X‐ray Absorption in Chemical Analysis and Control

Abstract

Three X-ray processes are potentially useful in analytical chemistry: the diffraction of X rays by crystals, the emission of characteristic spectra, and the absorption of X rays by matter. The first process can establish structure and composition. The others establish composition only. In the past, the analytical chemist has used each process to a very limited extent, because there was no convenient way to measure the intensity of an X-ray beam with sufficient speed and precision. Of the three, he has logically found that X-ray difiraction is the most useful for the following reasons: first, he had many ways of determining composition, but few of establishing structure; second, it is often possible by X-ray methods to establish structure, but usually it is impossible to do quantitative analysis without making a precise intensity measurement. As a consequence, the unique contribution that X-ray absorption methods can make to chemical analysis and control, though long known to physicists, has not been generally appreciated.Owing to marked and relatively recent progress in measuring the intensity of X-ray beams, this situation is now changing. There are available today three X-ray detectors that convert X-ray beams into electric currents proportional in magnitude under the simplest conditions to the intensities of the beams. These devices are the ionization chamber, the Geiger counter, and the photoelectric X-ray detector. The ionization chamber, the oldest of the three, has found little or no recent application in chemical analysis or control, nor, in view of the generally greater suitability of the other two devices, is it likely to do so. The analytical chemist can proceed on the basis that the Geiger counter or the photoelectric detector, both of which will be discussed subsequently, are capable of meeting any reasonable requirements that he is likely to impose. He is thus in a position to realize the advantages of X-ray absorptometry, some of which accrue from the facts that it can be accomplished without noticeably altering the sample, and that a single measurement can often be made in a matter of seconds once the sample is in the beam.The advantages and limitations of X-ray absorptometry are in general deducible from the known characteristics of X rays. The reader is referred to several excellent This brief review, being largely qualitative, will present only the necessary minimum of fundamental information about X rays, and it cannot discuss all the advantages or limitations of X-ray absorptometry. (For a more complete treatment, see bibliographical reference 5 and others listed in the bibliography therein.)The outstanding characteristic of X rays, from which certain others derive, is their high energy, or small wavelength (near 1 i.). Because of this characteristic, the absorption of X rays usually 997 for a thorough discussion of these.X-Ray A bsorptioii Described.