Tuesday, July 2, 2013

Light Element EDS

The ability to resolve very light elements is entirely depending upon the pulse shaping time constants used. The top images shows the lower energy portion of an EDS spectrum of magnetite (Fe2O3) taken at 30 kV using a 16 µs shaping time constant. The Fe Lα peak is clearly visible to the right of the O Kα peak. There is a small shoulder for the C Kα peak since the sample was coated with graphite to suppress charging. The peak just above the Si Kα line is actually the Si K absorption edge. Using data of this quality one could, with the use of standards, quantitate oxygen using O Kα. The Cr Lα peak is shown near the O Kα peak to illustrate a common overlap with O Kα. Chromium is also a common trace contaminant in mineralogical magnetite.

The detection efficiency of the light elements is less than the higher X-ray energy elements, and thus one might be tempted to decrease the shaping time constant, TC, to increase the throughput of the EDS detector allowing one to perform EDS with more beam current and higher acquisition rates.  The second image shows the same energy region with a time constant of 0.5 µs.  The energy range is attenuated around 2 keV and all information regarding light elements is lost. This is an artifact of pulse processing.

The point of this brief example is very simple: use the larger TC's for light element studies! A TC as small as 8 µs should allow for the resolution of C Kα, and for O Kα one can go as low as 1 µs.

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