ATOMIC ABSORPTION SPECTROMETRY

The Second James 1. Waters Annual Symposium Recognizing Pioneers in the Development of Analytical Instrumentation was held at the 1991 Pittsburgh Conference and Exposition in Chicago. This year‘s symposium honored four pioneers in the field of atomic absorption spectroscopy: Alan Walsh, Boris l‘vov, S. R. Koirtyohann, and Walter Slavin. In the October 1 issue, l‘vov and Walsh recounted their efforts in developing AAS as an analytical method. Here Koirtyohann discusses these developments from an academic perspective and Slavin explores the reasons for the success of AAS.

S. R. Koirtyohann Department of Chemistry University of Missouri-Columbia Columbia, MO 6521 1

Viewed from a 35-year perspective, atomic absorption spectroscopy (AAS) is a tremendous success story. It is easy to forget that this success did not happen as soon as the idea was introduced. Individuals and companies had to invest considerable amounts of time, money, and talent, and not everyone who sought to take advantage of this new technology was equally successful. The purpose here is to review the historical facts and consider reasons for the variabil- ity in the degree of success achieved.

Early history If history were concerned only with what happened, we could begin by considering two papers published in 1955 (1’2). However, because the in- tent here is to understand (or at least speculate about) why certain things happened, we must look briefly at earlier years. Historical chapters in books by Twyman (3) and Slavin (4 ) are particularly helpful.

Optical spectroscopy can be traced to 1672, when Newton observed that sunlight could be separated into col- ors upon passage through a prism. Indeed, the word spectrum comes from Newton’s attempt to describe the ghostly appearance of dispersed sunlight. The first person to observe spectral features other than simple colors was Wollaston, who in 1802 described (but made no attempt to explain or characterize) numerous dark lines in the sun’s spectrum. Starting in 1817, Fraunhofer careful- ly mapped these lines that bear his name and designated some of the more prominent ones by letters, starting with A at the red end of the spectrum. Even today, one sees refer- ence to the sodium D lines, using Fraunhofer’s original designation. His observat ions also la id t h e groundwork for spectral observations in astronomy. Fraunhofer noted that light from Venus looked much like that from the Sun but that light from the bright star, Sirius, had a very different spectrum.

1024 A ANALYTICAL CHEMISTRY, VOL. 63, NO. 21, NOVEMBER 1,1991

Kirchhoff and Bunsen explained the origin of Fraunhofer lines in a classic series of papers published in 1859 and 1860. They not only ex- plained that Fraunhofer lines were caused by atomic absorption in the Sun’s atmosphere; they also estab - lished the general laws of emission and absorption of light and described the conditions needed for earthbound analytical observations using flames in both emission and atomic absorp- tion. Flame analysis became common in Bunsen’s lab, and several new ele- ments were discovered by spectral analysis in the early 1860s. However, the full analytical potential of both flame emission and atomic absorp- tion remained unexploited for many years.

Other significant events in the de- velopment of spectral analysis, to be discussed in greater detail below, in- clude:

The first quantitative spectral analyses by Lockyer in 1873

Hartley’s work on the quantita- tive analysis of solutions, beginning in about 1884 at the University of Dublin and continued by Pollok and Leonard well into this century

de Gramont’s studies at the Sor- bonne in the early 1900s

Twyman’s design of the first con- venient commercial spectrographs at Adam Hilger Ltd. from 1909 through 1912

The first US. journal paper on spectral analysis by Meggers in 1922

The development of flame spec- tral analysis by LundegArdh in Swe- den starting in 1929

Direct electrical detection of flame emission spectra by Jansen, Hayes, and Richter in the mid-1930s

Introduction of the Beckman DU spectrophotometer (the instrument generally credited with converting chemists to spectral thinking) in 1941 and the photomultiplier attach- ment for that instrument in 1953.

Modern AAS We begin with a “thumbnail” history covering the first three and a half de- cades of flame AAS. There were sev- eral distinct periods, as indicated in the box on p. 1026 A.

The induction period is best char- acterized by what did not happen.

0003-2700/91/0363- 1 024A/$02.50/0 0 1991 American Chemical Society

FROM AN ACADEMIC PERSPECTIVE Atomic absorption-the technique that soon would revolutionize ele- mental analysis-was ignored by near ly a l l pract ic ing analyt ical chemists. Walsh and a mere handful of people down under developed the method and demonstrated its utility time and time again-and still peo- ple stayed away in droves. Walsh traveled extensively, trying to pro- mote AAS, and was often frustrated by his inability to generate wide in- terest. His American friends have not let him forget that he once accu- rately described the United States as a country “underdeveloped” in AAS.

From 1962 through 1969 explosive growth occurred as AAS caught on and surged to the forefront of ele- mental analysis. The period opened, a t least from my perspective, with the 10th Colloquium Spectroscopi- cum Internationale in College Park, MD, in June 1962 and closed with the 2nd International Conference on Atomic Spectroscopy in Sheffield, England, in July 1969.

My own active participation in atomic absorption research nearly coincides with the beginning of this explosive period, which may explain why I think of it as the “fun time.” AAS symposia were spiced by fre- quent (usually polite) arguments about the best sources and atomizers. New techniques and applications were suggested almost as rapidly as most of us could follow. This high level of interest made a heady brew for those fortunate enough to be in- volved a t that time.

Such explosive growth could hard- ly be sustained, and 1969-76 was a period of relative stability (i.e., rela- tive to that of the previous years). AAS put on its work clothes and be- gan to generate data for other fields. Research was still being done, and improvements were still being made, but for anyone who had participated in the exciting growth period the slower pace was obvious.

In about 1976 the revolution in solid- state electronics began to make an impact on chemical instrumenta- tion, and there appears to be no end in sight. Most AAS customers today are a t least as interested in software as in hardware. Instrument through- put and, to a lesser extent, instru-

ment capabilities have been en - hanced whereas much of the tedium of routine analysis has been re - moved. One disadvantage of this rev- olution is tha t AAS systems are much more difficult to learn to use, especially for an old fogey like me. With older instruments, I could start with reasonably bright but totally untrained college freshmen and have them generating AAS data in 10-30 minutes, depending on the complex- ity of the work. Today 30 minutes hardly suMices for an introduction to the software manual, and without a working knowledge of the software, you can’t even turn the #@%$&%*-+ thing on!

Now let us return briefly to the fun time, because much of what we see in AA instruments today is a conse- quence of arguments resolved then. At least in certain areas of the mid- western United States, flames con- tinued to emit. Velmer Fassel and co- workers a t Iowa State University (5) and Ed Pickett and I a t the Univer-

sity of Missouri (6), along with a few others, pointed out repeatedly that the emission mode of operation had certain advantages and should not be abandoned. In the intervening years we have not changed our minds, but we seem to have made little progress in convincing others. Few people use flame emission today, even in cases where the advantages are significant and easily demonstrated.

During the fun time, detection lim- its were a source of great pride and it seemed that nearly every conference paper included a claim to have estab- lished a new record in the quest for zero. A discussion of interferences was sure to spark instant debate; many researchers argued that AAS was much less prone than atomic emission to effects from chemical in- terferences. The very existence of spectral interferences in AAS was vigorously denied by proponents of the technique. The confusion con- cerning interferences can perhaps be best conveyed by quoting from a pub- lication of the period (7): “Atomic ab-

sorption spectroscopy does not suffer from chemical interferences but (the) presence of large amounts of anions and cations can cause pronounced ef- fects on absorption.”

In other words, it is not the chemi- cals, but those blasted anions and cations that cause the problems. The au thors of t he above s ta tement should be defended, because they were not writing in their native tongue and undoubtedly had a lan- guage problem. The fact that such a statement got past two reviewers and a journal editor is harder to under- stand and is indicative of the confu- sion that prevailed.

By 1969 most misconceptions about AAS had been resolved. Spec- tral interferences, although rela- tively rare, were acknowledged, and chemical interferences were known to be primarily a function of the at- omizer, not the mode of observation. Atomic fluorescence had been intro- duced, and people already were be- ginning to wonder why it was not more popular. Automated back- ground correction systems for AAS were available and, in an important advance, the nitrous oxide-acetylene flame had nearly doubled the num- ber of elements to which AAS was applicable. The evolution of flame AAS was essentially complete. Fur- naces had been introduced and their evolution was just beginning, but that is another story.

Ideas that did not succeed This evolution did not proceed along a smooth pathway. Although numer- ous approaches to doing AAS were tried, most eventually were dis- carded. A few of these, listed below, will be discussed in detail.

Total consumption nebulizer- burners

Multipass optical systems Heated spray chambers Hot gas nebulization The tantalum boat Premixed oxygen-acetylene

Flame-in-tube designs In the 1950s emission flame pho-

tometry in the United States was dominated by the use of a combina- tion nebulizer-burner marketed by Beckman Instruments. The combus-

flames

ANALYTICAL CHEMISTRY, VOL. 63, NO. 21, NOVEMBER 1, 1991 1025 A

REPORT

tion gases were mixed only after they exited the burner, and solution aero- sol was sprayed directly into the re- sulting turbulent flame. Because the gases were not premixed, nearly any fuel-oxidant combination could be used safely. Oxygen-hydrogen and oxygen-acetylene flames were quite popular for emission work.

I t is not surprising that when AAS arrived on the scene many people tried to retain the simple and famil- iar burner. The short absorption pathlength was a disadvantage, how- ever, and workers tried various ways to overcome this deficiency. An opti- cal system that passed light from the primary source through the flame from a tandem array of three such burners as many as five times was developed commercially. The sound level was roughly comparable to that of a jet plane taking off.

This and other attempts to use total consumption burners failed not because of the short absorption path- length, however, but because large aerosol droplets were injected into the flame rather than separated in a spray chamber. Gradually it was shown that incomplete vaporization of analyte contained in large drop- lets, rather than the emission mode of operation, was the cause of many chemical interferences.

Conventional AAS burners waste about 90% of the sample solution. Many people sought ways to avoid that loss and enjoy an instant 10-fold improvement i n detection limits. Heating the spray chamber to vapor- ize much of the solvent increased sample transport and was successful if only detection limits were consid- ered. Unfortunately, memory effects were quite severe, and this method never became popular. Heating the nebulizing gas resulted in greater sensitivity, but at the cost of severe nebulizer clogging problems. (The hot gas caused salts to build up on the nebulizer tip because of solution evaporation.)

An attachment designed to im- prove detection limits by increasing the rate at which sample was intro- duced into the flame was marketed for a time. The sample solution was put into a tantalum boat with about a l - m L capacity. The boat was moved close to the flame for solvent evaporation and then inserted into the flame just below the light beam. Volatile analytes, such as lead, were quickly vaporized and atomized, re - sulting in a transient signal of a few seconds’ duration. Detection limits improved dramatically, but matrix effects also increased. The fatal flaw,

however, was instability in the shape and position of the boat. Repeated heating caused distortion of the boat and its holders. With each distortion increment came a change in atomiza- tion efficiency. More time was spent in recalibrating the system (chasing the working curve) than in running samples.

Before the nitrous oxide flame was introduced, several attempts were made to exploit the higher tempera- ture of the oxygen-acetylene flame in a premixed burner. The problem was the high burning velocity of the mixture, which caused those flames to have the nasty habit of flashing back into the spray chamber. The re- sulting explosion caused a loud re- port and often damaged the system.

Probably the most successful de- velopment was by Fassel’s group (8). The burner was water cooled, mas- sive, and rather complex, but it vorked quite well. Indeed, the design

A Brief Histor

1955 Methou described indeperl- dently by Walsh and by Alkemade and Milatz

development down under while United States remains an “under- developed country”

1962-69 Explosive growth-the “fun time” for those of us fortu- nate enough to have been there in person

1969-76 Relative stabilit! puts on its work clothes

1976-present Electronic revolu - tion-microprocessor and com - .puter enhancement of instrumen-

1955-62 The induction period,

1

Figure 1. Flame-in-tube designs. (a) Simplification of arrangement used by Fuwa and Vallee using only a hollow cathode source. (b) Continuum source background correction system using both a hydrogen lamp and a hollow cathode lamp. L, and L2 are lenses. (Adapted from Reference 10.)

probably would have gained accep- tance had a better idea, the nitrous oxide-acetylene flame, not come along. The paper describing the work contains a most unusual acknowledg- ment (8): “The authors would like to thank. . . Mr. J. 0. Rasmuson for his t imely confirmation t h a t oxy- acetylene flashbacks in this burner cause no serious damage to the burn- er or its surroundings.’’ One wonders if the undamaged “surroundings” were defined to exclude the eardrums of the operator.

The flame-in-tube design first de- scribed by Fuwa and Vallee (9) was not successful but led to another de- velopment that did succeed (IO). Both are shown in Figure 1. In the original design (Figure la), a quartz or Vycor tube about 1 cm in diameter and 10- 100 cm in length was placed on the optical axis, and the flame from a Beckman burner was directed into one end from a slight angle. The sys- tem gave extremely good sensitivity but suffered from high background absorption, as shown in Figure 2. There was an obvious need to com- pensate for the background, and this led to the development of the contin- uum source background correction system (Figure lb), which was later automated and incorporated into many commercial instruments. The original flame -in - tube design failed not because of background absorp- tion but because of the same matrix interference problems that plagued the total consumption burner.

Some unusual aspects of AAS history One of the most striking things about the history of AAS is the degree to which the method was “born ma- ture.” The operating principles of to- day’s flame AAS units are indistin- guishable from those of the one that Walsh first described. To be sure, re- finements have been made and com- puter technology has had significant impact. However, hollow cathode sources, the premixed air-acetylene flame formed on a slot burner, and double- beam optics with modulation and ac amplification were all a part of the earliest instruments. Many at- tempts were made to improve on these conditions, but most were gradually shown to be steps away from the optimum.

Contributions to the early develop- ment of AAS from the academic com- munity are conspicuous by their ab- sence. In the United States, only Pickett was active (to my knowledge) prior to the beginning of the explo- sive growth period, and his early

1026 A ANALYTICAL CHEMISTRY, VOL. 63, NO. 21, NOVEMBER 1,1991

work was never published. Jim Rob- inson probably was the first in the United States to do AAS, and cer- tainly he was the first to publish ex- tensively in this country (11, 12). He did his early work at Esso Research before moving to Louisiana State University in 1964. Both Robinson and Pickett used homemade, or at least home-assembled, AAS units that gave satisfactory performance. My own experience with the equip- ment at Missouri will be discussed later.

There are a few examples where the early commercial development of a technique was so totally associated with a single manufacturer when several others seemed to be in equal- ly strong initial positions. Perkin Elmer quickly dominated the mar- ket, especially in the United States, and largely through the company’s efforts AAS became an accepted technique. I will elaborate on P-E’s contributions in a later section.

Once accepted, AAS replaced com- petitive methods for elemental anal- ysis to an unprecedented degree. An atmosphere prevailed in which legit - imate problems with AAS were un- derestimated and its advantages over other methods overestimated. The confusion mentioned earlier regard- ing interferences prevailed for a time, and the virtual exclusion of flame emission continues today. Col- orimetric and polarographic methods are used for routine analyses only in special cases.

The replacement went beyond the optimum, at least in principle. It is legitimate to ask why. Operational simplicity for a laboratory seems to be a major part of the answer. AAS was the obvious choice for elements such as Zn, Cd, Hg, and Mg. It was also quite satisfactory, though per- haps not optimal, for a long list of others. The lab manager could sim- pli@ the operation by using this one technique for the broadest possible range of applications. In practice, it seems to have worked rather well.

Some disagreements with conventional wisdom It has been suggested tha t AAS should have been developed years, even decades, before it was. I dis- agree. It is true that the conditions for observation of atomic absorption were described in 1859, that astrono- mers routinely used AA spectra in establishing the composition and mo- tion of stars, that self-absorption and self-reversal were well known to emission spectroscopists as trouble- some phenomena to be avoided at all costs, and that an AA instrument for determination of mercury vapor con- centrations was described in the late 1930s. Why, then, did the advanta- ges of AAS as a general analytical technique remain unappreciated un- til 1953 when Walsh started the work that was published in 1955?

The answer, or a t least one answer, lies in the area of supporting technol- ogy. All new developments require

NaCl

I

II

Y

E

6 2

Figure 2. Absorption spectra of solutes in a long-path flame. (Adapted from Reference 10.)

supporting materials that often come from seemingly unrelated areas. Two examples will illustrate the point. First, Kirchhoff’s observations on atomic spectra were made possible by the burner that Bunsen had invented a few years earlier. For the first time, spectral behavior could be ob- served in a nonluminous, nearly transparent flame. The beginning of spectrochemical analysis and a series of related developments were made possible, or at least much easier, by the lowly burner that has been used by every chemist since Bunsen’s time. Second, the highly efficient diffraction gratings, on which mod- ern instruments depend, were made possible by industrial- scale high - vacuum technology, which devel- oped because of the tremendous commercial success of radio in the 1930s.

The development of AAS had to await the availability of photomulti- plier (PM) tubes. True, in principle it is possible to do AAS photographi- cally, but it is not nearly as conve- nient as emission (and goodness knows, that was bad enough). Other detectors were available, but hollow cathodes a re ra ther feeble light sources that would not have been very practical without the sensitive detection capability of PM tubes and their associated electronics, which were developed in the late 1930s. However, World War I1 intervened and PM tubes did not become rou- tinely available t o chemists until the late 1940s. As an example, the battery-powered PM attachment for the Beckman DU spectrophotometer was introduced, even later, in 1953 (13). Thus Walsh’s work followed, without great delay, the time when s u p p o r t i n g technology became available.

Was the seven-year period needed for AAS to really catch on an exces- sive length of time? It undoubtedly seemed so to those who realized its tremendous potential but were un- able to generate interest in the scien- tific community. However, if we look at how long it took for other methods to gain acceptance, we might recon- sider. The utility of arc-spark emis- sion spectroscopy for quantitative analysis was demonstrated as early as 1873 and was well established in several laboratories in the opening years of the twentieth century. Qual- ity commercial instruments were available only a few years later (1909- 12), but it was 1922 before the first paper on the method was pub- lished in the United States. Slavin (4) comments on the situation:

ANALYTICAL CHEMISTRY, VOL. 63, NO. 21, NOVEMBER 1,1991 1027 A

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Thus by 1920 all the conditions needed for a system of chemical analysis by spectroscopy ex- isted. We had excellent instru- men t s , good photographic emulsions, a power distribution network, and basic theory. However, chemists were very slow to take advantage of this powerful tool, even for simple quali tative identifications. They still relied on the classical instruments, the test tube, the blowpipe, the eye, and the nose.

Emission spectroscopy did not be- come popular until well over a decade later and, again, Slavin (4) does a better job of describing the situation than I could hope to:

Impetus for growth of the field, a t least in this country, was given by a series of summer conferences organized by G. R. Harrison a t the Massachusetts Institute of Technology. Begin- ning in 1933 and continuing to 1940 . . . these meetings were held for five days . . . in a small lecture hall seating about 80 persons. Only toward the end of

the period were there enough participants to fill the room, and more than half were organ- ic chemists working with the spectrophotometer, not emis - sion workers. . . . It was a truly egalitarian group; everyone was equal in his ignorance.

The story is much the same for flame emission analysis. LundegArdh started work in 1929 and eventually developed a completely automated system. Electrical detection of flame emission was described in the late 1930s. Developments undoubtedly were delayed by World War 11, but a full 20 years after LundegArdh’s work, Meggers would write (14): “the flame photometer appears suddenly (my emphasis) to have come into vogue for the quantitative determi- nation of sodium, potassium, and cal- cium in almost anything.”

After 20 years, the scientific com- munity suddenly woke up to the val- ue of flame emission analysis. Thus the induction period for AAS proba- bly was no longer than should have been expected.

Adam Hilger Ltd. was the first ma- jor instrument company to market

atomic absorption equipment under a n exclusive license to Walsh’s patent. This unit took the form of a kit consisting of a hollow cathode lamp and power supply and a pre- mixed nebulizer-burner assembly. It was designed to mount on one of the company’s existing UV spectropho- tometers, which used dc electronics. In this system, any flame emission would be detected and cause an error in the absorption measurement.

The electronics are often cited as the fatal flaw in the Hilger kit. Based on my experience, I disagree. The sec- ond atomic absorption unit I ever used was a modified Hilger kit mounted on a Beckman DU (dc electronics) spec- trophotometer in Pickett’s lab. The modification was important. The kit came equipped for an air-propane flame, which proved to be totally un- satisfactory. Within weeks after deliv- ery, pickett, who had used a Lunde- gArdh flame, made a new burner head, following Walsh’s design, and started burning acetylene.

By the time I arrived on the scene in 1963, the instrument was in rou- tine daily use and was a t least as convenient t o operate as the P-E Model 303, which eventually re -

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placed it. Its primary use was for the determination of nutritionally signif- icant elements (Ca, Mg, Cu, Zn, Fe, and Mn) in biological materials. Na and K were determined by emission, as they still should be today. The point here is not that dc electronics were desirable but that the modified Hilger unit worked well enough for enough elements that it could have been successful. It certainly outper- formed most methods commonly used at that time for the above elements.

If this kit worked so well, why did it not succeed? The most serious de- ficiency probably was the flame. Few users would have had the insight and inclination to modify the kit as Pick- ett did, and all reports indicate very poor performance using air-propane. In addition, Hilger made a limited investment in a marketing effort and the kit arrived on the market a bit too early. The induction period had not yet run its course.

The Perkin Elmer success Just a few years after the Hilger kit was introduced, the company agreed to a nonexclusive patent license, clearing the way for other businesses to enter the market. Several did, and

P-E was far more successful than the others. Why? Other companies that attempted to compete made one or more serious design errors, especial- ly with dc electronics and total con- sumption burners. P-E, however, by consulting extensively with Walsh and others at CSIRO, as well as with the few experienced users in this country, was able to produce the first optimized commercial atomic absorp- tion instrument.

This success was followed by ag- gressive marketing and a willingness to fill the education vacuum left by academia. Customer seminars, the Atomic Absorption Newsletter, a meth- ods manual unofficially but univer- sally called the AAS “Cookbook,” and field product specialists assisting customers with problems were im- portant components not only of the P-E success but of the success of AAS in general. Another reason for suc- cess was fortuitous timing. The in- duction period had run its course. Scientists had heard of AAS often enough from the group down under, and from a few other groups around the world, to be more receptive to the idea. Also, several companies that were in a position to become major

competitors concentrated on ideas from the “did not succeed” list dis- cussed earlier.

As significant as these factors are, they do not reach the heart of the matter. The real difference at P-E was the small number of talented and dedicated people who truly be- lieved in AAS and had the courage of their convictions. I hesitate to men- tion names because of the risk that equally deserving people will be ex- cluded. But fools rush in . . . ! In the early days, Dick Reiss was an impor- tant member of this group until he left P-E. Herb Kahn also eventually left P-E, but not until after its suc- cesses and his contribution thereto were well established. Walter Slavin, the son of Morris Slavin (whose work has been quoted extensively here), was in the thick of the early develop- ment and remains a major player in the still successful AAS business at the company. There may have been others but, if so, they were less visi- ble from my perspective.

What about the success of AAS as an analytical method? Why did it be- come so popular? The best simple ex- planation comes from Lundell’s pa- per, “The Chemical Analysis of

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Things as They Are,” published two decades before Walsh started his work (15): “There is no dearth of methods that are entirely satisfac- tory for the determination of ele- ments when they occur alone. The rub comes in because elements never occur alone, for nature and man both frown on celibacy.”

These noncelibate elements could be measured more quickly, more reli- ably, and less expensively by AAS than by methods commonly used in the late 1950s. AAS did a superior job of analyzing “things as they are.”

How were elemental analyses done before AAS? This question was asked by Zoe Gros- ser at a preliminary presentation of this paper to people at P-E. Her ques- tion is a good one because a genera- tion of fortunate analysts has grown up without being forced to consider some of the old problems. The answer depends on many specifics, of course, and my perceptions come from experi- ence in the analysis of agricultural and biological materials.

First, many fewer analyses were done, especially at trace levels. Those that were done usually required con-

siderably more chemical manipulation than is common today. Consider major constituents. Potassium in fertilizer was measured gravimetrically after separation from interferences and precipitation as K,PtCl,. It was a la- borious procedure even if the need to recover and reuse the platinum was not considered. The flame photometer was used for potassium in most other sample types, and later flame meth- ods were refmed to yield acceptable results for fertilizers. Calcium and magnesium were normally measured by two EDTA titrations, the first to measure calcium and the second to de- termine calcium plus magnesium. Magnesium was obtained by differ- ence. Good results usually were ob- tained for calcium, but those for mag- nesium left much to be desired- especially for cases of large Ca/Mg ratios. If I were to pick a single ele- ment for which AAS made the great- est difference, it would be magnesium.

For determination of trace ele- ments, the emission spectrograph was used extensively in large labora- tories that could afford one. Polaro- graphic methods were used for a few elements, notably Pb, Cu, Zn, and Cd. However, probably more trace

determinations were done spectro- photometrically or colorimetrically than by any other class of methods. Many books dealing exclusively with such methods were published; the classic work of Sandell (16) is one ex- ample. The methods lacked the spec- ificity inherent in the atomic spec- trum, and extensive manipulations- including pH control, extraction, back extraction, distillation, and ad- dition of masking agents-were the norm. In many cases, chemical sepa- rations were required to enhance sensitivity and to control interfer - ences. Keep in mind, too, that certi- fied biological reference materials for quality assurance did not exist. A skilled and patient analyst who had the ability to devise and carry out a quality-control scheme from the ground up was essential for reliable results. Contamination was an ever- present problem because of the num- ber of reagents and the amount of manipulation required. Indeed, high blanks probably were responsible for more profanity per hour in labs of the day than any other single factor. To- day, even though we typically work at concentration levels 1-2 orders of magnitude lower, the simplicity of

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highly specific procedures . and the availability of prepurified reagents make blank control much easier.

Reliable results were obtained in spite of the problems. I recall a group of food samples in about 1970 from which AAS and colorimetric results for lead were compared. Large dis- agreements were resolved in favor of the colorimetric values because workers using AAS had failed to cor- rect for background absorption.

The situation today The success of AAS continues. My feeling is that more elemental analy- ses currently are done by AAS than by any other method. Its only close competitor would be inductively cou- pled plasma emission. Instruments of ever - increasing complexity are sell - ing like hot cakes, and there appears to be no end in sight. The situation described by Slavin (4) is even more true today than it was in 1971, espe- cially if one includes the computer system with which the instruments interact: "In fact the possession of spectroscopic instruments (not neces- sarily their wise use) has become a status symbol. . . . A pecking order is developing among laboratory work-

ers on the basis of the size, number and elaborateness of optical appara- tus. Affluence breeds strange fruits."

References (1) Walsh, A. Spectrochim. Acta 1955, 7,

108. (2) Alkemade, C.T.J.; Milatz, J.M.W.

J. Opt. SOC. Am. 1966,45,583. (3) "an, F. The Spectrochemical Analy-

sis of Metals and Alloys; C. Griffin & Co. Ltd.: London, 1941; Chapter 1.

(4) Slavin, M. Emission Spectrochemical Analysis; Wiley -1nterscience: New York, 1971; Chapter 1.

(5) Fassel, V. A.; Golightly D. W. Anal. Chem. 1967,39,466.

(6) Pickett, E. E.; Koirtyohann, S. R. Anal. Chem. 1969,41(14), 28 A.

(7) Chan, Y. K.; Wong, P.Y. Talanta 1968, 15, 867.

(8) Fiorino, I. A.; Kniseley, R. N.; Fassel, V. A. Spectrochim. Acta 1968,23B, 413.

(9) Fuwa, K.; Vallee, B. L. Anal. Chem. 1963,35,942.

(10) Koirtyohann, S.R.; Pickett, E.E. Anal. Chem. 1965,37, 601.

(11) Robinson. J. W. Anal. Chem. 1960. . 32(8), 17 A. ' (12) Robinson, J. W. Anal. Chem. 1961,33,

1067. _ _ _ - . (13) Jarnutawski, R. J. Beckman Instru-

ments, Inc., personal communication, 1991.

(14) Meggers, W. F. Anal. Chem. 1950,22, 18.

(15) Lundell, G.E.F. Ind. Eng. Chem. Anal. Ed. 1933, 5, 221.

(16) Sandell, E. B. Colorimetric Determina- tion of Traces of Metals, 2nd ed.; Wiley-In- terscience: New York, 1950.

I S. R. Koirtyohann earned his M.S. degree and Ph.D. fiom the University of Missou- ri-Columbia, where he has been a faculty member since 1966. He joined Oak Ridge National Laboratory in 1958, working in emission spectroscopy with Cyrus Feld- man and becoming active in AA research. He has published about 75 research pa- pers and book chapters on AAS. His contri- butions include the definition of the nature of background spectra inflames and the de- velopment of continuum source background correction. He has also contributed to the understanding of atom formation inflames and firnaces and has wohed throughout his career on the application ofAA methods to practical problems.

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