eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

EDS Versus WDS

Electron transitions result in emission of X-rays from their influenced atoms. The emissions for each transition for individual elements are precisely known and when displayed in an energy histogram produce peaks at specific energies. The goal of qualitative analysis is the identification of elements excited by the electron beam in the specimen.

EDS (Energy Dispersive Spectrometer) is the most common microanalysis equipment seen on Scanning Electron Microscopes, as it is economical and easy to use. EDS is better suited to identifying specimens where the constituent elements are unknown. It should be clear that ED qualitative analysis is a valuable first step in a WDS analysis.

Of the two X-ray microanalysis techniques for qualitative analysis, Wavelength Dispersive Spectrometry (WDS) analysis is distinctly different from energy dispersive (EDS) analysis. WDS, a sequential analysis, is slower, as measurements are made one at a time. As a result, WDS is not ideally suited for element identification. Commonly, EDS is first used for element identification, for fast analysis of the entire spectrum, while WDS is then used to solve various spectral problems described elsewhere in this module.

Spectral overlaps are a common problem, which occur when emission lines from two different elements share the same energy. This leads to ambiguity and misidentification of elements in the specimen. WDS energy resolution The degree of sharpness that can be reached. is superior to EDS by a factor of 30, leading to improved peak separation. Other problems arise when attempting to measure low concentrations of elements in a sample. WDS is able to process higher numbers of X-ray events than EDS, leading to a greater sensitivity in this instance.

On a simplistic level WDS can be described as a radio that must be tuned to a specific location to intercept X-rays for an element emission. This implies that the analyst must already know what elements are present in the excited volume of the specimen—which is often not the case.

Most modern computer-based analytical systems for EDS include a feature that automatically identifies peaks for the user. Novice users trust that the automated peak identification software correctly identifies those peaks. More advanced users identify peaks manually, using rigorous qualitative analysis guidelines. This technique requires greater knowledge and skill on the analysts’ part.

See the table below to help clarify the discussed differences between the EDS and the WDS systems.

Differences in WDS and EDS

"Scanning Electron Microscopy and X-Ray Analysis" 3rd Ed., Goldstein, Joseph, Newbury, Dale, et al. Plenum Press, New York and London, Table 7.2. Comparison between Types of X-Ray Spectrometers, Chapter 7, p. 341.

Operating Characteristic

WDS
(crystal diffraction)

EDS
(silicon, energy dispersive)

Detectable Elements Variable, <30%
Detects Z ≥ 4
~100% for 2-16 keV
Detects Z ≥ 10 (Be Window);
detects Z ≥ 4 (windowless or thin window)
Resolution Crystal-dependent (5 eV) Energy-dependent (130 eV at 5.9 keV)
Maximum count rate (counts/s) 50,000 on an X-ray line Resolution-dependent, < 4000 over full spectrum for best resolution
Minimum useful probe size (nm) ~200 ~5
Typical data-collection time Tens of minutes Minutes
Spectral artifacts Rare Major ones include escape peaks, pulse pileup, electron-beam scattering, peak overlap, and window absorption effects


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