EDS
(energy dispersive spectrometry) - PEMC

Chemical analysis

  • The chemical composition of a sample can be determined using x-ray analysis.
  • This is done by collecting and analysing the characteristic x-rays that are emitted by the sample during electron bombardment.
  • X-rays can be detected using either Energy Dispersive Spectroscopy (EDS) or Wavelength Dispersive Spectroscopy (WDS) techniques.

X-ray Generation

  • When an electron from a K-shell is replaced by one from the next closest shell (L), the energy released is designated as a α x-ray.
  • When an electron from a K-shell is replaced by one from the second closest shell (M), the energy released is designated as a β x-ray.
  • Each shell has a different energy level and as an electron drops shells the excess energy is released as a photon (γ) which is the x-ray.
  • These events cause a unique energy release and can be detected in the SEM using an energy-dispersive spectroscopy (EDS) detector.

Varying Accelerating Potential

  • Decreasing the accelerating potential of the electron beam will reduce the interaction volume
  • This decrease will increase the resolution of chemical analysis
  • However, some x-rays may not be detected by standard EDS detectors and specialist EDS detectors may be needed, like a windowless detector
  • Increasing the accelerating potential of the electron beam will increase the interaction volume
  • This increase will decrease the resolution, however, it will enable analysis of structures deeper into the sample, i.e. printed circuit board below a protective polymeric coating.
Decreasing EHT
Extreme EDS
Mineral map of NWA 15189 where pink = Low-Ca pyroxene, orange = High-Ca pyroxene, dark green = Plagioclase and teal = silica

Low Voltage EDS

We have a range of EDS detectors that can carry out compositional analysis on a sample, including the Oxford Instruments Ultim Extreme detector that is designed to achieve high quality spatial resolution at low accelerating voltages. As a result, PEMC are able to carry out elemental analysis on samples at lower accelerating voltages than previously able, with better detection of light elements.  
This broadens the amount of samples we can carry out this sort of analysis on and is particularly useful in the elemental analysis of biological materials. The Oxford Instruments Ultim Extreme detector is installed on our JEOL IT800 FE SEM and can be used concurrently with the Ultim 65 mm2 EDS detector on the same instrument.