Quantitative Electron Probe Microanalysis

The recognition and understanding of the role of the microstructure in controlling the macroscopic behavior of a material has been a major theme of science and technology throughout the history of NBS/NIST. In materials science, the late 19th century saw the development of the sample preparation procedures we know today as “metallography” and “materialography,” and the emergence of optical microscopes with resolution performance at the physically defined optical limit. Applied to practical technological issues such as understanding the thermomechanical processing of steel to control its strength and ductility, the new microstructural science revealed a vast array of microstructural features, many of which were obviously compositional in nature. By 1912, the noted Harvard scientist Prof. Albert Sauveur could remark “To realize the practical importance of metallography, it should be borne in mind that the physical properties of metals and alloys—that is, those properties to which those substances owe their exceptional industrial importance—are much more closely related to their proximate than to their ultimate composition, and that microscopical examination reveals, in part at least, the proximate composition of metals and alloys, whereas chemical analysis seldom does more than reveal their ultimate composition” (Here the “ultimate composition” is what we today would refer to as the bulk composition, while the “proximate composition” refers to the local microstructural composition.) Sauveur continued in his enthusiastic and colorful prose: “Unfortunately the chemist too often is able to give us positive information in regard to the proportion of the ultimate constituents only, his reference to proximate analysis being of the nature of speculation. Ultimate analysis has reached a high degree of perfection in regard to accuracy as well as to speed of methods and analytical chemists have built up a marvelous structure calling for the greatest admiration, their searching methods never failing to lay bare the ultimate composition of substances. But how much darkness still surrounds the proximate composition of bodies and how great the reward awaiting the lifting of the veil!” The beginning of the solution to Sauveur’s dream came in 1949 with the first results from the electron probe x-ray microanalyzer (EPMA) by Raymond Castaing, a student of A. Guinier at the University of Paris. Castaing presented an extraordinary thesis that described both the complete development of a unique measurement tool for microstructural characterization and a detailed treatment of the underlying physics of electron/x-ray interaction with matter that would provide the framework for developing the new technique into a rigorous quantitative analysis tool. The first stage of this task took nearly 20 years and the contributions of many scientists from Europe, Japan, and the United States, including those of Kurt F. J. Heinrich and his colleagues at NBS. To evaluate the state of the development of quantitative analysis and to stimulate further progress in the measurement science of electron probe microanalysis, Heinrich organized a special workshop in June of 1967 that brought together the leaders of the field. The document of this gathering, NBS Special Publication 298, Quantitative Electron Probe Microanalysis [1], edited by Heinrich, became the “bible” of the rapidly developing field. It was understood from Castaing’s thesis work that a quantification scheme should proceed from the measurement of the x-ray intensities emitted by the unknown and, under the same beam conditions, from a simple standard suite consisting of pure elements and/or binary compounds. What was needed were correction factors based upon physical calculations of electron penetration and scattering, x-ray absorption, and consequent secondary x-ray fluorescence to convert relative measured x-ray intensities into relative concentrations. Extensive work to develop practical expressions for these factors and obtain the physical data (e.g., x-ray mass absorption coefficients, electron backscatter coefficients, etc.) was being done by researchers organized essentially along national lines. NBS Special Publication 298 brought together the various camps to present their work and the critical details on just how they proposed to implement these complex calculations. Most importantly, SP 298 contained the first robust tests based upon critical data sets that provided a rigorous comparison of the performance of different implementations of correction factors. Thus, the strengths and weaknesses of the algorithms under development could be accurately assessed, and future directions for continuing research could be sensibly planned, propelling the field forward. SP 298 also surveyed newly emerging areas of application, such as biological materials, where a solution was provided to the problem that radiation damage to the specimen limited the utility of the results. Changes in the electron bombarded region could be compensated through measurements of the high energy x-ray continuum.