Simultaneous or Sequential Determination of the Elements at All Concentration Levelsthe Renaissance of an Old Approach Velmer A. Fassel Ames Laboratory—U.S. Department of Energy and Department of Chemistry Iowa State University Ames, Iowa 50011
An award acceptance address provides the award recipient with a timely opportunity to contemplate, to predict, and to offer personal commentary on the nature and direction of the science in which he has been involved. In the remarks that follow I shall avail
myself of this opportunity. I shall mostly proceed in a serious manner but will occasionally punctuate my remarks with lighthearted commentary and historical reminiscences that are responsive to several provocative questions posed later in this address. On the occasion of this award symposium, I can boast enthusiastically about the fields of analytical chemistry and spectroscopy. As noted in recent editorial commentary in this JOURNAL (1, 2), the strength and stature of these disiplines, both in academia and in industry, have climbed
steadily during the past decade. Those of us who are now active analytical chemists or spectroscopists should therefore take pride in the contributions of chemical analysts in years past, but with the full recognition that leaders in these fields have often expressed concern, or chided their colleagues, or were critical of developing trends. In this context, it is appropriate to take a backward journey to 1933 when G. E. F. Lundell published a perceptive article that should be required reading for anyone striving to be identified as an analytical chemist.
Velmer Fassel (right) receives the ACS Award in Analytical Chemistry ( Fisher Award) from Gardner Stacy, President of the Society, at the ACS/ CSJ Chemical Congress which was held in Honolulu, Hawaii, April 1-6, 1979. This REPORT is based on Dr. A ward Address
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Fassel's
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This paper entitled " T h e Chemical Analysis of Things as They Are" (3), contained a number of classic sentences, including the following: " T h e determinator's salvation lies in the development of selective methods of analysis, and his final resting place will be a heaven in which he has a shelf containing 92 reagents, one for each element, where No. 13 is the infallible specific for Al, No. 26 the sure shot for Fe, No. 39 the unfailing relief for Y, and so on to U." T h e heavenly hopes expressed by Lundell have never been realized. Even today, the most optimistic analytical chemist sees little hope of ever reaching the Utopian state of specific reagents for each element, especially for those elements t h a t have nearly identical (the lanthanides) or very similar chemical properties, e.g., Hf and Zr. T h e thrust of Lundell's statement was to chide some of his analytical chemistry contemporaries for not giving adequate consideration to an important requirement of an acceptable analytical method, namely, sufficient selectivity so t h a t it would be useful for the "chemical analysis of things as they are." During Lundell's professional career—and occasionally even today—new analytical methodologies are proposed when it is quite obvious t h a t they suffer from interferences to such a degree that they are not useful for the analysis of "things as they are." Lundell aptly commented: There is no dearth of methods that are entirely satisfactory for the determination of elements when they occur alone. The rub comes in because elements never occur alone, for nature and man frown on celibacy. Methods of determination must therefore be judged by their 'selectivity.' It is in this respect that most methods are weak and improvements must come . . . . During the 46 years that have elapsed since Lundell's paper was published, these improvements have indeed materialized, and they have followed many different pathways. T h e remainder of my address will be devoted to a historical walk along one of these pathways.
Inductively Coupled Plasma— Atomic Emission Spectroscopy The Early Years. For the past 17 years my associates and I have devoted a fraction of our efforts to the development of the basic science, the investigative methods and the hardware for an analytical approach t h a t would eventually provide the capability of determining the chemical elements selectively, at all concentration levels, i.e., major, minor, and trace constituents, simultaneously if so desired, or in a rapid sequential manner, with a single analytical technique, and with accuracy and precision. A new analytical approach, usually identified as inductively coupled plasma—atomic emission spectroscopy (ICP-AES) has emerged from these studies. I use the term "new analytical approach" advisedly, because the AES portion of this technique is certainly not new; quantitative determinations have been made
via AES for at least 40 years. The use of ICP's as vaporization-atomizationexcitation cells is not new either. T h e first analytical studies of ICP's were launched independently 17 years ago, by Stanley Greenfield and associates at Albright and Wilson, Ltd., Oldbury, England, and by our research group at the Ames Laboratory, Iowa State University. T h e exciting possibilities offered by ICP's as vaporization-atomization-excitation-ionization (VAEI) cells for analytical atomic spectroscopy were first communicated ~ 1 5 years ago (4, 5), and a decade has passed since a landmark paper, as described by Barnes (6), was published (7). T h e analytical community, however, including academia, almost completely ignored these and subsequent papers until approximately five years ago. The surprising lack of interest in the ICP-AES approach to elemental determinations is reminiscent of Sir Allen Walsh's experience (8) following the publication of the pioneering papers in flame atomic absorption spectroscopy (AAS). His experience and ours are, I believe, excellent examples of how tacit acceptance of methodologies as they are, and perhaps a liberal sprinkling of mental paralysis, can influence the timely acceptance of new ideas. Because ICP-AES was originally conceived as an alternative approach for the determination of minor and trace constituents, it had to vie for attention at a time when AAS was experiencing its phenomenal growth and wide acceptance. T h e fact t h a t AAS provided a relatively simple, highly specific way for elemental determination at the minor or trace concentration level contributed so much to its popularity that the use of alternative techniques for performing the same tasks suffered precipitous declines. T h e technique that experienced the greatest decline in usage was atomic emission spectroscopy, which during the period from the mid1930's to the early 1960's was often, if not usually, the method of choice for multielement determinations at the minor or trace concentration level. Although spark-arc excitation AES retained its important role for compositional control in the metals industries, there is no doubt t h a t its use as a gen-
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eral analytical tool experienced a sharp decline from the mid-1960's to the early 1970's, whereas AAS was ascending its steep popularity curve. T h e interest of analytical chemists or spectroscopists in AES was further undermined when some AAS enthusiasts directed intellectual darts at the principle of observing free atoms in emission rather than in absorption. These darts consisted of undocumented, and usually theoretically unsound, claims regarding the alleged superiority of observing free atoms in absorption rather than in emission. T h u s , the claim that AAS should exhibit far superior powers of detection because the bulk of the free atoms in atomization cells were in the ground state in contrast to the far smaller number density in excited states, did not rest on sound theoretical bases. Neither did the claim that because the fraction of atoms in excited states was very small, the observation of free atoms in emission was subject to serious "excitation interferences" arising from collisional deactivation, preferential excitation, or energy transfer processes in general. Other assertions suggested that, in spite of recorded successes in the past, spectral line interferences were so inseparable, and temperature changes in the excitation cells so uncontrollable, that the observation of free atoms in emission for analytical purposes was surely destined for failure. These assertions were often read in amused astonishment by myself and others. Unfortunately, they were repeated often enough in the literature and advertising circulars to lead to their acceptance as fact by many analysts. T h e period from 1965 to 1970 saw the publication of a sufficient number of these unwise claims to fill part of an issue of the fictional journal Acta Rétracta. Regrettably—but also fortunately—that journal has never become a reality. I use the term "fortunately" because emanations from my pen in other contexts would have been acceptable candidates for publication in the journal! T h e publication of the scientific misdemeanors referred to above subsided rather abruptly after C. Th. J. Alkemade's perceptive paper entitled "Science vs. Fiction in Atomic Absorption Spectroscopy" appeared in 1968 (9). By that time the reputation of AES as a useful general analytical tool was sufficiently tarnished that its attractiveness as a research field in academia and as a commercial venture by instrument makers continued to decline. But decline in usage does not imply total stagnation in scientific development. Although basic research in analytical AES was confined to a limited number of academic, government, and industrial laboratories, sig-
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