Report H. A. Laitinen Dept. of Chemistry University of Florida Gainesville, Fla.
Analytical Chemistry in a Changing World It will be my purpose to examine some of the changes t h a t have occurred in the field of analytical chemistry in the past as it has adapted to changes in science and technology; to look at the major changes under way at the present time; and to speculate a little about the future. First of all, let us define analytical chemistry as the science of chemical characterization and measurement. T h e meaning of "characterization" has evolved, and will continue to evolve, and the instruments with which chemical measurements are made are likewise constantly undergoing development. It is, in fact, improvements in the theory and practice of chemical characterization and measurement t h a t constitute research in analytical chemistry. M a n y types of scientists and engineers, of course, make contributions to analytical chemistry. W h a t differentiates the analytical chemist is that he makes characterization and measurem e n t his goal, rather than a means to an end. To illustrate this point, a physical chemist may need to improve the sensitivity or time resolution or spatial resolution or frequency resolution of a spectroscopic measurement, and having done so, his attention is focused upon the use of the improved measurements for his own purposes. T h e analytical chemist seeks to systematize, generalize, and optimize measurements with the goals of widening the scope of applications, increasing the efficiency and reliability of measurements, and perhaps even of designing a commercial instrument. I should now like to turn to a brief look at the evolution of analytical chemistry to see if history might teach us a lesson or two as guidance for the future. Inorganic qualitative analysis was once at the forefront of chemical science, and it made important contributions to the discovery of elements 0003-2700/80/0351-605A$01.00/0 ©1980 American Chemical Society
and to our understanding of minerals and rocks. Certain techniques, such as blowpipe analysis, were once marvelous tools which have long since fallen into disuse-. Later, the development of systematic schemes for separations and confirmatory tests became an active research field. Selective organic reagents emerged early on the scene. Qualitative analysis gradually evolved to serve a largely educational role in the teaching of solution chemistry. In this role, it passed from the hands of analytical chemists to t h e teachers of inorganic and general chemistry. At the same time, analytical chemists continued its developm e n t for applications to analysis per se, with methods such as microscopic crystal tests, spot tests, and more recently the ring oven tests of Weisz. Qualitative analysis courses are dropping out of the teaching sequence as a natural consequence of competition for time in the curriculum. While there is some compensation in the form of subjects more in tune with contemporary science, it is a pity t h a t modern chemists often do not appreciate the power of the test tube, the spot plate and the microscope for quick and simple answers where these suffice. Of course, many modern methods provide both qualitative and quantitative information, so the distinction between these classical branches of analysis has become blurred. Turning to quantitative analysis, gravimetric measurements played a critical role in establishing the modern era of chemistry, which began with quantitative expressions of composition. By the middle of the 19th century, titrimetric methods were coming into vogue, as exemplified by the ironpermanganate titration (1846), the iron dichromate titration (1850), and the iodine-thiosulfate reaction (1843). These methods and others were checked against primary standards and by intercomparison, and proce-
dures of marvelous accuracy were developed empirically. No real basis of understanding existed until the turn of the 20th century, and the emergence of physical chemistry. It then became possible to apply concepts of solution equilibrium and reaction kinetics, and to make experimental studies of sources of error. We can mark the beginning of the modern era with research aimed at understanding the principles of quantitative analysis through studies such as buffer and indicator equilibria, coprecipitation phenomena, induced reactions, and the like. T h e motto "Theory guides, experiment decides," adopted by I. M. Kolthoff in the early 1920s, exemplifies this modern spirit. As we examine the history of analytical chemistry in the middle of the 20th century, we see t h a t it changed dramatically during the 1940s. World War II brought enormous demands for analyses of increased sensitivity, speed, and accuracy on samples of increasing complexity. Yet, because communications were disrupted between countries, and even within a single country because of secrecy requirements, these developments did not become generally known until the late 1940s. Not only were the new analytical methods adopted into the fold, but many older instrumental measurements which had not been generally accepted were now recognized as legitimate analytical methods. T h e essence of this second revolu-
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tionary period was not so much the instrumentation itself as the widened influence of fundamental concepts from all branches of science rather than just from chemistry. In turn, analysis began to contribute to a wider clientele. Kolthoff long ago spoke of analytical chemistry repaying its debt to physical chemistry through its contributions to basic knowledge. We can now generalize to include not only all the other branches of chemistry but other sciences and engineering as well. T o examine this infusion of scientific spirit into analytical chemistry requires looking in some detail at all the various areas separately, a task far beyond the limits of the present article. However, it is fascinating to examine a few examples, to observe how often gaps of surprising duration occurred in the development of modern day analytical methods. These gaps arose from one or more of a variety of causes, ranging from a lack of adequate theory, instrumentation, or materials to a lack of communication or even a lack of problems needing the type of information available. Let us find examples of gaps t h a t can be traced to such causes. Lack of theory Most types of instrumental analysis developed from discoveries in physics, and analytical applications were not hampered by lack of theory. Polarography, for example, found many empirical applications soon after its discovery by Heyrovsky in 1922. T h e theory of diffusion to the dropping electrode was worked out by 1934 by Ilkovic, and reversible electrode processes under diffusion control were well understood by the mid-1930s. Irreversible processes, and processes controlled by kinetics of reactions in solution remained poorly understood for nearly 20 years thereafter, until Koutecky in 1953 worked out the complex interaction between heterogeneous and solution kinetics and the diffusion process at the dropping electrode. These theoretical developments were essential to an understanding of the factors governing analytical response. Lack of Instrumentation T h e Raman effect was discovered in 1928, but the extremely weak signal and need for good wavelength discrimination made severe demands upon instrumentation. Photoelectric detection, introduced in 1946 to replace photographic plates, speeded up data acquisition, but the real breakthrough did not come until 1962 when the laser was introduced as a light source. An electrochemical example is represented by coulometry. Faraday's laws of quantitative electrolysis date
back to 1833, and the silver coulometer of T. W. Richards to 1902. For analytical work, coulometry takes two forms. Constant current coulometry involves the simplest of instrumentation, just a constant current source and clock, but it requires a method of ensuring 100% current efficiency and a method of detecting the end point. T h e former requirement is simplified and the latter requirement is removed if the potential is held constant and the current is allowed to decay toward zero. T h e electronic potentiostat was not introduced until 1942, by Hickling, and the accurate integration of slowly changing currents remained a challenge for years thereafter. T h e simple titration coulometer of Lingane (1945) reduced the measurement to an acid-base titration, b u t until operational amplifiers became commonplace in the 1950s, the electronic coulometer did not become a routine instrument. Of atomic absorption spectrometry, J. D. Winefordner (7) states: "It might seem t h a t atomic absorption came on the scene years, even decades, after its time. A careful look at developments in other areas shows, however, t h a t this is not altogether true. T h e practical development of atomic absorption as an analytical method could hardly have preceded the introduction of electrical detection systems." Lack of Materials T h e concept of using the enthalpy change to follow the course of a reaction is an old one, and the first titration of this type, using a mercury thermometer to measure temperature changes, was reported in 1913. T h e resistance thermometer dates back to 1871, b u t t h e thermometric titration did not become a practical reality until the development of the thermister in the late 1940s. By 1951, Rogers
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and H u m e had published a titration of this type. In the area of solid state ion-selective electrodes, Kolthoff and Sanders had published on silver halide membrane electrodes in 1937, but the method languished until two import a n t developments in 1966. T h e first was the Pungor "precipitate elect r o d e s " using solids embedded in silicon rubber, which of course could not have been done before the developm e n t of silicone polymers. These electrodes were not so important in themselves as they were in stimulating other solid state electrodes such as the silver sulfide composites. T h e other development, by Ross and Frant, was the most spectacularly successful of solid state electrodes—the fluoride ion sensor which depended on the development of doped single-crystal lant h a n u m fluoride. Lack of Communication T h e disruption of communications during World War II led to delays in the publication of many developments of importance to analytical chemistry. Among these, we might mention high frequency electronics, quantitative mass spectrometry, ion exchange separations, ultramicro techniques, nuclear techniques, advances in infrared and ultraviolet spectrometry, and many others. T h e immediate post-war period led to publications t h a t revolutionized analytical chemistry as we shall discuss in further detail below. B u t even without the intervention of war, publication in obscure or unconventional media can delay the exploitation of important discoveries. A good example is the publication by Martin and Synge of the principles of gas-liquid chromatography in the Biochemical Journal in 1941. True, World War II was on, but even after the war the method lay d o r m a n t until
the work was resumed by Martin him self in 1951.
Lack of Problems Needing Solution As far back as t h e 1940s a n d 1950s, electrochemical techniques existed for heavy metals in water a t concentra tions down t o 1 0 - 7 t o 10~ 8 M. Com peting techniques applicable to solid samples were usually preferred, a n d in fact even today elaborate schemes for collecting trace metals on solid sup ports are used, to " b e n d " t h e problem to meet the available method. In t h e early 1970s, interest in environmental chemistry, and especially in distin guishing between species of a given heavy metal in natural waters, greatly stimulated the application of electroanalytical techniques such as anod ic stripping analysis a n d differential pulse polarography.
Miscellaneous or Unknown Causes Sometimes the reason for long de lays seems to baffle even t h e experts. Walter Slavin (2), commenting on t h e slow adoption of atomic emission spectroscopy, stated: " T h u s by 1920 all the conditions needed for a system of chemical analysis by spectroscopy existed. We had excellent instru ments, good photographic emulsions, a power distribution network, a n d basic theory. However, chemists were very slow to take advantage of this powerful tool, even for simple qualita tive identifications. T h e y still relied on the classical instruments, t h e test tube, t h e blow pipe, t h e eye a n d t h e nose." Perhaps t h e clue lies in the last two sentences. Chemists preferred simple answers t o simple questions. I t all changed, especially for quantitative work, with t h e introduction of t h e di rect reading spectrometer made possi ble by photomultiplier tubes intro duced in the mid-1940s. Even so, t h e factor of h u m a n inertia enters into a reluctance to adopt new approaches. In the field of X-ray emission, t h e possibilities for chemical analysis had been recognized in t h e original papers of Moseley in 1913 a n d 1914 on atomic numbers, and by mid-1914 de Broglie had observed t h e phenomenon of X-ray fluorescence. T h e latter was in dependently discovered in 1925 and again in 1928, b u t lay d o r m a n t until 1948, when Friedman and Birks opened u p the modern era. X-ray emission by electron b o m b a r d m e n t was an active field in physics from 1914 until 1932, b u t little use was m a d e for chemical analysis. Birks (3) has commented t h a t in t h e 16-year pe riod 1932-48, Chemical Abstracts list ed only one publication on the subject. Although Borovskii' in Russia evi dently used t h e technique during t h e 1930s, it was not available in the Western world. T h e primary cause for
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