Links in this section:

Introduction
Evolution of WDS technique
Basics of WDS
Diffraction
Crystals
Detectors & Geometry
Comparison of EDS and WDS
Qualitative Analysis
Quantitative Analysis
Mapping
Summary

Evolution of the WDS technique

The development of WD spectrometers goes back long before ED detectors became widely available in the early 1970s.

The first electron probe microanalyzer (EPMA) was developed during the 1940s and used an optical microscope to observe the position and focus of the electron beam on the sample. Later, WD spectrometers were fitted to SEMs which allowed the specimen to be positioned more precisely under the electron beam and also made possible a visual picture of the distribution of a chosen element – the X-ray map.

On early commercial WDS spectrometers, all of the spectrometer mechanisms had to be moved by hand, and the operator had to physically exchange the crystals to cover the spectrum. The output from counters, recorded against time was sent to a chart recorder, and all the peak identification, peak and background measurements, and matrix corrections were done by hand.

The advent of desktop computers and precision stepper motors in the late 1970s made possible the mechanization of the spectrometer and the SEM stage. With these changes, WDS analysis became significantly less cumbersome. However, the initial setup for an analysis and the entire data reduction process were still tedious and time-consuming. Analysis parameters had to be optimized for each element and type of sample. Computers of that generation were not particularly user-friendly. Data for ED and WD were beginning to be combined, but automatic peak identification of the ED spectrum was not routine, and combined ED and WD analyses were unduly complicated.  The situation changed somewhat in the mid-1980s, when data processing became fully automated. However, setup for the analysis remained virtually unchanged.

 The current generation of WDS spectrometers with their advanced control and analysis software make the technique considerably easier to use. Multi-crystal spectrometers now change crystals on-the-fly rather than first moving to a specified position on the Rowland circle, flipping to the appropriate crystal, and moving back to the correct position on the circle for analysis of the desired element. Software has been developed for quick, easy qualitative analysis by WDS. Operating parameters for the detectors have been optimized and entered into the software so that analysis setup is quick for the vast majority of samples. Comparing WD and ED spectra and combining WD and ED analysis is now routine and easy.

Back | Next