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Wavelength-Dispersive X-Ray Fluorescence Analysis

X-Ray Spectrometry

  1. Ron Jenkins

Published Online: 15 SEP 2006

DOI: 10.1002/9780470027318.a6807

Encyclopedia of Analytical Chemistry

Encyclopedia of Analytical Chemistry

How to Cite

Jenkins, R. 2006. Wavelength-Dispersive X-Ray Fluorescence Analysis. Encyclopedia of Analytical Chemistry. .

Author Information

  1. International Centre for Diffraction Data, Newtown Square, USA

Publication History

  1. Published Online: 15 SEP 2006

Abstract

When an element is bombarded by high-energy particles, orbital electrons may be ejected creating inner orbital atomic vacancies. These vacancies may be filled by transition of outer level electrons giving rise to characteristic X-radiation. X-ray fluorescence spectrometry provides the means of the identification of an element by measurement of its characteristic X-ray emission wavelength of energy. The method allows the quantization of a given element by first measuring the emitted characteristic line intensity and then relating this intensity of element concentration. While the roots of the method go back to the early part of this century, where electron excitation systems were employed, it is only during the last 30 years or so that the technique has gained major significance as a routine means of elemental analysis. Wavelength-dispersive spectrometers employ diffraction by a single crystal to separate characteristic wavelengths emitted by the sample. Today, nearly all commercially available X-ray spectrometers use the fluorescence excitation method and employ a sealed X-ray tube as the primary excitation source. The first commercial X-ray spectrometer became available in the early 1950s and although these earlier spectrometers operated only with an air path, they were able to provide qualitative and quantitative information on all elements above atomic number 22 (titanium). Later versions allowed use of helium or vacuum paths that extended the lower atomic number cut-off to around atomic number 9 (fluorine). X-ray detectors used include the flow counter, the scintillation counter and the Si(Li) detector.

The X-ray method has good overall performance characteristics. In particular, the speed, accuracy and versatility of X-ray fluorescence are the most important features among the many that have made it the method of choice in over 30 000 laboratories all over the world. Most wavelength-dispersive spectrometers fall into two broad categories – single channel and multichannel. Single channel spectrometers are typically employed for both routine and nonroutine analysis of a wide range of products, including ferrous and nonferrous alloys, oils, slags and sinters, ores and minerals, thin films, and so on. These systems are very flexible but relative to multichannel spectrometers are somewhat slow. The multichannel wavelength-dispersive instruments are used almost exclusively for routine, high-throughout analysis where the great need is for fast accurate analysis, but where flexibility is of no importance.

Interelement (matrix) effects often complicate quantitative analysis by X-ray fluorescence. However, a wide selection of methods is now available for minimizing these effects, allowing excellent accuracy to be obtained in many cases. Detection limits are achievable down to the low parts per million (ppm) range and it is possible to obtain reasonable responses from as little as a few milligrams of material.