Simultaneous determination of chromium and

PAPER

www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry

Simultaneous determination of chromium and manganese in alumina by slurry sampling graphite furnace atomic absorption spectrometry using NbC and NaF as modifiers Alexandre Luiz Souza and Pedro Vitoriano Oliveira* Received 26th August 2009, Accepted 19th January 2010 First published as an Advance Article on the web 29th January 2010 DOI: 10.1039/b917373k This paper reports a method for the direct and simultaneous determination of Cr and Mn in alumina by slurry sampling graphite furnace atomic absorption spectrometry (SiS-SIMAAS) using niobium carbide (NbC) as a graphite platform modifier and sodium fluoride (NaF) as a matrix modifier. 350 mg of Nb were thermally deposited on the platform surface allowing the formation of NbC (mp 3500 C) to minimize the reaction between aluminium and carbon of the pyrolytic platform, improving the graphite tube lifetime up to 150 heating cycles. A solution of 0.2 mol L1 NaF was used as matrix modifier for alumina dissolution as cryolite-based melt, allowing volatilization during pyrolysis step. Masses (c.a. 50 mg) of sample were suspended in 30 ml of 2.0% (v/v) of HNO3. Slurry was manually homogenized before sampling. Aliquots of 20 ml of analytical solutions and slurry samples were co-injected into the graphite tube with 20 ml of the matrix modifier. In the best conditions of the heating program, pyrolysis and atomization temperatures were 1300 C and 2400 C, respectively. A step of 1000 C was optimized allowing the alumina dissolution to form cryolite. The accuracy of the proposed method has been evaluated by the analysis of standard reference materials. The found concentrations presented no statistical differences compared to the certified values at 95% of the confidence level. Limits of detection were 66 ng g1 for Cr and 102 ng g1 for Mn and the characteristic masses were 10 and 13 pg for Cr and Mn, respectively.

Introduction Alumina (Al2O3) is intensely used in the technological industry, especially in microelectronics as a substrate and packaging material,1 in medicine as a constituent of bioceramics,2 and due to their physico-chemical properties as a substitute of beryllium oxide ceramics in high-efficiency electronics.3 For all applications mentioned above, the alumina purity has to be higher than 99.9%.1,4 In the failure of which, physico-chemical properties, such as electrical resistivity, thermal conductivity, dielectric constant and thermal expansion can be significantly affected.1–4 On the other hand, the presence of Cr2O3 and MnO can decrease the surface resistivity and the secondary electron emission (SEE) of the original alumina insulator, which results in the improvement of the surface hold-off voltage.5 The incorporation of small amounts of Ca, Mn, or Cr in the bioinert material, as alumina, can influence tissue differentiation and osteogenesis.6 Additionally, like the electrical properties, the colored effects depend primarily on the impurities. Without impurities alumina is colorless. However, addition of transition metal ions (i.e. Cr3+) to alumina leads to spectacular colors, gem stones, and practical applications such as ruby lasers.7 Trace element determination in aluminium-based matrix have been performed by high sensitivity spectrochemical techniques,

Departamento de Qu´ımica, Instituto de Qu´ımica, Universidade de Saõ Paulo, C.P. 26077–05513-970 Saõ Paulo, SP, Brazil. E-mail: pvolivei@ iq.usp.br; Tel: +55 11-30918516

This journal is ª The Royal Society of Chemistry 2010

such as graphite furnace atomic absorption spectrometry (GF AAS),8–15 inductively coupled plasma optical emission spectrometry (ICP OES)16–21 and inductively coupled plasma mass spectrometry (ICP-MS).22,23 Acid digestion procedures for alumina analysis may have some characteristics that become their application hard in routine analysis: (i) the time spent for total digestion is too long (around 4 h), (ii) the possibility of contamination or analyte volatilization in each one of the successive steps, (iii) sometimes the incomplete sample decomposition and (iv) additionally, the final acidity of the digestion product could cause some drawbacks, depending on the analytical technique used for measurements. Notwithstanding, some of the methods recommended wet chemical decomposition with acids10,16–18 or alkaline fusion.16,20 Alternatively, direct solid analysis can be more attractive and, for high purity samples, presents advantages over the conventional procedures of dissolution: (i) shortening the sample preparation time (good for routine analysis), (ii) less sample contamination and decreased losses of analyte, (iii) prevents the use of hazardous and corrosive reagents, (iv) improve the detectability of method, and (v) it is a clean method of analysis.24,25 The majority of the direct methods mentioned above for the determination of impurities in alumina-based matrix use slurry8,9,11,14 and direct solid12,15 sampling for GF AAS and slurry sampling combined with electrothermal vaporization for ICP OES20,21 and ICP-MS.22,23 Also, a direct aluminium oxide analysis by continuous powder sampling into microwave induced plasma optical emission spectrometry (MIP OES) has been reported in the literature.26 J. Anal. At. Spectrom., 2010, 25, 675–680 | 675

Compared with ICP OES and ICP-MS, GF AAS is well suited for the direct analysis of solids.24,25,27 This technique combines a lot of advantageous characteristics, such as specificity and selectivity, the relative facility for sample introduction and the heating program that is one of the most important attributes of GF AAS. During the drying and pyrolysis steps it is possible to eliminate matrix components and to carry out the in situ sample pretreatment, such as digestion and, as proposed in this work, fusion.28–31 It has provided good analytical results with relatively inexpensiveness and robustness. Additionally, the availability of simultaneous atomic absorption spectrometry with conventional light source (SIMAAS)32,33 or with high-resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GFAAS)34,35 allowed the development of multi-element methods, improving the analytical frequency, reducing the cost related to instrument maintenance, sample and high purity reagents. Alumina is suitable for the direct solid analysis by GF AAS slurry sampling because it is available as a fine powder (particles size < 74 mm), eliminating the need for any further grinding and minimizing the risk of contamination. However, alumina is a complex matrix needing an optimized heating program for the direct analysis. Nevertheless, even using an optimized heating program, the use of a matrix modifier could be the way to decrease the complexity of the matrix and to facilitate the release of the analytes for atomization. The goal of this work has been to study a method for the determination of chromium and manganese in aluminium oxide by simultaneous graphite furnace atomic absorption spectrometry using direct slurry sampling (SiS-SIMAAS). Owing to the high capacity of aluminium oxide it reacts with the carbon of the tube at the temperatures used in the atomization step, NbC was proposed as a protective modifier for the graphite platform surface and NaF as a matrix modifier to improve the aluminium oxide dissolution and elimination during pyrolysis step.

Experimental Instrumentation A simultaneous atomic absorption spectrometer, SIMAA-6000 model (PerkinElmer Life and Analytical Sciences, Shelton, USA) equipped with a longitudinal Zeeman-effect background corrector, standard THGA-tubes with pyrolytically coated integrated platform, and solid state detector was used throughout in this work. The instrument was operated in 2-element simultaneous mode using hollow cathode lamps (Perkin-Elmer Intensitron) for Cr and Mn. An AS-72 autosampler was used to transfer reference solutions, slurries and matrix modifier from polypropylene cups to the graphite tube.

Table 1 Instrumental setting for the simultaneous determination of Cr and Mn by SIMAA-6000 Element

Cr

Mn

Wavelength Band pass Lamp current

357.9 nm 0.7 nm 25 mA

279.5 nm 0.7 nm 20 mA

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Argon 99.98% (v/v) (Air Liquide Brasil S/A, Saõ Paulo, Brazil) was used as the protective and purge gas. The instrumental conditions adopted for the analysis are shown in Table 1. The analytical signals were based on the integrated peak area (AA – BG). Field emission scanning electron microscope (SEM), Jeol model JSM 7401F, operating at an accelerating voltage of 5.0 kV was used for the topochemical inspection of the modified platform surface before and after modification with NbC. An X-Ray Diffractometer, Philips model MPD 1880, operating at 40.0 kV and 40.0 mA was used for the cryolite crystal identification in the inner surface of the graphite platform. An analytical balance, model Ohaus Adventurer - Analytical balances of Precision (Mettler Toledo, Saõ Paulo, Brazil) with analytical sensitivity until 0.0001 g was used to weight the samples. Reagents and samples All solutions were prepared with analytical-reagent grade unless otherwise specified. High-purity deionized water with a final resistivity of 18.2 MU cm was provided by a Milli-Q water purification system (Millipore, Bedford, USA). Nitric acid 65% (m/v) (Merck, Darmstadt, Germany) was purified in-house by distillation in quartz sub-boiling still (Marconi, Piracicaba, Brazil). The analytical reference solutions were prepared by successive dilution of 1000 mg L1 of Cr (K2Cr2O7 in H2O) and 1000 mg L1 of Mn (Mn(NO3)2 in H2O) from Titrisol standard solutions (Merck). The NbC covering of the graphite platform surface was thermally obtained from high purity 1000 mg L1 Nb (Nb2O5) (Spex, Eddison, USA). Sodium hydroxide 99.99% (m/m) (# 0.01 mg L1 of Cr and Mn) (Suprapur, Merck) and HF 48% (m/v) (# 0.1 mg L1 of Cr and # 0.02 mg L Mn) (Ultrapur, Merck) were used to prepare the NaF matrix modifier. The reference material Alumina Reduction Grade-699 from NIST (National Institute of Standard and Technology) was used for method development and to check the reliability of the entire proposed method. This material was available as a fine powder in which 95% of the particle size is less than 24 mm. Procedure All glassware, polypropylene flasks (Falcon tubes), and auto sampler cups were cleaned with detergent solution, soaked in 10% (v/v) HNO3 for 24 h, rinsed with high-purity deionized water, dried and stored in a closed polypropylene container. All solutions and sample manipulations were conducted in a class 100 laminar flow bench (Veco, Campinas, Brazil). The detailed procedure for graphite platform coating with niobium carbide is depicted in Table 2. The heating program was executed by 7 repetitive injections of 50 ml of the 1000 mg L1 of Nb stock solution. After, the surface of the thermally modified platform was analyzed by scanning electron microscopy (SEM). This permanent modifier was used throughout in this work. A solution containing 5 mg L1 of Cr + 10 mg L1 of Mn in 2.0% (v/v) HNO3 and alumina slurry were used to optimize the heating program. Pyrolysis and atomization temperature curves were simultaneously obtained for Cr and Mn in absence and presence This journal is ª The Royal Society of Chemistry 2010

Table 2 Heating program for the thermal modification of the graphite platform with NbC Heating program (T/ C, ramp/s, hold time/s)

Step

Procedure

1a

3

Pipette 50 ml 1000 mg L1 Nb Pipette 50 ml 1000 mg L1 Nb Run 6 times

4

Run 6 times

2b

a

Repeat 2 more times. min1.

b

(120, 5, 25); (150, 10, 60); (600, 20, 15); (1000, 10, 15) (150, 1, 10); (600, 10, 15); (1100, 10, 5); (1400, 10, 10); (1600, 5, 5) (120, 5, 25); (150, 10, 60); (600, 20, 15); (1000, 10, 15); (1400, 10, 5); (2000, 3, 2), (2200, 3, 2) (150, 1, 10); (600, 10, 15); (1100, 10, 5); (1400, 10, 10); (1500, 3, 5); (1600, 1, 1); (1700, 1, 1); (1800, 1, 1); 1900 (1, 1); (2000, 1, 1); (2200, 1, 1); (2300, 1, 1)

Repeat 3 more times; Ar gas flow rate: 250 ml

of the co-injected 0.2 mol L1 NaF. This matrix modifier solution was prepared from the neutralization reaction of 0.2 mol L1 NaOH with 0.2 mol L1 HF, both prepared in high-purity deionized water. Aliquots of 20 ml of the solutions or slurries were co-injected into the graphite tube with 20 ml of matrix modifier. The optimized pyrolysis holding time was studied considering the analytical and background absorbance intensity in presence of alumina slurry. The effect of aluminium concentration on the analyte atomization was investigated with solutions containing 5 mg L1 of Cr + 10 mg L1 of Mn in 2.0% (v/v) HNO3 and the following concentration of AlIII, from AlCl3 (Merck): (i) none, (ii) 100 mg L1, (iii) 200 mg L1, (iv) 400 mg L1, (v) 800 mg L1, and (vi) 1000 mg L1. In this case, aliquots of 10 ml of these solutions were injected into the graphite tube using only NbC as platform modifier. The analytical calibration solutions were prepared simultaneously for Cr (0.5 to 10 mg L1) and Mn (5 to 50 mg L1) in 2.0% (v/v) of HNO3. For the preparation of sample slurries,

aluminium oxide masses around 50 mg were directly weighed on to the Falcon tubes and suspended in 30 ml of 2% (v/v) of HNO3. The slurry was manually homogenized and a volume of 1.2 ml was transferred to the autosampler cup (total volume of 1.5 ml) for analyses. Before auto sampler pipetting, all samples were pumped with a micropipette several times to ensure completely slurry homogenization. All measurements were made with at least five replicates and based on integrated absorbance.

Results and discussion NbC as platform modifier At temperatures above 1500 C, aluminium oxide reacts with the carbon of pyrolytic platform forming gaseous products c.a. Al, Al2 and Al2O, causing a nonspecific background absorption.11,12,36 The background spectra of these aluminium species consist mainly of two absorption bands regions (200 to 225 nm and 235 to 280 nm). Additionally, these gaseous products can react with the carbon of the platform damaging the pyrolytic surface, decreasing the tube lifetime.11,36 This way, it is mandatory to find strategies to avoid these reactions allowing direct determination of impurities, such as Cr and Mn in alumina without laborious sample pretreatment. Niobium pentoxide reacts with carbon to form NbC (Nb2O5 + 7C / 2 NbC + 5CO) at 1800 C, which is one of the most thermally stable niobium compounds (m.p 3500 C).37 Due to the thermo-physical and chemical properties of Nb2O5, this reaction must occur during the heating of the graphite tube. However, NbC can react with Nb2O5 above 2000 C to form Nb following the reaction (5NbC + Nb2O5 / 7Nb + 5CO).37 These reactions confirm the consumption of the pyrolytic carbon of the platform, resulting from the formation of NbC and subsequent metallic niobium. This is the main reason for the short lifetime of the platform observed during analysis of highpurity niobium pentaoxide.37,38

Fig. 1 Scanning electron micrograph of a graphite platform: (Left) after direct introduction of 33.3 mg Al2O3 without surface modification (magnification of 2 000x), and (Right) modified with NbC after performing 100 atomization cycles with a 33.3 mg Al2O3 (magnification of 5 000x).


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Keeping this in mind, the initial steps (1 and 2) of the heating program developed for the platform modification (Table 2) did not exceeded 1600 C to promote NbC formation, avoiding its destruction by the second reaction. After depositing about 350 mg Nb onto the platform, two conditioning steps (3 and 4) were carried out six times each, in order to transform niobium oxides to niobium carbide (Table 2). After and before modification, the integrated platform was cut off to allow the topochemical study to confirm the presence and distribution of NbC film by X-Ray diffraction and scanning electron micrography (SEM). By X-Ray diffraction it was possible to identify niobium species, such as NbO0.4F2.6, NbO2, and NbC. The SEM images are shown in Fig. 1. In Fig. 1A (2 000 fold magnification) the damage of the graphite platform without modification can be seen, with 20 consecutive pipetting slurries (33 mg Al2O3) and atomization at 2400 C. After modification with NbC the contact between Al2O3 and carbon was reduced. Consequently, the graphite platform was less damaged, as depicted in Fig. 1B (at 5 000 fold magnification). In these images, white areas represent Nb compounds and black the pyrolytic graphite surface, attested by electron dispersive spectroscopy (EDS).

NaF as matrix modifier Regarding the compromise conditions for the simultaneous determination by SIMAAS, the pyrolysis and atomization temperatures obtained for Cr (5 mg L1) and Mn (10 mg L1) in the absence and in the presence of NbC as platform modifier were 1300 C and 2400 C for aqueous solution and 1400 C and 2400 C for alumina slurry sampling, respectively. In principle, these results indicated that NbC did not significantly change the thermal stabilization of Cr and Mn. After establishing the heating program using NbC as platform modifier, pyrolysis temperature of 1300 C and atomization temperature of 2400 C, Cr and Mn were determined in the reference material (alumina Reduction Grade-699). Recoveries based on the recommended values were 82% for Cr and 66% for Mn. The reason for the bias between the results might be due to matrix effects or inefficient atomization caused by occlusion of the analyte into the matrix. To investigate the aluminium effect over the analytical signals of elements, solutions containing 5 mg L1 Cr and 10 mg L1 Mn in 2.0% (v/v) HNO3 in absence and presence of increased

Fig. 2 Interference of Al on the analytical signals of Cr and Mn.


concentrations of AlIII (100 up to 1000 mg L1) was analyzed. The results are depicted in Fig. 2. It can be seen in this figure that the absorbance signals of Cr and Mn rise with the increase of the aluminium concentration. This effect was clearly more pronounced on the Mn absorbance signals and could be related to the spectral interference. As mentioned above, aluminium species, such as Al, Al2 and Al2O, can be simultaneously vaporized with the analytes species causing a nonspecific background absorption.11,12,36 The select wavelength of Mn (279.5 nm) is in the same absorption band region of the these aluminium species (235 to 280 nm). The analytical signals of Cr and Mn showed a difference between aqueous solution and alumina slurry, involving variations in the appearance time, absorbance and background signal profiles. Considering the high thermal stability of Al2O3 (mp 2054 C, bp 2977 C)39 and the relatively high temperature observed for Cr and Mn, the use of chemical modifier to increase the pyrolysis temperatures was not a good alternative. On the other hand, the possibility to achieve the dissolution of alumina to facilitate the release of elements and matrix evaporation showed to be the best choice. In order to decompose alumina to the formation of volatile compounds and minimize the interference observed over Cr and Mn atomization, NaF was investigated as the matrix modifier. For this purpose, aliquots of 20 ml of 0.2 mol L1 NaF solution obtained by the neutralization of 0.2 mol L1 of NaOH with 0.2 mol L1 HF, were co-injected with 20 ml of reference solutions and slurry. The hypothesis to explain action of NaF as a good matrix modifier is based on the fact that alumina is soluble as cryolitebase melt for the temperature range of 967 C to 1027 C.40 The first reaction that could be occurred is the formation of AlF3, as following equation:40 2NaF(l) + 34/3Al2O3 # Na2O.11Al2O3 + 2/3AlF3(s) The following reaction describes the formation of complex solutes proposed for the cryolite-base melt containing alumina:41 1/3Al2O3 + 4/3AlF3 + 2NaF # Na2Al2OF6 A step at 1000 C was optimized owing the formation of cryolite-base melt in situ onto the inner of the graphite platform using the heating program depicted in Table 3. The formation of cryolite-base in this condition was proved by X-Ray Diffraction. For this, 10 consecutive aliquots of 20 ml of 0.2 mol L1 NaF solution was co-injected with 20 ml of alumina slurry and the heating program (Table 3) was stopped at pyrolysis I step. Then, the graphite platform was cut off and analyzed by X-Ray diffraction. The compounds identified in this solid product were CaMg(CO3)2, SiO2, CaCO3, NbO0.4F2.6, Na5Al3F14, Al2O3, NbO2, AlF3 and NbC. For a suspension prepared using 50 mg/30 mL of Al2O3, a concentration of 0.2 mol L1 NaF was enough to reduce the aluminium interference and background signal. For higher concentration of NaF, intense background and blank signal were observed. Pyrolysis holding time was also optimized and 20 s was sufficient to ensure effectiveness of background correction. This journal is ª The Royal Society of Chemistry 2010

After the optimization of cryolite-base melt formation, pyrolysis and atomization temperatures were again optimized using 5 mg l1 Cr and 10 mg l1 Mn in 2.0% (v/v) HNO3 aqueous solution and Al2O3 slurry. Under of the compromise conditions, the best pyrolysis and atomization temperatures recommended were 1300 C and 2400 C, respectively. The final proposed heating program for the simultaneous determination of Cr and Mn in alumina slurries is showed in Table 3. Peak shapes and background absorption were also considered when choosing the furnace conditions. Typical atomization peak for Cr and Mn are shown in Fig. 3.

modifier and 0.2 mol L1 of NaF as matrix modifier. The slopes (b) and regression coefficients (r) observed for the calibration graphs were 0.00856 and 0.99952 s pg1 for Cr and 0.00692 and 0.98833 s pg1 for Mn, respectively. The characteristics masses of Cr (10 pg) and Mn (13 pg) were calculated from the calibration curves and based on the integrated absorbance. The limits of detection (LOD) were calculated considering the variability of 20 consecutive measurements of 2.0% w/v HNO3 as the blank solution, according to 3 sblk/b (sblk ¼ standard deviation of the blank and b ¼ calibration curve slope). The values obtained were 0.11 mg L1 (66 ng g1) for Cr and 0.17 mg L1 (102 ng g1) for Mn. The analytical frequency of the method was approximately 10 samples per hour. This estimation was made considering the surveys in triplicate, the heating program and time spent for the data acquisition. After 150 heating cycles performed with 33 mg of Al2O3 slurries, it was observed that a decrease of the Cr and Mn analytical signals of 18% and 12% with concomitant increase of the background signals occurred. Considering the simultaneous determination of Cr and Mn, it was possible to have up to 300 analytical results with the same atomizer, which lowers the costs associated with the replacement of graphite parts.

Analytical figures of merits

Analytical results

The non-linear calibration curves were obtained using aqueous solutions of Cr (0.5 to 10 mg L1) and Mn (5 to 50 mg L1) in 2% (v/v) HNO3 in presence of 350 mg of Nb (NbC) as platform

The trueness of the proposed method was checked by analysis of certified reference materials (Alumina Reduction Grade – 699 from NIST – National Institute of Standard and Technology).

Table 3 Heating program for the simultaneous determination of Cr and Mn by SIMAA-6000a Step

T/ C

Ramp/s

Hold/s

Ar flow/ml min1

Read

Drying I Drying II Pyrolysis I Pyrolysis II Atomization Cleaning

130 200 1000 1300 2400 2600

10 5 10 10 0 1

10 5 20 15 5 5

250 250 250 250 0 250

No No No No Yes No

a

Injection temperature 30 C.

Fig. 3 Absorbance signal for Cr (a–c) and Mn (b–d) using aqueous solution prepared in 2% v/v HNO3 (a–b) and Al2O3 slurry (c–d). All signals were obtained in presence of NbC and NaF modifiers.


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Table 4 Results for the simultaneous determination of Cr and Mn (n ¼ 3) in the Standard Reference Material (NIST 699 Alumina Reduction Grade) (% w/w) Cr2O3 Found value uncertainty Certified Values uncertainty

(% w/w) MnO 0.00016 0.00003 0.0002 0.0001

Results are presented in Table 4. The found Cr and Mn concentrations for the reference materials are in accordance to the acceptable range at 95% of the confidence level (Student’s t-test).

Conclusions The feasibility of using NbC as a platform modifier and NaF as a matrix modifier for the simultaneous determination of Cr and Mn in high purity alumina by slurry sampling and graphite furnace atomic absorption spectrometry has been demonstrated. The good results allow for the recommendation of the proposed method as a simple, rapid and accurate procedure for the routine determination of chromium and manganese. NbC showed to be a very good platform modifier that can be used also in solid sampling atomic absorption spectrometry (SS-GF AAS) and electrothermal vaporization (ETV). The tube lifetime was increased by 750% in relation to an pyrolytic carbon platforms, leading to a remarkable decrease in the variable analytical costs. The action of NaF as a matrix modifier was capital to decrease the complexity of the matrix and to facilitate the Cr and Mn release during the atomization.

Found value uncertainty Certified Values uncertainty

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

24 25 26 27

Acknowledgements We are grateful to Fundaçaõ de Amparo a Pesquisa do Estado de Saõ Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cientı´fico e Tecnol ogico (CNPq) for financial support. PVO is thankful to CNPq for the researchship provided and ALS is also thankful to FAPESP by the fellowship support.

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