Wavelength Dispersive X-ray Fluorescence (WDXRF)
Wavelength Dispersive X-ray Fluorescence (WDXRF) is one of two general types of X-ray Fluorescence instrumentation used for elemental analysis applications. In WDXRF spectrometers, all of the elements in the sample are excited simultaneously. The different energies of the characteristic radiation emitted from the sample are diffracted into different directions by an analyzing crystal or monochrometer (similar to the action of a prism dispersing different colors of visible light into different directions). By placing the detector at a certain angle, the intensity of X-rays with a certain wavelength can be measured. Sequential spectrometers use a moving detector on a goniometer to move it through an angular range to measure the intensities of many different wavelengths. Simultaneous spectrometers are equipped with a set of fixed detection systems, where each system measures the radiation of a specific element. The principle advantages of WDXRF systems are high resolution (typically 5 – 20 eV) and minimal spectral overlaps.
X-ray optics can be used to enhance WDXRF instrumentation. For conventional XRF instrumentation, typical focal spot sizes at the sample surface range in diameter from several hundred micrometers up to several millimeters. Polycapillary focusing optics collect X-rays from the divergent X-ray source and direct them to a small focused beam at the sample surface with diameters as small as tens of micrometers. The resulting increased intensity delivered to the sample in a small focal spot, allows for enhanced spatial resolution for small feature analysis and enhanced performance for measurement of trace elements for Micro WDXRF applications. Polycapillary collimating optics allow for efficient collection of the characteristic fluorescence X-rays from a small spot on the sample surface and redirection as a parallel beam to the analyzing flat crystal for energy dispersion.
Conversely, instead of using a polycapillary collimating optic and a flat crystal for fluorescence collection and dispersion, a doubly curved crystal monochrometer can be used to collect select emitted fluorescence from the sample and direct it to a detector.
Additionally, two doubly curved crystal optics can be used for monochromatic WDXRF with the advantage of very high sensitivity for a specific sample element of interest. The technology has been successfully used for measurement of low levels of sulfur in petroleum available in the XOS SINDIE analyzer.