Determination of trace elements in lithium niobate crystals by solid sampling and solution-based spectrometry methods

Analytica Chimica Acta 726 (2012) 1–8

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Determination of trace elements in lithium niobate crystals by solid sampling and solution-based spectrometry methods László Bencs a,∗ , Krisztina György a , Márta Kardos a , János Osán b , Bálint Alföldy b , Imre Varga c , Zsolt Ajtony d , Norbert Szoboszlai c , Zsolt Stefánka e,f , Éva Széles e , László Kovács a a

Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary Institute for Atomic Energy Research, Centre for Energy Research, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary c Department of Analytical Chemistry, Institute of Chemistry, Loránd Eötvös University, P.O. Box 32, H-1518 Budapest, Hungary d Institute of Food Science, University of West Hungary, H-9200 Mosonmagyaróvár, Lucsony u. 15-17, Hungary e Institute for Isotope Research, Centre for Energy Research, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary f Hungarian Atomic Energy Authority, H-1136 Budapest, Fényes Adolf u. 4, Hungary b

a r t i c l e

i n f o

Article history: Received 5 November 2011 Received in revised form 2 March 2012 Accepted 8 March 2012 Available online 15 March 2012 Keywords: LiNbO3 Solid sampling analysis Atomic spectrometry Three-point-estimation standard addition method Electrothermal vaporization

a b s t r a c t Solid sampling (SS) graphite furnace atomic absorption spectrometry (GFAAS) and solution-based (SB) methods of GFAAS, flame atomic absorption spectrometry (FAAS), inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) were elaborated and/or optimized for the determination of Cr, Fe and Mn trace elements used as dopants in lithium niobate optical crystals. The calibration of the SS-GFAAS analysis was possible with the application of the threepoint-estimation standard addition method, while the SB methods were mostly calibrated against matrix-matched and/or acidic standards. Spectral and non-spectral interferences were studied in SBGFAAS after digestion of the samples. The SS-GFAAS method required the use of less sensitive spectral lines of the analytes and a higher internal furnace gas (Ar) flow rate to decrease the sensitivity for crystal samples of higher (doped) analyte content. The chemical forms of the matrix produced at various stages of the graphite furnace heating cycle, dispensed either as a solid sample or a solution (after digestion), were studied by means of the X-ray nearedge absorption structure (XANES). These results revealed that the solid matrix vaporized/deposited in the graphite furnace is mostly present in the metallic form, while the dry residue from the solution form mostly vaporized/deposited as the oxide of niobium. © 2012 Elsevier B.V. All rights reserved.

1. Introduction High-purity lithium niobate (LiNbO3 ) single crystals are worldwide applied for manufacturing optical devices [1]. Transition metal elements, applied as dopants during the crystal growth, such as Cr, Fe and Mn are influential on the crystal optical properties [2,3]. Along with dopants, impurities can inadvertently incorporate into the crystal bulk from the starting materials, furnaces/utensils, and/or airborne particulate during powders processing and/or crystal growth. Therefore, the study of their possible segregation in the crystal bulk during the crystal growth process is important from both research and application points of view. The concentrations of dopants in high-purity crystal hosts generally are between 10−5 and 10−2 mol mol−1 , while impurities in

∗ Corresponding author. Tel.: +36 1 392 2222x1684; fax: +36 1 392 2223. E-mail address: [email protected] (L. Bencs). 0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2012.03.013

undoped crystals are present below this range. A further limitation to the chemical analysis is the low sample mass (a few grams only), cut from various parts of the crystal bulk. This microanalytical challenge requires instrumentation of high sensitivity and accuracy, which can, for instance, be provided by spectrochemical techniques [4–28]. Several types of crystals were the subjects of mostly solutionbased (SB) spectrochemical analyses, like LiNbO3 [4,5], lithium potassium niobate [4], potassium titanyl phosphate (KTP) [6–9], potassium gadolinium tungstate [8], lithium barium phosphate [10], yttrium aluminum garnet (YAG) [11], titanium dioxide [12], calcium fluoride [13,14], ␤-barium borate [15], Sr–Li–Ti-based oxide crystals [16], copper germanate [17], mercury iodide [18,19], lithium tantalate niobate [20], bismuth tellurite [21–26], zinc tungstate [27,28], and yttrium aluminum borate (YAB) [29]. In these analyses, dopants and/or trace impurities of the crystals (Al, K, V [4,5], Cr, Mn, Ni, Al, Fe, Mg, Na, W [6–8], Er, Eu, Gd, Ho, Nd, Yb [8], Ho, Tm, Ga [9], Cu [10], Nd [11], Al, Cr, Cu, Fe, Mg, Mn, Mo,

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Nb, Si, Te, V, W [12], Cd, Cu, Fe, Pb, Zn [13,14], Ce, Er, Eu, Nd, Al, Fe, Mg, Na [15], Cu, Ge, Si, Zn [17], Ag, Al, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Zn [18,19], Cu, Fe, Li, Ti, V [20], Cr, Mo, V, Ni, Cu, Er, Nd [21–26], Fe [27,28], Cr, Na [28], Ce, Cr, Dy, Er, Yb, Mo [29]) and in a few cases, the main components [4,5,10,16,29] were quantified by inductively coupled plasma optical emission spectrometry (ICP-OES) [4,5,7,8,10,15–18,20,22,23,25,29], flame atomic absorption spectrometry (FAAS) [6,10,15,23–25,29], direct current arc optical emission spectrometry [11–13], solid sampling (SS) graphite furnace atomic absorption spectrometry (GFAAS) [13], electrothermal vaporization (ETV) ICP-OES [14,22], inductively coupled plasma mass spectrometry (ICP-MS) [19], SB-GFAAS [21–28], X-ray fluorescence spectrometry [26] and flame atomic emission spectrometry [28]. In some of these analyses, the use of matrix-matching [8,11,13–15,27,28] and/or standard addition [4–6,15,18,19,28] was necessary for accurate calibration. In most of these analyses, solution-based ICP-OES methods were applied, and also GFAAS and ICP-MS, when lower limits of detection (LODs) were required. However, solid sampling has several advantages over solution-based methods, like the smaller sample mass demand, lower LODs, less risk of sample contamination, lower consumption of chemicals and shorter analysis time. By the elaboration of a GFAAS method, there is also the possibility to adapt the knowledge to GF-ETV-ICP-OES and GF-ETV-ICP-MS techniques. Probably the most critical issue in SS-GFAAS analyses is the means of calibration. Calibration against simple acidic standards can lead to inaccurate results [30,31], while the application of matrix-matching is not always possible, due to the lack of suitably pure matrix [30]. Solid certified reference materials with similar matrix-composition to that of the sample investigated can also be used for calibration [13,32], but they are expensive and hardly commercially available. In order to overcome these difficulties, Eames and Matousek [31] developed a standard addition method to determine Ag in quartzite rocks, which is based on weighing different amounts of samples, followed by spiking them with the same standard solution. Although a linear relationship was found between the absorbance and the sample mass, the results showed significant variation, responding to small changes in the slope of the absorbance–sample mass curve. Therefore, Minami et al. [30] developed the three-point-estimation standard addition method to the solid sampling determination of Cr in biological samples, which is based on a calibration curve derived from the equations of four absorbance–sample mass curves obtained by adding different masses of the analyte (i.e., spikes). This method was proven to be less susceptible to changes in the slope of calibration than the former [31], thus, it is applied in the present study for the solid sampling analysis of optical crystals. The X-ray absorption near-edge structure (XANES) method uses high intensity monochromatic X-ray beams mostly originating from synchrotron radiation. From the near edge part of the absorption spectrum one can deduce information on the chemical state of the element of interest. Changes in the charge distribution around a given atom in different chemical environments can alter the binding energies of the core electrons, so that the XANES spectrum shows a shift in the absorption edge. A more oxidative chemical environment results in deeper core state binding energies, which in turn cause an absorption edge shift toward higher energies in the spectrum. The order of magnitude of the shift is around 1–3 eV per valence change. The presence or absence of the possible pre-edge structure and the energies of the near-edge structures (the white line and the multiple scattering resonance peak) are also dependent on the oxidation state of the absorbing atom. The present work aims at the elaboration and/or optimization of spectrochemical methods for the determination of dopants and impurities in LiNbO3 single crystals that have been produced in

the Institute for Solid State Physics and Optics. For this purpose, SS-GFAAS and solution-based methods of GFAAS, FAAS, ICP-OES and ICP-MS methods are applied in a complementary manner. Moreover, electron probe X-ray microanalysis (EPMA) and XANES were applied to determine possible vaporization forms of the studied matrix compounds in the graphite furnace atomizer.

2. Experimental 2.1. Instrumentation All the GFAAS experiments were performed on a Carl-Zeiss model AAS-3 (Jena, Germany) atomic absorption spectrometer equipped with an end-heated EA-3 (Carl-Zeiss) graphite tube electrothermal atomizer. Continuum source background (BG) correctors, i.e., deuterium and tungsten halogenide lamps were applied for the correction of non-specific absorption in the UV and the visible spectral range, respectively. For solution sample introduction, sample aliquots of 20 ␮L were dispensed with an MPE (MLW, Germany) autosampler into pyrolytically coated graphite tubes (Elektrokohle, Germany). For solid sampling GFAAS, the hollow cathode lamps (HCLs) of Cr, Fe and Mn (Cathodeon, Cambridge, England) were operated at 7, 10 and 4 mA current, and the analytical lines of Cr 520.4, Fe 392.0, and Mn 403.1 nm were used with a spectral bandpass of 0.2, 0.4 and 0.2 nm, respectively. Alternatively, the less sensitive Mn 321.7 nm line with a spectral bandpass of 0.25 nm and a HCL current of 10 mA could also be utilized for the analysis of higher sample masses (2–5 mg). For the solution-based GFAAS method, the HCLs of Cr, Fe and Mn were operated at 3, 6.5 and 4 mA current, while the Cr 357.9, Fe 248.3, Mn 403.1 nm lines were applied with a spectral bandpass of 0.2, 0.15, and 0.4 nm, respectively. The reason for the usage of different (optimal) HCL working currents and bandpasses in the analysis of each individual element by the SS and SB methods was the application of alternative analytical lines, which required individual optimization of the optical conditions in each case, involving the check for the linearity of the calibration curve. The graphite furnace heating programs for SSand SB-GFAAS methods are indicated in Table 1. An IRCON Model 300C (Santa Cruz, CA, USA) pyrometer was focused onto the bottom of the graphite atomizer wall for temperature calibration. The pyrometer was pre-calibrated at low temperatures with the application of Fe-Ni and Pt-Rh thermocouples. Integrated absorbance (Aint ) was applied for the evaluation of GFAAS measurements, if not stated otherwise. The FAAS measurements were performed with the application of a Varian Model AA-20 spectrometer, fitted with an air-acetylene burner of 10 cm slit. The determinations were all optimized for the best sensitivity by adjusting the optical and flame parameters during introducing one of the sample solutions. The optimal operating conditions for the FAAS methods are listed in Table 2. The ICP-OES determinations were performed on a Labtam 8440 Plasmalab spectrometer equipped with a Paschen-Runge polychromator and a Czerny-Turner monochromator. The ICPMS measurements were performed on a high resolution, double focusing magnetic sector field inductively coupled plasma mass spectrometer (DF-SF-ICP-MS) equipped with a single electron multiplier (ELEMENT2, Thermo Electron Corp., Germany). The optimized conditions for ICP-OES and ICP-MS are listed in Tables 3 and 4, respectively. The fairly high solid load of the furnace during SS-GFAAS analytical cycles causes the deposition of a layer of matrix constituents on the graphite platform, of which composition was analyzed by EPMA and XANES. Dry residues of deposited solution samples were also studied by these X-ray techniques. Details of XANES measurements are described in Section 2.3.

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Table 1 Graphite furnace heating programs. Step

Temperature (◦ C)

Ramp (◦ C s−1 )

Hold (s)

Internal flow rate (cm3 min−1 )

Drying Drying Pyrolysis Atomization Clean-out

100/130 130/280 Variablea Variablea 2650

10 10 100 2650 2650

20–30 20–30 20–50 3–5b /3–7c 2–4

160 160 160 0b /280c 280

a For solid sampling analysis, the optimal Tpyr and Tat values are 1200 and 2500 ◦ C (Cr), 900 ◦ C and 2500 ◦ C (Fe), and 900 ◦ C and 2400 ◦ C (Mn), respectively (see optimal values for solution samples in Section 3.1). b Solution-based method. c Solid sampling.

Table 2 FAAS conditions. Oxidant flow Acetylene flow Flame observation height Nebulizer Liquid uptake rate Analytical lines (nm) Spectral bandpass (nm) Hollow cathode lamp current (mA)

3.5 units on arbitrary scale 1.5 units on arbitrary scale 6–10 mm (optimized for each element) Varian, pneumatic “adjustable” 4.8 mL min−1 Cr 357.9, Fe 248.3, Mn 279.5 0.2 (for Cr, Fe and Mn) Cr: 7, Fe: 12, Mn: 5

Table 3 ICP-OES conditions. Nebulizer RF forward power Reflected power Plasma observation height Outer argon (coolant gas) Intermediate argon (plasma gas) Inner argon (sample gas) Analytical lines (nm)

pneumatic 1.0 kW ∼1.0 W 13 mm 14 dm3 min−1 0.6 dm3 min−1 0.9 dm3 min−1 Cr (II) 267.716, Fe (II) 238.207, Mn (II) 257.610

Table 4 ICP-MS conditions. Sample introduction system Sample uptake rate RF forward power Cooling gas-flow rate Auxiliary gas-flow rate Nebulizer gas-flow rate Scanned isotopes

conical nebulizer ∼1 mL min−1 1.3 kW 15.4 dm3 min−1 1.13 dm3 min−1 0.95 dm3 min−1 52,53 Cr, 55 Mn, 56,57 Fe

For EPMA measurements, a PHILIPS 505 scanning electron microscope (FEI Philips, Peabody, MA, USA) equipped with a Link Si(Li) detector (Oxford Instruments, Cambridge, UK) was applied. Each sample was glued by an adhesive tape to an Al sample-holder and coated by a thin carbon layer to prevent charging and heating during the measurements. The analyses were performed at an accelerating voltage of 20 keV. To reduce absorption effects, the analysis points were selected in a section of the deposited particle that faced the X-ray detector. The measurement time was 200 s, resulting in typical, total spectrum integrals ranging from 25,000 to 270,000 counts. 2.2. Materials and methods All the chemicals used were of analytical grade (supplier: Reanal, Budapest, Hungary), if not otherwise stated. For the dilution of the sample and stock solutions, ion-exchanged and doubly distilled water was applied. The high-purity undoped and doped crystals were grown as described in details elsewhere [33]. As a first step of the chemical analysis, crystal samples, taken from various regions of the cylindrical bulk of the crystal, were

ground in an agate mortar. For weighing the powdered solid samples, a Sartorius model SE 2 microbalance (Göttingen, Germany) was applied. For this purpose and insertion of the powdered samples into the graphite furnace, curved graphite platforms (PerkinElmer, Überlingen, Germany) were applied. These platforms were obtained from PerkinElmer THGA tubes by mechanical removal. Then the graphite platforms were cleaned with the application of temperature conditioning and mixing carbon tetrachloride vapor into the stream of the internal Ar furnace gas [26]. For the GFAAS analysis of the solid samples, the three-pointestimation standard addition method was applied using aqueous standards preserved either with 0.05 mol L−1 HNO3 (Cr, Fe and Mn), or 0.24 mol L−1 HCl (Mn). The 10–20 ␮L aliquots of standard solutions of the analytes were dispensed onto the platforms before weighing of the solid samples, and dried gently on an electric hotplate to avoid sputtering of the solution. For the insertion of solid samples, around 0.05–0.2 mg (Mn), or 1–5 mg (Cr), or 0.5–1.5 mg (Fe) of the powdered sample was placed onto a graphite platform, weighed and then inserted into the graphite furnace. The graphite platform was inserted with a pair of PTFE tweezers at either end of the graphite furnace after unscrewing the caps of the optical windows. To assure the exact positioning of the platform in the middle of the graphite furnace, a pair of thin, yellow-colored plastic micropipette tips (Transferpette, Brand, Wertheim, Germany) with pre-marked distance scales were concurrently inserted through each end of the furnace-housing till they reached the sides of the platform. After positioning, the tips were removed and the ends of the furnace were again sealed with the optical windows. Blanks were measured after dispensing a proper aliquot of the blank solution to the clean platforms and drying them gently on an electric hotplate. According to the three-point-estimation standard addition method, first, four sets of weighed samples of various masses were spiked by using either one of three different standards or a blank solution, respectively. Linear least-squares fitting was applied to each set of the calibration points, which yielded four Aint –sample mass curves. One set included in determinations with 6–12 samples for proper confidence of the fitting. Then the calibration curve, which is the relationship between the Aint and the added amount of the analyte, belonging to a given sample mass, for example, 1 mg, could be constructed. The points of each calibration graph were calculated from the equations of the Aint –sample mass curves. For the digestion of the crystals, approximately 0.147 g of the powdered samples was weighed on a Kern model 770-14 (Balingen, Germany) electronic analytical balance. The samples were digested by melting them with a mixture of 2 g KCl and 2 mL H2 SO4 (96% m m−1 ) in a Pt crucible heated with a Bunsen burner under a fume hood. After cooling, the solidified melt was dissolved either in 10 mL of 8 mol L−1 HF, or 10 mL of 1 mol L−1 triammonium citrate (TAC), and diluted to a final volume of 100 mL. The stock solution of TAC was prepared from Suprapur 25% (m/v) ammonia solution and solid citric acid (Merck, Darmstadt, Germany). For ICPOES measurements, the fluoride content of the sample/standard

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solutions was neutralized with addition of a slight excess of boric acid. For the FAAS and ICP-OES analyses, the samples were measured directly, without any dilution, while for the GFAAS and the ICP-MS determinations, they were diluted 10–50 and 20-fold, respectively. For the EPMA and XANES studies on the vaporization form of the dissolved matrix during the GFAAS cycles, 20 ␮L aliquots of 0.01 mol L−1 LiNbO3 solution were dispensed onto regular L’vov graphite platforms (PerkinElmer, PerForm Ltd., Hungary). Then they were dried and pyrolyzed in the graphite furnace at various pyrolysis temperatures, ranging from 800 to 1600 ◦ C. 2.3. Micro-XANES measurements The XANES experiments were performed at the microfluorescence beamline L of the DORIS III synchrotron ring, which is operated by the Hamburg Synchrotron Radiation Laboratory HASYLAB at the German Electron Synchrotron DESY (Hamburg, Germany). The white beam of the bending magnet was monochromatized by a Si(1 1 1) double monochromator. A polycapillary half-lens (X-ray Optical Systems, E Greenbush, NY, USA) was employed for focusing a beam of 1 mm × 1 mm down to a spot size in the micrometer range (20 ␮m at 10 keV, 30 ␮m at 6.5 keV). The absorption spectra were recorded both in transmission and fluorescent mode, tuning the excitation energy near the K absorption edge of Nb (18,986 eV) by stepping the Si(1 1 1) monochromator. The X-ray fluorescence photons were detected by a silicon drift detector with a 50 mm2 active area (Radiant Vortex). The step size varied from 0.5 (edge region) to 2 eV (more than 50 eV above edge). The measuring time for each energy point was set to 5 s. Standards of known Nb chemical state were prepared from analytical grade chemicals of LiNbO3 and Nb2 O5 ground to microscopic particles. Pressed pellets containing around 1% (m m−1 ) Nb were prepared from the powdered standards using boric acid for dilution. A graphite furnace platform covered with a thin layer of NbC was used as an additional standard. The NbC standard layer was prepared from Nb2 O5 and graphite powder by mixing and then pyrolyzing them at a temperature of 1700 ◦ C on a graphite platform inserted into the graphite atomizer. The energy scale was calibrated using a high-purity Nb foil. The normalization of the XANES spectra was done using the ATHENA software package [34]. 3. Results and discussion

Fig. 1. Pyrolysis (1) and atomization (2) curves of 5.2 ␮g L−1 Cr in various media with platform atomization; A – HCl, B – HNO3 , C – HCl + LiNbO3 + TAC (concentrations in mol L−1 : acids: 0.02, LiNbO3 : 0.01, TAC: 0.1).

low pyrolysis temperatures (e.g., 800 ◦ C), the continuum source BG corrector could not properly compensate the high non-specific absorbance, originating from the vaporization of the quite large amounts of the matrix. Thus, these sections of the pyrolysis curves are rather fluctuating, instead of developing a plateau. The onset of the atomization curves is generally referred to as the appearance temperature (Tapp ). In HCl medium, the Tapp value of Cr is found to be 1300 ◦ C, while it is considerably increased in HNO3 and LiNbO3 –TAC media, i.e., to 1500 and 1700 ◦ C, respectively. For Mn, in HCl medium the optimal Tpyr and Tat are 800 ◦ C and 1400 ◦ C, respectively (Fig. 2). However, considering peak shape, reproducibility and the fact that for solid sampling analysis the graphite platforms were used, while pyrolysis and atomization curves were plotted without platform, 900 ◦ C and 2400 ◦ C were used as Tpyr and Tat , respectively. In HNO3 medium, the optimal Tpyr remains 800 ◦ C, but the optimal Tat is increased to about 2000 ◦ C. The atomization curve has a shoulder at 1500 ◦ C, i.e., closely to the optimal Tat in HCl medium. This indicates that in HNO3 medium Mn may vaporize from two different chemical forms, one of them is likely the same as in HCl medium. In the presence of LiNbO3 and TAC, the optimal Tpyr and Tat are 1000 ◦ C and 1700 ◦ C, respectively. The Tapp values in HCl, HNO3 and LiNbO3 –TAC media are 1000 ◦ C, 1000 ◦ C and 1200 ◦ C, respectively. For Fe, in HNO3 medium the optimal Tpyr and Tat were observed to be 900 and 2500 ◦ C, respectively, using platform atomization.

3.1. Optimization of the GFAAS conditions As a first step, the solution-based GFAAS techniques were applied to optimize the conditions for the measurement of solid (powdered) samples. In this context, the pyrolysis and atomization curves were recorded with the use of various sample solutions. An example of these plots for Cr is depicted on Fig. 1. The maximum allowable pyrolysis and the minimum necessary atomization temperatures (corresponding to values at which one reaches the plateau section of each curve) are generally referred to as the optimal pyrolysis and atomization temperatures, respectively, which terms are also used further on. In HCl medium, the optimal pyrolysis (Tpyr ) and atomization (Tat ) temperatures of Cr are 800 and 2400 ◦ C, respectively. In HNO3 medium, the optimal Tpyr is considerably increased, i.e., to 1400 ◦ C, while the optimal Tat remains essentially the same. In the presence of LiNbO3 matrix and TAC additive, however, the optimal Tpyr is as high as 1600 ◦ C, while the atomization curve does not reach a constant value (plateau) up to the maximum permissible temperature recommended for the heating of graphite furnaces (2650 ◦ C). Thus this temperature is considered to be optimal. At

Fig. 2. Pyrolysis (1) and atomization (2) curves of 5 ␮g L−1 Mn in various media with wall atomization; A – HCl, B – HNO3 , C – HCl + LiNbO3 + TAC (concentrations in mol L−1 : HCl(A): 0.24, HCl(C): 0.029, HNO3 : 0.29, LiNbO3 : 0.001, TAC: 0.01).

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Fig. 3. Variation of background absorbance at Cr 357.9 nm by the vaporization of 0.01 mol L−1 lithium niobate matrix present in 0.027 mol L−1 HCl plus 0.1 mol L−1 TAC medium as a function of the pyrolysis temperature (Tat = 2650 ◦ C).

3.2. Spectral and non-spectral interferences by the LiNbO3 matrix 3.2.1. GFAAS The non-specific absorption originating from the vaporization of matrix salts in HCl–TAC medium was recorded using increasing Tpyr values and a Tat of 2650 ◦ C at the Cr 357.9 nm line (Fig. 3). It is found that the BG absorbance decreases stepwise by increasing pyrolysis temperature, but also two well separable plateaus can be observed, i.e., the first is between 300 and 800 ◦ C, while the second is between 900 and 1200 ◦ C, corresponding possibly to the vaporization of chlorides and oxides of the matrix/modifier, respectively. The BG absorbance drops to negligible low levels over 1500 ◦ C pyrolysis temperature. In general, the BG corrector of the applied AAS apparatus could correct well this type of spectral interference, as is also manifest by the acquired results. Implying a 1250 ◦ C melting temperature for the solid crystals, one can assume that the vaporization characteristic can be similar to that of the solution samples, as the chloride salts of the samples are mostly hydrolyzed in the presence of TAC modifier. A similar effect of TAC was experienced in the analysis of dopant elements in bismuth tellurite crystals [21]. In solution-based GFAAS, the presence of matrix chloride salts (without TAC) causes signal suppression for the Cr analyte (Fig. 4) at LiNbO3 concentrations higher than 1 × 10−3 mol L−1 . Since the Aint signal of Cr decreases, therefore, it is supposed that the co-vaporization of the matrix chloride salts occurs. This type of matrix interference is a well-known phenomenon in GFAAS

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[35]. According to the EPMA analysis, the samples present in HCl medium, after pyrolysis on the graphite platform, consisted of the refractory main component of the crystal (Nb) and the residues of the digestion agents (K, S and Cl), even at as high pyrolysis temperature as 1200 ◦ C. To overcome this interference, the addition of TAC chemical modifier was applied, which was also proven to be useful for the preservation of the sample and standard solutions during longterm standing. As can be seen in Fig. 4, the addition of TAC has the advantage of enhancing the Cr signal (Aint ), at LiNbO3 concentrations higher than 1 × 10−4 mol L−1 . A similar enhancement effect of TAC on the Cr signal has been also observed in connection with the analysis in bismuth tellurite matrix [36], but interestingly only for peak height absorbance. Although platform atomization was applied in this interference study, the heating rate of the furnace could have played an important role. Namely, the analyte evaporates at a higher temperature in the presence of TAC (and the matrix) than without the modifier, which is likely due to formation of refractory compounds of Cr (e.g., carbides). The modifier undergoes thermal decomposition in the pyrolysis step and forms a carbonaceous layer, which can adsorb the analyte, delaying its vaporization. This is also manifest in the twice longer appearance time of the Cr absorbance transients, experienced with TAC in Bi2 TeO5 matrix [21]. Because of this, Cr can vaporize in a graphite furnace of more homogenous temperature distribution, which eventually results in a doubly increased concentration of the analyte in the optical path, as compared to the samples without TAC. 3.2.2. ICP-OES and ICP-MS The optimization of the determination of Cr, Fe and Mn by ICPOES was done by selecting the most sensitive analytical lines of the analytes. The Nb constituent of the crystal matrix and the analytes concerned produce line-rich spectra in ICP-OES. Therefore, in order to avoid any line coincidence, possible spectral overlaps between the matrix constituents and analytes were studied by gradually stepping the wavelength of the ICP-OES spectrometer in the vicinity of the analytical lines concerned. For line identification, the Boumans line coincidence tables were applied [37]. In the case of the ICP-MS method, the samples could be measured after 20-fold dilution, which served to prevent clogging of the spray-chamber of the nebulizer, due to the fairly high original matrix concentration (0.01 mol L−1 ). On the other hand, due to the high sensitivity of this technique, only sample solutions below 1% (v/v) and 1 mg L−1 for the matrix and the analyte, respectively, should be introduced into the ICP-MS. The medium resolution (4000) of the DF-SF-ICP-MS was found to be high enough to overcome the polyatomic interferences occurred on all isotopes of the analytes (Cr, Fe and Mn) at the low resolution mode. 3.3. XANES studies

Fig. 4. Matrix effects of the lithium niobate solution on the determination of Cr in various media; A – HCl + HF + LiNbO3 , B – HCl + HF + LiNbO3 + TAC (concentrations in mol L−1 : acids: 0.027, LiNbO3 : 0.01, TAC: 0.1).

The fairly high solid load of the furnace causes the deposition of a layer of matrix constituents, of which composition was characterized by XANES. The oxidation state of Nb in the deposited layer of solution samples (pyrolysis residues) was investigated using microXANES at different positions of a regular graphite furnace platform (Fig. 5). No significant difference was observed in the Nb K-edge XANES spectra collected at different positions along the platform (i.e., 2, 5 and 8 mm from either side). The absorption edge shift of around −10 eV compared to the spectra of LiNbO3 and Nb2 O5 indicates that the oxidation state of Nb is much less than +5, likely close to the metallic state. This finding suggests that the fairly slow vaporization of Nb from LiNbO3 takes place via its metallic form, rather than the carbide.

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Fig. 5. Micro-XANES spectra of various chemical forms of niobium (solution sample) in the deposited sample residue layer after pyrolysis at different positions (i.e., 2, 5 and 8 mm from either side) of the graphite platform (against reference samples of Nb2 O5 and LiNbO3 ).

Fig. 7. Absorbance–sample mass curves of chromium for the solid sampling GFAAS analysis of a lithium niobate crystal.

3.4. GFAAS analysis of solid samples The layers of the matrix deposited during solid sampling analysis are also studied (Fig. 6). The Nb metal foil and the NbC standard shows absorption edge shifts of −7 and −4 eV, respectively, compared to the spectra of LiNbO3 and Nb2 O5 . Regarding the curved platform with the deposited sample layer in the inside (sample 0), no significant difference was observed in the Nb K-edge XANES spectra collected from different positions along the length of the platform. The absorption edge shift of around −7 eV compared to the spectra of LiNbO3 indicates that the oxidation state of Nb is much less than +5, Nb is likely close to the metallic state in this sample. However, the post-edge oscillations in the spectrum indicate that the first coordination shell is similar to that in NbC. As the spectrum cannot be reconstructed by a simple linear combination of Nb metal and NbC standard spectra, separate metallic and carbide phases are unlikely to be present. The XANES spectra collected from different positions of samples 1, 2B and 3 are also not significantly different from each other, despite the difference in treatment, i.e., (1) sample run with the normal SS furnace heating program, (2) sample dried and pre-treated at 1400 ◦ C and (3) after normal run a halogenation method was applied, as in Ref. [26], respectively. This means that these techniques were rather ineffective in the removal of high amounts of metallic species off the graphite platform. However, they are different from the spectra collected from Sample 0 and more similar to that of NbC, both regarding the edge shift (around −4 eV) and the shape of the spectrum.

Fig. 6. Micro-XANES spectra of various chemical forms of niobium species and solid samples deposited and pyrolyzed on graphite platforms (sample Nos. 0, 2B and 3).

For the elaboration of a direct solid sampling method, the results obtained from the optimization of the SB-GFAAS method were utilized. Because of the considerably higher sample loading in the furnace compared to SB methods, the maximum flow rate of the internal furnace gas during atomization and less sensitive analytical lines were selected for the determinations. Typical absorbance–sample mass curves of Cr are depicted on Fig. 7. As can be seen, all four curves are practically parallel and have proportionally increasing intercepts with the amount of the standard added, which assures a linear relationship between the Aint signal and the sample mass on addition of aqueous standards. The correlation coefficient (R) of each Cr curve is not worse than 0.81. For the calibration curve of Cr (i.e., each point corresponds to 1 mg sample mass), the slope and the intercept of the linear fit are 0.345 s ␮g−1 and 0.0167 s, respectively (R = 0.9941). Examples of absorbance–sample mass curves for the solid sampling analysis of an Fe-doped crystal sample are presented on Fig. 8. As can be seen, the Aint vs. sample mass graphs for Fe are near parallel and lie approximately the same distance from one another, as it was also found for Cr. The correlation coefficient of each curve is not worse than 0.36. For the calibration curve of Fe (each point corresponds to 1 mg sample mass), the slope and the intercept of the linear fit are 0.597 s ␮g−1 and 0.0725 s, respectively (R = 0.9995). In spite of the sometimes low R values and the fairly wide variation in the slopes of the Aint –sample mass curves, the R data of the calibration curves are always better than 0.91 and their slopes in

Fig. 8. Absorbance–sample mass curves of iron for the solid sampling GFAAS analysis of a lithium niobate crystal.

L. Bencs et al. / Analytica Chimica Acta 726 (2012) 1–8

7

Table 5 Comparison of the LOD data of various methods applied in this study. Limit of detection in ␮g g−1 (and in ␮mol mol−1 )

Element

SS-GFAAS

SB-GFAAS a

Cr Fe Mn a b

b

Estimated

3/S

Wall

Platform

11 (31) 27 (72) 4.1 (11)

4.7 (13) 17 (45) –

0.19 (0.54) 2.0 (5.3) 2.2 (5.9)

0.13 (0.38) 1.8 (4.7) –

FAAS

ICP-OES

ICP-MS

18 (52) 20 (53) 12 (32)

3.5 (10) 15 (40) 3.7 (10)

0.03 (0.09) 0.64 (1.7) 0.016 (0.04)

Estimate from the analytical results. Calculated from 3 of the blanks (n = 7–13) and the slope (S) of the calibration curve.

parallel analyses are the same within the experimental error. This is in line with the observation of Minami et al. [30] and their explanation that each point of the calibration curve is an average value, unlike the points of the Aint –sample mass curves, which correspond to the calibration method of Eames and Matousek [31]. 3.5. Analytical performance SS-GFAAS does not require the digestion/dissolution of the sample and as a consequence, the sample is not “diluted”. This can offer lower values of the limit of detection (LOD) referring to the solid crystal. On the other hand, the three-point-estimation standard addition method requires the optimization of the parameters as the function of the concentration, i.e., measuring a sample containing relatively high analyte concentration (for example, which can be determined by SB-GFAAS) can be done using either a less sensitive analytical line, maximum internal furnace gas flow and small sample masses. This means, in other words, that in many cases, intentionally less sensitive conditions have to be chosen. Obviously the best attainable LOD values could be obtained by using the most sensitive conditions, for instance, using the most sensitive resonance line and interruption of the internal Ar gas flow during atomization. Under these conditions, samples with considerably lower analyte concentrations can be analyzed than the crystal samples investigated in this work. Because of this, the estimated LOD values do not express the real boundaries of SS-GFAAS applied with the three-point-estimation standard addition method. However, the flexibility of the method is obvious: selecting an alternative analytical line, varying the internal furnace gas flow and inserting various sample masses allow the determination of trace elements in crystals over a concentration range of several orders of magnitude. By the present SS-GFAAS method, the total analysis time of one crystal sample is around 8 h, which is much shorter than that of the solution based methods. The LODs of SS-GFAAS were estimated in a simple manner. The result of every individual analysis (i.e., the dopant concentrations determined in the crystal samples) were multiplied by the ratio of

sample mass chosen for the calculation of the points of the calibration curve and the maximum sample mass, which can still be dispensed on the platform (i.e., around 5 mg in LiNbO3 analysis). As a comparison, in some cases, the LODs were also calculated according to the 3/S relationship, where S is the slope of the calibration curve and  is the standard deviation of 7–10 blanks. The LODs of SB methods were also calculated by the 3 procedure (Table 5). For SS-GFAAS, the LODs calculated by the two procedures are in the same range, although considering repeated determinations, the LODs calculated from the analytical results are in good agreement (e.g., 10 vs. 11 ␮g g−1 for Cr), whereas the LODs acquired by the use of blanks differ significantly (e.g., 17 vs. 44 ␮g g−1 for Fe). For Mn, the LODs of SS- and SB-GFAAS methods are comparable (Table 5). For the SB-GFAAS techniques, atomization from a graphite platform sample holder resulted in better reproducibility for all three analytes than applying vaporization from the graphite furnace wall. For the FAAS and ICP-OES methods, the application of matrixmatched calibration solutions was necessary to obtain accurate results. The relative standard deviations (RSDs) of the determinations for the three analytes were typically 6.7%, 8.5%, 6.8% and 4.0% for the solution-based GFAAS, SS-GFAAS, ICP-OES and FAAS methods, respectively. For Fe analysis by ICP-MS, generally, higher RSDs were found, due to the necessary sample (matrix) dilution, thus the relatively low levels of the analytes and the high blank values. On the other hand, the ICP-MS method exhibits much lower LODs than the other spectrochemical methods applied in this study (Table 5). 3.6. Analytical results The dopant concentrations obtained by the SS-GFAAS method are in a fairly good agreement with the results of solution-based methods and the reproducibility of the SS-GFAAS method is also sufficient (Table 6). By using the highly sensitive ICP-MS method, it was also possible to quantify the Cr, Fe and Mn trace impurity content of undoped LiNbO3 , which was found to be 3.9 ± 0.1, 73 ± 2 and 0.036 ± 0.001 ␮mol mol−1 , respectively. It is to be noted that the ttests at 95% significance level has shown deviation of the average

Table 6 Dopant concentrations and standard deviations (SDs) in lithium niobate crystals determined with various spectrochemical methods. Crystal No.

Sample type

Dopant contenta (mmol mol−1 )

Measured dopant concentration (±SD) in the crystalb (mmol mol−1 )

FAAS

ICP-OES

– – 0.43 ± 0.49 ± 0.85 ± – 1.07 ± –

0.10 ± 0.029 ± 0.39 ± 0.54 ± 0.85 ± – 0.97 ± –

SS-GFAAS

Solution-based GFAAS Wall

179709 179709 170101 170101 170101 140101 179911 179911 a b c

T B T B R T T M

2.0 Cr 2.0 Cr 1.0 Fe 1.0 Fe 1.0 Fe 0.1 Fe 1.0 Mn 1.0 Mn

0.04 0.02 0.04 0.02

0.02 0.002 0.06 0.10 0.03 0.04

0.14 ± 0.030 ± 0.34 ± – – 0.084 ± 1.13 ± 0.55 ±

0.01 0.003 0.02c

0.007 0.04 0.01c

0.11 ± 0.033 ± – – – 0.090 ± – 0.50 ±

ICP-MS

Platform 0.003 0.004

0.003 0.01

0.12 ± 0.033 ± – – 0.86 ± 0.091 ± – –

0.002 0.001

0.04 0.001

– 0.036 ± 0.001 – – – 0.099 ± 0.009 – 0.67 ± 0.01

Concentration applied in the crystal growth melt. Average results of triplicate determinations, if not stated otherwise. Duplicate determinations; T – top of the crystal, B – bottom of the crystal, R – crystal growth residue from the crucible, M – middle of the crystal (upper part).

8

L. Bencs et al. / Analytica Chimica Acta 726 (2012) 1–8

values in a few cases. This was rather due to the inhomogeneity of some crystal samples cut at nearby sections of the vertical axis of the host crystal. 4. Conclusions Solid sampling GFAAS, solution-based GFAAS, FAAS, ICP-OES and ICP-MS methods could be well applied for the determination of Cr, Fe and Mn elements in lithium niobate single crystals. The vaporization data derived from the solution-based GFAAS are suitable for the optimization of the solid sampling GFAAS methods. The vaporization of lithium niobate matrix has been observed by XANES to occur in the forms of oxides and elemental (metallic) species, rather than the carbide. Excellent agreement has been found between the dopant concentrations attained with methods of solid sampling GFAAS, solution-based GFAAS, FAAS and ICP-OES. Although the FAAS and ICP-OES methods are less sensitive than the GFAAS methods, they generally provide better precision, when they are applied to the determination of higher dopant concentrations in lithium niobate crystals. The ICP-MS method has been proven to be sensitive enough to determine the trace impurity content of lithium niobate crystals. The great advantage of SS-GFAAS over the solution-based methods is that it does not require any sample preparation step, i.e., less labor intensive, thus much faster and cheaper. The developed solid sampling method can easily be adapted to ETV-coupled techniques as well as to up-to-date, automatized, multi-element SS-GFAAS systems, which incorporate the accessories for sample/standard weighing/dosing in one compact instrument, facilitating a faster spectrochemical analysis. Acknowledgments The authors express their thanks to Zsuzsanna Szaller and Gyula Matók for their help in the preparation of crystal samples. The support from the Hungarian Scientific Research Fund (OTKA) under the project of F67647 is gratefully acknowledged. The research leading to these XANES results has received funding from the European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement of N◦ 226716. The XANES measurements were carried out at the light source DORIS III at DESY, a member of the Helmholtz Association (HGF). The authors thank Karen Appel for her assistance in using beamline L. References [1] V. Gopalan, N.A. Sanford, J.A. Aust, K. Kitamura, Y. Furukawa, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, Elsevier, Amsterdam, The Netherlands, 2001, pp. 57–114.

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