X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

I. Introduction
SECTION IV
WAVELENGTH DISPERSIVE X-RAY
FLUORESCENCE (WD-XRF)
Wavelength Dispersive X-Ray Fluorescence Spectrometry (WD-XRF) is the oldest
method of measurement of X-rays, introduced commercially in the 1950’s. This name is
descriptive in that the radiation emitted from the sample is collimated with a Soller collimator,
and then impinges upon an analyzing crystal. The crystal diffracts the radiation to different
extents, according to Bragg’s law, depending upon the wavelength or energy of the Xradiation.
This angular dispersion of the radiation permits the sequential or simultaneous
detection of X-rays emitted by elements in the sample. Simultaneous instruments normally
contain several sets of analyzing crystals and detectors; one is adjusted for each desired
analyte in the sample. These instruments tend to be very expensive, but efficient for the
routine determination or preselected elements.
WD-XRF is a technique that has become indispensable when fast, accurate elemental
analysis is needed, as when controlling a melt in a steel works or the raw mix at a cement
plant. One reason for its popularity in these applications is that its ease of use, and the
ruggedness of the equipment, allows quality results to be obtained in plant conditions by
operators without advanced analytical skills. Furthermore, its inherent precision, speed, and
simplicity of sample preparation can often eliminate many of the problems encountered with
solution based methods like ICP or Atomic Absorption spectroscopy.
WD-XRF spectrometers are usually larger and more expensive than other
spectrometers. Because the analyzing crystal d-spacing determines wavelength sensitivity,
they are usually more sensitive than other spectrometers. To overcome losses in X-ray optics
of the WD-XRF spectrometers and to maximize primary radiation intensity, X-ray tubes are
usually employed. The sample is usually held under vacuum to reduce contamination and
avoid absorption of light element characteristic radiation in air.
Typical uses of WD-XRF include the analysis of oils and fuel, plastics, rubber and
textiles, pharmaceutical products, foodstuffs, cosmetics and body care products, fertilizers,
minerals, ores, rocks, sands, slags, cements, heat-resistant materials glass, ceramics,
semiconductor wafers; the determination of coatings on paper, film, polyester and metals; the
sorting or compositional analysis of metal alloys, glass and polymeric materials; and the
monitoring of soil contamination, solid waste, effluent, cleaning fluids, sediments and air
filters.
II. Principle of WD-XRF
WD-XRF spectrometers measure X-ray intensity as a function of wavelength. This is
done by passing radiation emanating from the specimen through an analyzing diffraction