Anal. Chem. 1998, 70, 482-490
A Graphite Furnace Electrothermal Vaporization System for Inductively Coupled Plasma Atomic Emission Spectrometry 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 for inductively coupled plasma atomic emission spectrometry is presented. The transport losses of eight selected analytes in the individual segments of the device were determined by means of the radiotracer technique by applying amounts traced comparable to those to be determined in real samples. The results obtained from the radiotracer study were the basis for further improvement of the interface design, leading to considerable increase of the total transport efficiency, which finally was found to be between 26 (for Cr) and 57% (for Ga). The whole system consists of a graphite furnace vaporizer, a power supply, a gas flow box, and an autosampler with incorporated microbalance. The temperature program, gas flows, and autosampler functions are controlled by a data station which also provides the data acquisition and processing of the transient signals. The performance parameters of the developed system were evaluated using aqueous standard solutions. Absolute limits of detection for most analytes were between 0.1 and 1 ng, and for As, K, Ni and Pb, they were between 2 and 3.2 ng. Inductively coupled plasma atomic emission and mass spectrometry (ICP-AES and ICP-MS, respectively) involving sample digestion and often also analyte-matrix separation still represent the most important routine methods for the determination of trace elements in solid samples. However, in many instances, the application of these methods to the determination of low analyte concentrations is considerably limited by contaminations introduced into the sample preparation stage and by low nebulization efficiency. In addition, the digestion of many materials, such as high-performance ceramics, some metals, and geological materials is time consuming and requires the use of highly toxic acids at high temperatures and pressures. The problems resulting from the digestion procedure can be avoided by using direct sampling techniques. Several types of these techniques including direct insertion,1,2 powder insertion,3 (1) Salin, E. D.; Horlick, G. Anal Chem. 1979, 51, 2284-2286. (2) Ohls, K.; Sommer, D. Fresenius J. Anal. Chem. 1979, 296, 241-246. (3) Guevremont, R.; De Silva, K. N. Spectrochim. Acta B 1992, 47, 371-385.
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slurry nebulization,4-6 laser7,8 or spark9 ablation, and electrothermal vaporization (ETV) using metal filaments10,11 and graphite furnaces12-19 have been developed for ICP methods. In comparison with the other direct sampling techniques, ETV offers some essential advantages. Prior to the actual evaporation stage, samples can be introduced into the vaporizer directly as solids by means of a sample carrier, e.g., a graphite boat or a cup. An autosampler system with exact mechanical guidance and minimized sample handling is a substantial prerequisite for the reduction of the contamination risk during weighing and insertion of the sample into the vaporizer and of the time consumption of the analysis. Often a trace-matrix separation in the ETV device is possible, for instance, when a refractory compound is formed from the matrix, a refractory matrix remains unchanged on the platform after volatilization of the analytes, or the matrix is completely or partially removed in a thermal pretreatment step. Thus, compared to other sampling techniques, no considerable additional loading on the plasma occurs during measurement. The performance of the ETV-ICP methods is strongly dependent on the transport efficiency of the volatilized analytes from the ETV unit to the ICP torch. T. Kantor20 described the influence of various factors on the formation of stable aerosols, which proved to be decisive for efficient transport. The transport efficiency has been most frequently estimated from the measured sensitivities while diverse experimental (4) Min, H.; Xi-en, S. Spectrochim. Acta B 1989, 44, 957-964. (5) Docekal, B.; Broekaert, J. A. C.; Graule, T.; Tscho¨pel, P.; To¨lg, G. Fresenius J. Anal. Chem. 1992, 342, 113-117. (6) Lobinski, R.; van Borm, W.; Broekaert, J. A. C.; Tscho¨pel, P.; To¨lg, G. Fresenius J. Anal. Chem. 1992, 342, 563-568. (7) Arrowsmith, P. Anal. Chem. 1987, 59, 1437-1444. (8) Denoyer, E. R.; Fredeen, K. J.; Hager, J. W. Anal. Chem. 1991, 63, 445457A. (9) Hirata, T.; Akagi, T.; Masuda, A. Analyst 1990, 115, 1329-1333. (10) Barth, P.; Krivan, V. J. Anal. At. Spectrom. 1994, 9, 773-777. (11) Barth, P.; Hauptkorn, S.; Krivan V. J. Anal. Atom. Spectrom., in press. (12) Moens, L.; Verrept, P.; Boonen, S.; Vanhaecke, F.; Dams, R. Spectrochim. Acta B 1995, 50, 463-475. (13) Verrept, P.; Dams, R.; Kurfu ¨ rst, U. Fresenius J. Anal. Chem. 1993, 346, 1035-1041. (14) Kantor, T.; Zaray, Gy. Fresenius J. Anal. Chem. 1992, 342, 927-935. (15) Ren, J. M.; Salin, E. D. J. Anal. At. Spectrom. 1993, 8, 59-63. (16) Nickel, H.; Zagorska, Z. Spectrochim. Acta B 1995, 50, 527-535. (17) Fonseca, R. W.; Miller-Ihli, N. J. Appl. Spectrosc. 1995, 49, 1403-1410. (18) Voellkopf, U.; Paul, M.; Denoyer, E. R. Fresenius J. Anal. Chem. 1992, 342, 917-923. (19) Ediger, R. D.; Beres, S. A. Spectrochim. Acta B 1992, 47, 907-922. (20) Kantor, T. Spectrochim. Acta B 1988, 43, 1299-1320. S0003-2700(97)00834-2 CCC: $15.00
© 1998 American Chemical Society Published on Web 02/01/1998
parameters (gas flow, matrix, modifier) were changed in a sequence.15,21 Another approach has been based on the analysis of the deposit collected at the end of the transport tube after carrying out a number of replicate vaporizations of relatively high amounts (>10 µg) of analytes applied as aqueous solutions.22,23 As the transport efficiency depends on the total mass of the vaporized material, this technique may provide efficiencies considerably differing from those obtained at normal working conditions. Edinger and Beres19 showed that the nonlinearity of the calibration curve was an indicator for analyte loss. They improved the linearity of the calibration curves using transport modifiers. Verrept et al.24 calculated the relative transport efficiency as the ratio of the ICP-AES signals obtained by ETV and by nebulization. The reliability of these results has to be judged as questionable, as no exact data for the nebulization efficiency were available (it was assumed to be 1-2%) and the role of the solvent in liquid nebulization can hardly be estimated. Apart from the uncertainties, all these techniques provide information only about the total efficiency for the complete apparatus but none of them is capable of quantifying partial losses in different segments of the ETV setup. However, more detailed knowledge on the localization of the main loss is essential for further optimization of the experimental conditions. Therefore, in this work, the vaporization and transport efficiency was investigated by means of the radiotracer technique, which can differentiate between losses at different localizations of the ETV device used. On the basis of the results obtained by the radiotracer technique, the ETV apparatus and experimental conditions were optimized and the transport efficiency was essentially increased. EXPERIMENTAL SECTION Reagents and Radiotracers. 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. Doubly distilled water was used for dilution. Nitric and hydrofluoric acid supplied by Merck were of pro analysi grade, and the nitric acid was purified by subboiling distillation. The radiotracers 58Co, 51Cr, and 59Fe were obtained from Amersham Buchler GmbH & Co. KG (Braunschweig, Germany), and 76As, 64Cu, 72Ga, 42K, and 24Na were produced by irradiation of dried standard stock solutions in the FRM-1 reactor of the TU Mu¨nchen (Garching, Germany) at a thermal neutron fluence rate of 2.8 × 1013 cm-2 s-1 for 24 h. The nuclear characteristics of the radiotracers used are listed in Table 1. Instrumentation. The graphite furnace electrothermal vaporization (GF-ETV) system Model KS 10 was developed in a joint effort by the Ingenieurbu¨ro Schuierer (Ismaning, Germany) and our laboratory and is comercially available. It basically consists (21) Matousek, J. P.; Mermet, J. M. Spectrochim. Acta B 1993, 48, 835-850. (22) Park, C. J.; Van Loon, J. C.; Arrowsmith, P.; Frech, J. B. Can. J. Spectrosc. 1987, 32, 29-36. (23) Schmertmann, S. M.; Long, S. E.; Browner, R. F. J. Anal. At. Spectrom. 1987, 2, 687-693. (24) Verrept, P.; Galbacs, G.; Moens, L.; Dams, R.; Kurfu ¨ rst, U. Spectrochim. Acta B 1993, 48, 671-680.
Table 1. Most Important Characteristics for the Radiotracers Used
radioisotope 76Asc 58Co 51Cr 64Cuc 59Fe 72Gac 42Kc 24Nac
half-life
γ-linea (keV)
specific activityb (Bq/ng)
26.3 h 70.8 d 27.7 d 12.7 h 44.6 d 14.1 h 12.4 h 15.0 h
559.10 810.77 320.08 511.00 1099.25 834.03 1524.70 1368.53
104 34800 5.8 22.4 1.8 18.2 1.1 15.8
amt traced per vaporization (ng) 50 0.1 50 50 50 50 500 50
a Used for counting. b Corrected to the day when the experiments were performed. c Produced by irradiation with reactor neutrons.
of a graphite furnace with a longitudinally heated and pyrolytically coated graphite tube (length and outer and inner diameters 52, 10, and 8.5 mm, respectively) without pipetting hole, an AWD 10 autosampler workstation (Ingenieurbu¨ro Schuierer, Ismaning, Germany) with an integrated Sartorius M2P microbalance (Sartorius, Go¨ttingen, Germany), a power supply, a gas flow control module, and a data station. Pyrolytically coated platforms are used for sample insertion. For the purposes of ETV, several modifications had to be made on the furnace originally designed for absorption spectrometry to obtain a “flow-through” vaporizer (see Figure 1). One side of the furnace is equipped with a shutter driven by the autosampler which automatically opens the device for sample insertion and closes it for vaporization. A specially designed interface was fitted to the other side, connecting the graphite furnace contact via a PTFA tube (length and outer and inner diameters of 230, 6, and 4 mm, respectively) to the ICP torch. A component diagram of the whole system is shown in Figure 2. The volatilized analytes are transported by argon as a carrier gas from the furnace to the plasma. The option of adding an additional gas (for example, methane, Freon, etc.) to the carrier gas can easily be realized. The design of the interface shown in Figure 3d allows introduction of cold argon as bypass gas to the carrier gas flow behind the evaporation cell in order to enhance the formation of a stable dry aerosol.20 All gas flows are adjusted by mass flow controllers. Samples (typically 1-80 mg) are loaded manually into pyrolytically coated graphite boats, using a spatula for solids or a pipet for slurries and liquids. The loaded boats are reproducibly introduced into the furnace by the autosampler, where they function as L’vov platforms during the vaporization step. Furnace program, gas flow, and autosampler control as well as data acquisition and processing are performed by using a 486 personal computer equipped with two ADDA cards. The software was written in Power-BASIC (Kirschbaum Software, Emmering, Germany), partly by the Ingenieurbu¨ro Schuierer and partly in our own laboratory. High-temperature calibration of the furnace was carried out with an optical pyrometer Cyclops 152 (Land Infrarot GmbH, Leverkusen, Germany) while the drying temperature was measured with a PT 100 sensor (Sensycon GmbH, Hanau, Germany). A Jobin Yvon JY 70 Plus spectrometer equipped with both a THR 100 asymmetric Czerny-Turner-type monochromator with a Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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Figure 1. Schematic representation of the graphite furnace ETV device (Model KS 10), modified for ICP-AES purposes: (1) graphite tube, (2) graphite platform, (3) electric contacts, (4) cooling water chambers, (5) protecting gas chamber, (6) automatic shutter, and (7) ETV-ICP connecting interface (see Figure 3d).
Figure 2. Component diagram of the elaborated ETV-ICP-AES system, consisting of an AWD 10 autosampler workstation (Ingenieurbu¨ ro Schuierer) with integrated microbalance M2P (Sartorius), the ETV device shown in Figure 1 with power supply and gas control module, an AE spectrometer JY 70 Plus (Jobin Yvon), and a 486 data station.
focal length of 1 m and a JY 32 Plus polychromator of Paschen Runge type with 0.5 m focal length was used. As with the manufacturer’s software package the evaluation of transient signals was not possible, 16 selected analogous outputs of the amplifiers connected to the photomultiplier of the polychromator and additionally the analogous output of the monochromator were fed to two 12-bit ADDA converter systems controlled by the ETV data station. The 0.51-MeV γ-rays of 64Cu were counted with a well-type NaI(Tl) detector (2 × 2 in.) connected to a single-channel counting system, equipped with an automatic sample changer (Berthold GmbH, Munich, Germany). The γ-rays of all the other radionuclides were counted with a high-resolution γ-ray spectrometer system consisting of a high-purity germanium detector with an 484 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
Figure 3. Diverse ETV to ICP connecting interface designs used during the optimization of the transport efficiency: (a) simple aluminum-made fitting allowing no use of a bypass gas, (b) graphite fitting with quartz tube inlay having the same inner sizes as (a), (c) nozzle type aluminum-made interface with cooling bypass gas admittance, and (d) the same as (c) but with large inner diameter prior to the bypass gas entrance.
efficiency of 44% relative to a 3 × 3 in. NaI(Tl) detector, a resolution of 1.72 keV at the 1332-keV 60Co line, and a peak-toCompton ratio of 78:1, as well as a multichannel analyzer (both from Ortec GmbH, Munich, Germany). Procedure. Stock standards were mixed and diluted to contain 5 µg/mL of each analyte. A 10-µL portion of this
Table 2. Operating Parameters and Experimental Conditions Used for ICP-AES observation height (above the coil) plasma gas (argon) intermediate gas (argon) rf power
12 mm 14 L/min 0.3 L/min 1000 W
emission lines used element
wavelength (nm)
element
wavelength (nm)
Ag Al As Bi Ca Cd Co Cra Cu
328.068 308.215 193.695 306.772 317.933 226.502 228.616 317.716 324.754
Fe Ga K Li Mg Na Ni Pb
259.940 294.364 766.490 670.776 279.079 589.592 231.604 220.353
a
Detection with monochromator.
Table 3. Operating Conditions of the Electrothermal Vaporization Device step
time (s)
dry 1 dry 2 ash vaporize 1a max power vaporize 2 cool
30 25 5 5 0.6 10 40
a
platform temp (°C)
carrier flow (mL/min)
bypass flow (mL/min)
100 130 800 1700
500 700 400 400 400 400 300
30 300 300 300 300 300 30
2600 10
Data acquisition started for 15 s.
multielement standard solution was pipetted into the graphite boat previously cleaned by running the temperature program without loading. After pipetting, the boat was automatically introduced into the graphite tube of the vaporizer by the autosampler, the shutter was closed, and the run was started. The optimized operating parameters for the instrumentation are summarized in Tables 2 and 3. For the determination of the transport efficiency under conditions similar to those of the normal operation routine using the radiotracer technique, the whole ETV device with autosampler, furnace, power supply, gas control box, and data station was installed into a hood in our radiochemical laboratory. The aerosols were collected on two cylindrical cellulose filters of high retention ability, causing no significant back pressure. Portions (10 µL) of tracer solutions containing 5 µg/mL carrier were pipetted into the graphite boat for each run. The temperature program and gas flows were identical with those given in Table 3. For a more detailed description of the radiotracer experiments, see the section The Radiotracer Study. RESULTS AND DISCUSSION Optimization of ICP Parameters. As the vaporization and the excitation of the analytes is realized in two separate steps, the ICP and the ETV parameters can be optimized independently from each other. The operating parameters of the ICP were optimized to reach the best compromise conditions for multiele-
ment measurement using the described ETV device. From the 30 elements available in the polychromator system, the elements Ag, Al, As, Bi, Ca, Cd, Co, Cu, Fe, Ga, K, Li, Mg, Na, Ni, and Pb were chosen and the monochromator was set to a Cr line. Thus, these 17 elements can be determined simultaneously. The emission lines and the parameters used are listed in Table 2. For the search of the polychromator center position, the emission from a Cu hollow cathode lamp on the 324.754-nm Cu line obtained was used. The monochromator was set to the maximum of the 317.716-nm Cr line (which was regularly checked for drift) using a Cr hollow cathode lamp. Optimization of the ETV Parameters. First experiments were made with a simple aluminum-made adapter fitting to the outlet graphite contact. This first approach (see Figure 3a), which did not allow work with a bypass gas, was useful for getting some basic information on required parameter settings for the furnace program. Some interesting observations made with this setup have to be mentioned. As graphite tubes of large size (see the section Instrumentation) are used in this type of furnace, high temperatures are required for a complete release of the analytes with high boiling points and to avoid memory effects occurring especially with the carbide-forming elements, such as Ca, Cr, and Li. Increasing the tube temperature above 2700 °C (the platform temperature being ∼100 °C lower) resulted in production of large amounts of graphite particles. This phenomenon was also observed earlier while the temperature of the graphite furnace and of the platform was calibrated by using an external optical pyrometer. At temperatures above 2600 °C, a cloud of graphite particles (visually observable) made an accurate measurement of the temperature impossible. For quantification of the graphite loss, a new graphite tube was weighed, and after carrying out 10 runs in a sequence, it was weighed again. This procedure was repeated five times. Applying a volatilization temperature of about 2600 °C (tube temperature ∼2700 °C), after execution of 10 runs, the weight loss was 2.6 ( 0.5 mg, but with a tube temperature of ∼2900 °C, the graphite loss increased to 20 ( 3 mg whereby, with increasing total number of runs, the weight loss increased slightly, too. No significant weight loss of the platform was observed. Kantor20 assumed that the presence of particles acting as condensation and adsorption nuclei can enhance the transport efficiency. Therefore, the observed formation of fine graphite particles during the volatilization step could be on principle considered to be helpful. On the other hand, high particle density led to considerable gravitational deposition along the transport path (after ∼60 runs with 2900 °C, the interface was obturated) and, due to the large carbon amount reaching the plasma, to alteration of the excitation conditions. Consequently, a maximum vaporization temperature of 2600 °C (platform temperature) has proved to be a good compromise, reducing the graphite particle production to a useful degree, although a thin graphite layer in the connecting PTFE tube was still built up with time. In processing aqueous standard solutions, part of the evaporated water condensed in the PTFE connection tube. Using a carrier gas flow of 700 mL/min, it took ∼3 min until the oxygen signal at 777.193 nm, used for monitoring the water introduction into the plasma, decreased to the baseline (the baseline change on the Cr line at 317.716 nm can be used as well). As the residual Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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Table 4. Percentage Reduction of the Sensitivities by Using the Compromise Gas Flow of 700 m/min in Relation to Those of the Optimum Transport Gas Flow (100%) for Each Analyte
Figure 4. Emission signal profiles of an aqueous solution containing 20 ng of Ag, 100 ng of Pb, 50 ng of Mg, 50 ng of Fe, and 100 ng of Co.
water might support the graphite deposition, and also in order to shorten the time of the drying steps, the PTFE tube was heated electrically to ∼80 °C. Taking this measure led to a considerable reduction of the graphite deposition (only minimal deposition was observed on the colder ends of the PTFA tube) and a shorter drying time of only 50 s. For simultaneous multielement determinations, a compromise had to be found for both the temperature program for efficient vaporization of analytes with very different boiling temperatures and the transport gas flow. Thus, after two drying steps and one ashing step, the first vaporization step at 1700 °C followed during which the highly volatile elements such as Bi, Cd, and Pb were released completely and Ag and As to some extent. Then, the temperature program was continued by a short (0.6 s) maximum power push to speed up the heating of the furnace to the desired 2600 °C, at which the elements with high boiling point including Al, Ca, Co, Cu, Fe, Mg, Ni, and the alkaline elements were released. The different appearing times and release curves of some elements are shown in Figure 4. As analytes of very different vaporization behavior can be measured simultaneously, the temperature program listed in Table 3 is well suited for multielement analysis. As both the transport efficiency and the plasma temperature have an influence on the integrated emission signals, the optimum total gas flow through the ETV device was very different for the individual analyte elements. To optimize the gas flow for multielement determinations, first the optimum flow for each individual element was estimated. According to the data listed in Table 4, three groups of analytes with very similar behavior could be identified. The alkaline elements and to some extent also Mg and Al require high gas flows (between 1000 and 1400 mL/min) to achieve optimum conditions, mainly due to the lower plasma temperature derived. The LODs of alkaline element could be improved by cooling the torch with an additional gas flow, which, however, drastically worsened the sensitivity of other analytes. For the elements Ag, As, Bi, Ca, Co, Cu, Fe, Ga, and Ni, gas flows between 500 and 800 mL/min were found to be the optimum ones, while Cd and Pb required a low flow rate of ∼300 mL/min, probably because of their high volatility and as they require higher energies for excitation.20,24 Considering all these aspects, a total 486 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
element
total optimum gas flow (mL/min)
sensitivity reduction at 700 mL/min (%)
Ag Al As Bi Ca Cd Co Cu Fe Ga K Li Mg Na Ni Pb
800 1000 500 500 500 300 500 800 500 800 1000 1400 1100 1200 500 300
3 11 12 8 16 67 16 7 9 3 21 57 9 36 13 27
Table 5. Relative Sensitivities (%) Achieved in Processing 50 ng of Each Analyte Using the Interface Made of Graphite with Quartz Tube Inlay (Figure 3b), the Nozzle-Type Interface (Figure 3c), and the Large Size Nozzle-Type Interface (Figure 3d)a interface design depicted in element Ag Al As Bi Ca Cd Co Cr Cu Fe Ga K Li Mg Na Ni Pb
Figure 3b 68 108 73 133 93 75 67 96 80 66 66 83 85 100
Figure 3c
Figure 3d
142 133 124 128 131 136 125 123 140 123 137 127 160 138 141 124 91
221 200 168 189 244 227 238 202 241 217 214 185 239 199 205 252 145
a Normalized to the sensitivities achieved with the simple adapter fitting (Figure 3a), which were taken as 100%.
transport gas flow of 700 mL/min was chosen for further development of the multielement method. With this gas flow, the relative sensitivity compared to that obtained with the optimum gas flow, which was taken as the 100% basis, was for six elements greater than 90%, for five elements between 80 and 90%, for two elements between 70 and 80%, and only for Cd, Li, and Na was below 70%. It was expected that not only the interface design but also the material used could influence the transport efficiency. A comparative study with the interface described above (see Figure 3a) and an interface consisting of a graphite fitting with a high-purity quartz tube inlay (see Figure 3b), having the same inner sizes, showed that the graphite particles were deposited to a higher degree on the quartz than on the aluminum surface. Therefore, for most elements studied, the analyte losses were significantly larger using
Figure 5. Schematic illustration of the transport path of vaporized analytes including the ETV device shown in Figure 1, the nozzle-type interface shown in Figure 3c, and a 230-mm PTFE connection tube. The complementary table gives percentage retention of the analytes in the individual segments of the device determined by the radiotracer experiment.
Figure 6. Percentage distribution of 60Co, 51Cr, and 59Fe in different parts of the graphite tube. Also given are maximum temperatures estimated at parameter settings summarized in Table 3 and the length of the segments.
the quartz tube (see Table 5), i.e., by 15-34% for Al, Ca, Cr, Cu, K, Li, Mg, Na, and Ni, than those observed with the metal interface. No difference was observed for Pb, and an enhancement of the signal areas occurred only for As (+8%) and for Cd (+33%). These three analytes are volatilized at relatively low temperatures at which no significant graphite particle emission from the tube wall occurs. Thus, the transport efficiency of these three elements was not affected by the graphite powder deposition, but the adapter material may play a certain role. For As and Cd, the retention on a metal surface was obviously higher than on the surfaces of the compound interface. However, the behavior of the other analytes suggested use of an interface made of aluminum. It was reported12,14,15 that an admixture of a cold gas to the carrier gas behind the vaporization cell had a positive effect on the transport efficiency. Obviously, the quick cooling down of the sample vapors supports the formation of condensation nuclei. For the manufacture of such an interface (see Figure 3c), aluminum was found to be a suitable material. A ratio of the carrier and bypass gas flow of 400 to 300 mL/min (see Table 3), giving a total flow rate of 700 mL/min, has proved to be the optimum compromise for all analytes studied. As no aluminum blank was observed and more efficient performance of the ETV was achieved, this interface was chosen for the determination of the transport behavior of the analytes.
Radiotracer Study. The radiotracer technique has proved to be an extremely valuable tool in examining the physicochemical behavior of analyte elements in each step of an analytical process.25,26 Labeling the sample system with radioisotopes in the same chemical form and in comparable amounts as the investigated analytes, a simple, rapid, and highly accurate determination of the distribution of the analytes in the segments of the transport path of the ETV device could be carried out by detecting the characteristic radiation of each radioisotope. Using the high-resolution γ-ray spectrometer and a radiotracer solution mixture containing 5 µg/mL each of As, Co, Cr, Fe, Ga, and Na and 50 µg/mL K (because of low specific activity) enabled a simultaneous multielement screening of the distribution of the analytes. For this purpose, 10 µL of the labeled solution was pipetted into the graphite boat and the furnace program (see Table 3) was started. After three runs with 10 µL of the tracer solution, each collecting filter was decomposed in a 15-mL PE flask with 5 mL of concentrated nitric acid containing 50 µL of hydrofloric acid, 5 mL of water was added, and the resulting 10-mL solution was counted with the γ-ray spectrometer. As only ∼1% of the radiotracers reached the second filter, the loss by penetration (25) Krivan, V. J. Anal. At. Spectrom. 1992, 7, 155-164. (26) Krivan, V. In Treatise on Analytical Chemistry, 2nd ed.; Elving, P. J., Krivan, V., Kolthoff, I. M., Eds; John Wiley & Sons: New York, 1986; Part I, Vol. 14, pp 339-417.
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Figure 7. Slightly nonlinear calibration curves obtained for the analytes Ag and Na and calibration curves with more pronounced curved course obtained for Cd, Bi, and Pb.
through the filter system could be neglected. As 64Cu could be measured with the required sensitivity only via the unspecific annihilation radiation at 511 keV resulting in interference by other radiotracers, the Cu transport efficiency was determined separately, prior to the group labeling experiment. A 10-µL aliquot of a 64Cu tracer solution containing 5 µg of Cu/mL was pipetted into the graphite boat for each run and the same procedure as described above was performed, except that for measurements the single-channel γ-detector was used. After nine repeated executions of the furnace program with 10 µL of the tracer solution, the ETV device was disassembled and each part that came into contact with the analytes was counted individually for the γ-rays of the radioisotopes traced. As a reference value, 10 µL of the multielement tracer solution was counted. The different counting geometry of the individual parts was adequately considered. The results on retention of the analytes in different sections of the vaporizer and transport setup obtained by means of the radiotracers are given in Figure 5. No detectable residual activity 488 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
Table 6. Estimated Total Transport Efficiency after the Final Optimization of the ETV Device element
transport effic (%)
element
transport effic (%)
As Co Cr Cu
37 35 26 29
Fe Ga K Na
37 57 40 35
was found on the platforms. As can be seen from the complementary table in this figure, the elements with high boiling points, i.e., Co, Cr, Cu, and Fe, were retained by more than 40% in the graphite tube while the more volatile elements As, Ga, K, and Na were retained by a much lower percentage. For the elements Co, Cr, and Fe, in a special series of experiments, we also estimated the distribution of the residue along the graphite tube. For this purpose, after excecution of nine vaporization cycles using aqueous solutions of these elements labeled with 60Co, 51Cr, and 59Fe, the tube was cut into five parts (see Figure 6), which were then separately counted for radioactivity of these radioisotopes.
The results obtained are given in Figure 6. As expected, only a negligibly low activity fraction (1-2%) was detected in tube part 1 located before the platform in the direction opposite to the gas flow, reaching it by diffusion. The deposition increased with decreasing temperature in the gas flow direction from 3 to 4% on the hottest tube center (part 2), where the platform was placed, reaching the maximum value (79-85%) on the coolest last section of the graphite tube (part 5). The distribution of the deposited labeled analytes along the graphite tube correlated well with the temperature gradient within the tube measured in a separate study. Furthermore, the results of the radiotracer experiments showed that the highest percentage retention of the analytes occurred on the interface unit consisting of the graphite contact 2 and the aluminum tube (see Figure 5). Consequently, in order to achieve a substantial improvement of the total transport efficiency, first the retention on the interface unit had to be minimized. Final Modifications of the Interface. The results obtained from the radiotracer experiment showed that the interface design played a key role for the transport efficiency. As already discussed, the bypass gas was used to support the formation of stable aerosols and, thus, to avoid the adsorption of the gaseous analytes on the contact surface of the device. However, with the design shown in Figure 3c, this attempt was achieved only to a low degree. The addition of the cold bypass gas before the conical reduction of the the inner diameter of the transport path (from 8.5 mm in the graphite tube to 3.8 mm in the aluminum tube) could not significantly reduce the retention of the gaseous analytes on the cold impact surface (∼60 mm2) directly behind the graphite contact 2 (see Figure 5). Therefore, in this segment, the largest percentage (up to 70%) of the analytes was lost. In addition, the diameter reduction gave rise to a considerable back pressure causing turbulence, which further increased the probability of interaction of the analytes with the cold surfaces. It could be expected that setting the diameter reduction (which is necessary because of the diameter of the inner torch tube, ∼4 mm) behind the bypass admittance nozzle would increase the space for nucleation and that the bypass argon, if introduced under an extremely small angle, would create a shield layer to the impact surface. Thus, these two modifications should improve the transport efficiency. On the basis of these considerations, the interface unit shown in Figure 3d was designed. Indeed, it was proved in spectrometric measurements that, for all elements investigated (see Table 5), an increase in sensitivity of 36-100% was achieved when the interface shown in Figure 3d was used instead of that represented in Figure 3c. As can be seen from Table 6, by this means, the total transport efficiency was essentially enhanced, too. System Performance. The analytical performance of the improved ETV device was tested by using aqeous standard solutions and the optimized parameters listed in Tables 2 and 3. For the analytes Al, As, Ca, Co, Cu, Fe, Ga, K, Li, Mg, and Ni, the slope of the calibration curve was linear throughout the whole range monitored (up to 150 ng of each analyte) having correlation coefficients better than 99.9%. For Ag and Na, the dependence of the integrated emission signals on the analyte mass applied
Table 7. Absolute Limits of Detection Achieved with the Improved ETV-ICP-AES System and Comparison of the Relative LODs Obtained with the ETV and with a Meinhard Nebulizer Using the Compromise Conditions Listed in Table 2 and 3 ETVa
nebulizerb
element
abs LODs (ng)
rel LODsc (ng/mL)
rel LODs (ng/mL)
Ag Al As Bi Ca Cd Co Cr Cu Fe Ga Kd Lid Mg Nad Ni Pb
0.11 0.20 2.30 1.00 0.90 0.90 0.54 0.30 0.10 0.12 0.50 3.20 0.90 0.30 1.00 2.70 2.00
4 7 80 34 30 30 18 10 4 4 17 107 30 10 34 90 68
20 100 180 160 30 6 20 3 12 11 90 880 35 182 330 25 150
a Based on 3σ of 7 blanks. b Based on 3σ of 10 blanks. c Applying a 30-µL liquid sample. d Without additional cooling of the ICP torch.
showed a slightly curved course in the low-mass region (see Figure 7a,b). The deviations from linearity was much more pronounced for the analytes Cd, Bi, and Pb, which have low boiling point temperatures (see Figure 7c-e). However, nonlinear calibration curves have already been reported19,20 and have been related to the increasing transport efficiency with increasing analyte amounts. The absolute (ng) and the relative (ng/mL) limits of detection, calculated on the basis of three times the standard deviation of the blank of water (n ) 7) and of the sensitivity obtained by applying an analyte amount of 10 ng are listed in Table 7. Absolute LODs better than 1 ng were achieved for Ag, Al, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, Li, Mg, and Na. Applications of this ETV-ICP-AES system to analysis of a number of high-purity materials are in progress. CONCLUSIONS A radiotracer study undertaken for eight analytes and performed under real working conditions proved that the major part of the analytes was retained in the interface unit. On the basis of this study, a novel interface was designed allowing the admittance of a cool bypass gas under a very small angle. By this means, significant enhancement of the total transport efficiency of the analytes was achieved, ranging from 26 to 57%. With the presented graphite furnace ETV system, a significant improvement of the ICP-AES performance with regard to several aspects could be achieved. It can be used for multielement analysis of liquid, slurry, and solid samples, being particularly advantagous for the latter. Depending on the material density, sample portions up to 80 mg are applicable for one analysis cycle. Moreover, using the autosampler, simple, fast, and reproducible handling of the samples is possible. This makes a high sample throughput (up to 30 samples/h) possible and minimizes the contamination risk. Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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Owing to the high transport efficiency and low contamination risk, absolute detection limits better than 1 ng were achieved for 13 of the 17 elements studied. ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (Bonn, Germany). The authors gratefully acknowledge the provision of irradiation facilities free of charge by the
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Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
FRM Reaktorstation Garching, Technical University of Munich, Garching, Germany. Received for review August 1, 1997. Accepted October 23, 1997.X AC9708349 X
Abstract published in Advance ACS Abstracts, December 15, 1997.
