WDS Quantitative Analyses
The EPMA is one of the primary micro-analytical techniques used to quantify the concentration of specific elements in solid materials. The technique is capable of measuring the concentration of all elements with an atomic number greater than 4 (Beryllium), some at concentrations less than 10 ppm. Quantitative measurements are made by measuring the intensities of specific X-ray lines from an element, or elements, of interest in an unknown sample, and comparing the intensities with the X-ray intensities measured in well characterized standard materials.
EPMA WDS versus SEM EDS
Energy Dispersive Spectroscopy (EDS or EDX) is a common technique used to determine the chemical composition of solid materials. EPMA WDS and EDS both work on similar principals: a high energy electron source is used to bombard a solid material causing electrons to be ejected from inner electron shells leaving a vacancy. Once created, electrons from a higher energy orbital will transition to a lower energy level to fill the inner vacancy and release energy in the form of an X-ray photon, which, importantly, is equal to the energy difference between the two orbitals, and is unique to specific elements. EDS systems work by measuring all of the X-ray photons being emitted from a sample while the sample being bombarded with a high energy electron beam and results in an energy histogram. In contrast, WDS systems utilize diffracting crystals with specific interplanar spacing (d-spacing), and allow for individual X-ray photons to filtered, and delivered to an X-ray detector. Wavelength-dispersive spectrometers have an order of magnitude better spectral resolution than EDS spectrometers, eliminating many common energy interferences encountered with EDS. In addition, the increased spectral resolution and ability to move and measure off-peak X-ray intensities allows for the X-ray continuum near an X-ray line of interest to quickly be measured and accurate backgrounds to be modeled. From a practical perspective, the differences discussed above result in EPMA WDS having much lower detection limits and better accurancy and precision than EDS.