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by Karl Loren
The History of X-Ray Fluorescence Spectroscopy
The history of X-ray fluorescence dates back to the accidental discovery of X-rays in 1895 by the German physicist Wilhelm Conrad Roentgen. While studying cathode rays in a high-voltage, gaseous-discharge tube, Roentgen observed that even though the experimental tube was encased in a black cardboard box the barium-platinocyanide screen, which was lying adjacent to the experiment, emitted fluorescent light whenever the tube was in operation. Roentgen's discovery of X-rays and their possible use in analytical chemistry went unnoticed until 1913. In 1913, H.G.J. Mosley showed the relationship between atomic number (Z) and the reciprocal of the wavelength (1/l) for each spectral series of emission lines for each element (Willard 341). Today this relationship is expressed as:
c/l = a (Z-s)2
where a is a proportionality constant and s
is a constant dependent on a periodic series.
Mosley was also responsible for the construction of the early X-ray spectrometer. His design centered around a cold cathode tube where the air within the tube provided the electrons and the analyte which served as the tube target. The major problem experienced laid in the inefficiency of using electrons to create x-rays; nearly 99% of the energy was lost as heat.
In the same year, the Bragg brothers built their first x-ray analytical device. Their device was based around a pinhole and slit collimator. Like Mosley's instrument, the Braggs ran into difficulty in maintaining efficiency. Progress in XRF spectroscopy continued in 1922 when Hadding investigated using XRF spectrometry to analyze mineral samples. Three years later, Coster and Nishina put forward the idea of replacing electrons with x-ray photons to excite secondary x-ray radiation resulting in the generation of an x-ray spectra. This technique was attempted by Glocker and Schrieber, who in 1928 published Quantitative Roentgen Spectrum Analysis by Means of Cold Excitation of the Spectrum in Ann. Physics.
Progress appeared to be at a standstill until 1948, when Friedman and Birks built the first XRF spectrometer. Their device was built around a diffractometer, with a Geiger Counter for a detection device and proved comparatively sensitive for much of the atomic number range.
Within a decade, XRF became an important method of analysis for elements with atomic numbers greater than twenty-two. It might be noted that XRF spectrometers have progressed to the point where elements ranging from Beryllium to Uranium can be analyzed.
Although the earliest commercial XRF devices used simple air path conditions, machines were soon developed utilizing helium or vacuum paths, permitting the detection of lighter elements. In the 1960’s, XRF devices began to use lithium fluoride crystals for diffraction and chromium or rhodium target x-ray tubes to excite longer wavelengths. This development was quickly followed by that of multichannel spectrometers for the simultaneous measurement of many elements. By the mid 60’s computer controlled XRF devices were coming into use. In 1970, the lithium drifted silicon detector (Si(Li)) was created, providing very high resolution and X-ray photon separation without the use of an analyzing crystal. An XRF device was even included on the Apollo 15 and 16 missions.
Since the 1940’s, analytical chemists have studied the possibility of using layered synthetic microstructures (LSM’s) as the diffracting structures for XRF instruments. These are made by depositing successive layers of atoms or molecules on a smooth substrate. The spacing and compositions of the layers are designed to give the best possible diffraction characteristics. Henke and Barbee have both separately demonstrated that LSM’s can substantially improve the sensitivity of XRF devices, making possible the detection of carbon, boron, and even beryllium.
Meanwhile, Schwenke and co-workers have fine tuned a procedure known as total reflection x-ray fluorescence (TRXRF), which is now used extensively for trace analysis. In TRXRF, a Si(Li) detector is positioned almost on top of a thin film of sample, many times positioned on a quarts plate. The primary radiation enters the sample at an angle that is only slightly smaller than the critical angle for reflection. This significantly lowers the background scattering and fluorescence, permitting the detection of concentrations of only a few tenths of a ppb.
This history was adapted from:
Bertin, Eugene Principles and Practice of X-Ray Spectrometric Analysis; Plenum Press: New York, 1970, p 1-2, 114-115.
Jenkins, Ron X-Ray Fluorescence Spectrometry; John Wiley and Sons: New York, 1988, p 51-53, 78-83, 87.
For a list of all the Nobel prizes awarded for work on x-rays, click here http://xray.uu.se/hypertext/nobelprize.html.
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