GRAPHITE FURNACE ATOMIC Boris V. L’vov Department of Analytical Chemistry Leningrad State Technical University Leningrad 195251 USSR
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The Second James 1. Waters Annual Symposium Recognizing Pioneers in the Development of Analytical Instrumentation was held at the 1991 Pittsburgh Conference and Exposition in Chicago. This year‘s symposium honored four pioneers in the field of atomic absorption spectroscopy: Alan Walsh, Boris l‘vov, S. R. Koirtyohann, and Walter Slavin. Here l‘vov and Walsh recount their efforts in developing AAS as an analytical method. In the November 1 issue, Koirtyohann will discuss these developments from an academic viewpoint and Slavin will explore the reasons for the success of AAS. 924 A
During the past 35 years I have had the privilege to participate in or witness many events that have resulted in the evolution of atomic absorption spectrometry (AAS). In accordance with the theme of the Waters Symposium, I would like to address these events, some of which may seem strange or irrelevant today in the context of the problems of applying and implementing new ideas in analytical instrumentation, As with living organisms, analytical methods evolve over time, and AAS is no exception. Although we speak about two independent branches of AAS that differ in methodology and are used to solve different analytical problems, we must remember t h a t both flame a n d graphite furnace (GF) AAS grew out of the same tree created through the efforts of Sir Alan Walsh. Personally, I divide the evolution of GFAAS into four periods: birth and infancy (1956-65), commercial realization (1966-75), stagnation and revival (1976-84), and absolute analysis (1985-90). Birth and infancy The events that stimulated my interest in GFAAS were described in detail in a paper written on the occasion of the Silver Jubilee of the method (1). My interest in AAS was prompted by Walsh’s famous 1955 paper (2).At that time, I was working in the isotope laboratory of the State Institute of Applied Chemistry in Leningrad. I had some experience in using atomic emission spectrosco py and recognized the difficulties associated with matrix effects and the need for standards that are similar in composition to the sample. It is no wonder that I was impressed with Walsh’s idea (2) of developing absolute AAS methods that would be free of matrix effects and the need for calibration. In my mind, the flame was inappropriate for this purpose because of
ANALYTICAL CHEMISTRY, VOL. 63 NO. 19, OCTOBER 1, 1991
incomplete analyte atomization. The traditional technique of producing a stationary vapor column in the King graphite furnace by evaporating an excess of metal was also inappropriate and, based on my experience with the King furnace a t Leningrad University, I knew that it would require a power supply of about 100 kW. (Because of the power requirements, most researchers performed their experiments at night.) Fortunately, some unused commercial equipment for fractional distillation of impurities from refractory materials was available, and after I replaced the crucible with a horizontal graphite tube, this equipment was suitable for my first experiments. At that time I believed that the only appropriate means for absolute measurements involved complete evaporation of the sample in a predetermined volume-forming, a s i t were, a cuvette. However, after the first experiments with sample evaporation from the furnace wall, I realized that it would be impossible to ret a i n t h e sample vapor inside a graphite tube during furnace heat ing, and I decided to evaporate the sample from an additional electrode introduced into a preheated cuvette (Figure 1). To accelerate the electrode heating, I used a dc arc (3)and, later, ohmic resistance of the electrode furnace contact (4).To increase the atom residence time in the cuvette, I used tubes with caps at the ends as well as a sheath gas pressure of a few atmospheres ( 4 ) . The major goal of this research was to determine the conditions suitable for absolute measurements, regardless of the technical and methodological difficulties. Although we a t tained this goal ( 4 ) , operating the instrument required much experi ence. We understood only later that our efforts to accelerate the sample evaporation and reduce the vapor diffusion were unnecessary because the duration of sample evaporation in a n isothermal furnace does not affect the absorption peak area. At the same time, the experience gained and some of the ideas proposed or realized during this period-including development of radio frequency powered electrodeless dis 0003-2700/91 /0363-924A/$02.50/0 0 1991 American Chemical Society
ABSORPTION SPECTROMETRY charge lamps (EDLs), use of pyrolytic graphite for tube coating, development of an automated version of the D,-corrector proposed earlier by Koirtyohann and Pickett (51, and theoretical and experimental substantiation of the integrated absorbance (peak area) measurementturned out to be fruitful for further GFAAS development. All of these ideas were subsequently implemented in commercial instrumentation. Commercial realization In the mid-l960s, 10 years after beginning our investigations, we u n derstood t h a t distribution of t h e graphite cuvette to analytical laboratories was improbable. A subsequent attempt to build the entire instrument ourselves and offer it for sale was unsuccessful. The only instrument built (Figure 2) was installed at t h e laboratory of t h e Institute of Medical and Biological Problems in Moscow (6). It was resold soon after that and subsequently disassembled. A later attempt (in the 1970s) to manufacture the graphite cuvette in combination with a Saturn-1 spectrometer (developed in the Severodonetsk branch of the R&D Bureau for Automation) had the same unhappy ending. Hans Massmann played a decisive role in the further fate of GFAAS. Massmann became interested i n graphite furnaces in the early 1960s, after reading our first publication in Russian (3). Because the work had he not yet appeared in English started working with the graphite furnace before other Western scientists, and in 1965 he presented the results of his studies a t the Reinstoff Symposium in Dresden (8). I n Massmann’s furnace design (Figure 31, he renounced the introduction of the sample into a preheated tube on a n additional electrode and returned to the idea I had rejected after my first experiments: evaporation of samples directly from the furnace wall. As expected, we lost the main advantage of the graphite cuvette: analytical results that are independent of the matrix composition. Another key point in the history of commercial GFAAS instrumentation was the First International Confer-
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ence on Atomic Spectroscopy, held in Prague in 1967. This was my first time abroad, and I was happy to meet there with Massmann, C.T.J. Alkemade, Velmer Fassel, Walter Slavin, and other scientists whom I knew before only by reputation. I was flattered by Slavin, who asked my opinion about the possibility of commercial production of graphite furnaces. I wanted very much to establish business relations with Perkin Elmer, but I was aware of the troubles these contacts would cause when I returned to my institute, which was closed to foreigners. I was also afraid that the comparatively complex procedures necessary for analysis with the graphite cuvette would discourage people from using the furnace. Despite my doubts, however, I recommended that Slavin use the Massmann design as a prototype for the commercial furnace. The sensitivity of the Massmann furnace was only slightly inferior to that of the graphite cuvette, and the Massmann
furnace permitted introduction of larger volumes of solutions-including organic solutions-and their preliminary pyrolysis. Slavin left Prague to attend a Perkin Elmer technical planning meet ing in the Bodenseewerk plant on the shore of Lake Constance in southern Germany. By the end of that weeklong meeting, Slavin’s colleagues a t Bodenseewerk had agreed to a p proach Massmann with the idea of building a prototype furnace (9). Bernhard Welz, who had just started working at Bodenseewerk Perkin Elmer, recalls visiting Massmann in the spring of 1968; six months later he started working on applications with the first prototype in the Bodenseewerk laboratory (10). In April 1970 Perkin Elmer introduced the first commercial graphite furnace under the trade name HGA70, and in June 1970 Manning and F e r n a n d e z , close c o l l e a g u e s of Slavin’s, published a paper on the use of this instrument for characterization of biological materials (11).
This paper confirmed a 100-fold gain in sensitivity compared with that of flame AAS and the ability to analyze extremely small samples. The introduction of this first commercial instrument began a new period in the development of GFAAS, this time as a new technology in analytical instrumentation. Stagnation and revival As expected, the first experiments with the commercial atomizer based on the Massmann furnace design revealed a high level of nonselective interferences and strong matrix effects. As analytical applications of t h e graphite furnace expanded, these shortcomings became more obvious. Despite improvements in the atomize r design-including reducing the size of the graphite tube, flushing the tube with gas, and automatic sampling of the solution into the furnace-spectral interferences, calibration instability, a n d m a t r i x effects remained major obstacles to further development of the method. Statements such as this were typical: “It has become apparent for some time that the greatest barrier to the acceptance of flameless atomization as a normal tool in atomic absorption spectroscopy is its susceptibility to matrix interferences’’ (12). In the mid-l970s, interest in the graphite furnace began to wane, as shown by the smaller number of pub lications in the field (1).The future of the method was in jeopardy. Because these trends were related to dissatisfaction with the Massmann furnace, I felt uncomfortable about having recommended it to Slavin. Then, in March 1975, I was invited to head the Department of Analytical Chemistry of the Leningrad Polytechnical Institute (LPI). An attractive aspect of this transfer was the possibility of closer contacts with colleagues abroad and attendance at international meetings. I met with Sabina Slavin and with Lennart Sjodell, Perkin Elmer’s USSR sales manager, at a seminar in Novosibirsk in April 1975 and proposed a collaboration between Perkin Elmer and LPI. At the end of that year, an official protocol on cooperation w a s signed, a n d i n autumn 1976 Perkin Elmer made
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
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available to us a Model 603 spectrometer with an HGA-76 furnace. For the first time in my 20 years of work in AAS, a n excellent commercial instrument was available! Upon receiving a n invitation to attend the 6th International Conference on Atomic Spectroscopy in Phil-
adelphia in 1976, I decided to talk about the current situation in GFAAS. Convinced t h a t the main source of trouble was the temporal nonisothermality of the atomizer in the course of sample evaporation, I formulated the problem in the following way: “Are there any prospects for
Figure 1. Original graphite cuvette. Components: 1 , cuvette; 2, movable electrode with sample; 3, counter electrode; and 4, arc gap. (Adapted with permission from Reference 3.)
Figure 2. AAS instrument for use with the graphite cuvette. (Adapted with permission from Reference 6.)
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the improvement of the now popular atomizers while preserving a t the same time the inherent simplicity of their analytical procedure?” (13). To solve this problem, I decided to test two techniques: sample evaporation from a platform and pulsed furnace heating by capacitive discharge. The idea of the platform came from my previous experience in measuring the furnace temperature using a graphite disc placed perpendicular to the tube axis. Because it took longer to heat the disc than to heat the wall, pyrometric measurements were difficult. In the analytical case, however, it was just what we needed. To check the efficiency of the technique, I asked Larissa Pelieva, who worked at the Severodonetsk Bureau for Automation and was later one of my graduate students, to perform some experiments on a commercial HGA-74 furnace. (I did not yet have any commercial instruments in my lab.) These experiments met my expectations, and the platform furnace was “born” (Figure 4). To ensure support of the Severodonetsk instrument manufacturers in possible commercial production of the platform, I included A. I. Sharnopolsky, head of the department where Pelieva worked, in the author list of our platform paper (14).Alas, this tactic did not stimulate in any way the commercial production of platforms in the USSR; moreover, a t a joint Perkin Elmer-LPI seminar on AAS in 1983, Sharnopolsky severely criticized the platform furnace system. He was not alone in this criticism: “atomization from a platform has not brought the commercially available atomizers a great deal closer to the ideal case described by L’vov” (15). The one person who immediately recognized the significance of the platform for GFAAS and, by the way, proposed this name in place of the original “support,” was Slavin. On his recommendation, Perkin Elmer manufactured a commercial version of the platform in 1978, and in 1979 Slavin and Manning concluded that “the addition to the graphite furnace of a thin pyrolytic graphite plate (L’vov platform) on which the sample is deposited makes it possible to atomize the sample a t more nearly constant temperature conditions” (16). Subsequent events surpassed even my most optimistic expectations. Using the platform furnace and other techniques, Slavin and co - workers (17) developed the analytical system now known as the stabilized temperature platform furnace (STPF). I t eliminated or substantially reduced
REPORT matrix interferences and, in most cases, made it possible to perform analysis by a simplified calibration with reference solutions containing only the analyte and the modifiers. The STPF system did not become universally accepted very quickly. Tedious work was needed to clarify the advantages of the system over traditional methods, specifying the
Figure 3. Original Massmann furnab=. (Adapted with permission from Reference 8.)
significance of each condition and, most important of all, demonstrating its merits in particular situations by analyzing real samples. As a result of the efforts of Slavin and colleagues, as well as the support of several leading researchers such as Koirtyohann (18),this system has finally gained universal recognition. The STPF system not only improved the reputation of GFAAS but also generated new interest in the method, such as the use of tantalum platforms in the atomization of alkaline-earth and r a r e - e a r t h metals (19).In addition, the STPF system was increasingly used for direct analysis of solid samples as slurries, which can be analyzed with the same simplicity of calibration as solutions. The automation of experimental techniques that started in the mid1970s also played a part in the successful implementation of the plat form and the STPF system in GFAAS (see Table I). Indeed, the use of the platform would not have been so efficient without autosamplers, fast heating of the furnace, fast electronics, Zeeman effect background cor rectors, and experience in matrix modification. From this point of view, the solution to the furnace problem appeared at just the right time.
Toward absolute analysis Despite all the changes in the tech-
nology and methodology of GFAAS during the 30 years required to go from t h e graphite cuvette to the STPF system, we did not abandon our goals of eliminating matrix interferences and stabilizing calibration methods as well as developing a calibration method based on fundamental constants and actual measurement conditions. The first two problems were solved during development of the STPF technique, which opened the door to the solution of the last problem, absolute calculation of sensitivity. By the mid-l980s, the theory of formation of the analytical signal was fairly well developed, and the fundament a l c o n s t a n t s r e q u i r e d for t h e calculation (oscillator strengths, damping constants, and hyperfine structure of the analytical lines) were available. In addition, experimental data on the characteristic masses of a large group of elements, which could be used for comparison with the calculations, had been obtained. In spring 1985 we organized the Fifth Seminar on AAS at LPI. When thinking about a possible topic for my presentation, I turned my attention to the theoretical calculation of sensitivity of the HGA furnace for the first time in the 15-year existence of these devices. The results turned out to be more than just enL For the 30 elecouraging (Figure 5).
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Figure 4. Platform in a graphite tube.
Table 1. STPF components 0
Year Techniaue
PrODOSed Realized
Platform furnace Internal absorbance Pyrocoated tubes Autosampling Fast heating Fast electronics Matrix modifiers Background correction by D, lamp by ac Zeeman
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1977 1968 1963 1972 1974 1975
1978 1976 1978 1975 1976 1978 1975
1965 1975
1968 1981
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Figure 5. Correlation of experimental and calculated characteristic masses.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1,1991
ments measured under the STPF conditions, the mean value of the ratio m,(calc)/m,(exp) was 0.90 with a standard deviation of 0.25 (20). We were very inspired by these results, and after five years of subsequent work, including critical selection, calculation, and measurement of a number of fundamental constants as well a s checking and updating the experimental values of m,, we were able to reduce the standard deviation of t h e measurement to 10% (21).These data were confirmed by Frech and Baxter (22). I t is not surprising that GFAAS has come much closer to absolute analysis than other spectrochemical techniques, including flame AAS. This success can be attributed to the unique properties of the isothermal graphite tube atomizer, which ensures complete sample atomization; a known vapor column geometry; and a known and temporally constant residence time of atoms in the analytical zone, regardless of the matrix composition of the material. I believe that practical realization of absolute analysis is limited not by any technical reasons or unaccept ably high errors but by a purely psychological barrier resulting from a lack of belief in its possible attainment. Therefore promotion of this approach should be started, as recommended by Holcombe and Hassell (23),with “education of the regulatory agencies, the plant engineers, and the analysts.” The availability of calculated Values of sensitivity is an efficient tool for revealing instrumental and methodological errors associated with incorrect selection of analytical conditions or preparation of calibration solutions, for checking the quality of manufactured instrumentation, and for evaluating the ultimate possibilities and choosing the proper ways to improve the techniques.
Conclusions
I would like to stress that scientists, engineers, and analysts of many countries have taken and continue to take an active part in the development of this method. In my opinion, however, during the past 20 years, Perkin Elmer has been an indisputable leader in the field of GFAAS instrumentation, and I am privileged to have had a productive and pleasa n t cooperation with this company for nearly a quarter of a century. But I would like to make a number of more general conclusions as well. First, ascending a staircase is easier if you go in small steps. (From this
point of view, the progression from the graphite cuvette to the presentday STPF technology through the Massmann furnace and the platform was the optimal strategy.) Second, most instrumental methods grow out of the needs of r e searchers, who eventually are rewarded with more precise, sensitive, and reliable instruments. Thus applications of the method, previously used only in research, promote efficient use of it in fundamental studies. (The latest achievements in absolute analysis are the best proof of this.) Finally, success in implementation of new ideas depends not only on their merits but also on the talent of the people selecting and transforming these ideas into the technical policy of a company. (Meeting such people is more difficult than finding a good idea. I was lucky in this respect .)
References (1) L'vov, B. V. Spectrochim. Acta 1984, 398, 149. (2) Walsh, A. Spectrochim. Acta 1955, 7, 108. (3) L'vov, B. V. Spectrochim. Acta (Engl.
Trans.) 1984, 39B, 159; Inzh. Fiz. Zh. 1959, 2(2), 44. (4)L'vov, B. V. Atomic Absorption Spectrochemical Analysis (in Russian); Nauka: Moscow, 1966. ( 5 ) Koirtyohann, S. R.; Pickett, E. E. Anal. Chem. 1965,37,601. (6) Katskov, D. A.; Lebedev, G. G.; L'vov, B. V. Zauod. Lab. 1969,35, 1001. (7) L'vov, B. V. Spectrochim. Acta 1961, 17, 761. (8) Massmann, H. Presented at the Second International Symposium on "Reinst stoffe in Wissenschaft und Technik"; Dresden, East Germany, 1965; paper 297. (9) Slavin, W. Spectrochim. Acta 1984,39B, 139. (10) Welz, B. In 20 Years ofGFAAS by Perkin-Elmer; Bodenseewerk Perkin- Elmer GmbH: Uberlingen, Germany, 1990, p. 14. (11) Manning, D. C.; Fernandez, F. At. Absorpt. Newsl. 1970, 9, 65. (12) Aggett, J. Presented at the Fifth International Conference on Atomic Spectroscopy, Melbourne, Australia, 1975. (13) L'vov, B. V. Spectrochim. Acta 1978, 33B, 153. (14) L'vov, B. V.; Pelieva, L. A.; Sharnopolsky, A. I. Zh. Prikl. Spektrosk. 1977, 27, 395. (15) Gregoire, D. C.; Chakrabarti, C. L. Anal. Chem. 1977,49,2018. (16) Slavin, W.; Manning, D. C. Anal. Chem. 1979, 51,261. (17) Slavin, W.; Manning, D. C.; Carnrick, G. R. At. Spectrosc. 1981, 2, 137. (18) Koirtyohann, S. R.; Kaiser, M. L. Anal. Chem. 1982,54, 1515 A.
(19) L'vov, B. V. J. Anal. At. Spectrom. 1988, 3, 9. (20) L'vov, B. V.; Nikolaev, V. G.; Norman, E. A.; Polzik, L. K.; Mojica, M. Spectrochim. Acta 1986,41B, 1043. (21) L'vov, B. V. Spectrochim. Acta 1990, 458, 633. (22)Frech, W.; Baxter, D. C. Spectrochim. Acta 1990,45B, 867. (23) Holcombe, J. A.; Hassell, D. C. Anal. Chem. 1990,62, 169R.
Boris K L'vov received a degree in physics from Leningrad University in 1955 and perf rmed his pioneering work in graphite firnace AAS at the State Institute of Applied Chemistry in Leningrad. Since 1975 he has been head of the Department of Chemistry at the Leningrad State Technical University. His research goals are to eliminate matrix inteflerences and develop a calibration method based onfindamental contants and actual measurement conditions.
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