Wavelength dispersive X-ray fluorescence (WDXRF) is one of the two general types of X-ray fluorescence instrumentation used for elemental analysis applications. In a WDXRF spectrometer, all elements in the sample are excited simultaneously. The different energies of the characteristic radiation emitted by the sample are diffracted in different directions by an analyzing crystal or monochromator (similar to how a prism disperses different colors of visible light in different directions). By placing a detector at a certain angle, the intensity of X-rays of a certain wavelength can be measured. Sequential spectrometers use a moving detector on a goniometer to move it through a range of angles to measure the intensity of many different wavelengths. Simultaneous spectrometers are equipped with a set of fixed detection systems, each of which measures the radiation of a specific element. The main advantages of WDXRF systems are high resolution (typically 5 - 20 eV) and minimal spectral overlap.

X-ray optics can be used to enhance WDXRF instruments. For conventional XRF instruments, the typical focal spot size on the sample surface ranges in diameter from a few hundred micrometers to a few millimeters. Polycapillary focusing optics collect X-rays from a divergent X-ray source and guide them to form a small focused beam on the sample surface with a diameter as small as a few tens of micrometers. The resulting increased intensity delivered to the sample in a small focal spot can enhance the spatial resolution for small-feature analysis and the trace element measurement performance for micro-WDXRF applications. Polycapillary collimating optics allow efficient collection of characteristic fluorescent X-rays from a small spot on the sample surface and redirect them as a parallel beam to an analyzing flat crystal for energy dispersion.

Conversely, a doubly curved crystal monochromator can be used to collect specific fluorescence emitted from the sample and guide it to the detector, instead of using polycapillary collimating optics and a flat crystal for fluorescence collection and dispersion.

In addition, two doubly curved crystal optics can be used for monochromatic WDXRF, a technique that has very high sensitivity for a specific element of interest. This technique has been successfully applied in the XOS SINDIE analyzer to determine low sulfur content in petroleum.

Source: XOS — Wavelength Dispersive X-ray Fluorescence (WDXRF).