Anal. Chem. 1999, 71, 849-854
Multielement Analysis of Graphite and Silicon Carbide by Inductively Coupled Plasma Atomic Emission Spectrometry Using Solid Sampling and Electrothermal Vaporization Uwe Scha 1 ffer and Viliam Krivan*
Sektion Analytik und Ho¨ chstreinigung, Universita¨ t Ulm, D-89069 Ulm, Germany
An improved graphite furnace electrothermal vaporization device, equipped with an autosampler system for precise and almost contamination-free introduction of solid samples, was used for the simultaneous determination of Al, Ag, As, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Na, Ni, and Pb in graphite and silicon carbide by inductively coupled plasma atomic emission spectrometry. The volatilization of most analytes could be significantly improved by addition of Freon 1,2 to the argon carrier gas. Calibration was performed using aqueous standard solutions. Owing to the extremely low blanks, large applicable sample portions (up to 16 mg), high transport efficiency, and the addition of Freon 1,2, extraordinarily low limits of detection in the range 5-250 ng/g were achieved. The accuracy was checked by comparison of the results with those obtained by various other methods. For most analytes in both matrixes, the comparison led to very good agreement of the results. Owing to its unique chemical and physical properties, graphite, especially synthetic graphite obtained by high-temperature treatment of carbons, has become an important material in various technological fields.1-4 Due to the inertness to irradiation and chemicals, graphite is used as moderator and structural material for nuclear reactors. The highly refractory character combined with the high electrical and thermal conductivity made high-purity graphite indispensable as a material for crucibles in various analytical instruments, for electrodes, insulators, conductors, and high-resistance engineering parts in space and aeronautic technology. Due to its lubrication properties, graphite is widely used as a friction modifier in breaking aggregates and carbon brushes of electric motors. It is also the outgoing material for a number of modern high-tech materials such as carbon fibers, carbon-resin bonded composites, hard metals, lightweight alloys, and highperformance carbon ceramics. (1) Criscione, J. M.; Volk H. F.; Smith, A. W. AIAA J. 1966, 4, 1791-1797. (2) Vohler, O.; von Sturm, F.; Wege, E. Ullmann’s Encyclopedia of Industrial Chemistry; VCH Verlagsgesellschaft: Weinheim, 1986; Vol. A5, pp 98-158. (3) Taylor, H. A., Jr.; et al. Kirk-Othmer: Encyclopedia of Chemical Technology; John Wiley & Sons: New York, 1991; Vol. 4, pp 949-1116. (4) Bo¨gershausen, W.; Cicciarelli, R.; Gercken, B.; Ko¨nig, E.; Krivan, V.; Mu ¨ llerKa¨fer, R.; Pavel, J.; Seltner, H.; Schelcher, J. Fresenius’ J. Anal. Chem. 1997, 357, 266-273. 10.1021/ac980821a CCC: $18.00 Published on Web 01/20/1999
© 1999 American Chemical Society
Silicon carbide, which is produced technically from quartz and carbon at temperatures of 2000-2300 °C, has become one of the most important ceramic materials for modern industrial applications.5-7 Due to its high thermal strength and refractory character, silicon carbide is widely used as a construction material for engines, turbines, heat-transfer systems, and nuclear and fusion reactors, as semiconductors at higher temperatures, and for numerous other special purposes. The growing application of these high-tech materials in various fields of technology and science enhances the demand for strictly controlled properties which are often directly correlated to the contents of trace impurities.4,8,9 Therefore, powerful, rapid, and reliable analytical methods are required for trace characterization of these materials. The analysis of solid high-purity materials with conventional solution methods requires the decomposition of the samples. This leads to the introduction of considerable blanks, especially for the ubiquitous elements, to high time consumption, and often also to sample dilution, worsening the limits of detection (LODs). For analysis of graphite by solution electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma atomic emission and mass spectrometries (ICP-AES and ICPMS), several procedures have been reported for wet digestion with acids using open, closed pressure, or microwave oven techniques.4,10-13 However, with most of these common decomposition techniques, no complete digestion of graphite could be achieved. Therefore, direct instrumental methods, such as glow discharge emission and absorption spectrometry (GD-AES and GD-AAS),14,15 direct (5) Salmag, H.; Scholze, H. Keramik, 6th ed.; Springer: Berlin, 1982/83; Parts 1 + 2. (6) Broekaert, J. A. C.; Graule, T.; Jenett, H.; To ¨lg, G.; Tscho¨pel, P. Fresenius’ J. Anal. Chem. 1989, 332, 825-838. (7) Dienst, W. KFK-Nachr. 1989, 21, 245-252. (8) Whalen, T. J. Cer. Eng. Sci. Proc. 1986, 5, 1135-1143. (9) Schwier, G. Mater. Tech. (Paris) 1983, 71, 179-183. (10) Hashitani, H.; Yoshida, H.; Adachi, T.; Izawa, K. Bunseki Kagaku 1986, 35, 911-915. (11) Kawakami, O.; Takeya, M.; Sayana, Y. Abstracts of 51st Analytical Chemistry Symposium; The Japan Society for Analytical Chemistry: 1990; pp 135136. (12) Watanabe, K.; Takashima, K. Abstracts of 51st Analytical Chemistry Symposium; The Japan Society for Analytical Chemistry: 1990; pp 431-524. (13) Koshino, Y.; Narukawa, A. Analyst 1993, 118, 827-830. (14) Pan, C.; King, F. L. Appl. Spectrosc. 1993, 47, 2096-2101. (15) Pan, C.; King, F. L. Appl. Spectrosc. 1993, 47, 300-304.
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current arc emmision spectrometry (dc-AES),4,16-18 and instrumental neutron activation analysis (INAA),4,19,20 have frequently been employed. However, with GD and dc spectrometric methods, good accuracy can be achieved only by using solid graphite standards for calibration. INAA is, due to high accuracy achievable even at extremely low concentrations, a very important reference method, but it is not suitable for routine analysis. Slurry sampling (SlS) has proved to be a very useful sample introduction technique for ETAAS, avoiding the problems occurring with decomposition, but it is applicable only to analysis of fine powders from which stable suspensions can be prepared.21 For silicon carbide, fusion with various oxidative salt mixtures has been the most frequently used digestion technique.22 However, the application of fusion digestion to trace analysis of silicon carbide is seriously limited by the introduction of blank and high salt content, which usually give rise to interference and/or loading effects in the plasma. Complete decomposition of silicon carbide was achieved with a mixture of concentrated hydrofluoric, nitric, and sulfuric acids (1:1:1) by heating for 12 h at 240 °C in a closed system.23 Silicon carbide is a very favorable matrix for INAA,23-25 too, but as mentioned for graphite, this method is unsuitable for routine analysis. For further processing to the final products, silicon carbide is usually required in the form of a fine powder. Thus, sample introduction can be accomplished by slurry sampling to avoid sample decomposition.26,27 Because the digestion of these materials is extremely difficult, the availability of a solid sampling technique for a multielement determination method would significantly improve the present state of development of the analytical methodology for these materials. In this work, a method for direct analysis of graphite and silicon carbide using solid sampling combined with an improved electrothermal vaporization (ETV) device for ICP-AES is described. EXPERIMENTAL SECTION Samples and Reagents. The investigated graphite sample was supplied by Lonza G+T Ltd. (Sins, Switzerland) as material Type PD5. The median particle size was about 20 µm, but the grain shape was irregular and scaly.21 The sample was well characterized in an interlaboratory collaborative study.4 The silicon carbide powder, Type S 933, was obtained from Elektroschmelzwerk Kempten (Kempten, Germany). The typical average particle diameter was at the submicrometer level, and the particle size did not exceed 5 µm.27 (16) Gorbunova, L. B.; Kuteinikov, A. F.; Avdeenko, M. A.; Murashkina, V. N. Zavod. Lab. 1975, 41, 178-179. (17) Maillard, P.; Ades, C. Method Phys. Anal. 1968, 4 (3), 262-267. (18) Schroll, E.; Huber-Schausberger, I.; Janga, I.; Spatzek, H. Mikrochim. Acta 1968, 3, 649-659. (19) May, S.; Pinte, G. J. Radioanal. Chem. 1969, 3, 329-343. (20) Mukhamedshina, N. M.; Yankovskii, A. V. Zavod. Lab. 1972, 38, 10991101. (21) Scha¨ffer, U.; Krivan, V. Spectrochim. Acta B 1996, 51, 1211-1222. (22) Docekal, B.; Broekaert, J. A. C.; Graule, T.; Tscho ¨pel, P.; To ¨lg, G. Fresenius’ J. Anal. Chem. 1992, 342, 113-117. (23) Franek, M.; Krivan, V. Fresenius’ J. Anal. Chem. 1992, 342, 118-124. (24) Marunina, N. I.; Chernova, A. I.; Bogatikov, B. F. Zh. Anal. Khim. 1976, 31, 1146-1149. (25) Vandergraaf, T. T.; Wikjord, A. G. Rep. At. Energy Can. 1973, AECL-4581, 1-19. (26) Barth, P.; Krivan, V. J. Anal. At. Spectrom. 1994, 9, 773-777. (27) Docekal, B.; Krivan, V. J. Anal. At. Spectrom. 1992, 7, 521-528.
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Standard stock solutions in nitrate form (1.000 ( 0.002 g/L) obtained either from Merck (Darmstadt, Germany) or from Alfa (Karlsruhe, Germany) were used for the preparation of multielement standard solutions. Twice-distilled water was used for dilution. The nitric acid of “pro analysi” grade, supplied by Merck, was additionally purified by subboiling distillation. Argon of 4.6 grade for the ICP and of spectrometric grade for the ETV were supplied by MTI Industriegase AG (Elchingen, Germany), and Freon 1,2 was obtained from Linde AG (Mu¨nchen, Germany). Instrumentation. A GF-ETV system, model KS 10, from Ingenieurbu¨ro Schuierer (Mu¨nchen, Germany) was coupled to an ICP-AE spectrometer, JY 70 Plus, from Jobin Yvon (Longjumeau, France). The instrumental setup was recently described in more detail elsewhere.28 The addition of Freon 1,2 to the carrier argon flow was performed by means of a flow meter, type RAGL 41, obtained from Rota Yokogawa (Wehr, Germany). Procedure. The entrance slit of the polychromator was calibrated using a Cu hollow cathode lamp (λ ) 324.754 nm). The monochromator was set to the peak maximum of the emission line of a Cr hollow cathode lamp (λ ) 267.716 nm). The platform was cleaned by running the furnace program with no sample. After cooling, the platform was transported by the autosampler to the built-in microbalance for taring. Sample portions between 5 and 12 mg of graphite powder and between 2 and 16 mg of silicon carbide powder were loaded manually onto the platform by means of a tantalum spatula. After weighing, the platform with the sample was inserted into the graphite furnace, the shutter was closed, and the furnace program was started. After each analysis cycle, the platform was taken out of the furnace, and the sample residue was removed by blowing it out with an air flow. Except for loading the sample on the platform and removing the residue, all these operations were performed automatically by the autosampler. To elucidate the effect of a halogenating reagent, the above procedure was performed also with addition of 3 mL/min Freon 1,2 to the argon carrier gas. The blank values were determined by carrying out seven complete analysis cycles, including taking out the platform from the graphite furnace, its transport to and from the balance, and its introduction into the furnace, but without application of any sample. For calibration, a multielement aqueous standard solution with appropriate analyte concentrations was mixed from standard stock solutions. Aliquots of 5, 10, and 15 µL of this multielement standard solution were pipetted onto the platform and measured three times each using the same parameters as for the samples. The optimized operating parameters and experimental conditions used are listed in Tables 1 and 2. RESULTS AND DISCUSSION The analysis of graphite and silicon carbide reported in this work represents the first application of an improved graphite furnace ETV device for ICP-AES we described recently.28 The reported novel interface, designed on the basis of a radiotracer investigation undertaken for the analytes As, Co, Cr, Cu, Fe, Ga, (28) Scha¨ffer, U.; Krivan, V. Anal. Chem. 1998, 70, 482-490.
Table 1. Operating Parameters and Experimental Conditions Used for ICP-AES observation height (above the coil) plasma gas (argon) intermediate gas (argon) rf power element
Emission Lines Used wavelength (nm) element
Ag Al As Bi Ca Cd Co Cra Cu a
12 mm 14 L/min 0.3 L/min 1000 W
328.068 308.215 193.695 306.772 317.933 226.502 228.616 267.716 324.754
wavelength (nm)
Fe Ga K Li Mg Na Ni Pb
259.940 294.364 766.490 670.776 279.079 589.592 231.604 220.353
Detection with monochromator.
Table 2. Operating Conditions of the Electrothermal Vaporization Device temperature (°C) step
time (s)
dry 1 dry 2 ash vaporize 1a max power vaporize 2 cool
30 25 5 5 0.6 10 40
a
no Freon
with Freon
100 130 800 1700
100 130 600 1200
2600 10
2200 10
carrier flow bypass flow (mL/min) (mL/min) 500 700 400 400 400 400 300
30 300 300 300 300 300 30
Data acquisition started for 15 s.
K, and Na, allowed the admittance of cool bypass gas along the tube wall. By this means, a significant enhancement of the total transport efficiency of the analytes, ranging from 26% to 57%, was achieved without the addition of any carriers. Sample volumes up to 30 µL can be applied. Using the autosampler, simple, fast, reproducible, and almost contamination-free sample introduction is possible. In this work, Freon 1,2 was used as a halogenating reagent to improve the analyte release from the matrix and to match the vaporization behavior of the analytes from the matrix and from aqueous standard solutions used for calibration. Thus, utilization of this ETV system for simultaneous multielement analysis of the analytically difficult matrixes graphite and silicon carbide by ICPAES should lead to an essential improvement of performance, allowing the complete release of the analytes from the matrixes to be achieved. Utilization of Freon 1,2 as Modifier. Gaseous halogenating reagents such as Freon 1,2 have been used as modifiers in ETV to improve the volatilization behavior of some analytes.29-36 Freon (29) Kirkbright, G. F.; Snook, R. D. Anal. Chem. 1979, 51, 1938-1941. (30) Kantor, T. Spectrochim. Acta B 1988, 43, 1299-1320. (31) Kantor, T.; Zaray, Gy. Fresenius’ J. Anal. Chem. 1992, 342, 927-935. (32) Zaray, Gy.; Kantor, T.; Wolff, G.; Zagorska, Z.; Nickel, H. Mikrochim. Acta 1992, 107, 345-358. (33) Zaray, Gy.; Leis, F.; Kantor, T.; Hassler, J.; To ¨lg, G. Fresenius’ J. Anal. Chem. 1993, 346, 1042-1046. (34) Ren, J. M.; Salin, E. D. Spectrochim. Acta B 1994, 49, 555-566.
Figure 1. Emission signals obtained for Li with and without the addition of Freon in processing (a) aqueous standard solution and (b) graphite sample.
1,2 decomposes at temperatures higher than 700 °C, forming CF2 radicals, CF3Cl, CF4, C2F4, and Cl atoms. Therefore, at these temperatures, Freon 1,2 is able to react with most of the analyte elements to form chlorinated compounds which are much more volatile than the elemental species. By this means, the vaporization temperature can be considerably reduced, the release of the analytes from the samples can be improved, and, in some instances, the reaction of carbide-forming elements with the furnace material can be diminished. This should lead to an improved transport efficiency and, thus, to a higher sensitivity. Indeed, for the elements Al, As, Ca, Co, Cu, Fe, K, Li, Na, and Ni, the addition of Freon 1,2 led to a considerable improvement of sensitivity. Taking the integrated peak areas obtained without addition of Freon as 100%, improvements from about 20% (Al) up to ∼300% (Li) were obtained in processing aqueous standard solutions. As an example, Figure 1a shows the emission signals obtained on processing an aqueous solution of Li with and without addition of Freon. The normalized integrated area of the signal measured in the absence and in the presence of Freon was found to be 48.8 and 196 (mV‚s)/ng, respectively. For Ag, Ga, Mg, and Pb, no significant change in sensitivity was observed. Unexpectedly, for Bi and Cr, the addition of Freon 1,2 caused a decrease of sensitivity. The reduction of the sensitivity of Bi by about 30% is probably caused by loss during the thermal pretreat(35) Ren, J. M.; Salin, E. D. Spectrochim. Acta B 1994, 49, 567-575. (36) Alaray, J.-F.; Hernandez, G.; Salin, E. D. Appl. Spectrosc. 1995, 49, 17961803.
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Figure 2. Emission signals obtained for (a) Ca, (b) Fe, and (c) Ni in processing graphite with and without the addition of Freon 1,2. Table 3. Results Obtained in Analysis of Graphite Using SoS-ETV-ICP-AES with and without Freon 1,2 (Mean of Seven Replicates ( 1 SD) and Comparison with Those of Other Methods content (µg/g) SoS-ETV-ICP-AES
a
element
without Freon
with Freon
Al Ag As Bi Ca Co Cr Cu Fe Ga K Lia Mg Na Ni Pb
4.2 ( 0.6
