Wavelength Dispersive X-ray Fluorescence Spectrometry (WDXRF)

Introduction

The wavelength principle behind WDXRF is that upon irradiation of a material with high-energy x-rays, its atoms will produce secondary (or fluorescent) x-rays that are characteristic of each element. Through measurement of the magnitude of these spectroscopic wavelengths, the concentration of the involved elements can be calculated. In comparison to energy dispersive X-ray fluorescence (EDXRF), separation of these wavelengths is performed, and detection is possible using diffraction crystals in WDXRF. In contrast, in EDXRF, no such separation and detection is possible, which means that WDXRF gives better spectral resolution and background noise, and accuracy in results. The flexibility and the rugged nature of WDXRF have made it indispensable in quality control and process touchpoints as well as research facilities.

Principle

The techniques used in WDXRF are based on the movement of X-rays on the sample, whereby excitation of the inner electron shells emanates from the atoms in the material. These X-rays are transmitted into a crystal monochromator, which disperses them and separates the X-rays into wavelengths through an angle dependent upon the Bragg angle. A detector can then count each wavelength, and the spectra can be used in the identification and quantification of the elements that are present.

Methodology

Sample preparation should be carried out first, and it can be either the grinding of solid samples to form powders, pressing of pellets, or fused bead techniques to guarantee homogenization. A sample in preparation is now put in the spectrometer chamber, which is bombarded with primary X-rays produced by an X-ray tube. The fluorescent X-rays are subject to a diffraction crystal, which only diffracts wavelengths onto the detector. The measurements are generally done under vacuum to reduce the effect of air absorption. Quantitative accuracy is gained through calibration by use of certified reference materials.

Instrumentation

A typical WDXRF system is made up of an X-ray tube as the major source of radiation, a sample holder, a wavelength-dispersive crystal spectrometer, and either a single or more detectors. Wavelength separation is essential, whereby the crystal monochromator (usually formed of substances like lithium fluoride, LiF, or pentaerythritol, PET) plays a significant role. WDXRF uses detectors such as proportional counters operated by gas flows (light elements) and scintillation counters (heavy elements). The current systems use computer-controlled goniometers to profile the wavelength and in-built software to process the data, calibration, and reporting.

Strengths

WDXRF has outstanding spectral resolution, providing the ability to separate closely spaced elemental peaks. It gives good precision and accuracy in both major and minor element analysis and low detection limits of most elements. It is a non-destructive method, normally involves light sample preparation, and has a large scope of study of samples. It can also be extensible and is most suitable for a continuous quality control procedure in industries.

Limitations

Although WDXRF is potent, it also has a couple of limitations. Its machinery is quite costly, with the initial costs of establishment and maintenance being high. Due to the low fluorescence yield and great absorption, analysis of very light elements (e.g., beryllium or boron) is difficult. Homogeneity sample-wise is essential, and adjustment in the particle size or even density may give rise to analytical mistakes. The procedure can also be calibrated inappropriately or with suitable reference materials that cannot be present to all types of samples. Also, the method lacks portability, and so it can only be applied in laboratories.