Volume 54, Number 12, 2000 APPLIED SPECTROSCOPY 17190003-7028 / 00 / 5412-1719$2.00 / 0q 2000 Society for Applied Spectroscopy

accelerated paper

Enhancement of Aluminum, Titanium, and Iron in GlassUsing Pre-ablation Spark Dual-Pulse LIBS

DIMITRA N. STRATIS, KRISTINE L. ELAND, and S. MICHAEL ANGEL*Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South Carolina 29208

In this paper, we report the � rst enhanced emission for elements ina nonmetal or nonconducting matrix, glass, with the use of a pre-ablation spark. The glass samples used in this work are prototypesof samples used to immobilize inorganic waste at the Savannah Riv-er Site Vitri� cation Facility. We have found that using a pre-abla-tion spark results in larger signal enhancements, 11- to 20-fold fortitanium, aluminum, and iron in glass compared to the metal underthe same experimental conditions. We also demonstrate that thismethod is more sensitive than single-pulse LIBS experiments forthe direct solid sampling of vitri� ed waste glass.

Index Headings: Dual-pulse LIBS; Pre-ablation spark; LIBS, laser-induced breakdown spectroscopy; Glass; Waste glass; Laser abla-tion; Laser-induced plasmas, LIPS.

INTRODUCTION

We previously reported a new dual-pulse laser-inducedbreakdown spectroscopic (LIBS) method that uses a pre-ablation spark to produce very large signal enhance-ments, as high as 40-fold, for a variety of metals.1 Here,we extend this work to include nonconducting samples,and compare the enhancement for metallic elements inglass and the metal. The glass samples used in this workare prototypes of samples used to immobilize inorganicwaste at the Savannah River Site Vitri� cation Facility.

Vitri� cation is widely used for the immobilization ofheavy metals and liquid radioactive waste.2 Studies haveshown that glass can be 10 000 times more durable thanother forms of containment and results in up to a 97%reduction in volume compared to the most common al-ternative storage method, cement.2 The form of glassused to immobilize the waste must be resistant to aqueousleaching as well as disintegration since this characteristicwill increase the surface area available for leaching.3 Cur-rently, nine nations, other than the U.S., have either cho-sen or are considering the borosilicate glass form forlong-term disposal of highly radioactive waste.2 There-fore, there is a need to rapidly monitor the deteriorationdue to devitri� cation or to radiation damage to ensureproper radioactive containment. Ideally, a method ofanalysis that minimizes sample handling, human expo-sure, and contamination is desired.

Received 6 July 2000; accepted 6 September 2000.* Author to whom correspondence should be sent.

Glasses are some of the most dif� cult samples to an-alyze.4–6 Glass dissolution is tedious and time consumingand can degrade the accuracy of the analysis. To avoidtedious sample preparation, a number of methods use la-ser ablation (LA) or direct solid sampling of varioustypes of glass samples into a measurement instrument.7–10

Laser ablation sampling offers a number of advantagesthat make it ideal for sampling solids. For example, thereis little or no sample preparation required and virtuallyany type of solid sample, including conducting and non-conducting samples, can be rapidly ablated.7 In addition,laser ablation sampling offers the ability to perform aspatially resolved analysis, which is important for hetero-geneous samples and for studying the behavior of theimmobilized material in glass over a long period oftime.10

Most glass analyses that use laser ablation samplingcouple the ejected material to a detection system such asinductively coupled plasma (ICP), mass spectrometry(MS), and ICP atomic emission spectrometry (AES).7–10

Russo and Mao have studied the in� uence of parameterssuch as laser pulse width and power density on the ele-mental analysis of prototypical waste glass samples byusing LA-ICP-AES; quantitative analysis was demon-strated by the use of silicon present in the glass as aninternal standard.7 The effect of laser wavelength on LA-ICP-MS analyses of waste glass samples has been inves-tigated by several groups. In these experiments, an ultra-violet laser was found to minimize elemental fraction-ation and hence produced ablated material that was morerepresentative of the original sample.8–10

An alternative to ICP-AES or MS detection is to di-rectly measure the atomic emission that results when alaser is focused on the sample of interest since a laser-induced plasma accompanies laser ablation of the sample.This method is known as laser-induced breakdown spec-troscopy and is very useful for determining the elementalcomposition of solids. LIBS has been recently re-viewed,11–19 and a number of different applications20–24

have been described. With LIBS, elemental analysis ofthe sample is accomplished by measuring the emissionof the excited atoms and/or ions that comprise the plas-ma. Directly measuring the emission from the laser-gen-erated plasma allows for a much simpler measurement

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FIG. 1. Schematic diagram of pre-ablation spark dual-pulse setup.

FIG. 2. LIBS emission spectra showing an increase in emission inten-sity for elements in a waste glass sample using a pre-ablation spark(upper) relative to single-pulse LIBS (lower).

and works well for remote and in situ measurements. Infact, a number of remote LIBS measurements have beendescribed, which use � ber optics to both deliver the laserto form the plasma and collect the resulting emission.11–15

Incorporating � ber optics minimizes exposure to contam-ination and radiation exposure when sampling in hostileenvironments. LIBS has been previously employed for insitu and on-line process analysis of major glass constit-uents during a vitri� cation process for � y and bottomashes from waste incineration.25,26 The use of LIBS toidentify and determine concentrations of elements duringa glass vitri� cation process has also been recently ex-plored.27

Although LIBS holds much promise as a remote anal-ysis technique, the method suffers from poor detectionlimits and reproducibility compared to alternative formsof elemental analysis. The processes involved in laser-induced plasma formation, ablation, atomization, and ex-citation are quite complex and dif� cult to reproduce. Toaddress these problems, some research groups are inves-tigating the use of dual-laser pulses for LIBS, where thelaser pulses are separated by a short delay time. In somecases, it has been shown that the use of dual-laser pulses,or dual-pulse LIBS, can lead to lower detection limits28,29

and enhanced emission signals,1,14,28–33 as well as im-proved internal standardization.34

Recently, we demonstrated a new dual-pulse LIBSmethod that produces both enhanced emission signals andmaterial ablation through the use of a pre-ablation laserspark.1 Signal enhancements as high as 40-fold wereachieved by using this pre-ablation spark dual-pulseLIBS method. In this technique, the � rst laser pulse isbrought in parallel to the sample and focused above it toform an air plasma. Although the plasma generated bythe pre-ablation spark heats the sample, it is suf� cientlyfar from the surface to ensure that sample ablation andemission are not observed. LIBS emission is observed inthe plasma that is created by the second laser pulse,which is focused on the sample. In all cases where en-hanced LIBS signals were seen, enhanced sample abla-tion also occurred.1

Our previous study focused on metal samples. In this

paper, we extend this study to nonconducting glass sam-ples showing greatly enhanced emission signals for dif-ferent metallic elements in waste glass. We also demon-strate the use of the pre-ablation spark dual-pulse LIBSfor quantitative analysis with increased sensitivity com-pared to conventional single-pulse LIBS.

EXPERIMENTAL

Dual-Pulse LIBS System. The pre-ablation sparkdual-pulse LIBS setup is shown schematically in Fig. 1.The experimental setup has been previously describedand will be reviewed brie� y.1 Independent � ring of thetwo laser pulses is achieved by externally controlling twoQ-switched Nd:YAG lasers (Continuum Surelite III, andQuantel Nd 580) by a delay generator (Stanford Instru-ments Model DG535) and a variable clock (Stanford In-struments Model SR250). For the experiments reportedhere, the time between laser pulses was varied from 0 to1000 ms with minimal jitter (65 ns), although all pointsmay not be shown in the illustrations. Both lasers wereoperated at 1064 nm at a repetition rate of 5 Hz.

The laser used for sample ablation, with approximately100 mJ/pulse, was directed normal to the sample surfaceand focused 1 to 2 mm beyond the surface by an f /4lens. The air spark laser beam, with approximately 210mJ/pulse, was directed parallel to the sample and focusedby an f /4 lens approximately 1 mm above the samplesurface. This beam was focused to a point above the sam-ple that coincided with the center of the sample plasmathat was generated by the ablation beam. However, it ar-rived at the sample approximately 2.5 to 50 ms beforethe ablation pulse, forming an air spark above the surface,prior to any sample ablation. The effect of laser pulsetiming was investigated by bringing in the pre-ablationspark from 1000 to 0 ms before the ablation laser pulse.Most experiments were conducted with the pulse arriving2.5 ms before the ablation pulse (a 22.5 ms pre-ablationspark), since this timing was found to be within the op-timal range producing large LIBS signal enhancements.For single-pulse experiments, the same ablation laser(100 mJ/pulse) and experimental conditions were used.

The f /4 lens that was used to focus the ablation pulse

APPLIED SPECTROSCOPY 1721

FIG. 3. Pre-ablation spark dual-pulse LIBS enhancements for alumi-num (A), titanium (B), and iron (C ), using a 22.5 ms pre-ablation spark(upper spectra), compared to the LIBS signal from the ablation laserpulse only (lower spectra).

FIG. 4. SEM images of craters produced from 50 consecutive sam-plings of the glass. Increase in glass ablation from using a pre-ablationspark is clearly seen upon comparison of the ablation laser-only exper-iment (A) to the use of a 22.5 ms pre-ablation spark (B).

onto the sample was also used to collect emission axiallyas shown in Fig. 1. An f /2 lens was used to focus thecollected LIBS emission into a 5 m long, 2 mm core-diameter optical � ber (Edmund Scienti� c Co. ModelD2551). The optical � ber was coupled to a 0.25 m, f /4spectrograph (Chromex Model 250IS/RF). The detectionsystem consisted of a gated intensi� ed charge-coupleddetector (ICCD) (Princeton Instruments, Model ITEA/CCD-576-S/RB-E) with Model PG-200 pulser and ModelST-138 detector controller. For laser pulse delay timingexperiments, the detector gate delay was � xed at 2 msafter the ablation laser pulse, and the gate width was � xedat 0.5 ms. However, for all other experiments, the detectorgate delay was � xed at 2 ms after the ablation laser pulse,and the gate width was � xed at 1.5 ms. For pulse delayvs. LIBS enhancement measurements, 20 accumulationswere taken (e.g., 20 laser pulse pairs). All other mea-surements were taken as the accumulation of 50 sequen-tial samplings for calculation precision. Five replicatemeasurements were taken from � ve different locations onthe sample surface, with no pre-shots used to prepare thesurface of the sample. Plasma images were acquired by

imaging the plasma directly with the ICCD. The imagesshown here were formed on a lead sample and were ac-quired at a collection delay time of 2.0 ms after the ab-lation pulse. Scanning electron microscope (SEM) im-ages were used to examine the ablation craters that wereformed during measurement of the glass samples.

Preparation of Samples. The waste glass samplesused throughout these studies were obtained from the Sa-vannah River Site Vitri� cation Facility. All waste glasssamples were prepared by using reagent-grade chemicals

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FIG. 5. The size and shape of the plasma change dramatically in the dual-pulse experiments compared to single-pulse LIBS. The plasma emissionis affected by the pulse timing with an optimal pulse delay time, 5 ms, producing a taller and more de� ned plasma compared to single-pulseplasmas.

in the oxide form. The oxides were individually weighed,mixed well, and placed into a Pt/10% Rh crucible. Thecrucibles were heated to 1400 8C at a rate of four 8C perminute in a high-temperature furnace. To ensure samplehomogeneity, we held the samples at the melt tempera-ture for four hours. After melting was complete, the sam-ples were removed from the high-temperature furnaceand poured onto a one-half-inch thick steel plate. Thisprocess typically formed a glass disk ; 0.3 to 0.5 in.thick, which cooled overnight. In these studies, a calibra-tion set of concentrations ranging from 0 to 0.7% (wt/wt) for Al, and Ti 3% (wt/wt) for Fe was used.

The metal samples investigated were primarily com-posed of the metal of interest. The iron sample contained99.9% iron, and the titanium sample contained approxi-mately 94% titanium and 6% aluminum. For aluminum,common aluminum ( ; 95% Al) stock was used.

Analysis of Data. The error bars on the plotted datarepresent one standard deviation of the � ve replicates.The emission signals were measured by using the 391.4,394.4, and 404.6 nm atomic lines for Ti, Al, and Fe,respectively. All calculations and plotting of data wereperformed by using Igor Version 3.10 (Wavemetrics Inc.)including the least-squares � ts.

Plasma Temperature Determination. The Boltzmannplot method was used to determine the excitation tem-perature for single- and dual-pulse LIBS experiments ofglass with the use of the following neutral iron lines:404.58, 411.85, 426.05, 430.79, and 432.58 nm.35 Thismethod assumes at least partial local thermodynamicequilibrium with the main limitation on the accuracy

caused by the uncertainties in the transition probabilitiesof the lines. To maximize the accuracy of the technique,we chose iron lines with as wide of range of upper energylevels as possible (4.44 to 6.58 eV). Five replicate mea-surements were taken from � ve different locations on theglass sample surface.

RESULTS AND DISCUSSION

This is the � rst report of enhanced emission for ele-ments in a nonmetal or nonconducting matrix, glass, withthe use of a pre-ablation spark. Laser sampling and anal-ysis of highly refractory samples, such as glasses, aredif� cult to achieve, particularly when using a long wave-length laser. Enhancements in the LIBS signals when us-ing a pre-ablation spark are clearly evident upon exami-nation of Fig. 2, which shows dual-pulse LIBS spectrawith the use of a 2.5 ms pre-ablation spark (upper spec-trum) and a single laser pulse LIBS (lower spectrum). A2.5 ms pre-ablation spark indicates that an air spark wasformed above the glass 2.5 ms before the ablation laserpulse reached the sample. The spectra shown here weremeasured from a waste glass sample that contained 0.7%aluminum and titanium and 3% iron. The atomic emis-sion signals were signi� cantly enhanced for all three met-als under investigation, although all lines were not equal-ly enhanced. The enhancements were 20-, 20-, and 11-fold for aluminum, iron, and titanium, respectively, rel-ative to single-pulse LIBS measurements. For allenhancement calculations the 391.4, 394.4, and 404.6 nmlines were used for titanium, aluminum, and iron, re-

APPLIED SPECTROSCOPY 1723

FIG. 6. LIBS signal for elements in glass, vs. delay time between laserpulses for pre-ablation dual-pulse experiments. Note: a negative timerefers to the air spark being formed prior to the ablation pulse.

spectively. These lines were chosen since there was min-imal or no interference from the other metals known tobe present in the waste glass sample.

It is also interesting to investigate the effect that theuse of a pre-ablation spark has on metal samples. Figure3 shows single- and dual-pulse LIBS of aluminum (A),titanium (B), and iron (C) in the metal samples. In allthree cases, the dual-pulse LIBS signal (upper spectra) iscompared to single-pulse LIBS (lower spectra). Under thesame experimental conditions, signal enhancements re-sulting from the use of a pre-ablation spark were typicallylower (4- to 6-fold) for iron, aluminum, and titanium met-al compared to the higher (11- to 20-fold) LIBS signalenhancements in the glass matrix. The titanium alloy con-tained a signi� cant amount of aluminum (Fig. 3B). Theenhancement of aluminum in the titanium alloy with thedual-pulse LIBS method was also 5-fold.

Figure 4 shows SEM images of the ablation cratersformed after 50 consecutive samplings of the same regionof the glass sample by using a single pulse (A) and byusing a 2.5 ms pre-ablation spark (B). As found with pre-vious pre-ablation spark experiments of metal samples,the signal enhancement observed is related to large in-creases in the amount of material ablated. Although notshown, enhanced sample ablation also accompanied thelarge signal enhancements observed for the metal sam-ples of aluminum, titanium, and iron. The size and shapeof the plasma also changes dramatically in the dual-pulseexperiments compared to single-pulse LIBS, and is af-

fected by the pulse timing (see Fig. 5). At an optimalpulse delay time, 25 ms, the plasma is much taller andmore de� ned than the single-pulse plasma under identicalconditions. This effect is observed for all pre-ablationspark dual-pulse LIBS experiments. These images lookqualitatively similar to results that have been reporteddescribing LIBS plasma imaging in reduced-pressure at-mospheres.36 Increased ablation was also reported in theprevious work at reduced pressure. It is also interestingto note that, for glass, the plasma temperature is similarfor the single-pulse and dual-pulse plasmas. Speci� cally,the plasma temperature determined by using the Boltz-mann plot method 35 was found to be 9800 6 1900 K forsingle-pulse LIBS experiments of glass, and 9200 6 900K when a pre-ablation spark was used.

The effect of laser pulse timing on the glass samplecontaining 2.5% Al and Ti was investigated over a widerange of pulse delays. Optimum laser timing was foundto be similar to results previously found for metal sam-ples (approximately 22.5 ms).1 Figure 6 shows the effectof LIBS signal on laser pulse delay for aluminum (A),titanium (B), and iron (C) in glass. It is clear from Fig.6 that the LIBS signal enhancements for all three metalschange dramatically from 0 to 21 ms but are relativelyinsensitive to laser timing from 20.5 to 250 ms. Theeffect drops off if the pre-ablation spark is brought in tooearly, more than 50 ms before the ablation laser pulse.Figure 7 compares SEM images of the regions left after50 consecutive samplings of the same region of the glasssample by using various pre-ablation spark timings. Thelaser pulse timings that result in the maximum signal en-hancements (250 ms to 20.2 ms) also result in the mostamount of material ablated. In addition, it is clear that ifthe pre-ablation spark is brought in too early (2100 ms)before the ablation pulse, less material ablation results,corresponding to a lower LIBS signal enhancements.

Calibration curves were constructed by measuring sin-gle- and dual-pulse LIBS for glass samples containing Aland Ti over a concentration range of 0 to 0.7%. The cal-ibration set also contained a � xed concentration of ironto be used as an internal standard. The intensity of the391.4 nm Ti and 394.4 nm Al lines was found to be linearwith concentration over the concentration range investi-gated for both single- and dual-pulse LIBS (see Fig. 8).A large increase in sensitivity is observed for both metalswhen a pre-ablation spark compared to single-pulse LIBSis used. Speci� cally, the sensitivity or slope of the tita-nium calibration exhibited a 10-fold increase with the useof a pre-ablation spark. An even larger increase in sen-sitivity is observed for aluminum, as shown in Fig. 8B.In this case, a 22-fold increase in sensitivity resulted fromusing the 2.5 ms pre-spark over the single ablation pulse.That is more than double what is observed for titanium.Although the pre-ablation spark measurements were notoptimized, the increased relative sensitivity observedshows much promise for detecting various elements inwaste glass samples.

One of the main problems with LIBS in general is thatit typically suffers from poor reproducibility. This prob-lem is due in large part to laser shot-to-shot variations.In addition, in this study, the surfaces of the glass sam-ples were not � at, and thus the signals tended to vary asthe samples were moved between measurements. This is

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FIG. 7. SEM images of the regions left after 50 consecutive samplings of the same region of the glass sample using various pre-ablation sparktimings. The laser pulse timings that result in the maximum signal enhancements (250 ms to 20.2 ms) also result in the most amount of materialablated.

particularly a problem for dual-pulse LIBS where the po-sition of the pre-ablation spark above the sample can af-fect the resulting signal enhancement. Hence, this impor-tant parameter needs to be controlled and optimized infuture experiments. However, using iron as an internalstandard signi� cantly improves the reproducibility of the

aluminum measurements by correcting for the shot-to-shot and sample to pre-spark height variations. Figure 9shows plots for aluminum with the use of dual-pulseLIBS after taking the ratio of the intensity of the alumi-num and iron lines. The r 2 value increases from 0.8644without the internal standard (Fig. 8B) to 0.9732 when

APPLIED SPECTROSCOPY 1725

FIG. 8. Calibration curves for titanium (A) and aluminum (B) in wasteglass showing large increases in sensitivity when using a 2.5 ms pre-ablation spark (circles) over a single laser pulse (squares).

FIG. 9. Calibration plots for aluminum using dual-pulse LIBS aftertaking the ratio of the intensity of the aluminum and iron lines.

iron is used as an internal standard. This approach worksvery well in this case because aluminum and iron showsimilar dual-pulse enhancements. It does not work as wellfor titanium, which is enhanced differently.

The reason for the pre-ablation spark enhancement isnot yet known and is currently under investigation. Onepossibility is that the pre-spark heats the sample, causingchanges in its optical properties, resulting in increasedcoupling of the ablation pulse to the sample surface.However, since the laser pulse timing is critical to themagnitude of signal enhancement, it is clear that the pre-ablation spark effect must be transient in nature. Anotherpossibility is that the pre-ablation spark in the air abovethe sample creates free electrons, which aid in the for-mation and propagation of the second plasma. Yet anoth-er is that the expanding shockwave creates a lower pres-sure environment into which the second plasma is

formed, which could result in larger plasmas and en-hanced material ablation as stated above in the Fig. 5discussion. Future studies to elucidate the mechanism ofenhancement are currently being planned.

CONCLUSION

We have found that the use of pre-ablation spark dual-pulse LIBS compared to single-pulse LIBS results inlarger signal enhancements for elements in glass than forthe metal under the same experimental conditions. Wealso show that iron is a useful internal standard for alu-minum because it exhibits similar enhancements in theglass samples but does not work as well for titanium,which shows a different enhancement. The increased rel-ative sensitivity observed when using a pre-ablation sparkshows much promise for detecting various elements inwaste glass samples.

ACKNOWLEDGMENTS

We would like to thank the U.S. Department of Energy for supportof this work under Grant Number DEFG0796ER62305 and the Of� ceof Naval Research for support of this work under Grant Number N0014-97-1-0806. We would also like to thank Savannah River Site Vitri� -cation Facility for preparation of the prototypical waste glass samples.Finally, we would like to warmly thank Dr. Morgan for helpful discus-sions pertaining to the data analysis.

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