370 Volume 56, Number 3, 2002 APPLIED SPECTROSCOPY0003-7028 / 02 / 5603-0370$2.00 / 0q 2002 Society for Applied Spectroscopy

Analysis of Aqueous Solutions by Laser-Induced BreakdownSpectroscopy of Ion Exchange Membranes

NORMAN E. SCHMIDT and SCOTT R. GOODE*Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South Carolina 29208 (S.R.G.); andDepartment of Chemistry, Georgia Southern University, Statesboro, Georgia 30460 (N.E.S.)

Elemental analysis of solutions can be achieved by concentratingand immobilizing the metal ions into a commercially available ionexchange polymer membrane followed by laser-induced breakdownspectroscopy. Two methods of sample preparation were investigat-ed: � ltering the solution through the ion exchange membrane withsuction, and placing the membrane in the solution and allowing theions to equilibrate with the membrane. The membrane was thenablated with the focused energy of a Nd:YAG laser at 1064 nm.The emitted light was collected by an echelle spectrometer througha � ber-optic cable and detected with an intensi� ed charge-coupleddevice (CCD). Ten different metals, most covered by the ResourceConservation and Recovery Act (RCRA), were studied. The con-centrations of barium, cadmium, chromium, cobalt, copper, silver,lead, mercury, nickel, and zinc can be measured simultaneouslywith limits of detection ranging from 2 mg/mL to 4 ng/mL. Thelinear range is 2–6 orders of magnitude depending upon the elementand sampling method. The major advantages of the technique arethe multielement capability and the ease of sample preparation.

Index Headings: Laser-induced breakdown spectroscopy; LIBS;Aqueous solutions; Sample preconcentration; Ion exchange mem-branes; Elemental analysis.

INTRODUCTION

A number of review articles1–8 indicate that the bulkof the applications of laser-induced breakdown spectros-copy (LIBS) has been for solid samples. In this tech-nique, a high-power pulsed laser is focused on a sampleto ablate and excite it. The radiative emission from theexcited atoms affords qualitative and quantitative analysisof the original sample.

LIBS has several inherent advantages. It is a multiel-ement technique requiring little or no sample preparationfor most samples. The sample size is small; the lasersamples an area less than 0.2 mm in diameter. Laser-induced breakdown spectroscopy also offers the potentialfor remote in situ elemental analysis. In many situations,costs, times, and dif� culties of sampling and transportingthe samples to the laboratory exceed those of the labo-ratory analysis. Extending laser-induced breakdown spec-troscopy to the elemental analyses of aqueous subsurfacesamples is the major goal of this study.

Many environmental groundwater analyses can be per-formed by remote LIBS by placing a pair of optical � bersinto a test well; one � ber can transport the laser pulse tothe sample and the second can send the LIBS emissionto the spectrometer.9 Remote analysis is particularly im-portant when the groundwater is contaminated. Not onlymust workers be protected, but also the excess ground-water must be containerized and disposed of as a hazard-

Received 21 May 2001; accepted 24 October 2001.* Author to whom correspondence should be sent.

ous material. Fiber-optic LIBS could be used in concertwith cone penetrometry to measure concentrations ofsubsurface elements of interest without drilling a well.

The elemental composition of hazardous or radioactivesamples can be determined by remote LIBS without car-rying samples to the lab, thus minimizing exposure to labpersonnel. In addition, transporting radioactive or otherhazardous samples often requires extensive packagingand/or transport permits that add expense and time to theanalysis.

However, LIBS has a number of limitations. The anal-ysis is not completely free of matrix effects.10 The emis-sion is affected by both the form of the metal (carbonate,oxide, nitrate, chloride) and the composition of the sam-ple (soil, sand, metal, gas). The fundamental causes ofsuch matrix effects have not been determined. Some ofthe possibilities for matrix effects include variations inthe melting point, boiling point, heat of vaporization, heatcapacity, and ionization energy of the analyte and thematrix elements. Another limitation to LIBS is that whilelimits of detection can reach 1 mg/mL, they seldom getmuch lower.11

In comparison to solid samples, little work has beendone by LIBS on the analysis of solutions. Watcher andCremers performed the LIBS of solutions contained with-in a glass vial.12 Radziemski et al. focused the laser beaminto bulk liquid solution13 and later performed time-re-solved LIBS of aerosols.14 Ng and co-workers performedLIBS on a falling liquid stream.15 Arca and co-workersfocused the laser beam onto the surface of water.16 Ar-chontaki and Crouch performed LIBS using an isolateddroplet sample introduction system.17

In this paper we report the results of preconcentratingand immobilizing the analyte from a liquid sample intoan ion exchange membrane. The ion exchange polymeris a poly(styrenedivinylbenzene) copolymer support func-tionalized by iminodiacetic acid groups. At a pH greaterthan 5, the carboxylate groups are negatively charged andthe membrane functions as a cation exchanger or metalchelator. Using the membrane for LIBS provides two im-portant advantages. First, the sample is concentrated intoa smaller volume. Second, the analyte atoms all enter intoa common matrix (the membrane), thereby eliminatingerrors due to speciation and matrix effects.

Suction � ltration is a common way in which mem-branes are used to preconcentrate samples. The ions areeasily removed from the solution and the membrane driesas air passes through the polymer. Only a few minutesare required to � lter samples as large as 100 mL. Animportant advantage of � ltering is that the limit of detec-

APPLIED SPECTROSCOPY 371

FIG. 1. LIBS spectra (single pulse) of an ion exchange membrane after� ltering a 10 mL solution containing 18 mg Ag, 14 mg Cd, 51 mg Cu,8.5 mg Ni, 36 mg Pb, 11 mg Cr, 61 mg Hg, 24 mg Ba, and 52 mg Znthrough the membrane.

tion can be extended to lower concentrations by increas-ing the volume of solution drawn through the membrane.

The second method of preconcentration is by passiveextraction, in which the ion exchange membrane isplaced directly in the solution of interest. The ions insolution equilibrate with active sites on the membrane,concentrating the analyte on the membrane. The mem-brane is then dried and analyzed by LIBS. This samplingmethod essentially eliminates sample preparation, but agreat deal of time is required for the membrane to absorbthe metal ions from solution. Results from both samplingmethods are discussed.

EXPERIMENTALA Nd:YAG pulsed laser (Model SLI-10, Continuum,

Santa Clara, CA) was used at 1064 nm. The laser wascontrolled by a pulse generator (Model 500A, BerkeleyNucleonics Corp, San Rafael, CA). Laser pulses wereapproximately 7 ns in width, with energy of 80 mJ. Spec-tra were collected with an echelle spectrometer coupledto an intensi� ed CCD detector (Mechelle 7500, Multi-channel Instruments, Stockholm, Sweden). Spectra wereanalyzed using the software provided with the spectrom-eter (Mechelle version 1.51) and Grams 32 software(Thermo-Galactic Industries Corp., Salem, NH). The de-lay after the laser pulse before spectra were collected wasset at 2750 ns. After the delay, spectra were collected for11 ms.

Ion exchange membranes were obtained from FisherScienti� c (Suwanee, GA). The particular membranesused were 47-mm-diameter 3M Emporey ExtractionDisks (3M Corp., St. Paul, MN) cut to � t the � ltrationapparatus. The membranes were pretreated according tomanufacturer’s instructions before use. In � ltration ex-periments the sample was passed through the membraneusing a locally fabricated 100-mL borosilicate glass fun-nel with an inside diameter of 8.2 mm. The funnel wasdesigned to � t a Milliporey � ltration apparatus. There-fore, in � ltration experiments the sample was preconcen-trated into a � at circle of membrane with a diameter of8.2 mm. The membrane was then af� xed to a piece ofbacking paper with cellophane tape and mounted on anX-Y stage. In � ltration studies no special effort was madeto dry the sample because passing air through the samplewith suction pulled most of the water out of the mem-brane and some drying occurred. In passive extractionstudies, the samples were dried by brie� y dabbing witha paper towel, along with some air-drying.

The mechanical X-Y stage was a Model 451P BallSlide (Del-Tron Precision, Inc., Bethel, CT). The stagewas moved using Model K92111-P2 Digital Linear Ac-tuated Stepper Motors. The mounting brackets to holdthe sample used on the X-Y stage were prepared in-house, as was the software used to control the movement.

Solutions were prepared from reagent grade salts anddissolved in 18.2 MV water unless otherwise speci� ed.The ground water was obtained from a 180 foot welllocated in Columbia, South Carolina, and was � lteredwith Whatman #1 � lter paper prior to use.

RESULTS AND DISCUSSIONSpectra. A typical LIBS spectrum from an ion ex-

change membrane with metal analyte atoms is shown in

Fig. 1. A single laser pulse is suf� cient to ablate themembrane; a second pulse forms a hole and shows onlyair emission lines. A single laser pulse ablates an area ofabout 1.2 3 1023 mm 2 (0.04-mm diameter), but colorchanges are observed in a larger area, so a space of atleast 2 mm is needed between sampling points to ensurethat each sample is not in� uenced by previous experi-ments.

Some of the distinguishing characteristics of the spec-tra shown in Fig. 1 include the hydrogen emission lineat 656 nm and a CN band in the 380–400 nm region.Both the H and CN are due to ablation of the polymermembrane, which is the source of hydrogen and carbon.The nitrogen source for the CN emission is the air in theroom. Some individual oxygen and nitrogen lines at742.6, 744.4, 747.0, 777.5, and 868.2 nm are also due tothe air. These lines do not interfere with the analysis ofmetal ions in the membrane and most metal atoms canbe easily quanti� ed. The other major lines shown in Fig.1 are due to the emission of analyte metal ions.

Filtration. In the � ltration experiment, samples weredrawn through the membrane with suction. The effect ofsample volume was determined by preparing several so-lutions, each with 191 ng of Cu(II) diluted to 5, 10, 20,. . . , 1000 mL. The solution concentrations ranged fromapproximately 38 to 0.19 ng/mL. Each solution wasdrawn through an ion exchange membrane and the mem-brane was analyzed by LIBS. Figure 2 shows the actualspectra at the copper 327.4-nm line. The observed peaksare clearly visible above a relatively small backgroundand have a reasonable signal-to-noise ratio. Figure 3shows the dependence of the copper emission at 327.4nm on solution volume. The error bars indicate the sam-ple standard deviations, which are relatively high becausethese samples contain analyte at about 5 times the limitof detection. Although the relative standard deviations arehigh, these results demonstrate that the volume of solu-tion in the range 5–1000 mL does not in� uence the meth-od. The data clearly indicate that the number of countsdepends only on the mass of metal in the solution. Forconvenience, a sample volume of 10 mL was chosen forthe remaining experiments.

Effect of pH. The effect of pH was determined bypreparing solutions with a constant metal ion concentra-tion in a 0.050 M ammonium acetate buffer. The resultsare shown in Fig. 4. This graph is based on the 510.6-nm emission from a sample with 19 mg of Cu(II). The

372 Volume 56, Number 3, 2002

FIG. 2. LIBS spectra of exchange membranes. (A) Blank; (B) 10.0 mLof 19.1 ng/mL copper solution (191 ng); (C ) 1000 mL of 0.191 ng/mLcopper solution (191 ng).

FIG. 3. Signal from 191 ng of copper at 327.4 nm obtained from so-lutions ranging from 5 mL of 38 ng/mL to 1000 mL of 0.19 ng/mL.Error bars represent the sample standard deviation of 4–6 replicates.

FIG. 4. Effect of pH on the observed copper emission. Error bars rep-resent the sample standard deviation of 4–6 replicates. The line is theaverage of the samples with the � ve highest pH values.

100% recovery line shown in Fig. 4 is the average re-covery for samples with pH greater than or equal to 4.The error bars indicate the standard deviations. Resultsshow that as long as the pH of the solution � ltered re-mained greater than or equal to 4, the amount of copperretained on the � lter remained constant. Recoveries werepoor at a pH of less than 3 because the carboxylategroups in the iminodiacetic acid become protonated andmetal ions are no longer extracted from the solution. Theminimum pH depended on the speci� c metal: nickel andlead required a pH greater than 5; zinc, cadmium, andcobalt required a pH greater than 6. Solutions of pHgreater than 9 were not checked because basic solutionsare not likely to be used extensively for metal ion anal-ysis due to the formation of metal hydroxides. The man-ufacturer of the membranes recommends a pH of 5.3, butour results indicate that a pH in the range 7–9 is optimalfor the elements measured in this study.

In� uence of Timing. The delay time and integrationtimes were adjusted to optimize the signal-to-noise ratio.Compromise conditions were used because each elementhad different optimum timing requirements. The copper510.6-nm line had the best signal-to-noise ratio with a

delay of 3000 ns and a gate of 20 ms. The best conditionsfor studying the silver 328.1-nm line required a delay of1500 ns and a gate of 20 ms, and the chromium 520.9-nm line required a delay of 3000 ns and a gate of 10 ms.The conditions used in the rest of this work, 2750-nsdelay and 11-ms gate, represent a good compromiseamong these different optima.

Reproducibility of Analysis. The relative standard de-viation of replicate analyses (n 5 12) of the emissionfrom the 510.6-nm copper peak was 14.5%. Each anal-ysis was performed at a different location on the mem-brane. Therefore, one possible source of imprecision isnonuniform sample deposition. Another source of impre-cision is the 5–10% � uctuation in the output of the Nd:YAG laser.

Calibration Graphs. Calibration plots were construct-ed for the ten metals studied. A sample calibration curvefor the copper 510.6-nm line is shown in Fig. 5. The errorbars indicate the standard deviations of the signals ob-tained. This calibration curve is fairly typical for the oth-er metals studied. This curve has a correlation coef� cient(r 2) of 0.95 using 20 data points in the calibration. Thecounts are uncorrected for background emission, whichis signi� cant at 510.6 nm. The slopes of the calibrationgraphs (sensitivity) vary by almost three orders of mag-nitude, with copper being the most sensitive and zincbeing the least sensitive. At very high concentrations ofmetal (not shown in Fig. 5) the signal becomes nonlinear.These effects are due to instrumental parameters such assaturating the detector and physical effects such as self-absorption. The limits of detection for the elements stud-ied are given in Table I. The linear range of most of these

APPLIED SPECTROSCOPY 373

FIG. 5. LIBS calibration graph for the determination of copper ob-tained by � ltering 10 mL of aqueous solution through the ion exchangemembrane. Error bars represent the sample standard deviation of 4–6replicates.

TABLE I. Results of � ltration experiments.

Elementl

(nm)

FiltrationLODa

(mg/mL)

FiltrationLOD(mg)

Passive extrac-tion LODb

(mg/mL)

BariumCadmiumChromiumCopperCopperLeadMercuryNickelSilverZinc

493.4361.1520.9324.7510.6405.8435.8353.0328.1472.2

0.130.210.130.00950.00421.12.00.310.430.85

1210.10.04

1020349

1410.0340.08

1013221

a Assumes sample volume of 10.0 mL. Concentrations decrease as vol-ume increases up to 1000 mL. The LOD is the concentration or massthat provides a signal equal to 3 times the standard deviation of thebackground.

b Based on 1 day of extraction. The LOD improves for longer extractiontimes.

FIG. 6. LIBS calibration plot for the determination of copper obtainedby passive extraction of aqueous solution ions into the ion exchangemembrane. Concentrations vary from 6400 mg/mL to 6.4 3 1024 mg/mL. (v) Extraction after 1 day. (V) Extraction after 28 days.

elements was 2 orders of magnitude or greater. The con-centration detection limits reported in Table I are poorerthan those reported for ICP emission by a factor thatranges from 10 for copper to 4000 for barium. The LIBSdetection limits are based on a 10-mL sample and im-prove by a factor of 100 if a 1-L sample is used becausesignals are mass and not concentration dependent. Thisimprovement means that the LIBS system can detect low-er concentrations of copper than ICP and is within a fac-tor of 40 for the worst case, which is barium.

Efforts were made to analyze selenium and arsenic bythis technique, but the primary selenium and arsenicemission lines are too far into the UV region for the � ber-optic collection system. The emission lines in the visibleregion were not useful for concentrations of environmen-tal importance, so arsenic and selenium were excludedfrom further study.

The calibration curves show a non-zero intercept. Theintercept appears to be due to continuum emission, whichcould be minimized by increasing the delay time, sub-tracting the adjacent spectral background, or subtractingthe reagent blank signal.

Passive Extraction. To test the ef� ciency of passiveextraction, pretreated extraction membranes were placedfor a given length of time in the solution to be analyzed.The membranes were removed from the solution, driedwith a paper towel, and then LIBS was performed. It wasfound that approximately one day was needed to extractmetal ions into the membrane.

When the passive extraction method was applied to aseries of solutions containing nine metal ions with vary-ing concentrations, a linear calibration graph was ob-tained for each ion after one day of extraction (19 h). Thelimits of detection for this technique are listed in TableI. In many ways, the calibration curves of these elementsresemble those of the � ltered solutions. The limits of de-tection are approximately the same. However, the linear

range of the technique was found to be greater usingpassive extraction. For example, the linear dynamic rangefor copper was approximately six orders of magnitude.

A long-term study of the use of passive extractionLIBS to detect metal ions was conducted. In this study,several membranes were placed in the solution of copper(II) and fresh solution was added daily to emulate placingthe membrane into a large volume system such as a lakeor aquifer. After varying lengths of time a membrane wasremoved from the solution and analyzed by LIBS. Cali-bration graphs for copper (II) solutions after one day and28 days are shown in Fig. 6. The limit of detection isapproximately 10 ng/mL Cu(II) at 510.6 nm. The sensi-tivity (slope of the calibration plot, Fig. 6) remained con-stant over the four weeks of the study, although the cal-ibration graph shifted to higher counts over time as themembrane absorbed more copper from the solution.

Membranes placed in high concentration Cu(II) solu-tions did not increase in signal after day 1, thus indicatingthat copper occupied all the available exchange sites.Membranes in solutions having lower concentrations ofcopper showed continued increases in signal over time.At a copper concentration of 60 mg/mL, the membranebecomes saturated with copper after approximately four

374 Volume 56, Number 3, 2002

weeks. At any given length of time, the mass of metal inthe membrane is proportional to the bulk concentrationas long as the membrane is not saturated.

Matrix Interferences. The effect of common matrixcomponents on the quantitative results was determinedthrough several experiments. Natural local groundwatersamples have modest levels of sodium, calcium, and iron.Experiments showed that calcium concentrations up to200 mg/mL had no signi� cant effect upon the signals ofthe various metal ions studied, except barium, in whicha calcium concentration of 30 mg/mL reduced the bariumsignal to one-half the value observed from a solutionwithout calcium added. Additional increases in calciumfurther decreased the barium signal to less than one-fourth the initial value. The exact cause of this decreaseis uncertain. However, it is believed that calcium effec-tively competes with barium for active sites on the mem-brane, preventing barium from binding to the membrane.The information provided by the manufacturer shows thatthe binding constant for calcium is greater than that ofstrontium. Barium is not listed, but from periodic trends,it is reasonable to infer that the binding of barium is lessthan that of strontium and that calcium could displacebarium in the ion exchange membrane. The manufacturerstates that lead, copper, cadmium, cobalt, and iron havestronger binding constants than calcium and that the se-lectivity of the membrane roughly follows the EDTAcomplex formation constants. The binding of the othermetals studied (chromium, mercury, nickel, silver, andzinc) must also be stronger than that of Ca12.

The effect of added sodium was small. Most elementswere unaffected, and the signals from solutions of Cd,Hg, Ni, Zn, and Cu showed only minor decreases (lessthan 30%) after adding 30 mg/mL Na1.

Analyses of the test metals diluted in distilled waterand groundwater were compared to determine the cu-mulative effects of contaminants and concomitants foundin local groundwater. The groundwater showed no effectexcept for a 30% depression of the barium signal, mostlikely due to the presence of calcium in the ground water.It should be noted that the ground water contained ap-proximately 2 mg/mL of iron, which did not affect thedetermination of the other ions.

CONCLUSION

Using extraction membranes for laser-induced break-down spectroscopy of the metal ions in solutions allowsrapid sample analysis with minimal sample preparation.The technique determines the absolute mass of analyteextracted into the membrane. A large volume of solutioncan be � ltered through the membrane to measure verysmall concentrations or a smaller volume can be used todetermine a solution of higher analyte concentration. Incomparison, the passive extraction method also has lowlimits of detection, but the time scale of the experimentchanges from about one minute to about one day.

ACKNOWLEDGMENTS

This research was supported in part by the U.S. Department of En-ergy (DE-FG07-96ER62305) and the U.S. Department of Defense(N00014-97-1-0806-1).

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