Continuous Sample Introduction with Graphite Atomization Systems for Atomic Absorption Spectrometry Tibor Kantor’, S. A. Clyburn2, and Claude Veillon3 Defiartment of Chemistr,v, University of Houston, Houston, Texas 77004
A heated graphite furnace atomization system is described for use in atomic absorption spectrometry. The furnace is operated continuously, and desolvated sample aerosol is introduced continuously. Graphite components are coated with a layer of pyrolytic graphite for reduced porosity and increased durability. Gradual deterioration due to oxidation is completely compensated for by a continuous pyrolysis treatment. With continuous sample introduction and furnace operation, several beneflts are realized. Reproducibility of measurement is improved several-fold. A relatively simple furnace power supply can be used. Transient signals need not be dealt with. Steady-state conditions for the vaporization and atomization processes prevail, and these can be investigated under controlled conditions and temperatures. Thus, matrix effects can be studied in detail and the results of these studies used to advantage.
The earlier nonflame atomization systems which employed continuous sample introduction, such as the dc arc plasma jet (1, 2), rf plasma torch (3, 4 ) , and cascade arc ( 5 , 6) have proved to be effective primarily in atomic emission spectroscopy (6, 7). Woodriff and Ramelow (8) first used continuous sample introduction with a graphite furnace for atomic absorption measurements. Recently, an electricallyheated platinum tube furnace has been used by Black e t al. ( 9 ) for atomic fluorescence spectrometry, and Murphy et al. (IO ) developed a graphite tube furnace in this laboratory for the same purpose. This furnace system has been used with continuous sample introduction for atomic fluorescence spectrometry with a single continuum source and is described elsewhere 1\11 ). The graphite furnace atomization system used in this investigation was designed for the purpose of measuring atomic absorption with continuous sample introduction. For long-term stability of the atomization system when operating for long periods a t high temperature, the heated el-
’ Present address, Inijtitute for General and Analytical Chemis-
try, Technical University, Budapest 1111,Hungary. Present address, k‘arian Instrument Div., Los Altos, Calif. 94022. Present address, Biophysics Research Laboratory, Harvard Medical School, Peter Bent Brigham Hospital, Boston, Mass. 02115. Author to whom reprint requests should be sent.
(1) M. Margoshes and B. F. Scribner, Spectrochim, Acta, 15, 138 (1969). (2) S.E. Valente and W. G. Schrenk, Appl. Spectrosc., 24, 197 (1970). (3) R . Mavrcdineanu and R. C. Hughes, Spectrochim. Acta, 19, 1309 (1963). (4) G. W. Dickinson and V. A. Fassel, Anal. Chem., 41, 1021 (1969). (5) M.Riemann, Fresenius’ 2.Anal. Chem.. 215, 407 (1965). (6) M. Marinkovic and T. J. Vickers, Appl. Spectrosc., 25, 319 (1971). (7) C. Veillon and M. Margoshes, Spectrochim. Acta, Parf B, 23, 503 (1968). (8)R. Woodriff and G. Ramelow, Spectrochim. Acta. Part B, 23, 665 (1968). (9) M. S. Black, T. H. Glenn, M. P. Bratzel, and J. D. Winefordner, Anal. Chem., 43, 1769 (1971). (10) M. K. Murphy, S. A. Clyburn, and C. Veilton, Anal. Chem., 45, 1468 (1973). (11) S. A. Clyburn. 6 .R. Bartschmid, and C. Veillon Anal. Chem., 46, 2201 ( 1 974)
ements must not appreciably change because of oxidation and/or sublimation of the graphite. In other words, the voltage-current-temperature characteristics should be constant for an indefinite period. This stability was readily achieved with the methane pyrolysis treatment described elsewhere (12). There are several important considerations in the use of nonflame atomization systems. These include sensitivity, reproducibility, and matrix effects. Sensitivity is perhaps the least important consideration because with present systems, determinations of many elements are possible in the ng to pg range, and very careful sample handling is required to work a t these levels. The system used in this investigation exhibited absolute detection limits (based on a 1-ml sample volume) usually within an order of magnitude of the conventional method employing discrete, small-volume samples (spike method). With conventional graphite atomization systems, which must employ the spike method, the reproducibility is frequently on the order of 7-lo%, and even lower values are claimed in many cases. However, the authors were unable to obtain reproducibilities better than 7% under any conditions, using 10-pl autopipets with a measured reproducibility of 1%. With continuous sample introduction, the reproducibility was in the 2-4% range. With the spike method, one is dealing with a transient signal and atomization conditions, (i.e., temperature) which contribute to the poor reproducibility and matrix effects, respectively, observed in many cases. Matrix effects and other high temperature reactions and equilibria can best be studied under steadystate conditions, which are best achieved by continuous sample introduction into an atomization cell operating a t constant temperature and conditions. The system described here has considerable potential for this purpose, once the gas temperature in the furnace is accurately known.
EXPERIMENTAL Measurement System. A conventional atomic absorption instrumental set-up was used. Commercially available single-element hollow cathode lamp sources were operated a t approximately one-half their maximum rated current. Fused silica lenses (25-mm diameter X 100-mm focal length) were used to focus an unmagnified image of the hollow cathode on the graphite cuvette opening and then onto the monochromator entrance slit. Source radiation was modulated a t 330 Hz by a mechanical chopper, detected, amplified by a phase-sensitive (lock-in) amplifier tuned to the chopping frequency (Model 120, Princeton Applied Research, Princeton, N.J.) and displayed on a 25-cm potentiometric recorder. A 1-m Czerney-Turner grating monochromator (Model 1704, Spex Industries, Metuchen, N.J.) was used in this study and had a reciprocal linear dispersion of 8 A/mm in the first order. For all of the measurements, a monochromator spectral bandwidth of 0.5 A was employed. In practice, the wavelength setting was peaked with equal entrance and exit slit widths (0.05 mm) and then the exit slit opened to 0.06 mm to minimize any drift in the wavelength setting. Sampling and Gas Handling Systems. These are shown schematically in Figure 1. A Veillon-Margoshes sample introduction
(12) S . A. Clyburn, T. Kantor. and C. Veillon, Anal. Chem., 46, 2213 (1974).
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Figure 1. Schematic of the sample introduction, gas handling, and atomization systems
(1) Pneumatic nebulizer, (2) heated glass spray chamber, (3) Friedrich condenser (modified),(4) graphite tube furnace, (5)sheath gas duct, (6) regulation and measuring devices for nebulizer and sheath gases, and (7) regulation and measuring devices for methane
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Figure 3. Furnace wall temperature (circles)and applied voltage (triangles) as a function of applied power 0, V ,argon: 0 , V,nitrogen
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Figure 2. Graphite furnace for continuous sample introduction
system (13) was used to feed desolvated sample aerosol continuously into the graphite atomization cell (see below). Rotametertype meters were used to monitor the gas flow rates, which were adjusted by needle valves in series with (and downstream from) the calibrated rotameters ( 1 4 ) . As shown, methane in any proportion can be introduced into the argon or nitrogen gas streams fed to the cuvette interior (sample) or exterior (sheath). In practice, the valve controlling the flow to the nebulizer is usually opened completely so that the nebulizer orifice controls the gas flow rate through the sample introduction system. Further details and operating conditions of the system are indicated elsewhere (11, 12) and discussed below. Graphite Atomization System. The slot-rod atomization device described previously (10) was modified for this study. Dimensions were enlarged and a clamping arrangement was provided to hold the cuvette between two graphite tubes in spring tension, much like the Varian Techtron Model 63 system. As shown in Figure 2, the electrode clamps are water-cooled, as is the upper portion of the sheath gas duct. A quartz chamber with appropriate openings is placed over the graphite elements and simply rests on the sheath gas duct. This easily removable chamber greatly reduces entrained air reaching the heated elements. Sheath gas is introduced into the bottom of the duct, which is filled with small graphite rings. These rings serve two purposes: to shield the duct from the intense radiation when operating a t high temperatures, and to render the sheath gas flow laminar. All graphite parts were machined from spectrographic purity rod stock (Grade UF4S, Ultra Carbon Corp., Bay City, Mich.). The protective sleeves are 12.7-mm 0.d. and the cuvette and inlet tubes
(13) C. Veillon and M. Margoshes, Spectrochim. Acfa, Part 6, 23, 553 (1968). (14) C. Veillon and J. Y. Park, Anal. Chem., 42, 684 (1970).
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are 7.9-mm 0.d. The i.d. of the cuvette and the large portion of the inlet tubes is 5.2 mm, while the small portion of the inlet tubes has an i.d. of 2.5 mm. The graphite aperture stop (5.0 mm) determined the primary source beam diameter and prevented wall radiation from reaching the entrance slit. The purpose of the sleeves is to protect that region of the inlet tubes from oxidation by air diffusion in the relatively cool region near the clamps and these sheath openings. The temperature in this region (600-900 "C) is sufficient for gradual oxidation of the graphite but not high enough for the deposition of a protective layer of pyrolytic graphite (see below). Power Supply. Since the atomization system is operated continuously a t the desired temperature, a simple high-current, lowvoltage ac power supply can be used. The supply used for this purpose is described elsewhere ( I 1 ). Operation a n d Pyrolysis Treatment. The general principle of the pyrolysis treatment has been described (121, and only procedures unique to this furnace system will be described here. The graphite elements are purified (see below) and coated with a layer of pyrolytic graphite. This pyrolytic coating is formed by operating the cuvette a t a wall temperature of about 2000 "C with a methnnejinert gas mixture (2-3% methane) fed into the inner (sample) and outer (sheath) regions of the graphite tubes. This deposits a hard, nonporous layer of pyrolytic graphite on the surfaces of the heated elements, increasing their resistance to oxidation, reducing memory effects, and making the wall temperatures more uniform (15). In addition, the support tubes and cuvette are welded together a t their point of contact by this highly conductive coating, eliminating any effects of changing contact resistance with changing temperature, and, thus, making the voltage-currenttemperature characteristics of the system stable and highly reproducible. By monitoring the inner cuvette wall temperature with an optical pyrometer ( 1 2 ) and increasing the applied voltage stepwise (in about 200 "C increments), the onset of pyrolysis is evidenced by an increase in current through the graphite elements. The deposition of the pyrolytic coating begins a t a wall temperature of about 1400 "C and reaches its maximum rate a t about 2000 "C. Following this initial pyrolysis treatment, a low methane concentration (-0.8%) was maintained in the sample and sheath flows to just compensate for oxidation by residual water vapor and, a t high operating temperatures, air diffusion into the sheath gas. This procedure permitted the system to be operated continuously for long periods with complete stability and indefinite tube life (12). Depending on the operating temperature and sample content, the methane concentration could be adjusted to just compensate for the oxidation. When operating a t temperatures below about 2000 " C ,methane need not be added to the sheath gas. (15) B. V. L'vov. "Atomic Absorption SpectrochemicalAnalysis," Adam Hilger, London, 1970, p 206.
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A negligible absorbance (
