Use of polytetrafluoroethylene slurry for silica matrix removal in ETAAS direct determination of trace cobalt and nickel in silicon dioxide powder Wang Fuyi, Jiang Zucheng* and Peng Tianyou Department of Chemistry, Wuhan University, Wuhan 430072, China Received 6th January 1999, Accepted 8th April 1999 A novel method for the direct determination of trace Co and Ni in SiO2 powder by slurry sampling ETAAS was developed. At high temperature of the graphite furnace, a PTFE slurry in HNO3 was used as a fluorinating reagent to convert the silica matrix into high-volatility fluoride, which was subsequently evaporated by selective vaporization prior to the atomization of analytes.In this case, the severe interference of the matrix on the vaporization and atomization of analytes was reduced significantly, and chemical attack of the excess of silica matrix on the graphite tube was also minimized. The proposed method was successfully applied to the determination of trace Co and Ni in SiO2 powder with aqueous calibration and minimum chemical pre-treatment.For the direct analysis of high purity SiO2 powder, the detect limits of Co and Ni were 18.8 and 35.0 ng g-1, respectively. The analysis of NIES CRM2 Pond Sediment confirmed the reliability of the approach. High purity siliceous materials are widely applied in optical fluorine content, moderate decomposition temperature, very low inorganic impurities, lack of corrosion and easy utilization.waveguide fibre, glass, semiconductor and ceramics manufac- In our more recent studies, a PTFE slurry was also used as a ture. Any trace impurities (such as Cu, Ni, Cr and Fe) in these chemical modifier for the determination of chromium and materials strongly influence the quality of the products.1,2 It cadmium by ETAAS.24–26 It was concluded that the addition is therefore desirable to be able to determine accurately and of PTFE could improve the sensitivity for the refractory quickly the contents of trace impurities in siliceous materials element chromium, increase the maximum ashing temperature for quality control.for the volatile element cadmium and eYciently reduce the Several techniques have been used to determine trace chemical and spectral interferences of co-existing components impurities in siliceous materials, such as inductively coupled in the determination of Cr and Cd. plasma atomic emission spectrometry (ICP-AES),3,4 induc- In this work, a PTFE slurry as a fluorinating reagent was tively coupled plasma mass spectrometry (ICP-MS)5,6 and employed to convert the silica matrix and analytes (Co and Ni) atomic absorption spectrometry (AAS).7,8 However, in most in SiO2 into fluorides of various volatilities at high temperature cases, chemical pre-treatment is necessary to remove the silica of the graphite furnace.The high-volatility silicon fluoride was matrix prior to the determination. Since chemical preevaporated prior to the vaporization and atomization of ana- treatment leads to a long analysis time and the risk of lytes.The action of PTFE not only minimized the deterioration contamination and loss of analyte, the direct analysis of solid of the graphite tube, but also reduced substantially the inter- siliceous materials has received increasing attention.9,10 ference of the silica matrix on the atomization of analytes. In In ETAAS, there are two approaches for the direct this case, the vaporization and atomization processes of analytes introduction of solid samples, i.e., direct solid sampling and were free from the matrix.Moreover, the eVect of particle size slurry sampling. Owing to its advantages, slurry sampling has on the analytical performance was also minimized. With cali- been widely applied for the practical analysis of solid bration using aqueous standards, the proposed method was samples.9,11 The main problems related to the direct analysis used to determine directly Co and Ni in SiO2 powder.Only of siliceous samples by slurry sampling ETAAS includes12–17 minimum chemical pre-treatment was necessary prior to the (1) the severe physical/chemical and/or spectral interferences measurements and the determined values were in good agree- resulting from the excess of silica matrix and (2) the rapid ment with those obtained by the standard additions method. deterioration of the graphite as a consequence of chemical NIES CRM2 Pond Sediment certified reference material with attack by silicon at high temperatures.The deleterious eVect a high silica content was used to evaluate this method. of silicon on the graphite tube could be minimized by adding graphite powder.18 In this case, the lifetime of the graphite tube was increased considerably. However, interference of the Experimental silica matrix on the vaporization and atomization of analytes Apparatus still exists and causes diYculties in the direct measurement of trace elements in siliceous samples.Therefore, during the A WFX-IF2 atomic absorption spectrometer with a deuterium preparation of slurry samples of siliceous materials, HF and/or background corrector and a WF-4A electrothermal graphite HNO3 are added to the slurries to remove partially the silica furnace system (Beijing Second Optics, Beijing, China) was matrices from samples.13–17 employed. The operating conditions for AAS and the graphite In recent years, polytetrafluoroethylene (PTFE) slurry as a furnace are given in Table 1.fluorination modifier has been successfully applied to electrothermal vaporization (ETV) ICP-AES for the determination Standard solutions and reagents of refractory elements (such as rare earth elements, Ti and Mo) in high purity materials and biological and environmental Stock standard solutions (1 mg mL-1) of cobalt and nickel samples.19–23 Compared with the general fluorinating reagent were National Reference Solutions (10% HCl, GSBG 62021–90 and GSBG 62022–90) from the National Center for HF, the PTFE slurry has the special advantages of high J.Anal. At. Spectrom., 1999, 14, 963–966 963Table 1 Operating condition for ETAAS Wavelength/nm Co 240.7; Ni 232.0 Spectral bandwidth/nm 0.4 HCL current/mA Co 2.5; Ni 3.5 Deuterium lamp current/mA (Co) 42; (Ni) 45 Graphite furnace— Temperature/ °C Ramp/s Hold/s Dry 120 20 30 Ashing 1200 10 20 Atomization 2700 0 10 Flow rate of argon 500 mL min-1; 100 mL min-1 during atomization Analysis and Testing of Steel Materials.Working standard solutions were prepared by diluting these stock standard Fig. 1 (I ) Ashing curves and (II ) atomization curves of Co and Ni solutions. A 60% m/v PTFE slurry (Shanghai Organic in aqueous solutions and PTFE slurries. Chemistry Institute, Shanghai, China) was commercially available. A 0.5% m/v aqueous solution of plant glue (kindly EVect of silica matrix on the analytical signals provided by Professor Zhang Xiangcha) was employed as a stabilization reagent during the preparation of slurry samples Fig. 2 shows the dependence of the Co and Ni absorption of SiO2 powder. Nitric acid (suprapure) and silicon dioxide signals in suspensions on the content of SiO2. It can be seen (suprapure) were supplied by Shanghai reagent factory that without PTFE the silica matrix produced an increasing (Shanghai, China) and silicon dioxide powder for analysis by suppression of the analytical signals when its concentration Wuhan Industry University (Wuhan, China).Doubly distilled was increased. However, this matrix eVect could be reduced water was used throughout. significantly by adding the PTFE slurry. In the presence of 6% m/v PTFE the analytical signals remained constant until Slurry sample preparation the content of SiO2 was higher than 15 mg mL-1. Typical signal profiles of Co and Ni in aqueous standard Portions (5–100 mg) of SiO2 powders (particle size 74–97 mm) solutions and in SiO2 slurries are shown in Fig. 3. The resultant were transferred into 5 ml calibrated flasks and 0.5 mL of 60% graphs indicated that (1) without PTFE, the silica matrix not PTFE slurry and 0.4 mL of nitric acid (1+1) were added in only delayed the appearance time of absorption peaks, but turn, then diluted to 5 mL with a 0.1% aqueous solution of also caused bi-peak profiles with lower absorption signals plant glue.The resulting mixtures were dispersed with an [Fig. 3(A) and (B)] and (2) the addition of PTFE greatly ultrasonic wave vibrator for 20 min. The flasks were shaken changed this situation, similar profiles and peak heights being vigorously prior to any analysis. obtained for Co and Ni either in aqueous standard solutions For a calibration with the standard additions method, the or in SiO2 slurries [Fig. 3(C) and (D)]. In other words, the slurries prepared as described above were spiked with approvaporization and atomization behaviours of Co and Ni in priate amounts of aqueous standard solutions.In the resulting both cases were coincident. This result provided the possibility standard series the content of SiO2 powder was 12 mg mL-1 of the determination of Co and Ni in SiO2 slurries with and spiked Co and Ni ranged from 0.05 to 0.3 and from 0.1 calibration using aqueous standards. to 0.6 mg mL-1, respectively. For calibration against aqueous standards, the aqueous standards, in which the concentrations Influence of ashing time on analytical signal and background of Co and Ni varied from 0.1 to 0.3 and 0.2 to 0.6 mg mL-1, respectively, were prepared as slurries containing 6% PTFE At an ashing temperature of 1200 °C, the influence of ashing and 2% HNO3 by the procedure mentioned above.time on the intensities of the analytical and background signals were investigated and the results are shown in Fig. 4. It can Recommended procedure be seen that the analytical signals slowly increase with increase in ashing time and reach a maximum up to 20 s and then A 20 mL volume of solution or slurry samples was deposited remain constant in the range 20–50 s, whereas the background on the wall of the pyrolytic graphite-coated graphite tube with intensities do not change when the ashing time is shorter than a micropipette.After being dried and ashed, the analyte was 50 s. Hence a 20 s ashing time was selected for the direct vaporized and atomized.The absorption signals of the analyte analysis of SiO2 powder. and background were collected simultaneously by the incorporated microcomputer and the dependence of the integrated absorbance on atomization time was then plotted graphically. The peak height was used for quantification. Results and discussion Influence of PTFE on the vaporization and atomization processes of Co and Ni The ashing curves and atomization curves of Co and Ni with and without 6% PTFE were investigated and are presented in Fig. 1. Under the same atomization conditions the introduction of PTFE in 2% HNO3 slightly reduced the absorption signals; PTFE had no obvious influence on the maximum ashing temperature and the minimum atomization temperature of Co and Ni. Therefore, similar ashing/atomization curves were observed in the absence and presence of PTFE. An ashing temperature of 1200 °C and an atomization temperature of Fig. 2 Influence of concentration of silicon dioxide in slurries on cobalt and nickel absorption. 2700 °C were selected for subsequent experiments. 964 J. Anal. At. Spectrom., 1999, 14, 963–966Fig. 3 Typical signal profiles of Co and Ni in (a) aqueous solutions and (b) SiO2 suspensions in the (A, B) absence and (C, D) presence of PTFE. The values in parentheses are the peak areas. Co, 3 ng; Ni, 5 ng; SiO2, 120 mg; PTFE, 6%. Mechanism of suppression of matrix interference by selective vaporization at an appropriate ashing temperature prior to the vaporization and atomization of the Co and Ni It has been reported12 that the silica matrix caused a very analytes.In this case, the analytes (Co and Ni) could be strong background at a wavelength of 240 nm when the atomized freely from the silica matrix, and their atomization atomization temperature was higher than 2350 °C. This back- behaviours are similar to those in aqueous solutions (Fig. 3). ground absorption may be caused by SiO molecules evaporat- On other hand, since most of the silica matrix is removed prior ing at high temperature. In the present study, the strong to atomization of the analytes, the deleterious eVect resulting background absorption mentioned above was also observed from chemical attack of the silica matrix at high temperature at 240.7 nm on evaporating an SiO2 suspension under the is also minimized considerably.conditions given in Table 1. However, the strong signal was almost eliminated on adding 6% PTFE to the slurries (Fig. 5). EVect of particle size on the analytical signals Previous work19,20 indicated that at the high temperatures oVered by a graphite furnace, a PTFE slurry could convert the The particle size of the solid materials used to prepare a slurry can influence the stabilization, deposition and atomization silica matrix and analytes into fluorides of various volatilities. Among the fluorides, silicon fluoride (bp -86 °C) is one of the eYciency of the sample, which in turn can influence both accuracy and precision.9 In this paper, the eVects of the particle most volatile compounds and could be evaporated prior to the vaporization and atomization of most of analytes by selective size of the SiO2 powder on the relative absorbance of Co and Ni in SiO2 slurries with PTFE were investigated, and the vaporization.The results presented in Fig. 5 again confirm that the silica matrix could be converted by the fluorinating reagent results are given in Table 2.The results obtained indicate that PTFE into high-volatility fluoride and subsequently removed Fig. 5 Signal profiles of silica background absorption in the (a) absence and (b) presence of PTFE. During the atomization step, the Fig. 4 Dependence of Co and Ni absorption and background signals flow of argon was stopped. SiO2, 120 mg. The operating conditions are given in Table 1. in SiO2 slurries on ashing time. J. Anal. At. Spectrom., 1999, 14, 963–966 965Table 2 EVect of particle size of SiO2 powder on analytical signals Particle size/mm >300 300–200 200–150 150–125 125–97 97–74 <74 Relative abs.a of Co 0.80±0.19b 0.85±0.12 0.89±0.069 0.91±0.059 0.96±0.041 0.98±0.028 1.00±0.025 Relative abs.a of Ni 0.63±0.14 0.60±0.10 0.86±0.072 0.82±0.054 0.90±0.020 0.95±0.030 0.99±0.024 aThe relative absorbances of Co and Ni in PTFE slurries are 1.00 without SiO2; the concentration of SiO2 in suspensions is 12 mg mL-1; Co and Ni are 0.15 and 0.25 mg mL-1, respectively.bMean±SD (n=3). Table 3 Results for the determination of Co and Ni in silicon dioxide the recoveries are higher than 90% with RSDs of 2.0–5.9% powder and NIES CRM2 when the average particle size of the sample is smaller than 125 mm. This tolerable particle size is as large as four times Cobalt/mg g-1a Nickel/mg g-1a that reported in the literature.13,14 Obviously, the decreased influence of particle size on the analytical signals is attributed Aqueous Standard Aqueous Standard calibration additions calibration additions to the chemical modification of PTFE.It is PTFE that converts the silica matrix into volatile fluoride, which makes the silica Silicon dioxide 2.92±0.18 3.13±0.15 8.39±0.32 8.10±0.52 matrix evaporate prior to the atomization of analytes and NIES CRM2 25.27±2.20 —b 40.87±1.84 —b allows the analytes to be atomized under the conditions of aMean±SD (n=6). bCertified values: Co 27±3 mg g-1; Ni less matrix.However, when the average particle size of the 40±3 mg g-1. sample is larger than 125 mm, the recoveries of the analytes gradually decrease owing to slower vaporization of the silica matrix as a consequence of incomplete fluorination. Meanwhile, the RSDs slowly increase to 14–19% because of References the poorer pipetting eYciency. Therefore, the particle size of 1 J.W. Mitchell, Pure Appl. Chem., 1982, 54, 819. SiO2 powder should be smaller than 125 mm for direct analysis, 2 J.Dolezal, J. Lenz and Z. Suleck, Anal. Chim. Acta, 1969, 47, 517. which is easily met in practical analysis. 3 M. T. Larrea, I. Grome-Pinilla and J. C. Farinas, J. Anal. At. Spectrom., 1997, 12, 1323. Sample analysis 4 S. Mann, D. Geilenberg, J. A. C. Brockaert and M. Jansen, J. Anal. At. Spectrom., 1997, 12, 975. With either aqueous standards or the standard additions 5 G. Tsoupras, Analusis, 1996, 24(9/10), M23. method for calibration, the contents of trace Co and Ni in 6 H.Naka and H. Kusayssu, Bunseki Kagaku, 1996, 45, 1139. SiO2 powder were determined and the results are given in 7 X. C. Zhou, F. Y. Wang and J. Y. Lihua, Huaxue Fence, 1997, 33(5), 207. Table 3. It is found that the results obtained by above methods 8 X. H. Wen, L. Z. 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The tolerable particle size of solid samples in this method is as large as four times the value in the literature. Paper 9/00198K 966 J. Anal. At. Spectrom., 1999, 14, 963&nda