ORIGINAL PAPER

Determination of sub—ppm levels of boron in ground watersamples by laser induced breakdown spectroscopy

Arnab Sarkar & Suresh K. Aggarwal & K. Sasibhusan &

D. Alamelu

Received: 16 September 2009 /Accepted: 20 November 2009 /Published online: 8 December 2009# Springer-Verlag 2009

Abstract Laser induced breakdown spectroscopy (LIBS)was applied to the determination of sub-ppm levels of boronin ground water samples using spectroscopically pure graphiteplanchets as solid support. The data obtained by LIBS agreedwell with those from ICP-AES. No spectral interference due tothe possibly interfering elements Fe, Cr, Al and Mo wasobserved. The detection limit was 0.01 µg.g−1 for boronusing the B(I) 249.773 nm emission line. The method isconsidered to be promising for the rapid determination ofboron, with an acceptable degree of accuracy and without theneed for elaborate sample treatment, preconcentration andpurification steps.

Keywords Boron . Laser induced breakdown spectroscopy(LIBS) . Ground water . Spectral interference

Introduction

Boron (B) is an essential element for plants and itsdeficiency affects plant growth and yield in many parts ofthe world. At the same time, an excessive amount of B istoxic to plants and limits the crop productivity in manyenvironments [1]. B is often found in rocks and soil and isslowly leached into the ground water. B can also bereleased into the environment during manufacturing ofcommercial products such as pesticides. Because boron iswidespread in our environment and in the food chain, weall have some amount of this element in our bodies. It is

estimated that daily intake of B, on an average, is 10–25 mgthrough the food consumed. In animal studies, ingestion ofhigh levels of B affected the testes and sperm of males, andcaused birth defects in the offspring of exposed pregnantfemales. These reproductive and developmental effectsoccurred at much higher levels of B than are commonlyfound in drinking water. There is still some uncertainty aboutthe health effects of low levels of B in human beings and,therefore, limits of B concentration in drinking water havebeen fixed in a conservative way to protect human health. Thesafe concentrations of B in drinking water and irrigation waterare indicated to be belowmg.dm−3 (i.e. less than μg.g−1 of B)in the guidelines on water quality [2]. It has been alsoreported that B/Cl amount ratio in ground water sample is animportant factor for identification of the contaminationsource of salinity in ground water [3, 4]. Thus determinationof B in ground water samples is of great interest.

For the determination of elements of low atomic numbers(i.e. low Z), only a few analytical methods are available.Further, for elements like B where adventitious contaminationof sample is quite possible during any sample pretreatment,there is a need to develop and validate methods which do notinvolve elaborate sample preparation steps. Inductively cou-pled plasmamass spectrometry (ICP-MS), inductively coupledplasma atomic emission spectrometry (ICP-AES), thermalionization mass spectrometry (TIMS) and high performanceliquid chromatography (HPLC) have been reported for thedetermination of B in ultra-pure water at ng.L−1 levels [5–7].But these methods are not suitable for on-site analyses. Inaddition, due to the non-availability of certified referencematerials for B in ground water sample, there is a need todevelop different methodologies based on independentphysico-chemical principles to enhance confidence in the data.

Laser-induced breakdown spectroscopy (LIBS) being asimple, rapid, and minimally destructive analysis method

A. Sarkar : S. K. Aggarwal (*) :K. Sasibhusan :D. AlameluFuel Chemistry Division, Bhabha Atomic Research Centre,Trombay, Mumbai 400 085, Indiae-mail: skaggr2002@rediffmail.come-mail: skaggr@barc.gov.in

Microchim Acta (2010) 168:65–69DOI 10.1007/s00604-009-0259-7

without any complicated sample preparation procedure isattractive in real time quality control of industrial processesand environment monitoring [8, 9]. It involves the focusingof a high-intensity laser beam on a sample to create plasma.By spectrally analyzing the emission originating from theplasma, the elemental composition of the sample can bequantified. It was, therefore, considered worthwhile todevelop LIBS for monitoring of trace contaminant such asB in ground water.

Analysis of liquid samples by LIBS is less favored due tothe inherent draw-backs of the method such as splashing,surface ripples, quenching of emitted intensity and a shorterplasma life-time [10]. In our previous work, we showed theapplicability of using solid sample support namely Whatman-42 membrane filter paper, for analysis of liquids [10, 11].However for B, Whatman-42 filter paper could not be used asit contains around 1 µg.g−1 B [12]. Since B concentration inground water is expected to be a few hundred ng.g−1, suitablesolid support free of B was also identified in the present work.

There have been few reports of B determination in solid byLIBS, viz., Kurniawan et al. quantified B in glass using thesame methodology using direct irradiation of glass [13]. Uhlet al. applied LIBS for quantification of B in treated wood[14]. Fichet et al. showed the application of standard additionmethodology in UO2 and PuO2 samples for B analysis, butdue to high background, the detection limits quoted werevery poor [15]. B also can be determined directly in liquids;however, only one example (on Ni support) has beendescribed in the literature by Pardede et al. in liquid matrixby LIBS [16]. The LOD reported in this method iscomparable to that achieved in the present work, but themethodology employed is very complicated, has high samplerequirement and is difficult to apply for in-field purpose. Inthis method, using 250 mL sample solution, element ofinterest was deposited on a supra-pure Ni plate which wasthen irradiated with laser in a vacuum chamber (1–10 torr).

This paper describes LIBS methodology developed forthe determination of B in ground water samples, usingspectroscopically pure graphite planchets as a solid support.The results were compared with those obtained bycommonly used method of ICP-AES. Further, the detectionlimits achieved by LIBS were compared with thosereported previously for other matrices. It was demonstratedthat LIBS can be directly applied to the ground watersamples without any pre-concentration or pre-treatment.

Experimental

Instrumentation

The Spectrolaser 1000M, from M/s Laser Analysis Tech-nologies Pvt. Ltd (now known as XRF Scientific), Victoria,

Australia was used in this work. The details of the instrumenthave been described elsewhere [10, 11]. The instrument isequipped with a high power, Q-switched Nd—YAG laserwhich yields up to 200 mJ of pulse energy at thefundamental IR wavelength of 1,064 nm in a 7 ns pulsewidth with a repetition rate of 10 Hz. The laser is focused onto the sample by a plano-convex lens having a focal lengthof 5 cm. The laser spot area is estimated to be 1.96×10−7 m2.The radiated emission from the produced plasma is carried byoptical fibers to the spectrographs in Czerny–Turner (C–T)configuration having a spectral resolution of 0.6 nm at300 nm. ICP-AES analysis was done by using Jobin-Yvon“JY-50- Panorama” spectrometer. B(I) 249.773 nm was usedin ICP-AES analysis.

Sample preparation

Standard stock of 20 µg.g−1 aqueous solution of B wasprepared by dissolving known amount of boric acid (M/sMerck, www.merck.co.in ) in de-ionized Milli-Q waterhaving resistivity of greater than 18.2 MΩ-cm at 25°C. Thesolution was subsequently diluted to obtain B concentra-tions up to 0.1 µg.g−1. Ground water samples collectedfrom the states of Tamilnadu (TN1 and TN2) and Gujarat(GU1 to GU3), India were analyzed for determining theconcentration of B. Graphite planchets (Ted Pella Inc.,http://www.tedpella.com) of 32 mm diameter and 1.6 mmwidth were used as a solid sample support. Twenty-five μLof solution was transferred drop-wise to the graphiteplanchet and was evaporated to dryness using a hot airblower. Subsequently, the planchet was allowed to cool toroom temperature before mounting it on the sample holder.Spotting the ground water samples onto the planchet resultsin always the same surface area of deposit after drying thedroplet, but this may not be true with high level ofsurfactant. Since a very small amount of sample was loadedon the planchet, the surfactant effect will be very small andwas not considered in the present study. Triplicate analyseswere done for each of the calibration standards as well asthe ground water samples. Standard solutions of Fe, Cr, Aland Mo were also prepared by dissolving the Fe(NO3)3,K2Cr2O7, Al(NO3)3 and (NH4)6Mo7O24, respectively, innitric acid and then diluting with Milli-Q water to obtain aconcentration of 100 μg of each metal per gram of solution.

Results and discussion

Selection of substrate

Many factors that favored the choice of graphite planchet asthe substrate material. These are inexpensive and easy tohandle. A simple chemical composition with only a few

66 A. Sarkar et al.

emission lines, resulting in reduced spectral interferences,as shown in the Fig. 1a. It can be seen that apart from thetwo C(I) emission lines, three regions in the spectrumcannot be used for analytical purpose, i.e. CN radicalemission bands in the spectral region of 347–363 nm, 370–389 nm and 410–423 nm [17]. However, these regions arenot important for B determination. Graphite does not reactchemically with aqueous solution. The solution on dryinggets adsorbed on the planchet.

Selection of emission line

A typical B spectrum obtained by LIBS, in 190–270 nmemission range, using laser fluence of 50 J.cm−2 andacquisition delay of 1.5 µs, is shown in Fig. 1b. The twomost intense emission lines of boron, B(I) 208.957 nm andB(I) 249.773 nm, were identified to be interference free andwere used. The doublet for each of the lines selected wasnot completely resolved with the existing instrumentresolution (0.6 nm at 300 nm) available in our laboratory.Hence the gross intensity under the peak area was used forconstruction of calibration curves.

Normalization of the intensities of B(I) 208.957 nm and249.773 nm emission lines were done with respect to C(I)

193.091 nm and 247.856 nm emission lines, respectively.Carbon emission lines was chosen as internal normalizationlines since graphite was the sample support and afteradsorption of liquid sample over its surface, it can beconsidered as a graphite sample containing trace amounts ofB. So the C emission lines can be considered as matrixemission lines. The humic substances present in water (in ppmrange) will contribute negligibly to the integrated intensity ofcarbon emission line and hence were not considered.Normalization takes care of effect of variation of laser fluencefrom shot-to-shot.

Optimization procedure

Two important optimization parameters in LIBS analysisare laser fluence for plasma production and acquisitiondelay time. The integrated signal to noise intensity ratio(SNR) was chosen as the figure of merit for theoptimization procedure. The emission spectra wererecorded at different laser fluences (15 J.cm−2, 20 J.cm−2,35 J.cm−2, 40 J.cm−2 and 50 J.cm−2) to study its effect onthe signal to noise ratio (SNR) of B(I) 249.773 nm. Figure 2shows the dependence of SNR on incident laser fluence fortest sample containing B at a concentration of 1 µg.g−1. TheSNR was found to increase with increase in incident laserfluence from 15 J.cm−2 to 50 J.cm−2. At laser fluencehigher than 50 J.cm−2, the carbon emission line C(I)247.856 nm, used for normalization showed pixel satura-tion. Hence laser fluence was not increased further and wasrestricted to 50 J.cm−2 (i.e. irradiance of 7.1 GW.cm−2) forall analyses. Since same laser fluence was used in all

Fig. 1 LIBS spectrum of a graphite disk and b boric acid solution ona graphite planchet at a laser fluence of 50 J.cm−2 and acquisitiondelay of 1.5 µs

Fig. 2 The dependence of the SNR of the B(I) 249.773 nm emissionline on laser fluence and acquisition delay

Determination of sub—ppm levels of boron in ground water samples 67

analyses, the effect of self-saturation, if any, will be cancelledout and hence was not studied. Figure 2 also shows the effectof variation of acquisition delay on SNR at a fixed laserfluence of 50 J.cm−2. An acquisition delay time of 1.5 µs wasfound to give the best SNR for the B(I) 249.773 nm emissionline and was, therefore, selected for all the experiments. Thesignal acquisition gate window was fixed at 10 μs whichcannot be varied in the present instrument.

Effect of interfering elements

The possible spectral interferences expected for the emis-sion lines of B at B(I) 208.957 nm are due to Al(I)208.915 nm and Mo(II) 208.952 nm; and for B(I)249.773 nm line are due to Fe(I) 249.653 nm and Cr(I)249.630 nm [18]. 100 µg.g−1 of Al, Mo and Cr solution onanalyzing by LIBS, under the experimental conditionsoptimized for B, did not show any significant interferencein the two emission lines of B viz. B(I) 208.957 nm and B(I) 249.773 nm. 100 µg.g−1 of Fe solution was found tocontribute to the B(I) 249.773 nm emission line equivalentto a B concentration of 200 ng.g−1. The concentrations ofAl, Mo, Cr and Fe in the ground water samples determinedby ICP-AES are shown in Table 1. Since these concen-trations are less than µg.g−1, no pretreatment for theirremoval was necessary in the present work.

Calibration curves

The calibration curves obtained by plotting the normalizedintensities of the two selected emission lines of B versus thecorresponding concentrations of B are shown in Fig. 3.Each point, in these figures, corresponds to an average oftriplicate measurements and the error bars are the standarddeviations (±1σ). Both the emission lines B(I) 208.957 nmand B(I) 249.773 nm showed good linearity up to Bconcentration of 20 µg.g−1. The slope of the calibrationcurve for B(I) 249.773 nm emission line was approximatelyfive times higher than that for B(I) 208.957 nm.

B in ground water samples

Limit of detection (LOD) was calculated as 3 s/m, where m isthe slope of the calibration line and “s” is the uncertainty inthe intensity for the calibration point with the blank sample,i.e., only Milli-Q water. The LOD for the LIBS methodologyused for B quantification in ground water was found to be10 µg.L−1. The limits of LIBS for B determination are wellbelow the acceptable limits of B concentration in groundwater. Among the other reported methods for B determina-tion, Ramanjaneyulu et al. used prompt gamma neutronactivation analysis [19] to achieve LOD of 200 µg.L−1

using 15 g of sample size with 9 h counting, and Kumar et

Fig. 3 Normalized boron emission intensity as a function ofconcentration of boron at a laser fluence of 50 J.cm−2 and acquisitiondelay of 1.5 µs

Table 1 Concentrations of elements in ground water samples (givingspectral interference at boron emission lines in LIBS) determined byICP-AES

Element Concentration (µg.g−1) of the element in ground watersample

TN1 TN2 GU1 GU2 GU3

Fe <0.01 <0.01 <0.01 <0.01 <0.01

Cr <0.01 <0.01 <0.01 <0.01 <0.01

Mo 0.21 <0.02 0.12 <0.02 0.06

Al <0.07 0.09 <0.05 <0.05 <0.05 Fig. 4 Comparison of boron concentrations in ground water samplesdetermined by LIBS and ICP-AES

68 A. Sarkar et al.

al. [20] showed LOD of 100 µg.L−1 using flow-injectionanalysis method. Photometric azomethin H technique orfluorimetric methods had also been used for B determina-tion in the concentration range reported here [21]

Ground water samples were analyzed under the LIBSconditions identical to those used for the calibrationsamples. Figure 4 shows the results obtained for Bconcentration in the ground water samples by LIBS. Theresults obtained by ICP-AES are also shown in the Fig. 4for comparison purpose. For GU1 sample, the signal wasbelow detection limits for B(I) 208.957 nm and hence is notshown in the Fig. 4. As is seen, there is good agreement inthe results obtained by the two methods within theanalytical uncertainty of the method, which clearly showsthe applicability of LIBS for this work.

Conclusion

The solid sample support (graphite planchet) based LIBSmethod for the determination of sub-ppm amounts of B inground water samples is simple, rapid and does not require anypre-concentration or pre-treatment which eliminates the chan-ces of B contamination. These features would allow its use asan on-field real time analytical method for analysis of groundwater. The graphite planchet used in the developed methodol-ogy is highly pure and shows no significant contribution to Bsignal at both the wavelengths, thereby improving the LOD.The present studies are not aimed at replacing ICP-AES/MSwhich are useful analytical methods for multielementaldeterminations in ground water samples. However, thesestudies demonstrate the applicability of a LIBS method basedon an independent physico-chemical principle which has thepotential for field application in view of compact instrumen-tation, portable nature and freedom from requirements of largevolumes of high purity argon gas as in ICP-AES/MS.

Acknowledgements The authors are thankful to Dr. K. Shivanna,Isotope Applications Division (IAD), BARC for providing watersamples and members of the Spectroscopy Section in RadiochemistryDivision (RChD), BARC for providing the ICP-AES results. Authorsare grateful to Dr. V. Venugopal, Director of the Radiochemistry andIsotope Group at BARC, for his constant support and encouragement.

References

1. Sah RN, Brown PH (1997) Techniques for boron determinationand their application to the analysis of plant and soil samples.Plant Soil 193:15–33

2. vanLoon GW, Duffy SJ (2005) A global perspective, Environ-mental Chemistry, 2nd edn. Oxford University Press, Oxford, UK

3. Vengosh A, Heumann KG, Juraske S, Kasher R (1994) Boronisotope application for tracing sources of contamination ingroundwater. Environ Sci Technol 28:1968–1974

4. Vengosh A, Spivack J, Artzi Y, Ayalon A (1999) Boron, strontiumand oxygen isotopic and geochemical constraints for the origin ofthe salinity in ground water from the Mediterranean Coast ofIsrael. Water Resour Res 35:1877–1894

5. Wickham R, Godec R (2001) Proceedings of Semiconductor PureWater and Chemicals Conference. Hillsboro, USA, pp 15–33

6. Darbouret D, Kano I (2000) Ultrapure water blank for boron traceanalysis. J Anal Atom Spectrom 15:1395–1399

7. Gorenc B, Marsel J, Tramw G (1970) Application of Ion-Exchange to Mass Spectrometric Determination of Boron inMineral Waters. Mikrochimica Acta. 24–28.

8. Noll R, Bette H, Brysch A, Kraushaar M, Monch I, Peter L, SturmV (2001) Laser-induced breakdown spectrometry—applicationsfor production control and quality assurance in the steel industry.Spectrochim Acta Part B 56:637–649

9. Wainner RT, Harmon RS, Miziolek AW, McNesby KL, FrenchPD (2001) Analysis of environmental lead contamination:comparison of LIBS field and laboratory instruments. Spectro-chim Acta Part B 56:777–793

10. Alamelu D, Sarkar A, Aggarwal SK (2008) Laser-inducedbreakdown spectroscopy for simultaneous determination of Sm,Eu and Gd in aqueous solution. Talanta 77:256–261

11. Sarkar A, Alamelu D, Aggarwal SK (2008) Determination ofthorium and uranium in solution by laser-induced breakdownspectrometry. Appl Optics 47:G58–G64

12. Whatman filter paper website: http://www.whatman.com/CelluloseFilters.aspx

13. Kurniawan H, Nakajima S, Batubara JE, Marpaung M, OkamotoM, Kagawa K (1995) Laser-induced shock wave plasma in glassand its application to elemental analysis. Appl Spectrosc 49:1067–1072

14. Uhl A, Loebe K, Kreuchwig L (2001) Fast analysis of woodpreservers using laser induced breakdown spectroscopy. Spec-trochim Acta Part B 56:795–806

15. Fichet P, Mauchien P, Moulin C (1999) Determination ofimpurities in uranium and plutonium dioxides by laser-inducedbreakdown spectroscopy. Appl Spectrosc 53:1111–1117

16. Pardede M, Kurniawan H, Tjja MO, Ikezawa K, Maruyama T,Kagawa K (2001) Spectrochemical analysis of metal elementselectrodeposited from water samples by laser-induced shock waveplasma spectroscopy. Appl Spectrosc 55:1229–1236

17. Baudelet M, Guyon L, Yu J, Wolf JP, Amodeo T, Fréjafon E,Laloi P (2006) Spectral signature of native CN bonds forbacterium detection and identification using femto secondlaser-induced breakdown spectroscopy. Appl Phys Lett88:63901

18. vandeWiel H J (2003) Determination of elements by ICP-AESand ICP-MS. Horizontal — 19: National Institute of Public Healthand the Environment.

19. Ramanjaneyulu PS, Sayi YS, Nathaniel TN, Reddy AVR,Ramakumar KL (2007) Determination of boron in water samplesby chemical prompt gamma neutron activation analysis. JRadioanal Nucl Chem 273:411–414

20. Kumar SD, Maiti B, Mathur PK (1999) Determination of boronby flow injection analysis using a conductivity detector. AnalChem 71:2551–2553

21. Dietz F (1975) The boron concentration in water samples as anindicator of water pollution. gwf Gas- und Water Technologies:Water Wastewater 116:301–308

Determination of sub—ppm levels of boron in ground water samples 69