probe. The Raman spectrum of the Sr(NO 3)2 particle,shown in Fig. 5c, was obtained by summing 60 ten-sec-ond exposures using the same laser power and shows anintense Raman band at 1055 cm 2 1 and a weak band at735 cm 2 1, both characteristic of Sr(NO3)2. Although the

1152 Volume 52, Number 9, 1998

FIG. 6. (a) Raman image of the TiO2 particles shown in Fig. 5a measured with the spectrometer set to the strong 146 cm 2 1 band. (b) Ramanimage of the Sr(NO3)2 particle measured with the spectrometer set to the strong 1055 cm 2 1 band. (c) LIBS spectrum of the top TiO2 particle using® ve laser pulses, dominated primarily by Ti emission lines. (d) LIBS spectrum of the Sr(NO3)2 particle using ® ve laser pulses, dominated by twomajor Sr emission lines.

total exposure time used for this spectrum was relativelylong, these Raman bands were also observed with one-second exposures. The throughput of the Raman probewas found to be similar to other Raman probes developedin our laboratory.

Silica Raman background is especially a problem forRaman imaging. Unfortunately, high-quality in-line ® lterswere not available for the image guide during this work.To reduce silica Raman interference in the Raman im-ages, we used the 0.25 m spectrograph as a very lowresolution spectral ® lter by opening the slit to the samesize as the imaging ® ber (e.g., 1 mm). A Raman imageof the TiO2 particles in a sample like that shown in Fig.5a, obtained at 146 cm 2 1, is shown in Fig. 6a. The 146cm 2 1 line was used for Raman imaging of the TiO 2 par-ticles because of its high intensity and because there waslittle interference from silica Raman in this region. TheRaman image of the Sr(NO 3)2 particle, Fig. 6b, was mea-sured at 1055 cm 2 1. Both images were acquired by sum-ming 75 eight-second exposures using a laser power of400 mW at the sample. The particles shown in Fig. 5aare less than two millimeters apart and are easily resolvedin the Raman images in Figs. 6a and 6b. We are currentlymodifying the probe for Raman imaging to include high-quality in-line ® lters to overcome the problem of silicaRaman interference.

The microprobe was also used to measure the LIBSemission spectra of the TiO 2 and Sr(NO3)2 particles onthe surface of the pressed soil sampleÐ as shown in Fig.5a. Figure 6c shows the LIBS spectrum obtained for thetop TiO2 particle acquired by summing spectra from ® velaser shots (80 mJ of laser energy per shot at the sample).The LIBS spectrum in Fig. 6c is dominated by Ti emis-sion lines. The LIBS spectrum of the Sr(NO3)2 particle,

Fig. 6d, obtained under the same conditions, is dominatedby two strong Sr emission lines at 407.8 and 421.6 nm.The weaker emission lines shown are due to Ca, Al, andTi in the soil. The LIBS measurement is destructive, leav-ing small ; 1 mm ``craters’ ’ where the three particleswere ablated, but the soil around the particles is basicallyundisturbed because of the relatively small size of thelaser spot.

CONCLUSION

It has been shown that both LIBS and Raman spectracan be acquired by using a single ® ber-optic probe. Theaddition of an image guide to the probe makes it possibleto perform LIBS and Raman spectral analysis of sampleheterogeneity’ s remotely at any point within an imagedarea. The microprobe is also shown to be useful for trueRaman imaging. Although the results shown here arequalitative, it should be possible to use this probe forquantitative Raman and LIBS measurements. The com-bination of LIBS and Raman visual imaging is potentiallyuseful for remote and in situ elemental and molecularmeasurements in hard-to-access and hostile environ-ments. In the case of cone penetrometer measurements ofsoil contaminants, the ability to visualize the subsurfaceregion of interest, and to analyze minerals, inorganicsalts, and organic pollutants simultaneously, could be atremendous bene® t. This probe should also be very usefulfor the analysis of radioactive materials in hot-cell envi-ronments.

ACKNOWLEDGMENTS

This work was supported by the Department of Energy, Grant Num-ber DEFG0796ER62305. Partial support was also provided by the U.S.

APPLIED SPECTROSCOPY 1153

Department of Energy’s South Carolina EPSCoR Program through Co-operative Agreement Number DE-FCO2-91-ER75666. The portion ofthe LIBS work that was carried out at Los Alamos National Laboratorywas funded by Los Alamos National Laboratory through the Labora-tory-Directed Research and Development Program. Los Alamos Na-tional Laboratory is operated by the University of California for theU.S. Department of Energy. The authors wish to express their thanksto Dr. Stanley Knave of Savannah River Technology Center for provid-ing the ® lters and in-line ® lter assemblies, and Brian Cullum for hishelp in preparing some of the soil samples. The authors also wish tothank two anonymous reviewers for providing many helpful suggestionson a previous version of this paper.

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