X-rays are a part of the electromagnetic spectrum and as such have the properties of waves, and of particles of energy.  These two properties of the electromagnetic spectrum are inversely related via this simple expression:

E = 12.4 / λ

where

    E = energy (keV)

    λ = wavelength (A)

In wavelength dispersive spectroscopy, the wave properties of x-rays are used to identify and quantify the elements present in a sample.  Using the Bragg equation:

nλ = 2dsinØ

where

    n= the order of diffraction

    λ = x-ray wavelength

    d = interplaner d-spacing of diffracting crystal

    Ø = angle of incidence of x-ray on diffracting crystal

x-rays from individual elements are isolated from one another, and diffracted to a detector for counting. 

A ‘wavelength scan’ from a grain of anorthosite centered about the Si peak position is shown to the left.  Six peaks from 3 different elements can be resolved. The arrowed line illustrates the width of the Si peak from an EDS spectrum. None of the minor peaks shown on the WDS scan are resolvable via EDS.  Importantly, it is not possible to determine the concentration of the geologically important element Sr in a silicate mineral using EDS because of the much poorer spectral resolution of EDS.  Additionally, the minimum detection limit in WDS is much lower (10’s of ppms depending on the element and analytical conditions) compared to >2000ppm via EDS.

WDS is the method of choice for most all of the quantitative analyses we do.