Aotmci Allsoration SOectroscow 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.
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 p u r p o s e o f t h i s r e p o r t . i s t o s u g g e s t a n e w t e c h n i q u e f o r recordingabsorptionspectrawhichoffersmanyinterestingpossibilities. Themethodisbasicallysimpleandisillustratedinthediagrambelow.
Assumingthat the sample i s v a p o r i s e d b y t h e u s u a l m e t h o d s , e . g . flame, a r c , o r s p a r k , then the emissionspectrum is "removed"bymeans o f t h e c h o p p e r p r i n c i p l e . Thus, t h e a b s o r p t i o n s p e c t r u m o f t h e v a p o u r i s m e a s u r e d b y p a s s i n g t h r o u g h i t white l i g h t w h i c h i s c h o p p e d . The absorptionspectrumandtheemissionspectrumarethenscannedbya detector, t h e o u t p u t f r o m w h i c h i s a m p l i f i e d b y a n a m p l i f i e r t u n e d t o thesame f r e q u e n c y a s t h e c h o p p e r . T h u s theemissionspectrumproduces nooutputsignalandonlytheabsorptionspectrumisrecorded. Foranalyticalworkitisproposedthatthesampleisdissolved, a n d t h e n v a p o r i s e d i n a L u n d e g a r d h flame. Such flameshavealow temperature (2000°K) c o m p a r e d t o a r c s o r s p a r k s (5000"Kj andhave the advantage t h a t fewatomswouldbe e x c i t e d , t h e g r e a t m a j o r i t y b e i n g i n the g r o u n d s t a t e . T h u s a b s o r p t i o n w i l l b e r e s t r i c t e d t o asmallnumber
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ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8 , J U L Y 1974
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, I 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
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Stagnant or Pregnanl? “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 (50W”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 isolatedbut 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
Figure 2. Extract from report for AprilM a y 1952
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 a t the oscillograph was reduced t o 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 a t that time was working on infrared spectroscopy and was later to make important contributions to the development of atomic absorption methods of chemical anal-
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 A 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
42ndBimontlyReport.
April-May, 1952.
C . P . 1/14. Atomic AbsorptionSpectra
I n the previous r e p o r t t h e a p p l i c a t i o n o f atomic a b s o r p t i o n s p e c t r a t o s p e c t r o c h e m i c a l a n a l y s i s w a s suggested. T h e p o s s i b i l i t i e s o f t h i s approachhavebeenexploredandthe resultsobtainedtodatearemost encouraging. I n t h e p r e l i m i n a r y w o r k t h e a p p a r a t u s s h o w n b e l o w w a s u s e d . %48+
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T h e s o d i u m l a m p w a s o p e r a t e d f r o m 5 0 cycles/sec. a n d t h u s h a d analternatingoutputsothatitwasnotnecessarytouseachopper.
T h e D l i n e s fromthislampwereisolated-but notresolved fromeach other-bymeansofadirect visionspectroscopeandtheir i n t e n s i t i e s weremeasuredbymeansofaphoto-multiplier tube, theoutputfromwhich was r e c o r d e d o n a c a t h o d e r a y o s c i l l o g r a p h . A m p l i f i c a t i o n o f t h e s i g n a l w a s a c h i e v e d b y t h e A . C . amplifiersintheoscillograph. W i t h t h e s l i t widthusedthesignalgave full-scaledeflectionontheoscillographscreen AMeker f l a m e w a s i n t e r p o s e d b e t w e e n t h e s o d i u m l a m p a n d t h e e n t r a n c e s l i t ofthespectroscope. Whenasolutionofsodiumchloridewasatomisedinto theairsupply ofthe flamethesignalattheoscillographwasreducedto z e r o . The p r i n c i p l e o f t h e m e t h o d i s t h e r e f o r e e s t a b l i s h e d . No attempt
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
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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 ahsorption or rural absorptiun aa given by rhearea under theabsorption wavelmgth curvc.’’ Ar this juncture the pnssibilities uf measuring peak absorption were ohviouJy coming into consirlerarion. ‘l‘herc is then a gnp of four monrhi in m y reports. owing r o atisence from the Isborarorv on sick leave for much of rhis period. The nvxt reporr. for rhe period I)ecetiiher l9.L2-.January 195%refcri 10 the DOlrr sen-itrvitv n1)tained in the determination of copper hy 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 sen& tivity) 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 emitsharp lines and a low resolution spectrometer will then be sufficient. The first attempts a t producing hollow-cathode sources have not heen 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 7QQA
resolution required for atomic ahsorption measurements is, in effect, provided by the shaip line source. At this stage we had arrived a t the principle of the technique which, in due course, hecame 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 didnot commence the development of sealed-off hollowcathode lamps until December 1953January 1954, when we first became aware of the work of Dieke and Crosswhite ( I ) , 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
ANALYTICAL CHEMISTRY, VOL. 46. NO. 8. JULY 1974
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 he necessary, hut I think it will include the following items:. (a) Monochromator having a resolution of 1 A. (b) Discharge tubes and gas-circulating system. (c) Flame burner assembly. (d) Photomultiplier plus associated power packs. (e) Amplifiers having two homodvne rectifiers on the outnut side. (f) ootentiometer or , , Ratio-meter.,. ratio-recorder. (9) One or two choppers “It is still too early to make any definite claims for the method, hut i t certainly offers most exciting possibilities and will, I believe, prove particularly valuable in your work, provided the sample can he 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, hut 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.
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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 ahsorption spectrophotometer. As soon as our final patent specification was lodged on October 21,1954 (3),I submitted to Spectrochimica Actamyfirstpaper(4), inwhich1 discussed the factors governing the relationship between atomic ahsorption 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 (51, who had independently arrived a t the atomic ahsorption 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 he determined by flame photometry.’’ In 1956 and 1957 we published papers (6, 7) describing results obtained with our instrument, hut 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 a t this stage that we decided to arrange for the production of appropriate equipment in 702 A
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atomic obsorp’iion spectrophotom?ler
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 ahsorption equipment. That a new type of Australian industry was eventually created was, of course, cause for much satisfaction, hut 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 a t that time had a
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
total staff of five. During the period 1958-62, some 30 Australian lahoratories 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 hut he would make inquiries! In this period there was also a slightly increased interest by manufacturers in other countries. For me,
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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 peri704A
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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 ( I O ) stated in his review of the subject in 1962, “It still is.” In a review ( 1 I ) 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
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8 , JULY 1974
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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
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CIRCLE 240 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
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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 I,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
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ATOMIC FLUORESCENCE ’\ \ \
SYNCHRWUS DETECTION
OUTPUT SIGNAL Figure 7. Schematic diagram of nondispersive atomic flame fluorescence spectrophotometer
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ties have now been overcome, and it seems that the sputtering technique will be applicable to the analysis of a wide range of metals and alloys. It remains to compare the performance and range of these methods with those of other analytical techniques. In the meantime, I would not expect the scientific instrument manufacturers to be greatly interested in the simple sputtering cell shown in Figure 8. I would, however, like to think that some of them are musing on possible ways of embellishment to ensure that any commercial version will have an impressive price tag. We are now exploring the extension of our work to the vacuum ultraviolet for the determination of carbon, phosphorus, and sulfur. We also propose to extend our experiments to the analysis of powders and solutions. We are increasingly conscious of the important advantages of cathodic sputtering over thermal methods of atomization in isotopic analysis. Finally, I believe our recent work on cathodic sputtering takes us one step nearer the goal I discussed in my first paper on atomic absorption spectroscopy, the development of absolute methods of spectrochemical analysis. It would appear; therefore, that the subject has not really been stagnant, but merely pregnant, and has now given birth to new offspring on which I trust Bunsen and Kirchhoff will look with approval and regard as worthy descendants of their original brainchild. References (IG. )Dieke and H. M. Crosswhite, J. Opt. Sac. Amer., 42,433 (1952). (2) Australian Patent Application 23,041153 (No". 17, 1953). (3) Australian Patent Specification 163,586(Oct. 21, 1954). (4) A. Walsh, Spectrochim. Acta, 7, 108 (1955);Erratum, ibid., 252. (5) C. T. J. AlkemadeanlJ. M. W. Milatz,Appl. Sei. Res., B4,289 (1955). (6) J. P. Shelton and A. Walsh, Proc. XVth Congress IUPAC, 2, Iv-50, Lisbon, 1956. (7) B. J. Russell, J. P. Shelton, and A. Walsh, Spectrochim. Acta, 8, 317 (1957). (8) J . E. Allan, A m l y s t , 83,433 (1958). (9) D. J. David, ibid.,p 536. ( I O ) J. E. Allan, Spectrochim. Acta, 18, 605 (1962). (11) A. Walsh, Proe. Xth Colloquium Spectroscopicurn Internationale,p 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, Toronto, Canada, 1913. (14) C. T. J. Alkemade, Proe. Xth Calloquium Spectroscopicurn Intertiationale, p 143, Spartan Books, Washington,
Figure 8. Photograph of atomic absorption spectrophotometer incorporating sputtering chamber for atomization of solid samples
Alan Walsh is assistant chief of the IXvision of ('hemical Physics, Commonwenlrh Scientific and Industrinl Research Organimriim ~(:SIROJ, Melhourne, Ausrralia. He received his DSc from Manchester L'nivers i t y in England. Hi,is considered th+: "iather" of atomic ahsorptiim spectrosropy und exhibited a cornplrrt. apparatus for the technique in Murch l9.-)4. Hi. now classic paper appeared in Spcrrrochrmrco A r t u in 1925. He holds hniic parenrs in Australia. the U.S.,andother countrieb on ;atomic absorption spectroscopy. mulriple monochromaturi. and improvement j in grating moniduoina. tori. Onr ui his niajur contribution-. was the development of inrrnie, sta-
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(15) J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 161 (1964). (16) B. M. Gatehouseand A. Walsh, Spectrochim. Acta, 16,602 (1960). (17) D. S. Gough, P. Hannafard, and A. Walsh, ibrd., B28.197 (1973). 708A
ANALYTICAL CHEMISTRY, VOL 46, NO. 8, JULY 1974
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 development of absolute methods. This interest has resulted in the development of methods of atomization by using cathodic sputtering; and these, in turn, have led to the development of resonance detection and selective modulation techniques. More recently, he and his colleagues have adapted these techniques to the development of atomic absorption and atomic fluorescence methods for the direct analysis of solid samples.
