Report
Atomic Absorption T Instrumental refinements have given AAS its power as an analytical technique, but applications were the key to its acceptance
J a m e s W. Robinson Louisiana State University 472 A
he development of atomic absorption spectroscopy (AAS) from a physical phenomenon to a routine analytical procedure is a textbook case of the multiple steps necessary for this type of transformation. AAS had a signal intensity that was almost independent of absorption wavelength or the temperature of the system and had virtually no interelement absorption interferences—overcoming major problems in flame photometry and emission spectrography. But this phenomenon lay dormant for a number of years. Somewhere in our school days we learned about Fraunhofer's early observations and about the fact that mercury vapors absorbed some mercury emission lines. But no one suggested that these physical phenomena could provide the basis for a new analytical technique. No one, that is, until Alan Walsh (i) of Melbourne, Australia, and C.T.J. Alkemade (2) of The Netherlands independently published papers in 1955 demonstrating AAS as a manageable laboratory technique for quantitative elemental analysis. Each proposed a viable analytical procedure. Alkemade's method was more "analytical" in its approach in that a double beam was used and corrections for background were attempted. However, flame emission was used as a light source, which effectively limited the method to group I and II elements; these were already in the domain of inexpensive flame photometry. The optical system was chopped and consequently included modulation. Walsh used single-beam optics, deliberately modulated the system to eliminate interference from flame emission, and
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used a hollow cathode lamp as the light source (Figure 1). Hollow cathodes were stable and could be used for most metals and metalloids. Their inclusion in the system extended the method's analytical range to transition and other metals. The cathodes emitted very narrow lines, which greatly reduced background absorption and increased sensitivity. Each researcher used the flame to generate free atoms, a delightfully simple step. But Walsh's equipment, which brilliantly invoked several vital steps necessary to the evolution of AAS, included a long-path Lundegârdh burner. This burner was less noisy (both physically and signalwise) and gave a greater AA signal than the total consumption burner commonly used at that time. A disappointing reception
Walsh built the instrument and presented it to the scientific world. His enthusiasm was great, but that of his audience was not. A. C. Menzies was research director of Hilger Watts, based in London and one of Europe's biggest and best instrument manufacturers at that time. He visited Walsh's labs at CSIRO Melbourne but, after extensive discussions, offered only to develop an AA accessory to the Hilger Watts 0.25-m emission spectrograph (Figure 2). It would include no modulation, no background correction, and would provide only poor resolution. This was the status of AAS when I met Walsh and heard him present the firstever lecture on the subject in the United States at the LSU Symposium on Analytical Chemistry in 1958. I was enthused by the potential of the method. Among other things, I had run an 0003 - 2700/94/0366 -472A/$04.50/0 © 1994 American Chemical Society
A View of the Early Days emission spectroscopy lab and was all too familiar with the problems of atomization, variable backgrounds, choice of spectral line, spectral interferences, and resolution requirements—problems not yet confronted in AA but ones that would have to be addressed. A.J.P. Martin (the Nobel Laureate) was at the same LSU symposium. While the three of us were enjoying a little "refreshment" at my home, Walsh expressed his frustration concerning the lack of acceptance for AA. Martin explained how he had gone through the same phase. Mikhail Tswett had demonstrated chromatography—big yawn. Martin concisely and mathematically explained how it worked—second yawn. Then Martin separated and determined the amino acids—Nobel Prize. He explained that until important quantitative analytical applications are demonstrated, not predicted, no one is more than mildly interested. And until someone manufactures a commercially viable instrument, the method does not become widespread. The way ahead was clear—develop and publicize proven useful analytical applications. At the time, I was working at Esso (now Exxon) Research in Baton Rouge, LA, where research was confined to fuels and chemicals and didn't include analytical chemistry. Although developing and modifying existing techniques for the analysis of highly variable research samples was a way of life, making long-term commitments to new and untried analytical techniques was not. However, I had managed to squeeze out some time for "nonproduction" developments in the analytical labs, and total analytical output and
quality had improved dramatically—the bottom line in industry. After several meetings and votes, my proposal to work full time on AAS was approved. There was nothing available commercially for AAS. A Perkin Elmer Model 13 IR-UV spectrophotometer offered the spectral range required, and it was chopped, which provided modulation (Figure 3). We were blessed with a glassblower (Bob Wagner, now deceased) who leaped at the chance to make hollow cathodes for many different elements. We were ready to go. Establishing basic parameters The first order of business was to use flame atomization to determine how many elements exhibited AA Inherent in this study was the choice of absorption line for each element. Grotrian diagrams were not commonly available at the time (Walsh was trying to determine carbon by AAS
when I first met him). The vital importance of using transitions originating from the highly populated ground state was understood, but the question was, which were these lines? We decided to examine reversible strong emission lines and found Brode's Chemical Spectroscopy (3) most useful. Our early results showed that AAS had widespread application to the analysis of most metals and metalloids (4) and reached far beyond flame photometry but worked with equal simplicity. The sensitivity seemed to be in the part-per-million range, and important elements such as Zn, Cd, Sn, Pb, Fe, Ni, and V could be determined with ease. At that time these elements were very difficult to quantify. For many of them, only time-consuming wet chemical methods could be used, and those required great skill and attention. However, to make the method reliably quantitative, several factors influencing
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Report the signal intensity had to be clarified and controlled. Required reading was The Spectroscopy of Flames by Gaydon (5) and Atomics and Thermodynamics by Taylor and Glasstone (6). -First, we examined the solvent effect. It was known that organic solvents en hanced flame emission signals. At that time, this enhancement was attributed to the Boltzmann distribution because the use of organic solvents increased the flame temperature, and higher tempera tures create more excited atoms. However, our studies of AA showed that organic solvents also enhanced ab sorption signals (7). Clearly, the Boltz mann distribution couldn't explain this phenomenon, because the increase took place in unexcited ground-state atom pop ulations. The reason had to be an increase in the total number of atoms generated, which would mean an increase in both excited and unexcited atoms. It became clear that atomization efficiency was a crit ical factor in the procedure and had to be controlled. We developed a mechanical burner that nebulized and fed the sample into the flame at a fixed rate independent of viscos ity, surface tension, and temperature (8). With it we demonstrated that combustible organic solvents (e.g., alcohol and hep tane) gave better results than noncombustible solvents such as water or CC14 (8). Unfortunately, the burner was stolen from a booth at an analytical conference, and we never built another. Atomization efficiency included both the rate of formation of atoms and the rate of their loss by processes such as oxida tion. The number of free atoms was the result of a dynamic equilibrium between these competing reactions that produced a steady-state concentration of free atoms. Understanding this equilibrium led us to address chemical interferences. The rate of atom formation depended on the chemical form (e.g., CaCl2 vs. CaS04) of the sample or on the predominant anion in the sample solution (9). Also, the rate of loss of free atoms depended on the chemistry of the atoms; for example, alu minum oxidized rapidly. Many studies have since been performed, and many Ph.D. theses have addressed these prob lems, and by and large the phenomenon is now understood. 474 A
It then became clear that correction for background absorption by the combus tion products in the flame would be neces sary. Running a blank was one option, but it was time consuming and often impossi ble to carry out because realistic blanks were unavailable. Measuring the back ground by using a nearby nonabsorbed emission line worked but also took time and was not completely accurate. Further more, the operator had to select the line (10). It has been my experience that this type of subjective selection should be avoided. Much better methods were developed later using the deuterium lamp or the Zeeman background correction system. The deuterium lamp technique, developed by Koirtyohann and Pickett (11) at the Uni versity of Missouri, has found universal acceptance, although it involves two light sources: the hollow cathode and the deu terium lamp. It also has complicated elec tronics to keep the readout for each light source equal to the other's and to ensure that both beams pass through the same part of the flame. The Zeeman background corrector has become the method of choice. It was first developed by Bob Dorsch (12), an engineer who was working for an Ameri can oil company. He didn't know that not all elements undergo normal Zeeman splitting (13). However, the instruments he designed that incorporated the correc tor successfully performed continuous plant stream monitoring of the concentra
Burner
Power source for hollow cathode
tion of certain elements. The first attempts at a commercial design placed the magnet around the hollow cathode, which unfor tunately became unstable. Then the mag net was placed around the atomizer, and the sample atoms were subjected to the field; this scheme worked. Zeeman cor rection is an absolute necessity for carbon atomizers because it has a fast enough response to follow the rapid "atomization" step and simultaneously correct for large background signals. Commercializing AAS
In 1962 Walsh came back to LSU and gave another exciting talk. We both went to the Pittsburgh Conference—one of the last to be held in Pittsburgh—and met with Perkin Elmer representatives Roy Sawyer, John Atwood, Marcel Golay, Dave Man ning, Herb Kahn, Dick Reese, and others. Clearly there were enthusiasts among PE's people, but there were also some skeptics. They argued that flame photom etry was already in use for group I and II elements and was inexpensive. Emission spectroscopy was commonly used for transition metals and metalloids. Would the development of AAS to displace these methods be justified or even successful? Finally, the meeting broke up without a conclusion or commitment on their part, and Alan and I went for a cup of coffee. Very dejectedly, he said, "It's like building a Hurricane [a great WWII fighter plane] and people asking, 'Will itfly?'" Shortly after this meeting, Atwood
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Figure 1 . Early atomic absorption spectrometer developed by Alan Walsh. (Adapted from Reference 4.)
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came to see our equipment at Esso Labs in Baton Rouge. We already had some routine analysis procedures in place, such as the direct determination of lead in gasoline. At the time, this was a mini-breakthrough. Tetraethyl lead in gasoline was difficult to determine because it was a volatile component in a volatile sample. Chemical treatment was almost impossible; such samples were also impossible to handle by emission spectroscopy. X-ray fluorescence was in its infancy. Our successful use of AA gave the edge to PE's "pro-AA" people, and they set to work designing a multipurpose commercial instrument—an essential step in the evolution of a new analytical technique. Later, Walter Slavin brilliantly headed up this group and developed the "Cadillac" of AA instruments. Varian and Aztec were also strong players in AAS. Jarrell Ash, guided by Fred Brech, emphasized specialized applications and simultaneous multielement determinations, which contributed significantly to progress in the field. In another development, we built perhaps the first multielement hollow cathode spectrometer for the determination of Fe, Ni, and V in crude oil—another very important analysis in the oil industry. After distilling off the small gasoline fraction in crude oil, a part of the remainder can be passed through a "cat [catalytic] cracker" to form more gasoline. Unfortunately, Fe, Ni, and V are present in crude oil as porphyrins, and if they are present at concentrations above 2 ppm, they become poisons to the Pt catalyst in the cat crackers. H2 is then produced instead of gasoline—a real process "upset." H2 is not desirable and is hard to get rid of safely. Furthermore, the whole catalyst bed is destroyed at a cost in materials of several million dollars or more, and time and product are lost, among other problems. Ideally, the oils were monitored before cracking, but in practice, the metals were determined by perchloric acid decomposition and emission spectroscopy. I have seen the porphyrins floating on top of boiling perchloric acid and refusing to decompose. Often the procedure took one or two days. Clearly this was an undesirable state of affairs, because there was only one control. By contrast, direct analysis using AAS required minimal sample preparation
Figure 2. Hilger Watts spectrophotometer adapted for AAS. (Adapted from Reference 4.)
and was a more attractive approach. The final development of a reliable real-time method took several years, but we laid the foundation. Convincing the analytical audience
In the early days, I spent a lot of time on lecture tours and visiting local ACS and Society of Applied Spectroscopy (SAS) groups, an activity that culminated in a Gordon Conference talk in 1960.1 had never been to a Gordon Conference before and was totally unprepared for the bloodbath that followed. At that time there were strong and vocal groups at Iowa State University and Cornell University vigorously championing emission spectroscopy and flame photometry as the methods of choice rather than AAS. I had only gotten to the third slide when a "friend" from Cornell said, "You're not going to give us that B.S. that Walsh has been spreading lately?" The remainder of the three hours was a most exhilarating experience. The fundamentals of flame atomic spectroscopy were examined vigorously; new problems and solutions were put on the table for review. This was exactly the reason for the Gordon Conferences. The audience participated, and the subject matter was laid bare. After three hours of "lecture-discussion," a rematch was scheduled. Finally, Jim Miller of American Can asked a leading AA skeptic, "I'm a simple analytical chemist, but I have a lab to run. If you were me, would you use flame photometry or AA?" The skeptic shuffled his feet
and then said gruffly, "AA." The phones were busy that night; the seeds of AA had finally taken root and begun to sprout. Flame AAS had become established as a method that was virtually free from spectral interferences and could be handled by analysts with a minimum of training—attractive features for industrial labs. By the time the Penn State meeting on atomic spectroscopy was convened in 1963, the technique was well established. Some very valuable applications were published in Australia, New Zealand, and the United Kingdom, which lent support, but most of the real development and commercialization effort took place in the United States. A number of manufacturers produced commercial instruments; the first was the PE Model 216 in 1961. But the PE 303, produced in 1963, was the acclaimed champion of routine analyses. Many publications, Ph.D. theses, and analyses followed the establishment of the method. The next important contribution was the nitrous oxide/acetylene flame. It was developed independently by Max Amos and John Willis (14), both from Melbourne. John Willis said he remembered his high school teacher saying that oxygen and nitrous oxide were the only gases that support combustion, so he decided to try N 2 0 with AAS. It was an instantaneous success. Elements that formed stable oxides in other flames could be determined at the part-per-million level. These included Al, Si, Be, Ti, W, and V, each of which was very difficult to determine by other techniques. But there were additional advantages
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Report
to the N 2 0/acetylene flame. It proved to be superior to other flames for the determination of most transition metals. Also, it was found in general to be superior for the determination of many metals by flame photometry and is now commonly used for that method as well. We wrote the first comprehensive book on the subject and published it in the United States in 1966 (15). It was well received, which reflected the widespread use of the method. Atomic fluorescence In our quest to understand the processes of excitation and absorption, we proposed the theory that background flame emission caused some atomic excitation with a concurrent increase in flame intensity (16). To prove this, we excited Mg atoms in an oxyacetylene flame with a Mg hollow cathode and, as anticipated, we observed atomic fluorescence. After extensive discussion with personnel from PE, it was decided that atomic fluorescence had great sensitivity, but quantitatively it would have all the problems of AAS and flame emission, and a reliable quantitative technique would be difficult to develop. After an SAS lecture in Gainesville, FL, Jim Winefordner recognized that many of the problems could be overcome by modulating the system. He and other researchers, including Tom West, demonstrated the high sensitivity of the method. But despite a Herculean effort on their part, no commercial instrument devoted solely to atomic fluorescence was developed, and the technique has never achieved the acceptance that perhaps it deserves.
atomized following a rigorous protocol, and analysis was completed in a batch process. The atom population never came to equilibrium and was therefore always suspect. In our method and Woodriff s, the sample was put onto a hot bed or tube of carbon. The carbon reduced metals to atoms and all organics or water to H2 and CO, which absorb only slightly and do not contribute significantly to the background noise. The gases were swept into a crosspiece and absorption was measured with the atom population at steady state, which improved accuracy. The method could be used on a continuous basis for liquid or gas samples and was sensitive enough to measure lead evaporating from a lump of lead metal. In retrospect, Woodriff s design was the best because he used a furnace, made entirely of graphite, that could operate at very high temperatures; ours was made of quartz. But because his design was not developed commercially, it was shelved. The opportunity is still there. These carbon atomizers were first publicized in 1969 at the conference in Sheffield. The manufacturers welcomed them with open arms and began production almost immediately. After all, superficially, here was a technique with sensitivi-
ties down to 10r14 g for some elements, and it appeared to be very simple to perform, à la flame AAS. The designs were based on the L'vov, West, and Massman models and included a program of dryash-atomize that required a different protocol for each element. But background levels were very high and variable, and some volatile elements were lost during ashing. Continuous background correction was vital. At no time was equilibrium reached in the atom population, and numerous variables affected the atomization efficiency. These parameters included the furnace heating rate and electrical resistance, the depth of sample penetration into the carbon surface, and sample composition, among others, and they varied from shot to shot. With all these variables, much greater skill was necessary on the part of the operator than was the case with flame AAS. The federal agencies in the United States jumped on the bandwagon and, in many cases, mandated carbon furnace AAS as the analytical method of choice. After all, from their point of view it was state of the art, had the highest sensitivity, and was "easy to run." To a legal mind, what could be more desirable? Many laboratories had to buy and use the spectrom-
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Carbon furnace AA
In 1961 Boris L'vov published the first paper on the use of carbon furnace AA. The increase in sensitivity was about 3 orders of magnitude, which was astounding. But the accuracy was terrible. Later, others, including Tom West, Hans Massmann, Ray Woodriff, and our group, developed alternate designs, and these were presented at the International Conference on Atomic Spectroscopy held in Sheffield, England, in 1969. The designs of L'vov, West, and Massmann were single-shot systems. A small sample was inserted into the furnace and 476 A
Figure 3. Modified Perkin Elmer Model 13 spectrophotometer. (Adapted from Reference 4.)
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following the same developmental path as eters to meet federal regulations. AAS and may be the next chapter in the But as Herb Kahn of PE put it, "The story of atomic spectroscopy. difference between flame atomizers and carbon atomizers is like the difference between watching tennis and playing tenReferences nis." Much later, the L'vov platform would (1) Walsh, A. Spectrochim. Acta 1955, 7,108. (2) Alkemade, C.T.J.; Milatz, J.M.N. /. Opt. alleviate some of the problems encounSoc. Am. 1955,45, 583. tered with carbon AAS. (3) Brode, W. R. Chemical Spectroscopy, 2nd Producing a reliable instrument that éd.; John Wiley and Sons: New York, 1943. corrected for backgrounds that were (4) Robinson, J. W.Anal. Chem. 1960,32, sometimes > 90% as well as for many other 17 A. variables was a monumental task, espe(5) Gaydon, A. G. The Spectroscopy of Flames; John Wiley and Sons: New York, 1957. cially considering that the atom popula(6) Taylor, H. S.; Glasstone, S. Atomics and tion rate never attained equilibrium. It was Thermodynamics; Van Nostrand: New a feat equivalent to balancing one needle York, 1942. (7) Robinson, J. W.Anal. Chim. Acta 1960, point on another, but some manufac23,479. turers, particularly Perkin Elmer, have (8) Robinson, J. W.; Harris, R. J. Anal. Chim. achieved success through truly outstandActa 1962,26,439. (9) Robinson, J. W.; Kevan, L. J. Anal. Chim. ing mechanical and electronic engineerActa 1963,28,170. ing. It would certainly have been easier (10) Robinson, J. W.Anal. Chim. Acta 1962, and more reliable to use Woodriff s con27, 465. (11) Koirtyohann, S. R.; Pickett, E. E.Anal. tinuous atomization design. The future AAS has now reached full bloom. Predictable extensions, such as GC/AAS and HPLC/AAS, have been demonstrated to be valuable for speciation analysis, and new applications are appearing regularly. Using a carbon tee piece and interfacing with a gas chromatograph, we were able to predict the fate of tetraethyl lead after a shipload ended up in the Adriatic Sea, decomposed, and entered the food chain (i 7). Using flame AAS with a much more efficient flame atomization system, we have been able to speciate the interaction between Zn and Cd in kidney tissue—an important health issue (18). Now, the boundary conditions of this physical phenomenon are established, and its uses as an analytical method have been studied. The tune is written and it just remains for us to sing it. What is the future of atomic spectroscopy? Perhaps the new "best method" for elemental analysis is inductively coupled plasma mass spectrometry (ICPMS). ICPMS has very high sensitivity (10~15 g or better) and few interferences; determines all elements, metals and nonmetals alike; performs simultaneous multielement analysis with ease; can get by with very small samples if necessary; can be used to analyze solid, liquid, or gas samples; and provides new information based on isotopic distribution. ICPMS is now
Chem. 1966,38, 585. (12) Dorsch, R., personal communication, 1969. (13) Hadeishi, T.; McLaughlin, R. D. Science 1971,274,404. (14) Amos, M. D.; Willis, J. B. Spectrochim. Acta 1963,22, 1325. (15) Robinson, J. W. Atomic Absorption Spectroscopy; Marcel Dekker: New York, 1966. (16) Robinson, J. W.Anal. Chim. Acta 1 9 6 1 , 24, 254. (17) Robinson, J. W.; Vidaurreta, L. E.; Wolcott, D. K.; Kiesel, E. Spectrosc. Lett. 1975,8,491. (18) Chang, P. P.; Robinson, J. W. /. Environ. Set. Health 1993, A28,1147.
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James W. Robinson is a professor of chemistry at LSU (Baton Rouge, LA 708031804), where he has taught since 1964. After receiving his Ph.D. in 1952 from the University of Birmingham (U.K.), he worked as a senior chemist at Esso Research and as a technical adviser at Ethyl Corp. Research before joining LSU. A Fellow of the Royal Society of Chemistry, Robinson has authored three books on atomic spectroscopy and instrumental analysis and currently serves as executive editor of Spec-
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Analytical Chemistry, Vol. 66, No. 8, April 15, 1994 477 A
