dual-pulse laser-induced breakdown spectroscopy in bulk aqueous solution with an orthogonal beam...

Dual-pulse laser-induced breakdown spectroscopy inbulk aqueous solution with an orthogonal beam geometry

William Pearman, Jon Scaffidi, and S. Michael Angel

Use of dual-pulse laser-induced breakdown spectroscopy with an orthogonal spark orientation is pre-sented as a technique for trace metal analysis in bulk aqueous solutions. Two separate Q-switchedNd:YAG lasers operating at their fundamental wavelengths are used to form a subsurface, laser-inducedplasma in a bulk aqueous solution that is spectroscopically analyzed for the in situ detection of Ca, Cr,and Zn. Optimizing the key experimental parameters of proper spark alignment, gate delay �td�, gatewidth �tb�, and interpulse timing ��T� allowed experimentally determined detection limits of the order ofmicrograms per milliliter and submicrograms per milliliter. We present supporting evidence of asampling mechanism that involves the formation of a cavitation bubble with the first pulse �E1� followedby analysis of that bubble with a second pulse �E2�. The plasma created by E2 contains the analyticallyrelevant information from the aqueous sample and often represents �250-fold enhancement over a singlelaser pulse with energy equal to E1 alone. © 2003 Optical Society of America

OCIS codes: 300.0300, 300.2140, 300.6210, 300.3660.

1. Introduction

Laser-induced breakdown spectroscopy �LIBS� is aspectrochemical method of analysis, first reportedby Brech and Cross in 1962,1 in which a laser isused to ablate and atomize material from a sampleand form a plasma. Atomic and ionic emissionfrom this plasma can then be used to identify andquantify elements within the sample.2 LIBS hasbecome quite a popular form of analysis over thepast few decades,3–9 and it has been reviewed manytimes in the literature.2,10–13 This increase in pop-ularity is in part due to the relative simplicity ofLIBS specifically with respect to the lack of anyneed for detailed sample preparation,2 and thisstrength makes LIBS an amazingly versatile ana-lytical technique. It is one of the few techniquesthat can be used for noncontact elemental analysis,which makes LIBS uniquely suited to measure-ments of hazardous materials and materials indifficult to reach locations.14–27 Furthermore, be-cause sampling is virtually nondestructive, LIBS is

The authors are with the Department of Chemistry and Bio-chemistry, University of South Carolina, Columbia, South Caro-lina 29208. The e-mail address for S. M. Angel [email protected].

Received 20 February 2003; revised manuscript received 11 July2003.

0003-6935�03�306085-09$15.00�0© 2003 Optical Society of America

useful for such unique applications as the analysis ofpriceless works of art and archeological relics.28–32

Other applications that can benefit from the uniqueadvantages of LIBS include environmental,33–39

industrial,14–16,34,35,40–47 geological,33,36–38 planetary,33

art,39–53 medical,54,55 and dental56 measurements.Also, many researchers have recently coupled LIBSwith other techniques such as inductively coupledplasma–mass spectrometry.23,52,53,57–59

LIBS has the potential to be useful for measure-ment of dissolved species in aqueous solutions foranalysis of contamination in ground water anddrinking water60,61 or for the monitoring of traceamounts of Fe in the boiler water of thermoelectricpower plants.62,63 Although numerous analyticaltechniques exist for the detection of trace analyte inaqueous samples such as inductively coupledplasma–optical emission spectrometry,64 UV–visible spectrophotometry,65 flame atomic emissionspectrometry,66 voltammetry67,68 and chromato-graphic methods, these techniques do not easily al-low for continuous, on-line, or in situ monitoring.LIBS alternatively requires only optical access tothe sample, and the measurement is usually fast.

In the case of LIBS measurements of aqueous so-lutions, many different sampling configurations havebeen studied, including the formation of the plasmaon the liquid surface12,21,66,69–71 in bulksolutions,46,72–75 on droplets,71,72 and in liquidjets.63,64,72,76–78 These configurations lack the sensi-tivity and reproducibility that is needed for trace

20 October 2003 � Vol. 42, No. 30 � APPLIED OPTICS 6085

analysis; also, in general, LIBS of bulk aqueous so-lutions when a single excitation laser is used is trou-blesome for several reasons. First, the analysis ofbulk solutions suffers from relatively short plasmalifetimes. The plasma lifetimes for laser-inducedplasmas in bulk liquids are typically less than 1�s73,74,78 compared with 5–20 �s in ambient air.8,71

Second, the intensity of the observed emission is re-duced, in part because of a decreased plasma temper-ature resulting from quenching.48,72,79,80 Finally,the sampling of turbid or colored solutions can causesignificant matrix effects for LIBS in bulk solutions.

In 1984 Cremers et al. showed that good sensitivityfor metals in solution can be achieved by LIBS mea-surements in the bulk aqueous solution using col-linear dual-pulse laser excitation.73 Bulk analysis isdefined as the formation of the plasma subsurface,and this minimizes problems associated with samplesplashing when measurements are made at the solu-tion surface or in liquid jets. In that study it wasnoted that optical breakdown of the solution by thefirst pulse forms a gaseous cavity, or cavitation bub-ble, and that excitation of this bubble when a secondlaser pulse is used produces greatly enhanced analyteemission. Since this discovery, several groups havebegun to investigate use of dual-pulse LIBS in theanalysis of aqueous systems.8,46,63,78,79

In the aqueous-phase LIBS research described byCremers et al., the best results were obtained whenthe dual-pulse lasers were collinear because of theinability to form a reproducible plasma with orthog-onal pulses.73 In recent research in our laboratorywe have begun to investigate use of dual-pulse lasersin an orthogonal orientation for determination ofmetal ions in bulk aqueous solutions with the ideathat this configuration would make optimization ofspark positioning easier. We found that this geom-etry, combined with emission collection orthogonal toboth pulses, allows better optimization of pulse align-ment and provides higher signal enhancements thana collinear configuration for the measurement of sev-eral dissolved species in bulk aqueous solution. Inthis paper we describe this technique and report pre-liminary detection limits for Ca, Cr, and Zn.

2. Experiment

The basic dual-pulse system has been described indetail previously4–9 and comprises two lasers, an op-tical fiber collection system and a fast spectrographwith a gated intensified CCD detector. TwoQ-switched Nd:YAG lasers �Continuum Surelite IIIand Quantel Nd 580� operating at 1064 nm with arepetition rate of 5 Hz were used for excitation with5-cm focal-length lenses. The lenses were mountedon micrometer stages allowing precise control ofbeam overlap in the sample cell. A variable clock�Stanford Instruments Model SR250� with a delaygenerator �Stanford Instruments Model DG535� wasused to generate control triggering of the lasers andcollection electronics. In some cases a fast photo-diode �Thorlabs, Inc. DET210� set to detect the firstlaser pulse was used to trigger the detector. Analyte

emission was collected with a pair of f�2 lenses fo-cused onto a 2-mm-core-diameter, 0.51-N.A., lightguide �Edmund Scientific Co. Model O2551�. Theoutput of the light guide was focused with unit mag-nification onto the slit of a 0.25-m, f�4 spectrograph�Chromex Model 250IS�RF� with a 1200-groove grat-ing blazed at 500 nm. The data collection systemconsisted of a gated intensified CCD �Princeton In-struments I-Max 1024E� with a pulse timing gener-ator �Roper Scientific ST-133A�, we acquired and allspectra using a computer with WinSpec�32 version2.5.7.3 software.

The sample cell was fabricated from a 2-in. �5-cm�polyvinyl chloride cube with 1-in.- �2.54-cm-� diame-ter quartz windows on five sides �see Fig. 1�a�� withthe top left open for the convenient filling of solutions.The internal volume of the finished cell was �35 ml.Care was taken to ensure that the quartz windows ofthe cell were not placed near the focal point of thelasers to prevent pitting.

The key timing parameters are illustrated in Fig.1�b� and have been used previously in the litera-ture.81 E1 and E2 refer to the first and secondpulses, respectively, and �T refers to the delay timebetween these two pulses. The detector gate delaytd is always measured relative to the second laserpulse, and the gate width tb refers to the time duringwhich emission is integrated.

Sample solutions were prepared with high-puritystandards solutions �High-Purity Standards: Ca,1009-1; Cr, 100012-1; Zn, 10M68-2� that were dilutedto the desired final concentration. All concentra-tions are reported as parts per million �ppm� or partsper billion �ppb� by mass.

Laser-pulse energy optimization was not part ofthis study. For all experiments the energy of thefirst 9-ns pulse �E1� was approximately 95 mJ andwas focused to a spot size of approximately 2 �m.The second 7-ns pulse �E2� was approximately 175mJ and was also focused to approximately the samespot size as E1. Therefore the combined pulse en-ergy was �260 mJ. These pulse energies weretaken from the earlier research of Cremers et al.,73

which involved LIBS studies of metal samples in wa-ter. In that study it was determined that the max-imum analyte signals were obtained when the sum ofE1 and E2 were in the range of 260–300 mJ and thatan increase of E1 above 100 mJ did not increase thesize of the cavitation bubble.81

3. Results and Discussion

A. Initial Optimization with Oxygen Emission

For aqueous solutions we found strong oxygen andhydrogen emission using dual-pulse excitation withshort detector delay times �td�. Interestingly oxygenand hydrogen emission are difficult to detect with asingle laser pulse. Because the matrix of interest iswater, hydrogen and oxygen emission were obviouscandidates for optimization experiments. However,because the hydrogen lines are severely broadened,the strong 777-nm oxygen emission band was chosen

6086 APPLIED OPTICS � Vol. 42, No. 30 � 20 October 2003

for the optimization of key experimental parametersincluding alignment of the laser-induced plasmas,collection optics, delay time ��T� between the laserpulses, detector delay time �td�, and detector gatewidth �tb�. It was found that large emission en-hancements for dissolved metals species could be ob-tained following the optimization of the beamalignment and laser-pulse delay when we maximizedthe oxygen emission intensity.

B. Laser Beam Alignment

In dual-pulse LIBS measurements of bulk aqueoussolutions, a laser pulse is used to create a plasma ina vapor bubble formed by a previous laser pulse.For optimal emission signals, alignment between thetwo focused pulses is critical. We found that an or-thogonal beam configuration made beam alignment

more convenient than a collinear geometry, resultingin higher emission signals. It was also found thatdirect visual overlap of the two laser pulses in thesolution did not provide the highest emission signal.The following procedure was found to rapidly maxi-mize the emission intensity. We made the initialalignment by moving the positions of the laser focusuntil the spark created by each pulse visually over-lapped in the same x, y plane as shown in Figs. 1�a�and 2�b� �iii�. �Note: for safety this is best donewith a video camera focused on the laser spark posi-tion.� A helium–neon �He–Ne� laser was then sentthrough the collection optical fiber, and the fiber po-sition was aligned so that the focused He–Ne beamoverlapped with the center of the laser sparks withinthe sample cell as mentioned above. At this pointthe He–Ne was removed and the emission intensity of

Fig. 1. �a� Orientation and order of laser pulses into the sample cell with relative positions of spark alignment in the x, y plane and top�z� and bottom �z� collection. �b� Detail of timing scheme utilized for all experiments where E1 is the first pulse, E2 is the second pulse,�T is the interpulse timing, td is the gate delay, and tb is the gate width.

Fig. 2. �a� Effect of spark position for E2 on observed signal intensity of oxygen at 777 nm. Optimal spark position for E1 was previouslyidentified and held constant while E2 was moved in the direction indicated by the arrow in �b� through the focal plane of E1. Approximatespark overlap ��i�–�iv�� is shown corresponding to the observed signal intensity in �a� ��i�–�iv��.


the oxygen band at 777 nm was maximized with fur-ther adjustment to the focal position of the secondlaser pulse �E2�. Emission intensity was found todouble when the overlap of the two laser sparks wasoptimized as shown in Fig. 2�a�. This usually re-quired that the second pulse be moved approximately0.5–0.6 mm as shown by Fig. 2�b� �ii�. A plot ofemission intensity versus the position of E2 is shownin Fig. 2�a�.

C. Detector Gate Delay and Integration Time

Detector gating is primarily used for the purpose ofdiscriminating analyte emission from backgroundemission, with the delay time �td� having the biggesteffect. Thus for this study the detector integrationtime �tb� was kept constant at 15 �s and the detectordelay was adjusted to minimize background contri-butions to the signal. In the case of oxygen emis-sion, the intensity is extremely large even at a delaytime of 0.1 �s after the second laser pulse and wasdetectable only for approximately 2.5 �s after thesecond laser pulse. For all optimization experi-ments based on the 777-nm oxygen line, a 0.1-�sdetector delay time was used with a 15-�s integrationtime.

D. Laser-Pulse Delay Optimization

Previous dual-pulse LIBS studies of aqueous solu-tions indicated a strong dependence of emission in-tensity on the delay between laser pulses.63,73,82,83

Therefore the effect of �T on the emission intensitywas investigated over a range of 0–500 �s. The ox-ygen emission intensity was found to increase rapidlyas the delay between the two laser pulses waschanged from 0 to 30 �s, remaining constant to ap-proximately 350 �s and then rapidly decreasing from350 to 380 �s �Fig. 3�a��. This experiment suggeststhat the cavitation bubble persists for at least 350 �sbefore collapsing, consistent with published reportsof laser-induced bubble lifetimes in bulk solutions.84

However, this result is significantly larger than the

value of 18 �s that was reported with a collinearpulse configuration.73

Figure 3�b� shows oxygen emission at 777 nm byuse of a single 250-mJ pulse �ii� and for orthogonaldual-pulse excitation by use of optimized conditions�i� with �T � 240 �s, td � 0.1 �s, and tb � 15 �s.The oxygen spectrum in Fig. 3�b� �i� represents atleast a 315-fold enhancement over oxygen emissionfrom a single laser pulse.

4. Determination of Detection Limits for Ca, Cr, andZn

It was found that the same laser spark alignment andpulse delay times that maximized oxygen emissionalso worked well for Ca, Cr, and Zn in aqueous solu-tion. However, emission intensity was found to bemore dependent on the laser-pulse delay time, show-ing a clear maximum at approximately 150 �s �Fig.4�. Figure 4 shows a plot of Ca emission with the422.7-nm Ca I line as a function of laser-pulse delaytime. The signal changes over approximately thesame range of delay times as oxygen �compare withFig. 3�a�� but is more dependent on the actual �Tvalue. The delay time corresponding to the maxi-

Fig. 3. �a� Effect of interpulse timing on observed emission intensity for oxygen at 777 nm �td � 0.1 �s, tb � 15 �s�. The single point�Œ� at �T � 200 �s is indicative of the greatest observed signal with a single pulse. �b� Observed oxygen emission for dual-pulse LIBSof water ��T � 240 �s, td � 0.1 �s, tb � 15 �s� at 777 nm after optimization �i� compared with a single 250-mJ pulse �ii�.

Fig. 4. Ca emission intensity at 422.7 nm as a function of inter-pulse timing �T. The difference in the shape of this graph whencompared with Fig. 3�a� suggests a higher dependence on �T for Caas opposed to oxygen.


mum on this curve, 150 �s, was used to determine thedetection limits of the metal samples.

One problem with the analysis of aqueous solutionsby use of LIBS is the relatively intense oxygen andhydrogen emission, which constitutes a considerableportion of the overall signal at short detector delaytimes. Figure 5�a� shows a plot of Ca emission byuse of the 422.7-nm peak, compared with the back-ground signal �calculated as the average signal in-tensity near the peak of interest� as a function ofdetector delay time. �Note that both signals arehigh at td � 0 because of the long tb � 15 �s integra-tion time used.� Figure 5�a� shows clearly that, fortoo short a detector delay, the spectrum is dominatedby H� emission �Fig. 5�b��. Use of a longer detectordelay �td � 1.7 �s�, however, allows Ca emission to bemeasured with little H� interference �Fig. 5�c��.

For the quantitative determination of detectionlimits for Ca, Cr, and Zn, the previously determinedoptimal experimental conditions for Ca were used

�e.g., �T � 150 �s, td � 1.7 �s, and tb � 15 �s�.Table 1 contains a tabulation of the experimentalparameters as well as a comparison with previouslyreported results. We determined the detection lim-its for all elements using Eq. �1�85 where is definedas the standard deviation of the blank within thespectral range of the peak of interest and S is theslope of the calibration curve:

DL � 3 �S. (1)

A. Determination of

We determined the standard deviation of the back-ground using a deionized water blank. Five sets of25 spectra, each representing 50 laser shots, wereaveraged to yield five spectra �each an average of1250 laser shots�. These five spectra were subse-quently averaged to obtain a single blank spectrum.This final spectrum was then used to calculate bytwo separate methods within the peak area of inter-est. First, the standard deviation of a 50-point re-gion, equally distributed about the peak of interest,was determined. Second, using the same 50-point

Fig. 5. �a� Ca emission �upper curve� versus background �lowercurve� plotted as a function of gate delay td. �b� and �c� Represen-tative spectra for Ca at delay times of 0.3 and 1.7 �s.

Table 1. Experimental Parameters and Detection Limits for this Studya

and Comparison with Other Results

Optimization Results Current Study Literature

�T value for maximumoxygen emission

Range: 70–330 �s 18 �sb

Enhancement of oxygensignal with dualpulse

314-fold 50-foldb

Maximum gate delay�td� for detection ofoxygen emission

2400 ns 500 nsb

Cavitation bubble life-time

480 �s 650 �sc

Detection limitsCa2 �aqueous� 41.7–47.3 ppb 0.6 ppmd

�422.7 nm� 0.09 ppme

0.13 ppmf

Cr �aqueous� 1.04 ppm 0.1 ppmg

�425.4 nm� 20 ppmh

200 ppmd

Zn �aqueous� 17 ppm 1 ppmi

�472.2 nm�

aBulk excitation 2 in. � 2 in. � 2 in. cube �35 cm3, � � 1064 nm,E1 � 95 mJ, E2 � 175 mJ, 5-Hz repetition rate.

bLiterature data in Ref. 73, bulk excitation 20-cm3 cell, � � 1064nm, E1 � 30–76 mJ, E2 � 125 mJ, 10-Hz repetition rate.

cLiterature data in Ref. 84, � � 1064 nm, 3000-mJ pulse, 50-nswidth.

dLiterature data in Ref. 48, surface and laminar flow excitation,� � 1064 nm, 25–100-mJ pulse, 20-Hz repetition rate.

eLiterature data in Ref. 85, bulk excitation in a 20 mm � 10mm � 35 mm quartz cell, � � 532 nm, 5-mJ pulse, 20-Hz repetitionrate.

fLiterature data in Ref. 74, bulk excitation in a 46 mm � 12.5mm � 22.5 mm silica cell, � � 308 nm, 22-mJ pulse.

gLiterature data in Ref. 12, surface excitation, � � 1064 nm,400-mJ pulse, 1-Hz repetition rate.

hLiterature data in Ref. 83, surface excitation, � � 532 nm,60-mJ pulse, 1-Hz repetition rate.

iLiterature data in Ref. 80, deposited on a carbon planchet, � �1064 nm, fluence 35 J�cm2.


region, we calculated one fifth of the peak-to-peakvariation, yielding a second value for . For the de-termination of detection limits, both ’s were used,with the highest limit of detection ultimately re-ported.

B. Experimental Design and Data Handling

To determine the range of concentrations to use forcalibration curves and detection limit calculations,preliminary estimates of the detection limit weremade on the basis of the emission signals from con-centrated solutions. For example, Ca emission froma 400-�g�ml solution was measured and based on theresponse of a preliminary estimate of a sub-50-

�g�mL detection limit. This value of 50 �g�ml wasthen used as the maximum concentration in a seriesof standards that were used to make a calibrationcurve from which the reported detection limit wasdetermined. Once the concentration range was de-termined, five sets of 25 spectra were collected at thelowest, middle, and highest concentration levels andthree sets of 25 spectra were collected for all otherconcentrations. Following the same methodology asreported for the blank, a single spectrum was pro-duced for each concentration and used to generate thecalibration curve. We determined the detection lim-its by a least-squares fit using all the experimentallydetermined points to eliminate possible overaverag-ing effects. In the case of Zn, which gave weakeremission than Ca and Cr, five sets of 25 spectra werecollected for all concentrations with the data han-dling the same as mentioned above. Using this pro-cedure we determined the detection limits to be 47ppb, 1 ppm, and 17 ppm for Ca, Cr, and Zn, respec-tively. Representative spectra and calibrationcurves are shown in Fig. 6.

5. Conclusions

It is well known that lasers are capable of accuratelyproducing a highly spherical bubble in aqueous solu-tions at a well-defined position and time,86 and it hasbeen shown that sequential pulse excitation can pro-vide enhanced emission for reasons that are not com-pletely understood. As shown here, use of twoorthogonal laser pulses, bringing the second laserpulse into the bulk sample orthogonal to the firstlaser pulse, allows precise control of laser spark over-lap with resulting improvements in analytical sensi-tivity and limits of detection relative to single-pulseexcitation. Further research is needed to addressthe effects of turbidity and colored solutions. In ad-dition, as evident in the LIBS spectra for Ca, Cr, andZn, the H� �434.5-nm� and H� �486.1-nm� lines sig-nificantly influence the absolute detectability of tracemetals in these regions at low analyte concentrations.The effect of both H� and H� can be greatly minimizedat higher concentrations by detector gating, as shownin Figs. 5�b� and 5�c�. Although further research isrequired, orthogonal dual-pulse LIBS clearly hasgreat potential as an on-line monitor for measure-ments of dissolved species in bulk aqueous solutions.

We acknowledge the U.S. Department of Energyfor support of this research under grantDEFG0796ER62305 and the U.S. Office of Naval Re-search for support under grant N0014-97-1-0806.Some travel funds were also provided by Noel Mooreand the Continuum Laser Corporation.

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dual-pulse laser-induced breakdown spectroscopy in bulk aqueous solution with an orthogonal beam...

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