Atomic Absorption Spectroscopy — Alan Walsh Division of Chemical Physics CSIRO P.O. Box 160 Clayton, Vic, 3168, Australia
When Foil Miller invited me to participate in the Silver Anniversary Symposium on Great Moments in Analytical Chemistry, he suggested that I may care to indulge in some personal reminiscences and comments regarding the development and present status of atomic absorption spectroscopy. I hope the ones I have selected may illuminate, if not answer, the problem posed by the title of my address. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 6, 1974. Silver Anniversary Symposium on Great Moments in Analytical Chemistry and Applied Spectroscopy.
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I realize that anyone who reminisces is usually so decrepit that his memory is totally unreliable. I shall, therefore, try to avoid too many errors of fact by restricting my comments largely to matters which are documented in reports of CSIRO, in correspondence, or in publications. My initial interest in atomic absorption spectroscopy was a result of two interacting experiences: one of the spectrochemical analysis of metals over the period 1939-46; the other of molecular spectroscopy over the period from 1946-52. The interaction occurred early in 1952, when I began to wonder why, as in my experience, molecular spectra were usually ob-
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The purpose of this report .is to suggest a new technique for recording absorption spectra which offers many interesting possibilities. The method is basically simple and is illustrated in the diagram below.
Assuming that the sample is vaporised by the usual methods, e . g. flame, arc, or spark, then the emission spectrum is "removed" by means of the chopper principle . Thus, the absorption spectrum of the vapour is measured by passing through it white light which is chopped. The absorption spectrum and the emission spectrum are then scanned by a detector, the output from which is amplified by an amplifier tuned to the same frequency as the chopper. Thus the emission spectrum produces no output signal and only the absorption spectrum is recorded. For analytical work it is proposed that the sample is dissolved, and then vaporised in a Lundegardh flame . Such flames have a low temperature (2000°K) compared to arcs or sparks (500CTKJ and have the advantage that few atoms would be excited, the great majority being in the ground state . Thus absorption will be restricted to a small number
tained in absorption and atomic spectra in emission. The result of this musing was quite astonishing: there appeared to be no good reasons for neglecting atomic absorption spectra; on the contrary, they appeared to offer many vital advantages over atomic emission spectra as far as spectrochemical analysis was concerned. There was the attraction that absorption is, at least for atomic vapours produced thermally, virtually independent of the temperature of the atomic vapour and of excitation potential. In addition, atomic absorption methods offered the possibility of avoiding excitation interference, which at that time was thought by many to be responsible for some of the interelement interference experienced in emission spectroscopy when using an electrical discharge as light source. In addition, one could avoid problems due to self-absorption and self-reversal which often make it difficult to use the most sensitive lines in emission spectroscopy. As far as possible experimental problems were concerned, I was particularly fortunate in one respect. For several years prior to these first thoughts on atomic absorption, I had been regularly using a commercial infrared spectrophotometer employing a modulated light source and synchronously tuned detection system. A feature of this arrangement is that any radiation emitted by the sample produces no signal at 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 vapours. In an internal report for the period February-March 1952,1 suggested that the same type of modulated system (Figure 1) should be considered for recording atomic absorption spectra. The following extracts from that report may be of interest:
Figure 1 . Extract from report for February-March 1952 698 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
Stagnant or Pregnant? "Assuming that the sample is vaporised by the usual methods, e.g., flame, arc, or spark, then the emission spectrum is 'removed' by means of the chopper principle. Thus the emission spectrum produces no output signal and only the absorption spectrum is recorded." "For analytical work it is proposed that the sample is dissolved and then vaporised in a Lundegardh flame. Such flames have a low temperature (2000°K) compared to arcs and sparks (5000°K) and have the advantage that few atoms would be excited, the great majority being in the ground state. Thus absorption will be restricted to a small number of transitions and a simple spectrum would result. In addition, the method is expected to be sensitive since transitions will be mainly confined to those from the ground level to the first excited state." At this stage I was thinking of electrical discharges, as well as flames, as a means of atomization. It will also be apparent that initially I had not appreciated the difficulties which would be involved in recording atomic absorption spectra when using a continuum source. The next Bimonthly Report, for the period April-May 1952, includes the diagram shown in Figure 2 and describes our first experiment as follows: "The sodium lamp was operated from 50 cycles/sec. and thus had an alternating output so that it was not necessary to use a chopper. The D lines from this lamp were isolated— but not resolved from each other—by means of a direct vision spectroscope and their intensities were measured by means of a photomultiplier tube, the output from which was recorded on a cathode ray oscillograph. Amplification of the signal was achieved by the A.C. amplifiers in the oscillograph. With the slit-width used the signal gave full-scale deflection on the oscillograph screen. A Meker flame
was interposed between the sodium lamp and the entrance slit of the spectroscope. When a solution of sodium chloride was atomised into the air supply of the flame the signal at the oscillograph was reduced to zero. The principle of the method is therefore established." In retrospect, such optimistic naivety is quite incredible. This simple experiment gave me a great thrill, and I excitedly called in John Willis, who at that time was working on infrared spectroscopy and was later to make important contributions to the development of atomic absorption methods of chemical anal-
Report
ysis. "Look," I said, "that's atomic absorption." "So what?" was his reply, which was the precursor of many similar disinterested reactions to our atomic absorption project over the next few years. My report for June-July 1952 discusses the problems of recording atomic absorption spectra of flames with a continuum source and concludes that a resolution of about 0.02 À would be required; this was well beyond the best spectrometer available in our laboratory at that time. The report concluded as follows: "One of the main difficulties is due to the fact that the relations between
CHEMICAL PHYSICS SECTION
42nd Bimontly Report.
April-May, 1952.
C.P. 1/14. Atomic Absorption Spectra In the previous report the application of atomic absorption spectra to spectrochemical analysis was suggested. The possibilities of this approach have been explored and the results obtained to date are most encouraging. In the preliminary work the apparatus shown below was used.
The sodium lamp was operated from 50 cycles/sec. and thus had an alternating output so that it was not necessary to use a chopper. The D lines from this lamp were isolated-but not resolved from each other-by means of a direct vision spectroscope and their intensities were measured by means of a photo-multiplier tube , the output from which was recorded on a cathode ray oscillograph. Amplification of the signal was achieved by the A. C . amplifiers in the oscillograph . With the slitwidth used the signal gave full-scale deflection on the oscillograph screen. A Meker flame was interposed between the sodium lamp and the entrance slit of the spectroscope. When a solution of sodium chloride was atomised into the air supply of the flame the signal at the oscillograph was reduced to zero. The principle of the method is therefore established. No attempt
Figure 2. Extract from report for A p r i l May 1952 ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974 • 699 A
Figure 3. Schematic diagram of use of sharp line source to measure peak absorption
absorption and concentration depend on the resolution of the spectrograph, and on whether one measures peak absorption or total absorption as given by the area under the absorption/wavelength curve." At this juncture the possibilities of measuring peak absorption were obviously coming into consideration. There is then a gap of four months in my reports, owing to absence from the laboratory on sick leave for much of this period. The next report, for the period December 1952-January 1953, refers to the poor sensitivity obtained in the determination of copper by use of a continuum source and a monochromator obtained by placing a slit and detector on the focal wave of a Littrow spectrograph. The report states: "It is thought that this (poor sensitivity) is due to the low resolution of the Littrow spectrograph and to the excessive amount of scattered light at low wavelengths. It is hoped to overcome this difficulty by using a hollow-cathode source (copper cathode) as source. This will emit sharp lines and a low resolution spectrometer will then be sufficient. The first attempts at producing hollow-cathode sources have not been successful." This use of a sharp line source to measure peak absorption is illustrated schematically in Figure 3. In this case, the function of the monochromator is to isolate the required line for measurement from all other lines emitted by the source. The high
resolution required for atomic absorption measurements is, in effect, provided by the sharp line source. At this stage we had arrived at the principle of the technique which, in due course, became the generally accepted method of making the intensity measurements required in atomic absorption methods of chemical analysis. These early experiments, carried out in collaboration with John P. Shelton, were originally confined to hollow-cathode lamps through which argon was flowed by a closed circulating system. We did not commence the development of sealed-off hollowcathode lamps until December 1953January 1954, when we first became aware of the work of Dieke and Crosswhite (2), which had been published in 1952. The first person to express any interest in the application of the technique we had developed was John David, at the CSIRO Division of Plant Industry in Canberra; and on February 24th, 1953, he wrote to me a letter which began as follows: "I understand from several sources that you have in mind a new technique of spectrochemical analysis involving the measurement of the absorption by the analysis line of an element in a vaporized sample rather than the measurement of its emission. "I would very much appreciate any information which you may have and are prepared to give me regarding its application to analysis for traces of
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metallic and semi-metallic elements in plant ash, soil, mineral or similar samples." My reply, dated 27th February 1953, was as follows: "At the moment my work on atomic absorption spectra is still in the development stage and I cannot specify exactly what equipment will be necessary, but I think it will include the following items :(a) Monochromator having a resolution of 1 À. (b) Discharge tubes and gas-circulating system. (c) Flame burner assembly. (d) Photomultiplier plus associated power packs. (e) Amplifiers having two homodyne rectifiers on the output side. (f ) Ratio-meter, potentiometer or ratio-recorder. (g) One or two choppers. "It is still too early to make any definite claims for the method, but it certainly offers most exciting possibilities and will, I believe, prove particularly valuable in your work, provided the sample can be taken into solution. It may prove possible to extend the method to solids. "I should add that we are most anxious not to divulge any information to people overseas, so please regard this letter as confidential." In view of the almost complete lack of interest in our work over the next few years, this request for secrecy was quite superfluous. Later in 1953 I discussed possible commercial exploitation of our ideas with various instrument manufacturers in the United States and England, but the only person who viewed our work with enthusiasm was A. C. Menzies of Hilger and Watts Ltd., London; CSIRO arrived at a tentative exclusive licence agreement with that firm, based on the patent which we lodged in November 1953. The next significant event was the first public exhibition of a working atomic absorption spectrophotometer (Figure 4). It was demonstrated in March 1954 in Melbourne University as part of an Exhibition of Scientific Instruments, arranged by the Australian Branch of the (British) Institute of Physics. The apparent complexity of the instrument was due largely to its being of the double-beam type, which in our early experiments we regarded as essential because of the poor stability of many of our hollow-cathode lamps. The viewer was 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 no interest whatsoever during the three days it was on exhibition.
However, when Dr. Menzies visited Melbourne shortly afterward to assess its performance, he was sufficiently impressed for his firm to decide to produce, under licence to CSIRO, the first commercial atomic absorption spectrophotometer. As soon as our final patent specification was lodged on October 21, 1954 (3), I submitted to Spectrochimica Acta my first paper (4), in which I discussed the factors governing the relationship between atomic absorption and atomic concentration, and the experimental problems involved in making atomic absorption measurements. The paper was published early in 1955, at about the same time as the paper by Alkemade and Milatz (5), who had independently arrived at the atomic absorption method. Neither paper created any great impact, and Alkemade and Milatz did not pursue their work further, possibly because they regarded this method merely as one for determining "all metals usually to be determined by flame photometry." In 1956 and 1957 we published papers (6, 7) describing results obtained with our instrument, but these papers also created little interest. John Shelton wrote to me from London on March 5th, 1956, after giving a lecture on our work to the Institute of Physics, and reported that my first paper had given the impression that "the method was a scientific curiosity rather than a practical analytical method." When I gave a series of lectures on the subject at the Louisiana State University Symposium in 1958, the net result was that only one person, Jim Robinson, was stimulated into activity. He became a "hot gospeller" and in due course played an important part in stirring up interest in the United States. The surprising thing is that the appearance in 1958 of the papers by Allan (8) in New Zealand and David (9) in Canberra did not arouse any sizable impact, even though they described eminently successful applications of the technique. On the commercial side, Hilger and Watts had produced an instrument which did not incorporate a modulated source and therefore could not fully exploit the technique. Other instrument manufacturers subsequently perpetrated the same error. By 1958 there was no sign of any instrument manufacturer willing to produce the type of instrument which we thought desirable. This was most curious since by that time there was some interest by other Australian laboratories in possible applications of this technique. It was at this stage that we decided to arrange for the production of appropriate equipment in
Figure 4. Photograph of atomic absorption spectrophotometer demonstrated at Institute of Physics Exhibition, Melbourne, March 1954
Figure 5. Photograph of simple atomic absorption spectrophotometer produced commercially in Australia
Australia. The necessary items were manufactured by three small companies in Melbourne and then assembled by the user, according to our instructions. As it transpired, for the next few years the members of our research group were increasingly involved in supporting the commercial production in Australia of atomic absorption equipment. That a new type of Australian industry was eventually created was, of course, cause for much satisfaction, but it was inevitable that there was a substantial reduction in our research effort over a period of several years. Figure 5 shows a typical instrument produced in this manner, the electronic units having been produced to our specifications by a firm in Melbourne called Techtron Appliances Pty. Ltd., which at that time had a
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total staff of five. During the period 1958-62, some 30 Australian laboratories were equipped by these "do it yourself" units. While knowledge of the technique spread rapidly throughout Australian industry, there was one memorable exception. I recall the technical director of one of our biggest mining companies phoning CSIRO Head Office in the early 1960's and stating that he had just returned from South Africa where they were using a brand new instrument called an atomic absorption spectrophotometer. He wanted to know if there was anyone in CSIRO who knew anything about it. Our man in Head Office said he didn't know but he would make inquiries! In this period there was also a slightly increased interest by manufacturers in other countries. For me,
one of the "great moments" was in 1962 when I described to various members of staff of the Perkin-Elmer Corp. in Norwalk the impressive results which were being obtained by the laboratories in Australia which were by that time successfully using the technique. It was during these discussions that Chester Nimitz asked, rather tersely: "If this goddam technique is as useful as you say it is, why isn't it being used right here in the United States?" My reply, which my friends in Norwalk have never allowed me to forget, was to the effect that he would have to face up to the fact that, in many ways, the United States was an underdeveloped country! The Perkin-Elmer decision to embark on a large-scale project relating to the production of atomic absorption equipment was made shortly afterward. They were, in fact, guilty of overreacting, as witnessed by their subsequent claim that atomic absorption was "the greatest invention since the bed." It was also in 1962 that Techtron decided to market a complete spectrophotometer. Initially, this incorporated an imported monochromator, Australian production of an appropriate monochromator being delayed until 1965. By that time the ruling engine constructed in the CSIRO Division of Chemical Physics was in full operation and has since supplied all the master gratings required by Techtron. Other firms also were becoming increasingly interested, and from that time onward I do not think the technique has suffered any major setback. In 1965 its future was virtually secured by the development (downunder) of the nitrous-oxide flame which made the technique applicable to more than 65 elements. I might add that the development of this flame by workers in atomic absorption spectroscopy was most altruistic, since it put back on its feet emission flame photometry which, at that stage, appeared to be dying rapidly. The recent remarkable growth in the number of atomic absorption spectrophotometers produced is shown in Figure 6 and is indicative of the increasingly widespread acceptance of atomic absorption methods. In view of such growth, there would appear to be little justification for describing the subject as having been stagnant at any stage during the past decade. But the growth in the number of applications has resulted mainly from fairly straightforward extensions of a technique which originated more than 20 years ago. I believe it is no exaggeration to state that more than 99.99% of all analyses are still carried out by that original technique. In this respect the subject has been stagnant over an extended peri-
Figure 6. World sales of atomic absorption spectrophotometers
od. The situation is particularly depressing in view of the well-known limitations of existing flame methods, of which the most serious was described as follows in our first paper (3) describing our spectrophotometer: "By far the most serious difficulty in the absorption method is due to the difficulty in atomizing various elements. This problem of complete atomization of the sample seems to us to be the outstanding problem at the present time." As Allan (10) stated in his review of the subject in 1962, "It still is." In a review (11) I presented in that same year, I pointed out that when a flame is used some elements are only partially atomized, "thus resulting in loss in sensitivity and the possibility of chemical interference due to variations of the degree of atomization of one element with the concentration of other elements, radicals or compounds in the solution . . . . This type of interference is present to the same extent in emission and absorption methods and is responsible for serious limitations in flame methods." Several years later, exactly the same statement was being made, as if it represented the disclosure of a new fundamental truth regarding flame spectroscopy. It is not my intention to decry the use of a flame as an atomizer, since it is unlikely that any other method will match its enormous range of application, its speed of operation, or its remarkable convenience. Nevertheless, I remain convinced that the best
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means of extending the range of application of atomic absorption methods of analysis is by developing new methods of atomization. In some respects I imagine the above remarks present a gloomy picture of the present state of atomic absorption spectroscopy. Fortunately, there is a happier way of assessing the situation, and I want to mention briefly some aspects which lead me to think that this stagnation is more illusory than real. In the first place, socalled flameless methods of atomization, based on developments of the L'vov furnance, have in the last two years become of rapidly increasing importance, and their full potentialities have by no means been fully exploited. They now provide a remarkable ability to analyse extremely small amounts of material, and their importance, particularly in biochemical and clinical work, is already apparent. The technique is still in its infancy, and we can expect it to develop rapidly over the next few years. Of particular interest to me was the paper presented at the Toronto Conference last year in which Segar and Gonzalez {13) described, I believe for the first time, the coupling of a gas chromatograph to the graphite cuvette of an atomic absorption spectrophotometer. This combination may well make even more ubiquitous the remarkable techniques of chromatography. The recent development of improved methods for the operation of electrodeless lamps seems likely to
produce a new interest in the flame fluorescence methods since, as was first pointed out by Alkemade (14) and demonstrated by Winefordner and Vickers (15), they have striking potential advantages over atomic absorption spectroscopy. In this respect, it has always surprised me that workers in flame fluorescence have, in general, failed to exploit the fact that the fluorescence phenomenon provides its own monochromator. In the arrangement shown in Figure 7, for example, if the illuminating source is "pure," the only need to have any wavelength selection is to avoid excessive noise owing to radiation from the atomic vapour. I would also like to refer to our recent work on the development of atomic absorption and atomic fluorescence methods for the direct analysis of solids (16, 17). The sample is made the cathode of a low-pressure discharge, and the atomic vapour is produced by cathodic sputtering. The encouraging results obtained in the analysis of low-alloy steels have recently been published (17). In that paper we reported the difficulties encountered when the method was applied to the analysis of aluminumand zinc-base alloys. These difficul-
MODULATED ATOMIC SPECTRAL LAMP
ATOMIC FLUORESCENCE
SYNCHRONOUS DETECTION SYSTEM
OUTPUT SIGNAL
Figure 7. Schematic diagram of nondispersive atomic flame fluorescence spectrophotometer
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ties have now been overcome, a n d it seems t h a t t h e sputtering technique will be applicable t o t h e analysis of a wide range of m e t a l s a n d alloys. It re m a i n s t o compare t h e performance a n d range of these m e t h o d s with those of other analytical techniques. In t h e m e a n t i m e , I would not ex pect t h e scientific i n s t r u m e n t m a n u facturers to be greatly interested in the simple sputtering cell shown in Figure 8.1 would, however, like t o think t h a t some of t h e m are musing on possible ways of embellishment t o ensure t h a t any commercial version will have a n impressive price tag. We are now exploring t h e extension of our work t o t h e v a c u u m ultraviolet for t h e d e t e r m i n a t i o n of carbon, phosphorus, a n d sulfur. W e also pro pose t o extend our experiments t o t h e analysis of powders a n d solutions. W e are increasingly conscious of t h e i m p o r t a n t advantages of cathodic sput tering over t h e r m a l m e t h o d s of a t o m ization in isotopic analysis. Finally, I believe our recent work on cathodic sputtering takes u s one step nearer t h e goal I discussed in m y first paper on atomic absorption spectroscopy, the development of absolute methods of spectrochemical analysis. It would appear, therefore, t h a t t h e subject h a s not really been stagnant, b u t merely pregnant, a n d h a s now given birth t o new offspring on which I t r u s t Bunsen a n d Kirchhoff will look with approval a n d regard as wor thy descendants of their original brainchild. References (1) G. Dieke and H. M. Crosswhite, J. Opt. Soc. Amer., 42, 433 (1952). (2) Australian Patent Application 23,041/53 (Nov. 17, 1953). (3) Australian Patent Specification 163,586 (Oct. 21, 1954). (4) A. Walsh, Spectrochim. Acta, 7, 108 (1955); Erratum, ibid., ρ 252. (5) C. T. J. Alkemade and J. M. W. Milatz, Appl. Sci. Res., B4, 289 (1955). (6) J. P. Shelton and A. Walsh, Proc. XVth Congress IUPAC, 2, TV-50, Lis bon, 1956. (7) B. J. Russell, J. P. Shelton, and A. Walsh, Spectrochim. Acta, 8, 317 (1957). (8) J. E. Allan, Analyst, 83, 433 (1958). (9) D.J.David, ibid., ρ 536. (10) J. E. Allan, Spectrochim. Acta, 18, 605 (1962). (11) A. Walsh, Proc. Xth Colloquium Spectroscopicum Internationale, ρ 127, Spartan Books, Washington, 1962. (12) B. V. L'vov, Spectrochim. Acta, 17, 761(1961). (13) D. A. Segar and J. G. Gonzalez, paper presented at Fourth International Conference on Atomic Spectroscopy, To ronto, Canada, 1973. (14) C. T. J. Alkemade, Proc. Xth Collo quium Spectroscopicum Internationale, ρ 143, Spartan Books, Washington, 1962. (15) J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 161 (1964). (16) Β. Μ. Gatehouse and A. Walsh, Spectrochim. Acta, 16, 602 (I960). (17) D. S. Gough, P. Hannaford, and A. Walsh, ibid., B28, 197 (1973).
Figure 8. Photograph of atomic absorption spectrophotometer incorporating sputtering chamber for atomization of solid samples
Alan Walsh is assistant chief of the Division of Chemical Physics, Com monwealth Scientific and Industrial Research Organization (CSIRO), Melbourne, Australia. He received his DSc from Manchester Univer sity in England. He is considered the "father" of atomic absorption spectroscopy and exhibited a com plete apparatus for the technique in March 1954. His now classic paper appeared in Spectrochimica Acta in 1955. He holds basic patents in Aus tralia, the U.S., and other countries on atomic absorption spectroscopy, multiple monochromators, and im provements in grating monochroma tors. One of his major contributions was the development of intense, sta-
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ble, and inexpensive hollow-cathode discharge lamps. He has published over 60 papers in atomic, infrared, and Raman spectroscopy. Dr. Walsh is currently interested in what he considers the ultimate goal of spectrochemical analysis, the develop ment of absolute methods. This inter est has resulted in the development of methods of atomization by using cathodic sputtering; and these, in turn, have led to the development of reso nance detection and selective modu lation techniques. More recently, he and his colleagues have adapted these techniques to the development of atomic absorption and atomic fluo rescence methods for the direct anal ysis of solid samples.
