Comparison of decomposition procedures for analysis of titanium dioxide using inductively coupled plasma optical emission spectrometry

Microchemical Journal 71 Ž2002. 41᎐48

Comparison of decomposition procedures for analysis of titanium dioxide using inductively coupled plasma optical emission spectrometry Maria das Grac¸as A. Korna,U , Adriana C. Ferreiraa , Antonio ˆ C.S. Costaa , b , Claudineia Joaquim A. Nobrega ´ ´ R. Silvab,c a

Instituto de Quımica-Uni ¨ ersidade Federal da Bahia ᎐ Campus Uni¨ ersitario ´ ´ de Ondina, CEP 40170-290, Sal¨ ador, BA, Brazil b Departamento de Quımica, Uni¨ ersidade Federal de Sao ´ ˜ Carlos, Sao ˜ Carlos, SP, Brazil c Instituto de Quımica de Sao Carlos, Uni¨ ersidade de Sao Paula, Sao Carlos, SP, Brazil ´ Received 10 April 2001; received in revised form 14 August 2001; accepted 24 August 2001

Abstract The determination of impurities in titanium dioxide pigments, such as Al, Cd, Cr, Fe, Mn, P, Zn and Zr, is relevant because trace elements affect pigment properties. The critical step in the analysis of this pigment is the conversion of the solid sample to a representative solution. This study compared four acid decomposition procedures for TiO 2 for the determination of Al, P and trace impurities using inductively coupled plasma optical emission spectrometry. The decomposition procedures investigated involved acid digestion with: Ži. ŽNH 4 . 2 SO4rH 2 SO4 ; Žii. HFrH 2 SO4 ; Žiii. H 3 PO4 ; and Živ. HClrHNO3rHF. This latter mixture was tested in a microwave-assisted procedure with closed vessels. Comparing the procedures using conventional conductive heating, the procedure using ŽNH 4 . 2 SO4rH 2 SO4 was the most suitable for complete decomposition of TiO 2 samples, requiring approximately 30 min. Applying a paired t-test, it was shown that all strategies led to results in agreement at a 95% confidence level with those obtained using X-ray fluorescence. The accuracy for Cr, Fe, P and Zr was also checked using a certified reference material, and again all results were in agreement at a 95% confidence level. The performance of two ICP-OESs, one based on a mini-torch using a radial view configuration, and the other based on an axial view configuration, were compared. Both plasmas are intensely affected by matrix constituents. The mini-torch plasma is less able to cope with high amounts of solids; however this parameter also negatively affects the background level when using axial-viewed ICP-OES. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: TiO 2 ; Decomposition procedures; Inductively coupled plasma optical emission spectrometry ŽICP-OES.; Axial view; Radial view

U

Corresponding author. Fax: q55-71-235-5166. E-mail address: [email protected] ŽM.G. Korn..

0026-265Xr02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 1 1 9 - 9

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M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

1. Introduction TiO 2 is a white crystalline substance with chemical and physical properties more attractive than other white pigments, such as low toxicity, good chemical and physical stability, and the ability to scatter light due to its high refractive index Ž2.75 for rutile, 2.55 for anatase . w1x. These properties make this pigment an essential component in all good quality paints, and it has replaced former pigments based on Pb compounds. The adverse health consequences of using lead paints are well known. The increase in the industrial use of this pigment demands a product with contaminant level as low as possible, so it is important to establish procedures for quantification of low Ž0.0001᎐ 0.01%. and trace Ž1 ␮g kgy1 ᎐1 mg kgy1 . levels of elements. The properties of TiO 2 are markedly affected by impurities, and consequently these constituents also affect applications and economic value. For instance, titanium dioxide as constituent of advanced ceramics plays an important role in various fields of technology, but if used as oxygen sensor, capacitors or piezoelectric applications, its electrical properties of these ceramics are significantly affected by some metal impurities, such as Cr, Fe, V, Zr and Cu. Therefore, efficient analytical methods are required as routine analysis to determine the impurities in titanium dioxide. The analytical techniques most frequently used in the analysis of TiO 2 are atomic absorption spectrometry ŽAAS. w2,3x, spark-source mass spectrography w4x, X-ray fluorescence spectrometry ŽXRF. w5x and inductively coupled plasma optical emission spectrometry ŽICP-OES. w6x. ICP-OES was employed in this study due to its multielement detection capabilities and wide linear dynamic range, which allows simultaneous determination of major, minor and trace constituents. However, spectral interference is the principal limitation of trace analysis by ICP-OES for analytes present in samples with high concentrations of elements with line-rich emission spectra, such as Al, Fe, Ti, U, etc. Korn et al. w7x studied spectral interference in the determination of Cd, Co, Cr, Cu, Mn, Pb and Zn in iron ores

using a mini-torch ICP-OES. The extraction of Fe with MIBK was imperative to eliminate spectral interference. Most analytical techniques do not have the capability for direct analysis of solid samples, so a previous decomposition sample step is generally required to convert solids into a representative solution. The sample preparation step for determination of low concentration elements should be carried out carefully to avoid errors caused by partial decomposition, losses or contamination. The literature reports the use of some reagents to decompose TiO 2 samples: fusion with KHSO4 , K 2 S 2 O 7 or Na 2 CO 3 w2,8x, ŽNH 4 . 2 SO4rH 2 SO4 w9x, HF mixed with other mineral acids w8,10x and H 3 PO4 w11x. The amount of sample that may be used for analysis is restricted to a few g because of the difficulty in dissolving titanium dioxide pigment, and because of the tendency for hydrolysis to occur in titanium solutions. The large amounts of reagents that are necessary to dissolve the sample give rise to high blank values. Fusion procedures are best suited to this type of sample, but they were not investigated here due to the elevated concentration of salts in the final solution. The high concentration of dissolved solids negatively affects the sample nebulization and can cause clogging, can increase the background level, and can deteriorate the detection limit due to contamination of the blank w10x. Recently, fluorination-assisted electrothermal vaporization ŽETV. was employed as the sample introduction technique for the direct determination of trace amounts of impurities in titanium dioxide solid powder by ICP-OES. Selective volatilization between analytes and matrix was used to improved the sensitivity and reduce the matrix interference caused by a large amount of Ti w12x. However, references were not found for a systematic study as proposed by this work, where the analytical performance of four sample dissolution methods were evaluated, not only taking into account the precision, accuracy and limits of detection achieved, but also practical considerations, such as labor and time to perform the analysis of titanium dioxide. In the work developed, four procedures were studied for decomposition of TiO 2 using mineral

M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

acids: ŽNH 4 . 2 SO4rH 2 SO4 ; HFrH 2 SO4 ; H 3 PO4 ; and HClrHNO3rHF. This latter mixture was employed in a microwave-assisted procedure using closed vessels. The main advantage of microwave-assisted procedures is the possibility of speeding up chemical processes owing to the high pressure inside the closed vessels, which proportionates a high-temperature medium to improve the action of the acids. Considering nebulization effects and spectral interference, we compared the performance of two ICP-OESs, one based on a mini-torch using a radial view configuration, and the other based on an axial view configuration, Echelle optics and a charge-coupled device detector. Both plasmas are intensely affected by matrix constituents. The mini-torch is extinguished when introducing a solution high in dissolved solids. On the other hand, the plasma with an axial view configuration has increased detection limits due to increments in the background level.

2. Experimental

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Table 1 Instrumental parameters for ICP-OES Parameter

Frequency ŽMHz. Forward power ŽkW. Reflected Power ŽW. Observation height Žmm. Nebulizer Outer Ar flow Žl miny1 . Carrier Ar flow Žl miny1 . Intermediate Ar flow Žl miny1 . Sample uptake Žml miny1 . Integration time Žs.

ICP-OES Mini-torch

Vista axial

27.12 0.65 - 10 12 Concentric 7.5 0.8 0.8 2.3 1

27.12 1.2 0.9 ᎐ V-groove 15.0 0.9 1.5 1.0 5

ted. In the first, the analytes were dissolved in a medium similar to the sample, and in the other, they were added to the sample dissolved according to the relevant procedure. The concentrations of the analytes were the same in the both curves. Aliquots of 0, 0.5, 1.0, 2.5 and 5.0 ml of a multielement solution containing Zn, Cd, Fe, Mn, Zr, Ni and Cr Ž10 ␮g mly1 . and Al Ž200 ␮g mly1 . and P Ž81.5 ␮g mly1 . were transferred to 50-ml volumetric flasks.

2.1. Equipment 2.4. Decomposition procedures All measurements were carried out using two ICP-OESs. The measurement conditions are shown in Table 1. The main characteristic of the ARL model 3410 ŽDearborn, MI, USA. is the use of a mini-torch mounted on a radial view configuration. The Varian Vista ICP-OES ŽMelbourne, Australia. has an axial view configuration, an Echelle optical system and a solid-state detector. 2.2. Reagents All reagents were analytical grade. All dilutions were made with distilled᎐deionized water that was also used to prepare reference solutions by dilution of Titrisol Merck ŽDarmstadt, Germany. stock solutions of 1 g ly1 . 2.3. Study of matrix effects: solutions and strategies The methodology used in the study was the standard addition method. Two curves were plot-

2.4.1. Procedure 1: (NH4 )2 SO4 r H2 SO4 A 0.4-g amount of sample was weighed in a 100-ml beaker. To this was added 6.4 g of ŽNH 4 . 2 SO4 and 16 ml of concentrated H 2 SO4 . The mixture was heated for 30 min at 250⬚C to effect complete dissolution of the sample. After cooling, the digest was transferred to a 50-ml volumetric flask and the volume was made up with water. 2.4.2. Procedure 2: HF r H2 SO4 A 0.4-g amount of sample was weighed in a 100-ml PTFE beaker; 10 ml of concentrated HF and 16 ml of concentrated H 2 SO4 were added. The mixture was heated at 200⬚C on a sand-bath until evolution of SO 3 fumes. The mixture was then cooled before adding 10 ml of concentrated HF, followed by heating to 200⬚C on a sand-bath until SO 3 fumes were evolved. After cooling, the above procedure was repeated. The beaker was

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M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

then cooled, its walls were washed with 3 ml of deionized water and heating was repeated until evolution of SO 3 . The digest was transferred to a 50-ml volumetric flask and diluted to the mark with water. Complete sample decomposition took approximately 4 h. 2.4.3. Procedure 3: H3 PO4 A mass of 0.1 g of sample was weighed into a 125-ml conical flask and 2᎐3 ml of water was added to wet the sample surface. A 17-ml portion of concentrated H 3 PO4 was added, and the reaction flask was heated at 250⬚C for 15 min until the sample was completely decomposed. After cooling, the clean digest was transferred to a 50-ml volumetric flask and the volume was made up with water. 2.4.4. Procedure 4: HCl r HNO3 r HF (microwa¨ eassisted) A mass of 0.1 g of sample was transferred to a perfluoralkoxy closed vessel designed to support high pressure ŽMilestone, Sorisole, Italy. and 4 ml of aqua regia plus 3 ml HF was added to each vessel. The vessels were placed in an Ethos 1600 oven ŽMilestone. and the heating program implemented as shown in Table 2 Žprocedure recommended by Milestone for TiO 2 .. Afterwards, the vessel was cooled down to room temperature, the

Table 2 Microwave-assisted decomposition: heating program Step

Applied power ŽW.

Time Žmin.

1 2 3

300 500 Vent

5 15 5

digest was transferred to 50-ml volumetric flasks, a volume of 12 ml of 4% wrv boric acid was added to avoid HF attack to the quartz torch and the volume was made up with water. We have not tried to measure the pressure and temperature inside the reaction vessels; however an excessive increase is pressure is not expected when using a 0.1-g mass of an inorganic sample. None of the closed vessels with a resealing design opened in any of the experimental conditions evaluated.

3. Results and discussion 3.1. Matrix effects and selection of emission lines Most of these studies were carried out using the mini-torch ICP-OES. Optimum line selection for trace analysis implies the choice of prominent lines with no or lowest spectral interference and

Fig. 1. Spectral interference of NiŽII. line at 221.647 nm caused by emission line of SiŽI. at 221.667 nm.

M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48 Table 3 Wavelengths selected for determination of nine elements in TiO2 matrix Analyte

AlŽI. Cd CrŽII. FeŽII. MnŽII. NiŽII. PŽI. ZnŽI. ZrŽII.

ICP-OES ␭ Žnm. Mini-torch

Axial view

394.401 228.802 ŽI. 206.149 238.204 257.610 231.604 213.618 213.856 339.198

396.15 214.48 ŽII. 283.56 238.24 260.56 231.60 177.49 213.86 343.82

minimal background signals in the presence of the matrix. Since Ti is one of the elements that causes the most severe spectral interferences, line selection is a critical parameter in the analysis of pigments containing up to 90% TiO 2 using ICPOES. Therefore, in order to obtain a better knowledge of the elements causing spectral interferences, a systematic study was performed. The influence of major matrix constituents ŽAl, P, Si, and Ti. on the measurement of nine elements Ži.e. 27 lines. was studied w13x. The analytes investigated were: Al; Cd; Cr; Fe; P; Mn; Ni; Zn; and Zr. Selected spectral scans are shown in Fig. 1 to illustrate interference effects. Considering these results, the lines shown in Table 3 were selected. The most sensitive lines were chosen only when not affected by spectral interferences. The difficulties in the determination of trace

45

impurities in TiO 2 samples are not only associated with spectral complexity, but also with the method of converting the solid sample into a representative solution. Considering the application of different strategies for sample decomposition, the effect of sample medium on emission intensities should also be evaluated. Two sets of multi-element solutions were prepared containing all analytes, with or without Ti in each one of the decomposition media tested. The concentration of Ti in the solutions prepared reflects the final concentration of this element in diluted samples. Results in Table 4 show that there is no difference between the slopes of the calibration curves, with or without the Ti matrix, for each decomposition medium under consideration. We concluded that neither a previous step of matrix separation nor matrix matching is necessary. On the other hand, all measurements using axial view were based on calibration curves prepared using matrix-matched reference solutions, owing to the well-known spectral complexity generated when using a longer optical path w14x. Despite the higher amount of radiation entering the optical system, the axial-view system employed presented more flexibility for choosing the wavelengths measured. Additionally, the pre-optics interface purged with an end-on argon flow was effective in minimizing interferences caused by processes on the cold plasma fringe. However, matrix-matched solutions were used for calibration, taking into account the matrix effects on background signals that deteriorate limits of detection.

Table 4 Slopes of the calibration curves with and without Ti matrix, for ŽNH 4 . 2 SO4rH 2 SO4 , HFrH 2 SO4 , and H 3 PO4 media Element

Al Cd Cr Fe Mn Ni P Zn Zr

ŽNH4 .2 SO4rH2 SO4

HFrH2 SO4

H3 PO4

Without Ti

With Ti

Without Ti

With Ti

Without Ti

With Ti

0.7 8.3 0.6 7.7 51.0 3.4 0.6 11.8 10.5

0.7 8.8 0.6 8.1 52.9 3.4 0.7 11.8 10.9

0.7 6.5 1.3 4.4 33.3 1.7 0.4 8.3 8.5

0.7 6.9 1.3 4.8 34.2 1.8 0.4 9.0 8.1

0.8 8.9 2.5 7.4 48.1 3.0 ᎐ 12.6 9.5

0.8 8.7 2.6 6.9 46.7 2.9 ᎐ 12.0 9.6

M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

46

Limits of detection Ž3␴ blankrslope, n s 12. obtained with each of the four media studied for the digestion of TiO 2 are shown in Table 5. The limits of detection obtained with different procedures and measurements using mini-torch ICPOES were similar, but the sulfuric acidrammonium sulfate medium presented values slightly higher that could be explained by the effect of viscosity on transport efficiency. On the other hand, the plasma with an axial view configuration, used in Procedure 4, has an increased detection limit for some elements, due to increments in the background level. 3.2. Performance of sample decomposition procedures An overview of the performance of the decomposition procedures evaluated is presented in Table 6. The best performance was attained with procedures 1 and 4. Procedure 2 involved the use of high amounts of HF plus H 2 SO4 and is timeconsuming because HF must be neutralized to avoid attack of the quartz plasma torch. The conventional procedure of adding H 3 BO 3 cannot be employed because it extinguishes the mini-

Table 5 Limits of detection in ŽNH 4 . 2 SO4 rH 2 SO4 Ž1., HFrH 2 SO4 Ž2., H 3 PO4 Ž3., and HClrHNO3rHF Ž4. media Analyte

Al Cd Cr Fe Mn Ni P Zn Zr

Limit of detection Ž␮g ly1 . Procedure 1

2

3

4

108.9 14.2 22.2 15.2 17.4 27.1 106.9 23.4 21.6

58.2 5.1 11.3 9.1 1.7 17.0 65.1 4.7 9.2

132.5 6.2 17.7 8.3 2.9 25.5 ᎐ 4.1 12.4

3.2 46 170 100 50 430 1130 35 5.9

torch plasma. Procedure 3 based on H 3 PO4 was the fastest method, but owing to the viscosity of the acid, the digest must be diluted to circumvent viscosity effects, and consequently it was not possible to determine trace impurities in the diluted digests. Microwave-assisted procedure 4 was easily implemented and effective for decomposing all but rutile samples, which left some solid residues after cooling and dilution. The use of sulfuric acid

Table 6 Experimental conditions and performance of decomposition procedures Procedure

1 2 3 4

Parameters Sample mass Žg.

Acid volume Žml.

Time Žmin.

Residuesb

Suitable for ICP-OES

0.4 0.4 0.4 0.1

6.4a ; 16 30; 16 17 1.0; 3.0; 3.0

30 240 15 20

0 q 0 q

Yes Yes Noc Yes

a

Mass in g. 0, absence of residues; q, residues for rutile reference material. c Considering trace analysis requisites. b

Table 7 Determination of Fe and P in NIST titanium oxide standard reference material SRM 154b Element

Fe2 O3 Ž%. P2 O5 Ž%.

1

Procedure 2

4

Certified value

0.006" 0.001 0.04" 0.01

0.007" 0.001 0.04" 0.01

0.006 0.04" 0.00

0.006 0.04

95% confidence interval; n s 5; sample mass: 0.4 g for procedures 1 and 2 and 0.1 g for procedure 4.

M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

47

Table 8 Analytical results for two TiO 2 commercial samples Procedure 1

Procedure 2

Procedure 4

Sample 1 Al2 O3 Ž%. Cd Ž␮g gy1 . Cr Ž␮g gy1 . Fe Ž␮g gy1 . Mn Ž␮g gy1 . Ni Ž␮g gy1 . P2 O5 Ž%. ZnO Ž%. ZrO2 Ž%.

2.85" 0.06 - 5.00 - 5.00 17.83" 3.92 - 5.00 - 5.00 0.16" 0.02 - 0.06= 10y2 0.02" 0.00

2.96" 0.00 - 5.00 - 5.00 21.72" 4.79 - 5.00 - 5.00 0.18" 0.01 - 0.06= 10y2 0.02" 0.00

2.83" 0.09 - 2.05 - 26.70 22.70" 0.68 - 5.30 - 25.90 0.18" 0.01 - 0.12= 10y2 0.02" 0.00

Sample 2 Al2 O3 Ž%. Cd Ž␮g gy1 . Cr Ž␮g gy1 . Fe Ž␮g gy1 . Mn Ž␮g gy1 . Ni Ž␮g gy1 . P2 O5 Ž%. ZnO Ž%. ZrO2 Ž%.

1.04" 0.03 - 5.00 - 5.00 27.81" 2.75 - 5.00 - 5.00 0.22" 0.01 - 0.06= 10y2 0.01" 0.00

1.09" 0.02 - 5.00 - 5.00 30.42" 2.63 - 5.00 - 5.00 0.19" 0.00 - 0.06= 10y2 0.01" 0.00

0.98" 0.01 - 2.05 - 26.70 29.91" 0.64 - 5.30 - 25.90 0.21" 0.01 - 0.12= 10y2 0.01" 0.00

XRF

2.77 ᎐ 2.00 19.00 ᎐ - 1.00 0.16 - 0.10= 10y2 0.02

1.02 ᎐ 1.00 29.00 ᎐ 2.00 0.22 - 0.10= 10y2 0.01

95% confidence interval; n s 5; sample mass 0.4 g for procedures 1 and 2 and 0.1 g for procedure 4.

was avoided to circumvent an excessive increase in temperature that could reduce the lifetime of the closed vessels. On the other hand, the use of HF was essential because the decomposition was not complete when using only aqua regia. The addition of boric acid to avoid HF attack to the quartz torch did not disturb the axial plasma, but contributed to increase background levels. 3.3. Accuracy To evaluate the accuracy of the procedures developed, all results obtained for commercial samples were compared with those using XRF. Accuracy was also evaluated using one reference sample: titanium dioxide ŽNIST SRM 154b.. It should be mentioned that there is no reference material with certified values for trace constituents. The results are shown in Tables 7 and 8. Comparing the procedures based on conventional conductive heating, the best performance was attained with procedure 1 wŽNH 4 . 2 SO4rH 2 SO4 x. The addition of a common anion salt to H 2 SO4 increases the temperature of the reaction medium,

providing a faster and more efficient dissolution. A paired t-test showed that all results are in agreement at a 95% confidence level with those obtained using XRF. Procedure 2 using a HFrH 2 SO4 mixture was efficient for most samples, but the decomposition of rutile certified material was incomplete. This procedure presented the best repeatability; however, it was time-consuming and demanding to the analyst. Applying a paired t-test, it was shown that all results were in agreement at a 95% confidence level with those obtained by XRF. Despite the satisfactory accuracy and precision, the application of this procedure in routine analysis is limited by the 4-h decomposition time. As previously mentioned, procedure 3 using phosphoric acids was effective for sample decomposition, but the trace analysis was negatively affected by the large dilution needed to decrease the viscosity of the digests. Additionally, attack of the glassware at temperatures higher than 280⬚C could occur. This effect was reported by De Andrade and Costa in the determination of chromium in chromite samples decomposed with

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M.G. Korn et al. r Microchemical Journal 71 (2002) 41᎐48

H 3 PO4 w15x. The heating time must be controlled because H 3 PO4 undergoes condensation reactions that produce polyacids in equilibrium ŽH 4 P2 O 7 , H 5 P2 O 10 , P4 O 10 ⭈ 2H 2 O, P4 O 10 ⭈ 6H 2 O, etc.., which are highly viscous and can deeply erode glassware.

4. Conclusions The dissolution procedure using a mixture of ŽNH 4 . 2 SO4 and H 2 SO4 was effective for complete decomposition of TiO 2 samples prior to determination of impurities by ICP-OES. The procedure using a mixture of HF and H 2 SO4 is not convenient because it is time-consuming. The use of H 3 PO4 required a large dilution to overcome viscosity influences on the transport efficiency and this strategy negatively affected the limits of detection. Finally, the advantages of the microwave-assisted procedure are its fast dissolution of titanium oxide using a mixture of aqua regia and HF. The addition of H 3 BO 3 to avoid fluoride attack to the quartz torch strongly affects the measurements using ICP-OES with axial view configuration. The sensitivities attained with this arrangement were close to those achieved with mini-torch ICP-OES, despite the matrix complexity.

Acknowledgements The authors are grateful to the Conselho Nacional de Desenvolvimento Cientıfico e ´ ŽCNPq. by financial support and reTecnologico ´

searchships provided to M.G.A.K., A.C.S.C. and J.A.N. The authors A.C.F. and C.R.S. also express their gratitude to CNPq and Fundac¸˜ ao de Amparo ` a Pesquisa do Estado de Sao ˜ Paulo ŽFAPESP., respectively, for fellowships. We are also thankful to Millennium Inorganic Chemicals do Brasil for donating the samples. References w1x R.C. Hutton, Anal. Proc. 21 Ž1984. 317. w2x J.C. Meranger, E. Somers, Analyst 93 Ž1968. 799. ´ w3x K.W. Jackson, T.S. West, L. Balchin, Anal. Chem. 45 Ž1973. 249. w4x P.F.S. Jackson, J. Whitehead, Analyst 91 Ž1966. 419. w5x C.L. Denton, G. Himsworth, J. Whitehead, Analyst 97 Ž1972. 461. w6x J. Claessens, C. Vanoeteren, J.; Anal. At. Spectrom. 12 Ž1997. 1017. w7x M.G.A. Korn, H.V. Jaeger, A.C. Ferreira, A.C.S. Costa, Spectrosc. Lett. 33 Ž2000. 127. w8x Z. Sulcek, P. Povondra, Methods of Decomposition in Inorganic Analysis, CRC Press, Boca Raton, 1989. w9x M.G.M. Andrade, S.L.C. Ferreira, A.C.S. Costa, B.F. Santos, N.O. Leite, Anal. Lett. 26 Ž5. Ž1993. 1001. w10x D.S.R. Murthy, B. Gomathy, R. Bose, R. Rangaswamy, Atom. Spectrosc. 19 Ž1998. 14. w11x J.B. De Andrade, A.C.S. Costa, S.L.C. Ferreira et al., Talanta 44 Ž1997. 165. w12x P. Tianyou, D. Pingwu, H. Bin, J. Zucheng, Anal. Chim. Acta 421 Ž2000. 75. w13x R.K. Winge, V.J. Peterson, V. Fassel, M.A. Floyd, Inductively Coupled Plasma Emission Spectroscopy an Atlas of Spectral Information, 6th, Elsevier Science Publishers BV, 1993. w14x A. Montaser, D.W. Gologhtly, Inductively Coupled Plasma in Analytical Spectrometry, 2nd, VCH, New York, 1992. w15x J.B. De Andrade, A.C.S. Costa, Quım. ´ Nova 31 Ž1984. 1171.