Encyclopedia of Spectroscopy and Spectrometry

Atomic Spectroscopy, Historical Perspective *

C.L. Chakrabarti

Carleton University, Ottawa, Ontario, Canada The development of atomic spectroscopy, particularly graphite furnace atomic absorption spectroscopy (GFAAS) is reviewed from a historical point of view. Some details of the various instrumental methods used are presented, and a comparison is given with inductively coupled plasma mass spectrometry (ICP-MS). Atomic absorption; Flame; Graphite furnace; ICP-MS Professor BV L'vov, the inventor of the very powerful analytical technique known as graphite furnace atomic absorption spectroscopy (GFAAS), has rightly pointed out in the introduction of his definitive book entitled Atomic Absorption Spectrochemical Analysis that 'The discovery of atomic absorption and the history of research into it are integral parts of the entire history of spectroscopy and spectrochemical analysis'. Indeed, the early history of atomic spectroscopy, as far as spectrochemical analysis was concerned, consisted of the development of emission spectrochemical analysis, which was usually dependent on Fraunhofer lines (atomic absorption lines) for wavelength calibration. Following Newton's study of the spectrum of the Sun in 1666, there was a period of almost 136 years entirely concerned with emission spectroscopy. Only in 1802 did Wollaston report the presence of dark bands in the continuum emission spectrum of the Sun and, after a more detailed study by Fraunhofer (1814), Brewster (1820) was able to ascribe them to absorption of radiation within the Sun's atmosphere. Another 40 years passed before Kirchhoff and Bunsen showed that one of these dark bands in the emission spectrum of the Sun corresponded exactly to the yellow emission band obtained when sodium vapour is heated in a flame. This led Kirchhoff to deduce that the Fraunhofer lines in the solar spectrum are absorption lines of elements whose flame emission spectra would contain lines at exactly the same position in the spectrum. Their work enabled Kirchhoff to develop the fundamental relationship between emission and absorption spectra: any species that can be excited to emit radiation at a particular wavelength will also absorb radiation at that wavelength. Thus, Kirchhoff not only laid the foundations of atomic absorption methods of chemical analysis but also gave a striking example of their power. Indeed, it is difficult to imagine a more convincing and dramatic demonstration. Bunsen and Kirchhoff are thus rightfully considered to be founders of spectrochemical analysis. However, it is surprising that almost a century after the work of Bunsen and Kirchhoff, the potentialities of atomic absorption measurements remained unexplored and unsuspected. Why? As Alan Walsh has presented in his perceptive analysis of the reasons, it seems likely that one reason for neglecting atomic absorption methods was that Bunsen and Kirchhoff's work was restricted to visual observation of the spectra. In such visual methods, the sensitivity of emission methods was probably better than that of absorption. Not only was the photographic recording of the spectra more tedious but also the theory seemed to indicate they would only prove useful for quantitative analysis if observed under very high resolution. But possibly a more fundamental reason for neglecting atomic absorption is related to Kirchhoff's law (1859), which states that the ratio of the emissive power E and the absorptive power A of a body depends only on the temperature of the body and not on its nature. Otherwise radiative equilibrium could not exist within a cavity containing substances of different kinds. The law is usually expressed as where K ( ?T ) is a function of wavelength and temperature. This law is perfectly correct but unfortunately much and possibly most of the subsequent teaching concerning it has been misleading. In most textbooks and lessons on this subject the enunciation of the law is followed by a statement to the effect that 'this means good radiators are good absorbers, poor radiators are poor absorbers'. This statement is patently absurd without any reference to temperature or wavelength. This erroneous conclusion has been made, presumably, because Kirchhoff's constant, K , as he so clearly pointed out, is a function of wavelength and temperature. He was unable to find an analytical expression for it, and it was not deduced until 1900 by Kirchhoff's successor Max Planck, at the University of Berlin, as Planck's distribution function. Many spectroscopists were misled by the widely held but misleading assumptions regarding the implications of Kirchhoff's law. What is even more surprising is that numerous spectroscopists wrote papers on atomic emission methods and referred to the problems caused by the effects of self-absorption and self-reversal, and yet failed to make the small-step connection between atomic emission and atomic absorption. It was left to Alan Walsh, whose research experience in atomic emission spectroscopy and molecular absorption spectroscopy virtually compelled him to see the obvious connection. Walsh expressed this experience in his inimitable way in the following words: 'It appears to be true that "having an idea" is not necessarily the result of some mental leap: it is often the result of merely being able, for one sublime moment, to avoid being stupid!'. Walsh in 1953 and Alkemade and Milatz in 1955 independently published papers indicating the substantial advantage of atomic absorption methods over emission methods for quantitative spectrochemical analysis. Alkemade and Milatz considered only the selectivity in atomic absorption methods, but Walsh discussed general problems of development of absolute methods of analysis. During his early experiments with flame atomizer burners, Walsh encountered the problems which arise when measuring atomic absorption using a continuum source problems, which might have been responsible for the neglect of atomic absorption methods. The use of a sharp-line source such as a hollow-cathode lamp not only solved this problem, it also provided the atomic absorption method with one of its important advantages, that is, the ease and certainty with which one can isolate the analysis line. The concept of 'putting the resolution in the source' also permitted the use of a simple monochromator since the function of the monochromator, in such a case, is only to isolate the analysis line from the neighbouring lines and the background. GFAAS has been widely used as a spectrochemical trace-analytical technique during the last 40 years. L'vov pioneered most of the theoretical and experimental developments in GFAAS and provided a masterly treatment of the possibilities of GFAAS in a paper entitled 'Electrothermal atomization-the way towards absolute methods of atomic absorption analysis'. RE Sturgeon, who has been responsible for much of the development of GFAAS, presented a critical analysis of what it does and what it cannot do. AKh Gilmutdinov, who did much of the theoretical development of GFAAS in the 1990s, also elucidated the complex processes and reactions that occur in an electrothermal atomizer by digital imaging of atomization processes using a charge-coupled device camera in the author's research laboratories. Atomic spectroscopy in the three variations that are most commonly used in spectrochemical analysis, atomic absorption, atomic emission and atomic fluorescence, are all mature techniques, with their particular areas of strengths and weaknesses now well recognized. Many a battle has been fought and won to establish the superiority of one or the other technique over the rival techniques. It is sometimes claimed that inductively coupled plasma mass spectrometry (ICP-MS), which has the winning combination of high-temperature ionization of elements in a plasma, with the detection of the ions by mass spectrometry, is the ultimate trace-analytical technique that will triumph over the rival techniques. However, such claims are based on the limited applications of these techniques by practitioners whose allegiance to their own techniques gives more credit to their loyalty than to their scientific objectivity, as elaborated in the following paragraph. Some have also predicted the imminent demise of GFAAS as a trace-analytical technique. Such a prediction, based as it is on inadequate comprehension of the enormous complexities and extreme diversities of real-life situations requiring a variety of trace-analytical techniques which possess, some special capabilities not possessed by many analytical techniques, is destined to be false, as is shown in the following text. In the author's research laboratories in recent years, PhD students and adjunct research professors have been doing research on the chemical speciation of potentially toxic elements in the aquatic, the atmospheric and the terrestrial environment. Because freshwaters and soils are systems that are usually far removed from equilibrium, our research is mostly directed to the development of kinetic schemes of chemical speciation which require routine use of ICP-MS and GFAAS to measure the kinetics of metal uptake from aqueous environmental samples. As such, hundreds of determinations of trace and ultratrace elements are routinely made every week. For these determinations, the latest models of two ICP-MS and several GFAAS systems exist. The performance characteristics of ICP-MS and GFAAS for kinetic studies of real-life, aqueous environmental samples have been analyzed for a period covering 6 years from 1993 to 1998, and the following inescapable conclusions are derived. The analytical sensitivities of...