S. R. Koirtyohann Dept. of Chemistry and Environmental Trace Substance Research Center University of Missouri Columbia, Mo. 65211
Current Status and Future Needs in Atomic Absorption instrumentation I have been invited to discuss the current status and future needs of atomic absorption (AA) instrumentation. The talk could also be called, "What the manufacturers should have done for their customers but haven't yet offered." Before considering where atomic absorption spectroscopy is going, perhaps it would be well to look at where it has been. Table I presents a thumbnail history of the method. I have divided the first quarter century of analytical applications of AA into sevenyear periods. Most people in the field are aware of the independent publications of Walsh (1) and Alkemade and Milatz (2), which pointed out that atomic absorption measurements offered certain advantages over the flame emission mode of operation in common use at the time. The seven years from 1955 to 1962 are best characterized by what did not happen. Atomic Absorption—the technique that would soon revolutionize elemental analysis—was ignored by 99.44% of practicing analytical chemists. Walsh and a mere handful of people "Down Under" in Australia and New Zealand developed the method and demonstrated its utility time and time again—and still people stayed away in droves. Alan Walsh traveled extensively during this period trying to promote AA and was often frustrated by his inability to generate interest in the scientific community. His American
friends have never let him forget that he once accurately described the U.S. as an underdeveloped country so far as AA was concerned. The period 1962-69 was one of explosive growth. AA caught on and surged to the forefront of elemental analysis. The period opened, at least from my perspective, with the Tenth Colloquium Spectroscopicum Internationale, held in College Park, Md., in June of 1962 and closed with the 2nd International Conference on Atomic Spectroscopy held in Sheffield, England, in July, 1969. The opening of this period coincides with the beginning of my own active participation in atomic absorption research, which may explain why I think of it as the "Fun Time." It was fun because people disagreed about the best light sources and atomizers. Interferences and especially detection limits were topics sure to spark instant debate. AA symposia at the time were spiced by frequent, usually polite, arguments. New techniques and applications were suggested about as rapidly as most of us could follow. Today a similar atmosphere prevails at symposia on the inductively coupled plasma. Then as now the high interest level was a heady brew for those fortunate enough to be active in the area. The explosive growth of 1962-69 could hardly be sustained and 1969-76 represents a period of relative stability and the period when AA put on its
Table I. A Brief History of Atomic Absorption 1955
Method described independently by Walsh and by Alkemade and Milatz
1955-62
Development "Down Under" in Australia and New Zealand.
1962-69
Explosive growth—The Fun Time
U.S. an underdeveloped country 1969-76
Relative stability—AA puts its work clothes on
1976-???
Enter the microprocessor. AA will never be the same—and neither will anything elsel
736 A • ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
work clothes. Applications to other research fields in need of the data that AA could provide became the order of the day. Let me emphasize that we are talking about relative stability compared with the previous period. Research was being done, improvements were being made, but for one who participated in the exciting growth period, the slower pace was quite obvious. Enter the microprocessor! In about 1976 the revolution in solid state electronics began its dramatic impact on individual laboratory instruments. AA will never be the same! But what will? I feel out-of-date because my wrist carries an old fashioned analog watch, and just a few days ago I caught myself reaching for a pocket calculator to take the square root of 16. My crystal ball is much too cloudy to predict when this period will end or how we will be doing AA when it does. The impact of microprocessor-controlled instrumentation is already great and will soon be greater. Properly used, microprocessors allow us to easily do things that are difficult or impractical without them. As we shall see, they can also be misused. Now let's return briefly to the Fun Time because much of what we see in AA instruments today rests on the outcome of arguments resolved then. Table II lists some of the controversial topics. At least in certain areas of the midwestern U.S., flames continued to be emitted. Velmer Fassel and coworkers (3, 4) at Iowa State University and Ed Pickett and I at the University of Missouri, (5) along with a few others, pointed out repeatedly that the emission mode of operation had some distinct advantages and should not be discarded. The topic of interferences was sure to spark heated debate, with many arguing that AA was much less prone than emission to chemical interferences. The very existence of spectral interferences in AA was vehemently denied by proponents of the technique. The state of confusion regard-
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ing interferences can perhaps best be conveyed by quoting from a publication of the period (6): "Atomic absorption spectroscopy does not suffer from chemical interference but [the] presence of large amounts of anions and cations can cause pronounced effects on absorption." See—its not chemicals but all those blasted anions and cations that cause the problems! The authors of the above statement should be defended because they were not writing in their native tongue. Theirs was undoubtedly a language problem. The fact that such a statement got past two reviewers and a journal editor is harder to understand and indicates the confusion mentioned above. A topic related to interferences was that of atomizer design. Flame emission in the U.S. had developed almost exclusively with the well-known total consumption burner marketed by Beckman Instruments. The premixed burner design that dominates today did not gain prominence without other ideas such as multiple pass optical systems, multiple burners, and long absorption path lengths being examined and eventually discarded. By 1969 most of the misconceptions about AA were resolved. Spectral in-
terferences, while relatively rare, were acknowledged to exist, and chemical interferences were known to be primarily a function of the atomizer, not the mode of observation. The relative place of emission and absorption had been resolved at least in principal. In practice, I see very few people using flame emission today even in cases where the advantage is obvious to me. By 1969 atomic fluorescence had been described, and people were already beginning to wonder why it was not more popular. Automatic background correction systems for AA were available, the nitrous oxide acetylene flame had extended application to refractory elements, and furnace atomizers had been introduced. Atomic absorption was an established technique. But here we must be careful about definitions. I recently saw a list of words and what they really mean when applied in academic circles. It had the following entry: Established—Over the hill, coasting gracefully toward retirement. We have all seen people to which this definition applies but it certainly
doesn't in the case of AA. Instrument sales are brisk and AA maintains a dominant role in elemental analysis, which is only now being challenged by the inductively coupled plasma. Established but still working hard and not about to retire—gracefully or otherwise. What has been the track record of the instrument manufacturers in the development of AA? For the most part I feel that it has been quite good. Initially, vigorous promotion by manufacturers was a major force in popular acceptance of AA methods. Since then, instruments have been continuously updated and innovations such as background correction, electrodeless discharge lamp sources, furnaces, hydride generators and automation have been incorporated in timely fashion. Indeed, the number of makes, models and accessory items available today makes buying an AA unit almost as hard as buying a car. At least you don't have to take your spouse along shopping to select the color— yet! One piece of good news easily overlooked by users is that the manufacture of AA instruments has been profitable. The primary objective of an instrument company is not to provide us
Table II. Controversies in the 1962-69 Period 1. Emission vs. Absorption 2. Interferences a. Severity in emission and absorption b. Spectral in absorption 3. Atomizer Design. a. Total consumption vs. premixed flames b. Tri-flame burner (7) c. PANT burner (8) d. FuwaTube(S) 4. Detection limits
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980 • 737 A
with reliable, convenient, inexpensive instruments. It is to make money for the stockholders. And the profit is good from the customers' perspective because without it the company goes out of the AA business. Recently purchased instruments quickly become orphans. Then everybody loses. Is there any bad news? What have the manufacturers failed to do or done poorly? I continue to be disappointed by the absence of good commercial atomic fluorescence instruments or attachments. The nondispersive atomic fluorescence concept described by Larkins (10) and shown schematically in Figure 1 is particularly attractive. I saw one of these units in operation in Larkins's laboratory and was impressed by the elegant design simplicity as well as by its performance. Perhaps the design simplicity is part of the problem. A commercial instrument of this design would have to be inexpensive. So long as the more expensive instruments are selling well, manufacturers have little incentive to offer something at a reduced price. It is much the same as Detroit's reluctance to offer compact cars so long as
the bigger, more expensive, more profitable models were selling. All major AA instruments now have a flame emission capability, yet most have simple, easily correctable deficiencies when used in emission. For example, I recently used an AA unit for a student experiment that involved the effect of spectrometer slit width on emission line intensities and line/ background ratios. Accurate emission data were needed over about a 100X range of intensities. The only way we could vary the response by accurately known amounts was to use a variable range recorder. The instrument itself has no provision for known gain adjustments. No commercial flame instrument offers wavelength modulation for emission background correction, in spite of frequent descriptions of its utility in the literature. (11,12, 13) Other examples could be cited but the main point is that the AA instruments with which I am familiar are clearly optimized for absorption measurements. Though details vary from company to company and model to model, generally emission is unnecessarily treated like a poor relativi
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