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ICP emission precision to be limited by the fluctuations in sample transport and nebulization to about 1% . It is reasonable, therefore, that the precision is slightly worse with two ICPs. The long-term stability of the ICP provides an advantage over other sources used for AFS such as electrodeless discharge lamps, which must be carefully thermostated (23),and the Eimac short-arc xenon lamp (24),which has a lower intensity in the ultraviolet. Also, there was no significant difference in the precision when the measurements for zinc and sodium were carried out in a matrix containing 10 mg mL-l calcium. However, there was an increase in scattered radiation. The precision for the measurement of low concentration solutions was about 3-4 times worse than that for high concentration solutions. The scatter signal due to the calcium matrix at the zinc and sodium lines resulted in a 2-5 times increase in signal over the blank level, respectively. Even though scatter was observed due to the calcium matrix, it did not act as a significant noise source. The precision for the low concentration analysis was about the same with and without the 10 mg mL-’ calcium matrix. Consequently, if present, scatter can be subtracted out by using the two-line technique (201, assuming that the scatter level does not change appreciably in the wavelength vicinity of the atomic fluorescence line. While the fluorescence detection system is well optimized for a background shot-noise limited system, the optical transfer of radiation from the source ICP to the atomization ICP could be improved by at least an order of magnitude through the use of an ellipsoidal reflector (25,261placed behind the plasma to collect a much larger solid angle of emission. Indeed, based upon solid angle considerations, we are presently collecting less than 1 % of the source radiation using 50 mm diameter lenses. Also, a mirror placed behind the atomization ICP in the direction of the fluorescence monochromator might improve detection powers by two times and aspiration of higher concentration excitation solutions could also be used to increase the source intensity in some cases. However, clogging of the nebulizer and aerosol tube of the torch may result as was noticed in this work for aluminum and sodium. The use of demountable torches with large diameter aerosol tubes might improve the situation.
ACKNOWLEDGMENT The authors wish to thank Nicolo Omenetto of the University of Pavia, Pavia, Italy, and Edward Voigtman, Benjamin Smith, and Gary Long (University of Florida) for their helpful discussions and comments during this work. Registry No. SOz, 7446-09-5;BaS04,7727-43-7;Vz06,131462-1; SOz, 7631-86-9;copper, 7440-50-8. LITERATURE CITED Fassel, V. A.; Knlseley, R. N. Anal. Chem. 1974, 4 6 , 1110A. Barnes, R. M. Crlt. Rev. Anal. Chem. 1978, 7 , 203. Demers, D. R. Appl. Spectrosc. 1968, 22, 797. Mansfield, J. M., Jr.; Bratzel, M. P., Jr.; Norgordon, H. 0.; Knapp, D. 0.;Zacha, K. E.; Wlnefordner, J. D. Spectrochlm. Acta, Part B 1968, 2 3 8 , 389. Lowe, R. M. Spectrochlm. Actci, Part B 1971, 268. 201. Epsteln, M. S.; Nlkdel, S.; Omenetto, N.; Reeves, R.; Bradshaw, J.; Wlnefordner, J. D. Anal. Chem. 1979, 51, 2071. Fraser, L. M.; Wlnefordner, J. D. Anal. Chem. 1971, 4 3 , 1693. Uchlda, H.; Koslnskl, M. A.; Omenetto, N.; Wlnefordner, J. D. Spectrochlm. Acta, In press. Hussein, Ch. A. M.; Nickless, G. Paper presented at the 2nd ICAS, Sheffield, England, 1969. Omenetto, N.; Nlkdel, S.; Bradshaw, J.; Epsteln, J. S.; Reeves, R. D.; Wlnefordner, J. D. Anal. Chem. 1979, 51, 1521. Kosinskl, M. A.; Uchlda, H.; Wlnefordner, J. D. Talants, in press. Uchlda, H. Spectrosc. Lett. 1981, 14, 665. Demers, D. R.; Allemand, C. D. Anal. Chem. 1981, 53, 1915. West, A. C.; Fassel, V. A.; Knlseley, R. N. Anal. Chem. 1973, 45, 1587. Murayama, S. Spectrochlm. Acta, Part B 1970, 2 5 8 , 191. Borowlec, J. A.; Boorn, A. W.; Dlllard, J. N.; Cresser, M. A.; Browner, R. F.; Matteson, M. J. Anal. Chem. 1980, 52,1054. Alkemade, C. Th. J.; Voorhuls, M. H. Fresenlus’ 2. Anal. Chem. 1958, 163, 91. Greenfield, S.; McGeachin, H. M.; Smith, P. B. Anal. Chim. Acta 1987, 8 4 , 76. Uchlda, H.; Matsui, H. Bunko Kenkyu 1978, 2 7 , 110. Larklns, P. L.; Wlllls, J. 8. Spechochlm. Acta, Part B 1974, 2 9 8 , 319. Zander, A. T.; OHaver, T. C.; Kellher, P. H. Anal. Chem. 1977, 4 9 , 638. Boumans, P. W. J. M.; DeBoer, F. J. Spectrochim. Acta, Part 8 1977, 3 2 8 , 365. Browner, R. F.; Batel, B. M.; Glenn, T. H.; Rletta, M. E.; Winefordner, J. D. Spectrosc. Lett. 1972, 5 , 311. Cochran, R. L.; Hleftje, G. M. Anal. Chem. 1977, 4 9 , 2040. Shull, M.; Wlnefordner, J. D. Anal. Chem. 1971, 4 3 , 799. Benettl, P.; Omenetto, N. 0.; Rossl, G. Appl. Spectrosc. 1971, 25, 57.
RECEIVED for review November 5,1982. Accepted December 20,1982. This work was supported by AFOSR-F-49620-80C-0005.
Furnace Atomic Absorption Spectrometry Atomizer with Independent Control of Volatilization and Atomization Conditions Darryl D. Siemer Exxon Nuclear Idaho Co., CPP 602, Idaho Falls, Idaho 83402
A graphite furnace atomizer featurlng Independent control of the temperatures of both an atomization zone and a spatlally separate volatlllzatlon zone was constructed and characterized. Separate optically sensed temperature feedback controlled power supplies were employed to heat both zones. A number of previously documented matrix Interference problems observed In analyses performed with furnaces of conventional construction were found to be greatly reduced or ellmlnated with thls system.
Recent work in this laboratory has been directed toward
the improvement of the routine reliability (“ruggedness”)of trace element determinations in complex samples by graphite furnace atomic absorption spectrometry (GFAAS). These projects have included the modification of furnace power supplies to incorporate temperature feedback control (I);a study of the biases that the “slow” electronic signal processing circuitry commonly found in AAS spectrometers primarily designed for flame AAS impose onto GFAAS signals (2);and, finally, the modification of the atomizers themselves in order to increase the effective gas-phase temperatures experienced by volatile analyte elements “atomized” in several versions of Varian Techtron’s carbon rod atomizer (CRA).
0003-2700/83/0355-0692$01.50/00 1983 American Chemlcal Society
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The first approach to modified atomizers involved a version of the “cup-in-flame”technique described by L’vov (3). This technique is not really GFAAS because the CRA cup is only used to selectively volatilize the analyte into the flame which actually serves as the atomization volume. (See ref 2 for an example of this work.) The second approach involved “end-heating” of an elongated CRA atomizer tube using four rods pinched in pairs across the ends of the tube (4). The next modification involved a simple change in heating rod placement with respect to a CRA cup’s optical axis: instead of placing rods in the conventional across-the-bottom configuration, the rods were clamped across the top of the cup (5). A final paper described the development of both “cup” and “tube-cup” variations of the CRA featuring atomization temperatures for volatile analytes approximately a thousand degrees higher than are attainable with the standard design (6). All of these modified furnaces significantly reduced the gas phase related ”matrix effects” observed in the analysis of the types of samples for which they were designed. However, they also all share two drawbacks. First, they were deliberately designed so that the point within the atomizer upon which the sample is deposited heats more slowly than does the portion of the atomizer optically sampled by the AAS spectrometer. An unavoidable consequence of this is that the sample deposition point never achieves as high a temperature as it does in the standard furnace configuations. This restrids these furnaces to the determination of only volatile analytes and, in some instances, permits the buildup of an undesirably large amount of involatile matrix salt if the atomizer is used for a long series of determinations. A second problem is one shared by both these atomizers and the “L’vov platform-Massmann furnace” system suggested by L’vov (and subsequently popularized by the Perkin-Elmer Corp.). This weakness is that in order to significantly increase effective atomization temperatures, the maximum possible furnace heating rate must be used. When this is done, careful control of the volatilization process is lost. Consequently, there is often an increase in the degree of overlap between the volatilization profiles of the analyte and the matrix which serves to counteract the beneficial effects of raising the atomization temperature (7). Both this writer’s previous efforts and the pioneering work of L’vov (3) and, later, that of Lundgren and Johansson (€0, clearly indicated that a truly “rugged” and flexible GFAAS atomizer would have to feature atomization conditions truly independent of volatilization conditions. Physically, this means that the atomizer must possess two distinct zones and that each be heated independently of the other. This paper describes an updated version of Lundgren and Johannson’s basic furnace design possessing some new features likely to enhance its practical value. These features include a completely isothermal atomization zone (both spatially and temporally), temperature feedback control of both power supplies, an overall physical size compatible with most AAS spectrometers, and a simple design that does not require any part of the furnace to be moved during an analysis. EXPERIMENTAL SECTION Furnace. The graphite working parts of the furnace consist
of a Massmann tube furnace (atomizationzone) with a carbon rod atomizer (CRA) cup (volatilization zone) inserted into an access hole drilled in the center of the tube (Figure 1). The tube and cup were machined from 6.2 mm diameter AFXBQ graphite rod (Poco,Decatur, TX).Standard CRA rods (the same rods used with the Varian Techtron CRA 63/90 atomizer series) were used with the cups. The central portion of the Massmann tube possesses a greater wall thickness than does the rest of the tube. Additionally, two relatively large oblong holes (end holes) were drilled through the
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Figure 1. Graphlte parts of the atomizer: (A) 6.2 mm outer diameter
tube ends which are clamped into the electrode blocks; (5) holes limltlng the length of the atomization zone: (C) CRA cup machined to fit into a 4.1-mm hole in the bottom of a Massmann tube: (D) CRA rods. a
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Flgure 2. (Top) Top view of atomizer: (a) electode blocks; (b) Massmann tube; (c) water cooling lines; (d) holes in bottom of inert gas sheath system; (e) base plate.” (Bottom) Detail of an electrode block (for the Massmann tube): (f) steel screw to clamp the graphite tube into place; (9) neoprene rubber gasket: (h) copper folk (i) no. 4 welding cable to the power supply: (j)optical temperature sensor (of the CRA power supply); (k) nylon screw.
tube vertically (parallel to the axis of the CRA cup) 8 mm from the center of the tube at both ends. The variation in wall thickness and the presence of the holes serve to balance the PR heatproducing phenomena with the conductive heat loss to the ends of the tube and, therefore, provide a spatially isothermal atomization zone between the end holes. Volatilized analyte is swept out of the light path at these end holes by an argon (or argon/propane mixture) gas stream directed vertically upward through holes in a perforated plate situated under the graphite parts. In practice, therefore, the spacing between these end holes defines the effective length of the optical path (in this instance 1.6 cm). The tube was designed in this way to give a high (and unambiguous) effective optical-path temperature and to prevent the “double peaking” often observed in conventional Massmann tube furnaces (9). The graphite parts were clamped in electrode blocks machined from 19 mm thick aluminum plate. These four electrode blocks were mounted onto a 12.7 mm thick rectangular aluminum base plate with nylon screws (see Figure 2(top) for a top view of the system). Holes in the electrode blocks were drilled to accommodate the tube (or rods), hold-down screws (in which case the holes were then tapped), the cooling water passage, and the optical probe. Then each block was sawed laterally across the center line of the rod (or tube) hole to permit ready replacement of the graphite parts. Figure 2(bottom) is a side view of one these electrode blocks showing the power cable connectionand an optical temperature sensing probe. Electrical power connectionsto each block were made with three 2.2 cm wide, 0.127 mm (0.005 in.) thick strips of heavy copper foil sandwiched between the block and the 2 mm thick soft neoprene rubber gasket which serves to electrically insulate the block from the base plate. The rubber gasket-nylon screw combination also serves to allow the electrode blocks enough flexibility to prevent breakage of the graphite parts when they expand upon
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Figure 3. Schematic of the modlflcatlon to the M63 power supply: Q,, HEP 312 phototranslstor; IC,, SNE555; IC,-IC,, 741 operatlonal amplifier; QP,2N3904; D,, 1N4007; R,, RB, R, 20 kQ; R,, 10 kQ; R,, 1 M a , R,, 1 kQ; R,,, 20 Ma; R,,, 3 20 kQ; R12, 10 kQ ten-turn; R13, 5.1 kQ; R,,, 12 kQ; R15, 25 kQ one-turn; C, 0.001 pF; Cp 10 pF.
heating. The center line of the CRA rod holes is 7 mm below that of the Massmann tube. Power Supplies. The power supply used to heat the CRA cup during the “dry”, “ash”, and “volatilize” stages is a Varian Techtron M63 CRA power supply modified to provide both optical feedback control of the volatilization temperature and an interface with the power supply used to heat the Massmann tube. In operation, only the CRA power supply is “on” during the “dry” and “ash” portions of the experimental cycle. At the end of the “ash” cycle, the Massmann tube’s power supply is turned “on” and the CRA supply is shut “off”. When the Massmann tube reaches its preset atomization zone temperature, the CRA supply is turned back on to volatilize the sample. Both the heating rate and the final temperature of the CRA cup are essentially independent of the temperature of the Massmann tube. The circuitry added to the M63 power supply necessary to implement the desired functions is shown in Figure 3. It incorporates the simple, optically sensed, temperature feedback control circuitry described in a previous report (ref 1) with a triggerable switching circuit; this circuit prevents the CRA cup from being heated until the Massmann tube’s power supply outputs a signal indicatingthat the atomizationzone’s temperature is at the desired level. The four integrated circuit “chips” and the associated discrete circuitry were all mounted in a small prototype “bread board” and connected to the rest of the internal circuitry with jumper wires as described in ref 1. A schematic diagram of the power supply used to heat the Massmann tube is give in Figure 4. Because the supply serves only one function (i.e., to heat the Massmann tube to a desired temperature for a fied time), it is very simple. Basically it consists of a 2.4 KVA 1O:l step-down transformer (Signal Transformer Co., Inwood, NY) and a 25 A, 400 V, triac. Switching current to this triac is supplied by an optically coupled, zero-crossing trigger integrated circuit “chip” (IC 1). This IC is, in turn, controlled by a circuit similar to that added to the CRA 63 power supply described above. The control circuitry functions as follows: When the triggerable monostable multivibrator IC (SNE 555) is triggered “on” (by a negative-going pulse output from the CRA 63 power supply at the end of its “ash” cycle), its output goes “high” and drives current (through Rs) to the photodiode within the triac trigger IC. This IC then turns on the triac in series with the primary winding of the power transformer. However, when the temperature of the tube becomes so high that the output of the logarithmic amplifier (IC,) becomes negative with respect to the setpoint of potentiometer RI2,the comparator (IC,) will “short” the output of the timer IC to ground with transistor Q2, thereby shutting “off“ the power supply until the temperature of the graphite falls below the desired level. Some hysteresis (overshoot) was added to the temperature setpoint comparator circuit (with R9) in order to prevent the supply from “locking in” to the 60 Hz mains frequency and possibly switching significantlymore mains power half-cycles of one polarity than the other through the primary of the transformer. (If this should happen, the transformer core will saturate which results in overheating and/or a “blown”circuit breaker.) Pieces of fine-bore ceramic tubing (of the sort usually used as high-temperature insulation for thermocouple wires) blackened
Figure 4. Schematic of Massmann tube power supply: (A) f 8 V logic power supply; (6) 2.4 KVA transformer; (C) GE SC260 D triac; (D) trigger input from CRA power supply: IC,, MOC 3031; IC,, SNE555; IC,,, 741 operatlonal amplifier; R,, 56 0; R,, 100 Q;R3, 1 MQ; R,, 75 kQ; R5 Q;Re, 5.1 MQ; RI, Rlo, 1 kQ Re, 20 MQ; R,, 2 MQ; RjI, 18 kQ; R,,, 2 kQ ten-turn; R,,, 510 Q;C,, 0.01 pF 400 V; C,, 0.1 pF; C3, 1 pF; Q,, HEP 312 phototransistor; Q,, 2N3904; D,007.
with “Aqua Dag” (a suspension of colloidal graphite powder in ammonia water) were inserted through holes drilled in the electrode blocks to act as simple “light pipes” for the phototransistors used as temperature sensors (see Figure 2). The front ends of these tubular ”light pipes” were placed approximately 0.5 cm from the graphite surface whose temperature was to be regulated. The phototransistors were attached to the other end of the ceramic tubes with several sizes of black plastic “heat shrink” electrical insulation. To extend the range of temperature regulation upward, the effective apertures of these “light pipes” were reduced by inserting pieces of electrical insulation stripped from 28 gauge wire into the “cool” ends of the ceramic tubes. Instrumentation. The same atomic absorption spectrometer (Varian Techtron AA6 optical components with electronically “fast” homemade analog signal processing circuitry), HewlettPackard computer data acquisition and display system, Thermodot optical pyrometer, and homemade gas regulation equipment described in the previous papers was used for this project (2,5,6).
Reagents and Supplies. All solutions used were prepared with doubly deionized water and reagent grade (or better) chemicals. Solutions were manually pipetted with Oxford micropipets. These pipets feature uniform diameter plastic capillary tubing tips instead of the more common conical plastic variety. Micropipets featuring conical tips are not suitable because the sample aliquots must be placed into the CRA cup through the entire diameter of the Massmann tube. To do this with a conical tipped pipet would require a sample access hole so large that significant analytical sensitivity would be lost. RESULTS AND DISCUSSION The temperature setting potentiometers of the power supplies were calibrated by directing the light (temperature) sensors a t the Massmann tube and recording the voltage output of the logarithmic amplifier circuits in the temperature control circuits while that tube was heated. (This is done with the same programmable DVM-computer system used to measure the analytical signals.) The temperature-time response of the Massmann tube upon heating was established by similarly recording the output of the optical pyrometer aimed at the same point. A correlation of the temperatures indicated by the optical pyrometer with the voltages of the logarithmic amplifier at various points in time allowed the corresponding potentiometer voltages to be assigned definite temperature values. The range of temperatures controllable by the power supplies (as configured for this paper) was from 1250 to 2750 K
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Flgure 5. Heating rate of atomization zone (A) at the center and (B) at the end.
Flgure 7. Mercury signals at different atomization temperatures: (A) slgnal seen at 1279 K; (B) signal seen at 2641 K.
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for the Massmann supply and from 1200 to 2550 K for the CRA system. These control ranges can be readily adjusted in either direction by changing the effective aperture of the ceramic light pipes. Figure 5 shows the heating rates measured a t the center (A) and a t the inner edge of one of the end holes (B) in the Massmann tube. The hysteresis deliberately added to the power supply control circuitry is responsible for the approximately 40 K of “jitter” in the setpoint temperatures at the top of the plateaus. On these plateaus, the power supply periodically turns itself “on” and “off“ to maintain the desired temperature and “overshoots” about 20 K each time it switches. A series of experiments which illustrate the degree of independence achievable between the volatilization and atomization processes are depicted in Figure 6 In this and all other figures in this paper showing actual ”raw”analytical data, the computer’s data acquisition cycle was initiated at the instant that the Massmann furnace started to heat up. The signals shown in all of the figures were not subjected to any digital “smoothing routine” and represent unbiased instrument responses. In this particular experiment, 20 pg samples of cadmium were volatilized into the Massmann tube preheated to three different temperatures. The same volatilization power supply program was used in each case, viz., a temperature ramp at about 1K/ms to a final, stable, CRA cup temperature of 2000 K. It is important to remind the reader that this CRA volatilization program does not begin until the Massmann tube has reached the desired atomization zone temperature (at the times indicated by the letters A’, B’, and C’ on the temperature-time traces in Figure 6). The atomic absorbance signals observed a t the three successively increasing atomization temperatures are correspondingly labeled A, B, and C. As expected the magnitudes of the analytical responses decrease as the temperature of the atomization zone increases.
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Flgure 8. Cadmium signals observed at dlfferent atomization temperatures: (A) signal seen at 1640 K; (B) signal seen at 2104 K; (C) signal at 2632 K. The prlmed letters depict the point in time and the tube temperature at which the volatlllzatlon power supply was turned on In each case.
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Figure 8. Signal transients observed for lead at three spectral lines arlslng from different energy levels: (A) 100 ng of lead at 368.3 nm; (B) 100 ng at 280.2 nm; (C) 1 ng of lead at 283.3 nm.
This is due to the fact that each individual atom spends less time within the tube a t higher temperatures (residence time is diffusion controlled and gaseous diffusion constants are inversely related to temperature). Please note that in each instance, the interval between the initiation of the volatilization heating program and the appearance of the atomic absorption signal is essentially independent of the temperature of the atomization zone. This degree of independence is possible because the CRA rods clamped onto the cup act as efficient “heat sinks” preventing the volatilization zone from being significantly heated either by radiation from the tube or by conduction along the walls of the cup itself. Figure 7 shows the results of a similar experiment performed with mercury. Two-microliteraliquots of a 10 mg/L mercury solution (stabilized with 1% nitric acid and 0.01% sodium dichromate) were atomized at either 1280 K (A’, A) or 2640 K (B’, B). Figures 6 and 7 demonstrate that, with this furnace, an analyst has the freedom to choose atomization conditions based on whether achieving the maximum possible sensitivity (compatible with a cool atomization zone featuring long individual atomic residence times) or the greatest assurance of freedom from gas-phase interferences (hot atomization zone) is more important for solving the problem at hand. The flame spectroscopist routinely makes the same decision for similar reasons in choosing which flame to use. The gas-phase temperatures experienced by analyte atoms were independently determined by a “two-line”spectroscopic method using lead as the test element. The detailed procedure for doing this has been described in previous papers (5, 6). The results of a typical experiment are shown in Figure 8. The larger of the two absorption transients denoted by the solid lines is the signal seen from 100 ng of lead using the 368.3-nm line (originating from an energy state 7819 cm-’ above ground) and the smaller is the response seen for the same amount of lead at the 280.2-nm line (10650 cm-’ above ground). The dashed line is the signal seen from 1ng of lead
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Figure 10. Thallium in both nitric and perchlorlc acid: (A) 2.5 ng of TI in 0.16 M “0,; (B) 2.5 ng of TI in 0.5 M HC104.
observed at the 283.3-nm resonance line (ground state). The spectroscopic temperature (based on the relative sizes of the two nonresonance line signals) was 2060 K, which is in good agreement with the 2100 K temperature setpoint of the Massmann furnace. Similar experiments at other nominal temperature setpoints gave equally good agreement with measured graphite temperatures. In order to demonstrate the practical analytical advantages of this atomizer, a series of signal recovery studies were performed for several volatile analyte metals in matrices which have been reported to cause severe problems with conventional furnaces. In the following examples, the comparisons made between the new furnace and conventional systems were based on published reports of what this writer considers to be reliable research performed by experienced workers able to get the best possible performance from the particular atomizer used for the work. The general conclusions drawn about any particular manufacturer’s product will apply with equal force to other basically similar systems. Figure 9 depicts a complete, representative experiment showing the analytical signals seen from 2.5 ng of lead in either (a) 1% nitric acid or (b) an acidified “hard water” matrix. The matrix chosen was similar to those studied by Manning and Slavin in their 1979 paper describing the advantages of using a L’vov platform in the Perkin-Elmer HGA 500 version of the Massmann furnace (10). The matrix solution used for this example actually represents a “worst case” and contains all of the concomitants at the maximum levels of the range of concentrations listed in Manning and Slavin’s paper (Ca, 800 mg/L; Mg, 800 mg/L; SO4,5000 mg/L; C1,100 mg/L as well as 973 mg/L of sodium and 2650 mg/L of nitrate added in salt form). This solution was acidified with nitric acid to the 1%(v/v) level usually recommended for trace element studies. Five-microliter aliquots of the solution were used (a total of 51 pg of mixed salts) in the experiment. The furnace heating program consisted of a 40-s “dry” cycle at 95 “C, a 10 s “ash” cycle at approximately 400 “C, and a final CRA volatilization temperature of 2100 K. The temperature of the Massmann tube was 2700 K. The recovery of lead from the hard water matrix was 99.7% in this experiment. (This is based on the relative sizes of the integrated signals after correction for the small reagent blank.) The mean recovery for five replicates of the experiment was 100.4%. The shapes of the analytical signals are not identical; nor is it reasonable to expect them to be so. The shapes of GFAAS signals are largely determined by the volatilization characteristics of the analyte. These characteristics are in turn determined by the physical and chemical state of the analyte at the end of the “ash” stage. However, because the individual atomic residence times within the atomization zone of this furnace are independent of the volatilization characteristics
of the analyte, the time integrated atomic absorbance signals are not different. In other words, it is not necessary to experiment with different “matrix modifiers” and “ash”programs until a set of conditions are found under which the analyte in both the sample and the standard volatilizes in exactly the same way in order to get an unbiased analytical result. Koirtyohann, Glass, and Lichte (11)recently reported that the use of perchloric acid in sample preparation caused essentially total suppressionof thallium signals (as well as those of some other metals) when analyses were attempted with a conventional Massmann furnace (a Perkin-Elmer HGA 2100). Figure 10 depicta analytical signals seen with the dual-power supply furnace for thallium in both 0.16 M nitric acid and 0.5 M perchloric acid. A 10-s “ASH” program at about 250 “C was used prior to volatilizing the analyte into the preheated Massmann tube (2540 K). A 25:l argon to propane gas mixture at a flow rate of 8 L/min was used to sheath the graphite parts. As was the case for lead in the experiment described above, differently shaped atomic absorption signals are observed for thallium in the two different sample matrices. However, as explained previously both in this paper and in Sturgeon’s review article (12),the best measure of analytical response in isothermal furnaces is the integral, not the peak height. The mean recovery of thallium from the perchloric acid relative to from the nitric acid (three replicates of each experiment) was 103%. The pooled relative standard deviation for the experiments was 4.1% which makes a definite statement about the existence (or absence) of a matrix effect difficult to justify: if there is one, it is small. When the experiment was repeated with argon alone as the sheath gas, the apparent recovery of thallium in the perchloric acid dropped from 103% to 77%. The reason for this is unproven but is probably due to the presence in the CRA cup of an oxygen and/or chlorine residue formed when perchloric acid is evaporated from graphite (13). The covolatilization of this material with the analyte during the ensuing “atomize” cycle would explain the reported signal suppressions. The pyrolysis products of the propane used in the first set of experimental conditions are good “scavengers” for both chlorine and oxygen (the carbonaceous moiety for oxygen and the hydrogen for chlorine). The recovery of thallium signals from sample solutions containing MgClz was studied by Manning and Slavin using an HGA 2200 furnace modified so that samples were introduced into it on a tiny coil of tungsten wire inserted after the furnace had achieved thermal equilibrium (9). These experiments were repeated with this furnace with the results shown in Figure 11. A MgClz concentration of 4% (200 pg of the salt) was necessary to reduce the thallium signal recovery by 50% when an atomizationzone temperature of 2650 K was used. This concomitant concentration is 20-fold higher than that which the temporally isothermal system described
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
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Figure 11. Percent integrated thallium signal recovery as a function of the amount of MgCi, present in the sample aliquot.
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Flgure 12. Cadmium signal recovery In a raw urine matrix: (A) cadium in 0.01% “0,; (B) urine alone: (C) urine “spiked” with cadmium: (D) background “smoke” signals from urine (dotted line).
by Manning et al. could accommodate, even when their furnace was used at a nominal atomization temperature of 2973 K (9). The probable reason for the difference in performance is that the Massmann atomizer tube used in this study is spatially as well as temporally isothermal. In contrast, the HGA 2200 furnace, as do most commercial versions of the basic Massmann design, possesses cool ends through which the cloud of volatilized sample must pass when the furnace is used in the “gas stop” mode usually recommended when a “platform” or “probe” is to be used. Analyte atom-matrix concomitant chemical association occurs at these cool zones reducing the atomic absorption signals seen when matrix is present. Another consequence of the cool tube ends is that some of the molecular species which cause much of the nonatomic absorption background signal are also formed there. With the furnace described in this paper, these “background” signals are often much smaller than those observed in conventional Massmann furnaces. A graphic example of the practical utility of this furnace is depicted in Figure 12. Basically, what the figure demonstrates is a signal recovery of 97% when a 50-pg ”spike” of cadmium was added directly to a 5-hL aliquot of a pooled “raw” human urine specimen. (The mean recovery for six such experiments was 99.2% with a relative standard deviation of 3.4%.) No “matrix modification”or elaborate prior chemical or thermal sample pretreatment procedure was used. The sample aliquot was simply dried at about 95 “C, “ash”ed for 10 s a t approximately 300 “C in order to drive some of the more volatile organic compounds out of the cup, and finally volatilized into the preheated atomization zone (2500 K). The temperature of the CRA cup was raised a t a rate of approximately 300 K/s to a fiial temperature of 1500 K. The dotted line on the figure is the nonatomic background absorbance signal measured with the hydrogen continuum lamp. The initial “smoke” peak occurs when the matrix material volatilized from the CRA cup (and then condensing in the
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Massmann tube) during the “dry” and “ash” cycles is revolatilized as the tube’s temperature starts to rise. The final nonatomic absorbance peak is probably due to the bulk of the matrix salts volatilizing after the cadmium has left the tube. The preceding examples demonstrate the relative “ruggedness”of determinations performed with this furnace as compared to those done with systems which do not permit independent control of volatilization and atomization conditions. In the examples shown, no extensive effort was expended in attempting to discover the “optimum” set of analytical conditions for each analysis because the additional effort would not have resulted in significantly improved analytical results. When conventional atomizers are used for analysis, it is usually the research effort necessary to find a satisfactory set of experimental parameters (suitable “matrix modifier(s)” and “ash” program) that makes the overall procedure tedious and “difficult” not the final determination itself. The extremely high prices of the latest commercial GFAAS systems can be largely attributed to the degree of automation incorporatedinto them in order to make these “in situ” sample preparation steps less tedious and more precise. This furnace in its present state of development represents only the writer’s first attempt at implementingcontrol of both of the fundamental processes taking place in GFAAS determinations. However, it does combine the stable, high-temperature, atomization environment seen in L’vov (3,141and Woodriff (15,16) furnaces with the useful “in situ” thermal sample pretreatment capability and volatilization control characteristics of the commercially available furnaces. The following changes would further enhance the practical utility of this furnace. First, the electrode blocks should be made of nickel-plated brass instead of aluminum (which tends to oxidize). Second, the addition of temperature feedback control circuitry to the “ash” stage of the CRA power supply would be desirable. Third, the Massmann tube should be heated during the “ash” stage to prevent volatilized matrix material from condensing there. Fourth, and finally, better temperature regulation (f10 K) of the Massmann tube would permit the accurate routine analysis of a number of metals using nonresonance atomic lines (17).
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ACKNOWLEDGMENT The author thanks both Norm Graham and Hany Brinkley for help in the construction of the atomizer. He also thanks L. C. Lewis for providing both the helpful advice and moral support necessary for the continuation of this line of research. LITERATURE CITED (1) Siemer, D. D. Appl. Spectrosc. 1979, 33. 613. (2) Siemer, D. D.; Baldwin. J. M. Anal. Chem. 1980, 5 2 , 295. (3) L’vov, B. V. Spectrochim. Acta, Part 8 1978, 338, 153. (4) Siemer, D. D. Anal. Chem. 1982, 5 4 , 1659. (5) Siemer, D. D. Appl. Spectrosc., in press. (6) Slemer, D. D.; Lewis, L. C. Anal. Chem. 1983, 5 5 , 99. (7) Czobik, E. J.; Matousek, J. P. Anal. Chem. 1077, 5 0 , 2. (8) Lundgren, G.; Johansson, 0. Talanta 1974, 2 1 , 257. (9) Manning, D. C.; Siavln, W.; Myers, S. Anal. Chem. 1979, 51, 2375. IO) Manning, D. C.; Slavin, W. Anal. Chem. 1979, 51, 261. 11) Koirtyohann, S. R.; Glass, E. D.; Lichte, F. E. Appl. Spectrosc. 1981. 3 5 , 22. 12) Sturgeon, R. E. Anal. Chem. 1077, 49. 1255A. 13) Siavin, W.; Manning, D. C. Prog. Anal. At. Spectrosc. 1982, 5 , 243. 14) L’vov, B. V. ”Atomic Spectroscopy”; Isreal Program for Scientific Translation: Jerusalem. 1969. (15) Hagemann, L.; Mubarek, A.; Woodriff, R. Appl. Spechosc. 1079, 33, 226. (16) Hagemann, L.; Nichols, H. A.; Viswandham, P.; Woodriff, R. Anal. Chem. 1979, 5 1 , 1406. (17) Slemer, D.; Stone, R. Appl. Spectrosc. 1975, 2 9 , 240.
RECEIVED for review September 23,1982. Accepted January 12, 1983.
