THEDEVELOPMENTOF ATOMIC ABSORPTION METHODS OF ELEMENTAL ANALYSIS 1952-1 962 Alan Walsh CSIRO Division of Materials Science and Technology Locked Bag 33 Clayton, Victoria 3186 Australia
In the 1950s, research a t the Australian Commonwealth Scientific and Ind u s t r i a l Research O r g a n i s a t i o n (CSIRO) drew attention to previously unsuspected potential advantages of AA methods, as contrasted to the usual atomic emission (AE) methods of elemental analysis. Although early publications describing the method aroused little interest, the CSIRO AA spectrometer developed for making the required measurements was pat ented with a view to licensing suitable manufacturers. The first licensee, Hilger & Watts Ltd. in England, produced a n AA spectrometer by adding a n attachment to their Uvispek spectrophotometer. The fact that it lacked one of the main features of the CSIRO AA spectrometer-namely, a means for rejecting signals caused by flame emission-together with a general lack of interest in CSIRO’s research, led CSIRO to design a simple AA spectrometer suitable for manufacture in Australia. Locally made components, together w i t h s u i t a b l e monochromators, atomizers, a n d spray chambers imported from overseas, were assembled by the customer under CSIRO’s guidance. From 1958 to 1962 these “do-it-yourself” kits proved highly successful and created in Australian analysts a great enthusiasm for AA methods. In the meantime, some manufacturers in the United States had also made AA 0003-2700191 /0363-933A/$02.50/0 0 1991 American Chemical Society
I instruments by adding attachments to existing spectrometers. In 1962 Techtron Pty. Ltd. (Melbourne) and the Perkin - Elmer Corporation em barked on the production of instruments designed specifically for using flame AA methods, which, a century after their development by Kirchhoff and Bunsen, found rapidly widening acceptance. These events illustrate how basic research can have a direct industrial spin-off, and they also remind inventors t h a t profitable licensing requires rapid development of their inventions. Foundations The foundations of spectrochemical methods of elemental analysis were laid in 1859-60 by the classic works of Kirchhoff and Bunsen ( I , 21, who showed that the AE or AA spectrum of an element could serve as a fingerprint to permit the identification and/or detection of t h a t element. They made no comments regarding the relative merits of using AE or AA spectra, a n d t h e development of
quantitative spectrochemical meth ods of elemental analysis following these great discoveries was extreme ly slow. It was not until the 1930s that spectrochemical methods found any appreciable applications, and these were based almost entirely on microphotometric measurements of photographed emission spectra. During World War 11, however, the field was revolutionized, largely as a result of developments in the United States. Photoelectric methods of recording and measuring spectra began to replace photographic methods, permitting simultaneous multiele ment analysis and significantly im proving the precision and accuracy of analysis. While these remarkable advances were being made, methods based on AA measurements continued to be almost entirely ignored. Early in 1952 I had the good fortune to become interested in this strange neglect of Kirchhoff’s and Bunsen’s pioneering experiments in AA. My interest resulted from two interrelated experiences: the spectrochemical analysis of metals during the period 1939-46 and molecular spectroscopy during 1946-52. This interaction led me to wonder why, as was my experience, molecular spectra were usually obtained in absorption and atomic spectra in emission. The result of this musing was quite astonishing. I could find no good reason for neglecting AA spectra; on the contrary, it appeared that they might offer many vital advantages over AI3 spectra. The most intriguing of these potential advantages related to absolute analysis. My experiences with spectrochemical emission methods of ele mental analysis had convinced me
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1,1991
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REPORT that their main limitation arose from the necessity to base analyses on comparison of the intensities of lines in the spectrum of the sample with intensities of the same lines in spectra of reference standards of known composition. It seemed to me that the ultimate aim of spectrochemical research must surely be the development of absolute methods of analysis. But the prospect of achieving this with emission methods seemed slim. Although we did not explore further the fascinating possibility of absolute analysis, Boris L‘vov accepted the challenge and, as a result, created graphite furnace atomic absorption spectroscopy. In his recent work in collaboration with Walter Slavin and his colleagues a t Perkin Elmer, he has achieved absolute analysis for some determinations. (See the related article on p. 924A of this issue.) There seems little doubt that in the near future such methods will become applicable to a n increasing number of analyses. First experiments In my first thoughts about AA methods, I had no preconceived preference for the means to be used to generate an atomic vapor of the sample. After some consideration I decided that the choice would be between arcs and sparks, with which I was familiar, or flames, with which I had had no previous experience. Arcs and sparks appeared to have the widest range of possibilities. Flames were far simpler than arcs and sparks but had a limited range of applicability. In my first internal CSIRO report I proposed that our first experiments use a LundegArdh flame, in which the analyte solution is atomized into the air supply of the flame (3).We found such premix laminar flow flames much superior to the turbulent flames produced by direct-injection burners. These flames can opera t e a t t e m p e r a t u r e s of 17002400 “C, depending on t h e fuel/ oxidant combination. T h e g r e a t majority of atoms in the flame are in the ground state. Because the strongest absorption lines are those attributable to transitions from the ground state to a low energy level, absorption, in contrast to emission, is not critically dependent on temperature, or on the energy difference between the two levels involved in the transition. Herein lies the main difference between AA and AE.AA lines originate from transitions from the ground state, whereas emission lines originate from transitions from an excited state to some lower state. 934 A
In that first report on the development of AA methods I presumed that the light source would be one that emitted a continuum spectrum over an extended wavelength range, such a s a hydrogen or a tungsten lamp. However, when I performed my first AA experiment I decided to measure the absorption of radiation from a sodium lamp, operated a t the lowest power a t which the lamp could function properly, in order to avoid excessive line broadening resulting from self-absorption or self- reversal. Regarding possible experimental problems, I was particularly fortunate in one respect. For several years prior to these first thoughts on AA, I had been regularly using a commercial IR spectrophotometer with a modulated light source and a synchronously tuned detection system. Under this arrangement, any radiation emitted by the atomized sample does not produce a signal a t the output of the detection system. This experience had no doubt prevented the formation of any possible mental block associated with absorption measurements on luminous atomic vapors, and I suggested t h a t the same type of system could be used for recording AA spectra. I pointed out that if the sample were vaporized by the usual methods (such a s flame, arc, or spark), the emission spectrum could be “removed” by modulating the light from the source-but not from the vaporized sample-and using an ac detection system tuned to the modulation frequency. My first experiment, merely to obtain the feel of AA measurements, was a n extremely simple one. I passed radiation from a sodium lamp through an air-coal gas laminar flow flame into which I sprayed a water solution containing a few ppm of sodium. T h e e m e r g i n g b e a m w a s passed through a small spectrometer to isolate the sodium D lines, which fell on a photoelectric detector, the output from which was passed to an ac amplifier on a small oscillograph. Because the lamp was operated at 50 Hz, it produced alternating signals but no signals in the amplifier output resulting from unmodulated radiation emitted by the flame. The deflection of the cathode spot served as a measure of the signal resulting from sodium D lines emitted by the lamp and falling on the photodetector. The photograph in Figure 1 indicates clearly the pronounced absorption of sodium light as i t passes through the flame. This simple experiment gave me a great thrill, and I excitedly called in
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1,1991
Figure 1. Absorption in a flame of light emitted by a sodium lamp. Absorption occurs while a water solution containing a few ppm of sodium chloride is sprayed into the flame.
my colleague, John Willis, who a t that time was working on IR spectroscopy and was later to make important contributions to the development of AA methods of chemical analysis. “Look,” I said, “that’s atomic absorption.” “So what?’ was his reply, which was the precursor of many similar disinterested reactions to our AA project over the next few years. In January 1953 we obtained poor sensitivity for the determination of copper when using a continuum source and a monochromator made by placing a slit and a photomultiplier tube on the focal curve of a Hilger Littrow spectrograph. We concluded that the poor sensitivity was a result of the low resolution of the Littrow spectrograph a n d t h e excessive amount of scattered light a t low wavelengths. We proposed overcom ing this difficulty by using a copper hollow-cathode lamp (HCL) as the source. Because the HCL would emit sharp lines, a low-resolution spectrometer would then be sufficient. Figure 2 illustrates a typical HCL; its use as a sharp line source to measure peak absorption is illustrated in Figure 3. In this case, the function of the monochromator is to isolate the required line from all other lines emitted by the source. The high resolution required for AA measurements is, in effect, provided by the sharp line source. Patent development At this stage we had arrived a t the
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REPORT principle of the technique that in due course became the generally accepted method of making the intensity measurements required in AA methods of chemical analysis based on flame atomization. We then began preparation of our patent application covering our AA spectrometer. Lloyd Rees, the leader of CSIRO’s Chemical Physics Laboratories, in which the atomic absorption project was being undertaken, decided that we would also make every effort t o interest Australian industry i n the further development of our spectrometer with a view to undertaking its subsequent manufacture in Australia. In 1953 I visited laboratories in the United States and the United Kingdom and discussed with some spectroscopic equipment manufacturers our hopes for AA methods of analysis. U n d e r s t a n d a b l y , o u r work aroused limited interest because we had not yet published any results based on real analytical problems. I was nevertheless greatly encouraged when A. C. Menzies of Hilger and Watts Ltd., London, expressed great interest in our work on AA and promised to keep it in mind. (I have always been grateful for his moral support.) The next significant event was the first public exhibition of the working AA spectrophotometer (Figure 4) in March 1954 at Melbourne University as part of an Exhibition of Scientific Instruments, arranged by the Australian Branch of the (British) Insti-
tute of Physics. The apparent complexity of the instrument resulted primarily from its double-beam de sign, which in our early experiments we regarded as essential because of the poor stability of some of our early HCLs. (Viewers were possibly further confused by the optical path being in opposite directions on the instrument and on the explanatory diagram.) Whatever the reason, the instrument aroused little interest during the three days it was exhibited. However, when Menzies visited Melbourne shortly afterward to assess its performance, he was sufficiently impressed for his firm to decide to produce, under license to CSIRO, t h e first commercial AA spectrophotometer. As soon as our complete patent specification (5) was published on October 21, 1954, I submitted to Spectrochimica Acta my first paper on AA spectroscopy (6). It was published early in 1955, at about the same time as the paper by Alkemade and Milatz ( 7), who had independently devel oped the AA method. Neither paper created any great interest, however, and Alkemade did not pursue his work further. (It is sad indeed that Alkemade was unable to take his rightful place at the Waters Symposium. Not only was he a pioneer of AA and atomic fluorescence spectroscopy, but his studies of the chemistry and physics of flames resulted in an immense contribution to the understanding and development of flame spectroscopy.) I remember being surprised and disappointed by what appeared to be an almost total lack of interest in my
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Figure 2. Early type of HCL for use in AA measurement. (Adapted with permission from Reference 4.)
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Figure 3. Diagram of the use of a sharp line source to measure peak absorption.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
1955 paper. Nevertheless I felt that its publication some four years after we started our AA project was fairly good progress. We had given our reasons for investigating the potential of AA methods and had developed a simple and inexpensive method of making t h e necessary measure ments. I n addition, a prestigious spectroscopic instrument manufac turer had obtained a license to manufacture. In 1956 my colleague J. P. Shelton and I reported our latest progress at a conference in Lisbon (8)as well as at a meeting of the Institute of Physics in London, where the audience described our AA work as “extremely interesting” but of no practical significance. At each lecture Shelton stressed that the performance of our instrument was limited by the difficulty of completely atomizing the sample. He showed, however, that “the sensitivity was not, as in emission methods, a function of the excitation energy of the line. Thus zinc and sodium have comparable sensitivity in absorption, but in emission considerably fewer zinc atoms than sodium atoms are excited at a given temperature’, (8). I remain incredulous at the lack of interest in that simple fact. Our spectrometer at that time was a prism monochromator from a Beckman Model DU spectrometer, and we used a single-beam system that was perfectly satisfactory for a wide range of analyses. (Further results are given in Reference 9.) “Do-it-yourself” spectrometers Three events in 1958 had a great influence on our AA project. First, we learned that Hilger and Watts had produced a n AA instrument by adding a n AA attachment to a Hilger Uvispek spectrometer, but that the instrument did not include a modulated light source/tuned detection system to eliminate signals from radiation emitted by the atomized sample. We regarded this as an undesirable and unnecessary limitation to performance. Second, a representative of Perkin Elmer suggested to us that P-E would be seriously interest ed in becoming a licensed manufacturer of AA equipment if it could be shown capable of determining calci um in blood serum. Finally, we received copies of two papers describing highly successful applications of AA to important analytical tasks. In the first paper, Allan (IO)describes the determination of magnesium in plant material, soil extracts, drainage water, blood sera, and milk.
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REPORT Using the Hilger apparatus, Allan reported: “By its use magnesium determinations can be performed with the same ease as, and probably with greater reliability than, flame photometric determinations of sodium, potassium, and calcium.” Similarly, David ( 1 1 ) obtained a detection limit of 0.5 ppm for the determination of zinc in plant-digest solutions, whereas the emission methods available at that time were so insensitive as to be inapplicable. Both Allan and David used LundegArdh flames fueled by air-acetylene, and these two applications demonstrated to me most forcibly that although flames had notable limitations as atomizers, they would prove perfectly adequate for a wide range of AA analyses. As a result of these developments, we became greatly attracted to trying once again to arrange for production
in Melbourne of our AA spectrometer. Rut this time we abandoned all attempts to have any one manufacturer produce a complete instrument. Instead, we proposed that three different firms supply, following our design, the various components t h a t could be made in Australia. The remaining components would be supplied by overseas manufacturers. The essence of the design was to provide a n AA spectrometer t h a t would be inexpensive, simple to build, easy to operate, and consist of components that could easily be assembled. Our experience had taught u s t h a t a single-beam instrument would perform quite acceptably for a large range of analytical work. By arranging the optical components on a n optical bar, one could, if necessary, readily replace the atomizer-spray chamber-burner combination with
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Figure 4. The AA spectrophotometer displayed at the Institute of Physics exhibition, Melbourne, March 1954. 938 A
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REPORT improved means for converting a sample solution to an atomic vapor. Figure 5 shows a typical “working man’s’’ AA spectrometer assembled by the “do-it-yourself” method. The hollow-cathode power module on the front left provides a modulated dc supply, and the electrical module on the front right houses a regulated photomultiplier power supply, ac amplifier, rectifier, and read-out meter. Radiation from the HCL at the left is focused on the appropriate region of the flame and by a second lens onto the entrance slit of the small grating monochromator. In this example the monochromator, optical bar, mount ings for the HCL, lenses, and atomizer-spray chamber-burner assembly were standard Hilger fittings. We also assembled some instruments Figure 5. The “do-it-yourself” AA spectrophotometer. using Zeiss optical components and fittings. The two electrical units were manufactured by Techtron Pty. Ltd. Elmer in a cordial relationship that of Techtron Pty. Ltd. declared its in(which at that time had a staff of led to substantial mutual benefits. tention to market a complete AA five); the HCLs by Ransley Glass In brief, just over a century after spectrometer that would incorporate Ltd. (which had no previous experiKirchhoff and Bunsen described AA ence in vacuum technology); and the monochromators designed by CSIRO methods of elemental analysis, the and use diffraction gratings made on lenses, lens holders, and burner by methods were clearly on the verge of the ruling engine designed and conStuart Skinner Pty. Ltd., a small rapidly increasing acceptance. structed in the CSIRO instrument shop that made highly specialized workshop. This announcement was optical components. Conclusions tantamount to announcing the imWe were particularly fortunate in There are two important lessons to pending birth of an Australian specfinding t h a t t h e atomizer-spray be learned from this account of the troscopic instrument industry. chamber combination used in a simdevelopment of AA methods and the I n the meantime, Perkin Elmer ple, inexpensive emission flame phodifficulties encountered in convincing tometer made by Evans Electrosele had also made several instruments analysts and scientific instrument by supplying AA attachments for nium Ltd. in England provided a manufacturers of their potential. their UV-vis spectrometers. In 1962 surprisingly high performance at low First, it should be noted that this it decided to produce a completely cost and was ideally suited to our work originated i n a laboratory new instrument designed specifically purposes. When used in conjunction where scientists were encouraged to for use in AAS, which, as shown in with an elongated burner described study a subject at a basic level and Figure 6, soon found rapid and everby Clinton (12),the chamber combiwere not expected to have a specific increasing acceptance. CSIRO obvination provided a n excellent unit for goal for every set of investigations. I ously gave strong support to the Austhe air-acetylene and air-coal gas think this is a tremendously imporflames of the type used by Allan (10). t r a l i a n m a n u f a c t u r e r s b u t also t a n t point. Increasingly we find By mid- 1962 some 30 of these “docollaborated closely with Perkin young scientists being channeled into it-yourself” kits had been supplied to narrow areas of activities aimed only Australian laboratories. Others were at targets with good prospects of sucsold overseas, especially to researchcess. They are being given less and ers in New Zealand and South Afriless room to maneuver. Their work is ca. Without exception, the Australian being largely confined to answering purchasers became satisfied customquestions, ignoring the many lessons ers and generated nationwide interthat have shown that much successest in the new technique. The contriful research has its origin in asking bution of the HCLs (13), the design the right question. (14) and performance (15)of the simThe second lesson is that it is a misple spectrometer, and its application take for the scientist or the inventor to to the analysis of biological materials try to sell an invention by scientific (16)have all been described, and a and technical arguments rather than survey of some early applications in by a demonstration of how well it can Australia has also been presented fulfill the functions it claims to fulfill. (17). The licensee is not interested in how Commercial instruments at last clever the invention is; he or she merely wants to know what benefits In July 1962 some 75 users and pothe- invention affords the designer, tential users held a symposium in manufacturer, and user of the equipFigure 6. World sales of AA the CSIRO laboratories. At the end of ment in which it is incorporated. the symposium, the Melbourne firm spectrophotometers, 1958-1 974. 940 A
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References ( 1 ) Kirchhoff, G. R. Monats-bericht der Academic der Wissenschaften zu Berlin, Oct. 1859,662. (2) Kirchhoff, G. R.; Bunsen, R. PoggendofiAnnalen 1860, 110, 161. (3) Walsh, A. Anal. Chem. 1974,46,698 A. (4) Hruce, C. F.; Hannaford, P. Spectrochini. Acta 1971, 26, 207-35. ( 5 ) Walsh, A. Apparatus for Spectrochemical Analysis, Australian Patent Specification 163, 586, Application date: Nov. 17, 1953, published Oct. 21, 1954, accepted June 24, 1955. ( 6 ) Walsh, A. Spectrociiim. Acta 1955, 7, 108. ( 7 ) Alkemade, C.T.J.; Milatz, J.M.W. Appl. Sci. Res. 1955, B4, 289. (8) Shelton, J. P.; Walsh, A. Proc. 25th Intern. Cong. Pure Appl. Chem. Lisbon, 1956, IV-50, 3. (9) Russell, B. J.; Shelton, J. P.; Walsh, A. Spectrochim. Acta 1958, 8, 317. (10) Allan, J. E. Analyst (London)1958,83, 466. ( 1 1 ) David, D. J. Analyst (London) 1958, 83, 655. (12) Clinton, 0. E. Spectrochim. Acta 1960, 16, 985. (13) Jones, W. G.; Walsh, A. Spectrochim. Acta 1960, 16, 249. (14) Box, G. F.; Walsh, A. Spectrochim. Acta 1960, 26, 255. (15) Gatchouse, H. M.; Willis, J. €3. Spectrochim. Acta 1960, 16, 602. (16) Willis, J. H. Methok of Biochemical Analysis; Interscience Publishers: New York, 1963; Vol. 11, pp. 1-67. (17) Walsh, A. Feigl Anniversary on Analytical Chem istry; I3 i rm i ngh a m, E:ng l a nd , 1962; pp. 281-87.
Cholinesterases
Structure, Function, Mechanism, Genetics, and Cell Biology resenting the proceedings of the Third International Meeting on Cholinesterases. this inaugural volume in the Conference Proceedings Series offers a wealth of new information on current and future cholinesterase research, including important advances resulting from new concepts and methodologies such as monoclonal antibodies and molecular genetics. The volume's 49 full papers and 140 poster papers are divided into six sections covering:
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S i r A l a n Walslz studicd physics a t Manchester University under the direction of the late W. L B r a g . After working at the British Non-Ferrous Metals Research Association in London on the development of spectrochemical methods of analysis, he joined the Chemical Physics Section at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Melbourne, Australia, where his initial research was in the application of infrared spectroscopy to the structure of small molecules. He retired from fill-time work at CSIRO in 1977 and has since been a consultant with Perkin Elmer and a CSIRO Honorary Research Fellow. WalshS major contribution to analytical chemistry is his now-classic research in AA spectrometry, which was begun in 1952. Walsh has received many scientific awards, including thp Maurice Hasler Award of the US. Society of Applied Spectroscopy, the Talanta Gold Medal, and the Royal Medal of the Royal Society of Imdon.
Polymorphism and Structure of Cholinesterases Cellular Biology of Cholinesterases Gene Structure and Expression of Cholinesterases Catalytic Mechanism of Cholinesterases: StructureFunction Relationships of Anticholinesterase Agents. Nerve Agents, and Pesticides Pharmacological Utilization of Anticholinesterase Agents. Neuropathology of Cholinergic Systems Noncholinergic Roles of Cholinesterases
This volume will be of great interest to a broad spectrum of readers. including those interested in the evolution of cholinesterase catalysis. researchers developing agricultural chemicals. scientists seeking up-to-date information on the treatment of glaucoma and such neurological diseases as Alzheimer's disease and myasthenia gravis, those interested in the design of drugs to bind the enzyme itself or to cholinergic receptors, as well as those who follow the progress toward complete structure elucidation of cholinesterases. Jean Massoulie. Centre National de la Recherche Scientifique. Editor Francis Bacou, lnstitut National de la Recherche Agronomique, Editor Eric Barnard, Medical Research Council, Editor Arnaud Chatonnet. lnstitut National del la Recherche Agronomique. Editor Bhupendra P. Doctor, Walter Reed Army Institute of Research, Editor Daniel M. Quinn. University of Iowa, Editor Conference Proceedings Series 41 4 pages (1 991 ) Clothbound ISBN 0-8412-2008-5 $89.95 American Chemical Society
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