eTraining Introduction

Specimen Preparation

Leica Ultracut UCT



Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB


Veeco Dim 3000 AFM

Fluorescence Microscopes


Basic Science

Wavelength Dispersive Spectrometer

The WDS system consists of a detector and analyzing crystal. The analyzing crystal has special properties, the most important being a known spacing between its atomic planes, called the d-spacing. When the X-rays hit the analyzing crystal, the signal diffracts off the planes. The relationship between a particular crystal with known d-spacing and the signal diffraction is expressed through the Bragg equation. Bragg’s law relates these parameters to the angle θ at which X-rays are strongly scattered.

Bragg's Law: = 2d sin θ,

where λ is the X-ray wavelength and n is an integer.

No single crystal can diffract over the entire range of wavelengths needed to cover all of the elements, so several crystals are used in analysis. The beam-specimen interaction site, the crystal, and the detector are all positioned on a circle of fixed radius called a Rowland circle. Since the sample location is fixed, the detector and analyzing crystal are designed to move around the circle. This feature allows the detector and analyzing crystal to move in closer or further away from the specimen, depending on the wavelength to be measured. When the proper analyzing crystal is chosen and the spectrometer is correctly positioned for measurement of a particular element, X-rays emitted from the sample impinge onto the analyzing crystal. The diffracted X-rays are then directed towards the detector, amplified, and sent on to the computer for electronic analysis.

Open in a new window.


A WDS detector is known as a gas-filled or gas proportional counter. One variation on this type of detector consists of a gas-filled cylinder containing a tungsten wire running down the middle and a thin window through which electrons can enter the detector. When X-rays enter the cylinder, they ionize the counter gas, producing electrons which are then attracted to the wire. The electrons create a current when they hit the wire. In turn, this current signal is used to measure X-rays from the specimen. This signal (pulse) is amplified and passed onto a computer X-ray analyzer, where each pulse is counted to produce one or more X-ray spectral lines unique to that element.

WDS is more sensitive than the EDS system and will detect smaller elemental concentrations: 0.01 wt% vs 0.1 wt% respectively. Also, fewer distortions and artifacts occur in WDS spectra because 1) resolution for WDS is 4 to 10 eV and for EDS is 130 eV, and 2) the proper choice of analyzing crystals can be used to separate elemental peaks. A disadvantage of WDS is that it requires more skill and knowledge to operate, more time is required for WDS measurements, and WD systems are much more expensive than EDS. As a result one is more likely to encounter ED systems than WD.

Top of Page