Optics & Laser Technology 44 (2012) 30–34

Contents lists available at ScienceDirect

Optics & Laser Technology

0030-39

doi:10.1

n Corr

E-m

skaggr2

journal homepage: www.elsevier.com/locate/optlastec

Gallium quantification in solution by LIBS in the presence of bulk uranium

Arnab Sarkar, D. Alamelu, Suresh Kumar Aggarwal n

Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

Article history:

Received 8 February 2011

Received in revised form

4 May 2011

Accepted 6 May 2011Available online 31 May 2011

Keywords:

Gallium

Uranium

LIBS

92/$ - see front matter & 2011 Elsevier Ltd. A

016/j.optlastec.2011.05.010

esponding author. Tel.: þ91 22 25593740; fa

ail addresses: asarkar@ymail.com (A. Sarkar),

002@rediffmail.com (S.K. Aggarwal).

a b s t r a c t

Laser induced breakdown spectroscopy (LIBS) was developed for the quantitative determination of

gallium (Ga) in liquid matrix to provide an independent analytical approach. The methodology involves

use of highly pure graphite planchet as a solid support. Important parameters including laser energy

and acquisition delay were optimized to achieve the best signal to noise ratio during the LIBS analysis.

Calibration was obtained for several emission lines of Ga with different dynamic ranges. The effect of

large amount of uranium was also studied. Satisfactory results were obtained for Ga present at

percentage level in the U–Ga mixture solution without the need to separate Ga from the bulk of matrix.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Gallium, considered as one of the rare elements, is widelydistributed in nature. The concentration of gallium in silicate rocksis normally in the 10–100 ppm range [1]. Increase in use of galliumand its compounds in the electronics and medical industry hasposed questions concerning its toxicity in the environment. This isalso one of the driving forces for developing sensitive analyticalmethods based on different physico-chemical principles for Gaquantification from ultra-trace to percentage level concentrations.Among the different compounds of Ga, gallium arsenide (GaAs) isan important semiconductor and is used to make devices such asmicrowave frequency integrated circuits (i.e. MFICs), infrared light-emitting diodes, laser diodes and solar cells [2]. Ga-citrate is usedfor its antitumor activity [3]. The addition of Ga to Pu has beenreported for d-phase stabilization of Pu [4]. Accurate and precisedetermination of gallium in presence of actinides is required fordifferent applications in nuclear science and technology.

The various techniques used for the determination of gallium arevoltammetry [5,6], atomic absorption spectrometry [7,8], neutronactivation analysis [9,10], inductively coupled plasma atomic emis-sion spectrometry [11,12] and inductively coupled plasma massspectrometry [13,14]. The utility of neutron activation methods isrestricted by instrumentation cost, long exposure times, matrixinterference and availability of neutron source e.g. reactor. Inductivelycoupled plasma mass spectrometry and isotope dilution-thermalionization mass spectrometry require expensive instrumentation[13]. Sensitive determination of gallium can be achieved by using

ll rights reserved.

x: þ91 22 25505151.

fluorimetric methods [15,16], but extraction steps are often requiredto separate it from other ions that interfere during the analysis.

Laser induced breakdown spectroscopy (LIBS) is a promisingalternative for determination of major and minor elementalconstituents in any type of matrix and is gaining popularity dueto its potential for in-situ field analysis. The method is mostattractive for solid samples, but has inherent draw-backs such assplashing, surface ripples, quenching of emitted intensity, ashorter plasma life-time, etc. for liquid samples [17,18]. Thesedraw-backs can be overcome by using the solid sample supportmethod [18–20].

A few reports are available in literature on the application ofLIBS for quantitative analysis of Ga. Fichet et al. [21] reportedsemiquantitative analysis of gallium in both UO2 and PuO2 withLODs of 240 and 80 ppm, respectively. Smith et al. [22] demon-strated the applicability of LIBS for characterizing solid Pu–Ga alloyin the 34–400 ppm range for Ga in PuO2. Papazoglou et al. [23]used LIBS in combination with interferometry for in-situ depthprofiling of semiconductor wafer composed of AlGaAs and InGaAs.

In the present work, we report the optimization of differentparameters to determine Ga in solution by LIBS. In addition,effects of Ga concentration on different emission lines and of bulkU in Ga quantification are also presented.

2. Experimental

2.1. Apparatus

A LIBS instrument Spectrolaser 1000 M obtained from LaserAnalysis Technologies Pvt. Ltd, Victoria, Australia was used in thiswork. The details of the instrument are given elsewhere [18–20].

2400

200

400

600

800

2400

Inte

nsity

(a.u

.)

Wavelength (nm)250 260 270 280 290 400 410 420

Fig. 1. LIBS spectrum of Ga solution on a graphite planchet at a laser energy of

60 mJ and acquisition delay of 2.5 ms.

Table 1Characteristics of spectral lines employed for LIBS analysis of Ga.

lij Aij (s�1) Ej (cm�1) Term (j) Ei (cm�1) Term (i)

Ga(I) 245.007 nm 2.78�107 40,802.72 2D3/2 0 2P3/2

Ga(I) 265.987 nm 1.22�107 37,584.62 2S1/2 0 2P1/2

Ga(I) 271.966 nm 2.34�107 37,584.62 2S1/2 826.24 2P3/2

Ga(I) 287.423 nm 17�107 34,781.67 2D3/2 0 2P1/2

Ga(I) 294.363 nm 13.4�107 34,787.92 2D5/2 826.24 2P3/2

Ga(I) 294.417 nm 2.61�107 34,781.67 2D3/2 826.24 2P3/2

Ga(I) 403.297 nm 4.85�107 24,788.58 2S1/2 0 2P1/2

Ga(I) 417.203 nm 9.45�107 24,788.58 2S1/2 826.24 2P3/2

lij is the transition wavelength, Aij is the transition probability, Ei and Ej are the

energies of the upper and lower level, respectively.

A. Sarkar et al. / Optics & Laser Technology 44 (2012) 30–34 31

The system consists of a high power Q-switched Nd–YAG laser,which yields up to 200 mJ of pulse energy at the fundamental IRwavelength of 1064 nm in a 7 ns pulse width with a maximumrepetition rate of 10 Hz. The laser energy is focused to a spot sizeof 500 mm. Four C-T spectrographs along with 4 CCD camerasare used for the detection of signals in the wavelength range of180–850 nm.

2.2. Preparation of samples

Standard solutions of Ga and U were prepared by dissolution ofGa(NO3)3 (Sigma Aldrich) and U3O8, respectively. The stock solutionof Ga was prepared by directly dissolving the salt in 1 M HNO3

prepared from supra-pure HNO3 (diluted using deionized waterhaving resistivity of 18.2 MO cm�1 at 25 1C). In the case of uraniumoxide, the material was digested with supra-pure nitric acid. Thesolution was then evaporated to dryness and the dried residue wasre-dissolved in 1 M HNO3. The calibration standards for Ga wereprepared with concentrations ranging between 1000 and 1 mg ofGa/g of solution by suitably diluting the stock solutions.

Ten synthetic U–Ga mixtures were prepared by mixing stocksolution of U with freshly prepared Ga standards. The mixtureswere prepared in such a way that the Ga amount varied fromabout 0.5% to 20% with respect to the total metal content, but theconcentration of U in all the 10 mixtures was 341871% mg of U/gof the solution. This was done to unify the degree of spectralinterferences by U for all the U–Ga synthetic mixtures.

2.3. LIBS analysis

Sample analysis method is the same as that described in ourprevious work [19]. 20 mL of solution was transferred drop-wiseon to a graphite planchet (Ted Pella Inc., http://www.tedpella.com) of 32 mm diameter and 1.6 mm width and was evaporatedto dryness using a hot air blower. Subsequently, the planchet wasallowed to cool to room temperature and was then mounted inthe sample holder. Three independent analyses, using separatesample loadings, were done for each of the standards as well as ofthe synthetic mixtures under identical experimental conditions.A laser energy 60 mJ and acquisition delay time of 2.5 ms wereused. The resolution of the instrument was 0.6 nm at 300 nm.

3. Results and discussion

3.1. Selection of emission line for analysis

The LIBS spectrum recorded in the 180–850 nm wavelengthregion, contained almost all the strong emission lines of Ga.Typical LIBS spectrum of Ga obtained under LIBS analysis condi-tions is shown in Fig. 1. Among the several lines available from theemission spectrum, seven emission lines were found to berelatively spectrally pure and sufficiently intense. At the availableinstrumental resolution, Ga(I) 294.363 nm and Ga(I) 294.417 nmwere unresolved and observed as a single peak. So these two lineswere not used for the analytical purpose. Table 1 shows thespectroscopic data of the selected emission lines. Among these,Ga(I) 403.297 nm and Ga (I) 417.203 nm were previously reportedin LIBS work [21–23].

Though any Ga emission line can be used for quantification,but all of them are not useful in the presence of large amounts ofU. Uranium has numerous emission lines, many of them are notresolvable in the present instrument, thereby creating a band likespectrum. Many of the Ga emission lines are spectrally interferedby this band and hence cannot be used for analytical purpose.To select the most appropriate emission line for the quantification

of Ga, LIBS spectra of pure Ga and pure U were compared withthat of the U–Ga mixture as shown in Fig. 2 under identicalexperimental conditions. In Fig. 2, the concentration of U is �200times that of Ga concentration. From Fig. 2, it can be seen thatonly four Ga emission lines are clearly visible in the presence oflarge amount of U. Among these lines, only Ga(I) 403.297 nm andGa (I) 417.203 nm are spectrally pure. The Ga(I) 287.423 nm isstrongly interfered by U(II) 287.519 nm and the other emissionline is a doublet of Ga(I) 294.363 nm and Ga(I) 294.417 nm asstated previously. Hence for the determination of Ga in presenceof bulk U, only Ga(I) 403.297 nm and Ga (I) 417.203 nm wereconsidered.

3.2. Optimization of the experimental conditions

The characteristics of laser induced plasma, i.e., electrontemperature and electron density vary with time as well as theamount of energy deposited. Several reports demonstrate thistemporal nature [24,25]. Due to this temporal nature, the emissionlines intensity also changes and hence the conditions for analysismust be optimized for analytical purpose. This optimization iscarried out by studying the effect of laser energy (EL) andacquisition delay time (td) on the most intense emission line’ssignal to noise ratio (S/N) [26–28]. The width of the gate box onthe detector was 1 msec and this cannot be varied in the presentinstrument. Fig. 3 shows a 3-D plot of EL vs. td vs. S/N for theGa(I) 403.297 nm. It can be seen from Fig. 1 that the Ga(I) 417.203 nm is the most intense emission line, but since it isriding on a C2

þ swan band generated due to graphite laser ablation,the second most intense line (403.297 nm) was used for this

2400

500

1000

1500

2000

2500

3000

Wavelength (nm)

Inte

nsity

(a.u

.)

Ga U U + Ga

250 260 270 280 290 400 410 420

Fig. 2. Comparison of the LIBS spectra of the Ga solution with the U–Ga mixture-2

under identical conditions of analyses.

30

40

50

60

1.01.52.02.53.03.54.0

2

4

6

8

10

12

14

16

Ga(I) 403.297 nm

Delay (µs)

SNR

Ener

gy (m

J)

Fig. 3. Variation of signal to noise ratio (S/N) of Ga(I) 403.297 nm with laser energy

and acquisition time delay.

0.0

0.2

0.4

0.6

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

I Ga(

I) 27

1.96

6 nm

/I C(I)

247

.856

nm

CGa (ppmw)

I Ga(

I) 40

3.29

7 nm

/I C(I)

247

.856

nm

200 400 600 800 1000

0 200 400 600 800 1000

Fig. 4. Normalized Ga emission from LIBS as a function of Ga concentration.

A. Sarkar et al. / Optics & Laser Technology 44 (2012) 30–3432

study. Fig. 3 can be viewed as a compilation of three time zonesviz. early, middle and end time zone. The early and end time zonesare constituted by signal obtained at tdo1.5 and 43.5 ms wherethe S/N ratio increases with the laser energy until the plasmadensity becomes too high at an EL of �50 mJ. At still higher laserenergy, electron density in the plasma increases to a level wherethe absorption of laser energy by the electrons (inverse brems-strahlung) and through multiphoton ionization (mainly for shorterlaser wavelengths) is a dominant process. The process is betterknown as laser shielding. Under these conditions the analytesignal and hence S/N starts decreasing. In the range 3.5 ms4td41.5 ms, the analyte signal was found to increase till an EL of60 mJ and S/N was found to be the highest at a td of 2.5 ms. Sincesufficiently high S/N was obtained at 60 mJ energy, studies withhigher EL were not carried out. Hence, a laser energy of 60 mJ andan acquisition delay of 2.5 ms were chosen as the optimum LIBSanalysis conditions for Ga analysis using the present equipment.

3.3. Calibration curves

Standard solutions of the Ga were used to obtain the calibrationcurves of all the selected emission lines. To nullify the effect of shotto shot variations in the laser energy during the analysis the ratio ofthe analyte emission line intensity to the intensity of emission of aninternal standard was used and this ratioing methodology isreferred to as the normalization procedure in this application. Inthe present case, since graphite planchet was used as a solid samplesupport base material carbon emission lines will serve as the bestinternal standard. The calibration plots were obtained by plottingthe normalized Ga emission line intensity with respect toC(I) 247.856 nm as a function of the concentration of Ga in thesolution. Fig. 4 shows the calibration curves obtained usingGa(I) 403.297 nm and Ga(I) 271.966 nm. Among all the emissionlines selected as discussed in previous sections, Ga(I) 287.423 nm,Ga(I) 403.297 nm and Ga (I) 417.203 nm showed non-linear beha-vior in the higher concentration range (4100 mg/g). In this range,a non-linear equation (Eq. (1)), employed in our previous work, wasused to fit the calibration data [18].

y¼ y0þAeð�x=tÞ ð1Þ

where x denotes concentration of Ga, y stands for the normalizedemission line intensity, and y0, A and t are constants. At lowerconcentration range (o100 mg/g), a linear dependence was observed,and a linear equation (Eq. (2)) was used for obtaining the calibrationcurve.

y¼mxþc ð2Þ

where the variables m and c are the slope and the intercept of thecalibration curve, respectively. The non-linear behavior at higher

Table 3Comparison of analytical results with expected values for Ga determination in

pure solution and Ga/U mixtures.

Emission

wavelength

(nm)

Pure Ga solutions (mg/g) % Ga in U–Ga mixture

LIBS (A) Expected

(B)

A/B LIBS (C) Expected

(D)

C/D

265.987 593 (74%)

605

0.98 –

9.9

–

271.966 623 (74%) 1.03 – –

287.423 617 (74%) 1.02 – –

403.297 601 (74%) 0.99 10.1 (71%) 1.02

417.203 600 (74%) 0.99 10.1 (73%) 1.02

287.423 30 (77%) 32 0.94 – 1.5 –

403.297 33 (74%) 1.03 1.45 (75%) 0.97

417.203 30.5 (74%) 0.95 1.53 (77%) 1.02

Values in parentheses indicate relative standard deviation from replicate analyses.

0.000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

I Ga(

I) 40

3.29

7 nm

/I U(II

) 367

.007

nm

CGa/CU

0.03 0.06 0.09 0.12 0.15 0.18 0.21

Fig. 5. Calibration curve for the U–Ga mixture in aqueous solution.

Table 4Comparisons of LODs in different matrix for Ga determination.

Emission line used LOD (mg/g) Major matrix Ref.

Ga(I) 417.203 nm 240 UO2 [21]

80 PuO2 [21]

34 Pu [22]

2.7 U Present work

A. Sarkar et al. / Optics & Laser Technology 44 (2012) 30–34 33

concentrations is most often due to ‘self-absorption’. Self-absorptiontypically is observed in those emission lines in which the lower levelof the transition is the ground state or close to the ground state.Because of the characteristically high temperature and electrondensity gradients in the micro-plasma, the outer layer of the plasmawill be populated by ‘cool’ atoms, residing mostly in the groundstate. The central core of the plasma will contain a higher density ofexcited atoms. As these atoms decay to the ground state, the emittedphotons corresponding to the resonance transitions will have a highprobability of being absorbed by the ‘cooler’ atoms in the outerlayers, thereby reducing the observed intensity of the emission line.As the concentration of the atoms in the target sample increases, thenumber of ‘cooler’ atoms in the outer layer increases and self-absorption becomes evident. This is commonly observed in laser-produced plasmas at atmospheric pressure [29]. The most intenseresonant lines are expected to show self-absorption, but are preferredfor obtaining the lowest detection limits. An increase in acquisitiondelay time will reduce the self-absorption, but will degrade thedetection limit. Among Ga(I) 245.007 nm, Ga(I) 265.987 nm andGa(I) 271.966 nm, the first two transitions involve zero ground leveland the third one involves a level very close to the ground level(826.24 cm�1). Hence they will also show self-absorption but due tosmall transition probability, it would occur at a very high concentra-tion of Ga, which is out of the range of the present study. The smalltransition probability is also responsible for very low signal at Gaconcentration o100 mg/g and hence not suitable for analyticalpurpose. Table 2 gives the details of the calibration constantsobtained for different emission lines of Ga. All the emission linestabulated can be used for analytical purpose, depending on the Gaconcentration range. Table 2 also contains the limits of detection(LOD) from different calibration lines. The LOD was determinedaccording to the definition 3sB/s, where sB is the standard deviationof the blank and s is the sensitivity determined by the slope of thecalibration curve [30]. We determined sB from ten measurements ofblank signals under the same experimental conditions, where thesample was a graphite planchet with only deionized water. ForGa(I) 287.423 nm, Ga(I) 403.297 nm and Ga (I) 417.203 nm, the linearcalibration in the lower concentration range was used for the LODcalculation. Two standard solutions with Ga concentrations of 605and 32 mg/g were analyzed (triplicate analysis) by the developedcalibration curves. Table 3 shows the data obtained by differentemission lines of Ga(I). A good agreement between the expectedvalues and experimental values was obtained both in the high andthe low concentration range.

In the presence of bulk amount of U, none of the abovedescribed calibration curves was applicable due to heavy matrixspectral interference from U, as described in the ‘‘selection ofemission line for analysis’’ section. To study the possibility of Gaquantification in such mixtures, solutions of varying Ga amounts(20–650 mg/g) with constant U (341871% mg/g) was prepared andwere analyzed under the identical conditions of analysis as in thecase of pure Ga solution. The two spectrally pure Ga lines in the

Table 2Fitted parameters obtained from calibration curves for Ga along with the limits of

detection (LOD) values.

Wavelength y0 A t ma ca LOD

(mg/g)

Ga(I) 245.007 nm – – – 3.1�10�4�1.5�10�2 51

Ga(I) 265.987 nm – – – 3.4�10�4�1.9�10�2 61

Ga(I) 271.966 nm – – – 5.4�10�4�2.3�10�2 48

Ga(I) 287.423 nm 4.52 �4.51 2949.53 16.8�10�4�2.8�10�2 0.4

Ga(I) 403.297 nm 3.04 �3.03 1954.40 18.5�10�4�0.4�10�2 0.9

Ga(I) 417.203 nm 0.90 �0.89 1134.61 10.7�10�4�2.4�10�2 0.5

a Applicable in the o100 mg/g concentration region only.

presence of bulk of U were normalized against U(II) 367.007 nmand were used for calibration by plotting intensity ratio withrespect to Ga/U amount ratio. Fig. 5 shows the calibration curveobtained using intensity ratio of Ga(I) 403.297 nm and U(II)367.007 nm. The non-linear response of the signal ratio was fittedto the Eq. (1) to get a calibration curve. To verify this calibrationcurve, two U–Ga mixtures with 9.9% and 1.5% of Ga were analyzed(triplicate analysis). The results in Table 3 shows that the valuesobtained agreed very well with the expected values showing theapplicability of LIBS for Ga analysis in the presence of uranium.The LOD was found to be 0.0008 (CGa/CU) from the data given inFig. 5. Using the data of U concentration in these samples, the LODfor Ga translates into 3418�0.0008¼2.7 mg/g. Table 4 gives thedata of reported LODs of Ga in different actinide matrices. Owingto the complex emission spectra of U and Pu, these investigatorsobtained LOD’s for Ga in the range of 34–240 ppm.

A. Sarkar et al. / Optics & Laser Technology 44 (2012) 30–3434

4. Conclusion

LIBS was developed for the determination of Ga in pure aqueousas well as in the U–Ga mixture solutions. Five emission lines werefound to give calibration lines to cover different concentrationranges of Ga for different applications. The method is applicableover a wide range of concentrations and hence the problemsusually associated with dilution, pH maintenance, etc. as encoun-tered in other methodologies are not present in this method. Theeffect of presence of bulk of uranium was also studied and wasfound to generate heavy matrix spectral interferences. Galliumcontent in the 0.5–20% range in U–Ga synthetic samples wasdetermined with good precision and accuracy. It may be addedthat the work reported here is based on pure solution of Ga and U,the effect of the spectrally interfering elements should be con-sidered when analyzing real-life samples. Thus LIBS is a promisingtechnique for Ga quantification and also provides an independentapproach based on different physico-chemical principle.

Acknowledgments

Authors express their sincere thanks to Dr V. Venugopal,Director, RC&I Group for his constant support and encouragementin LIBS work.

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