Tuesday, July 2, 2013
Galena: An Illustration of EDS Spectral Overlaps
Having an accurate estimate of the EDS detector resolution (the FWHM of the peaks) as a function of energy, together with the energies and relative intensities of the Kα/Kβ and Mα/Mβ doublets allows one to attempt to deconvolute the Pb and S contributions to this slightly asymmetric peak. The result? Pb:S is determined, without standards using ZAF matrix corrections, to be 73:27.
as shown in a former application on this blog. Increasing the TC from 0.5 µs to 32 µs drops the peak FWHM for Mn Kα from ~ 263 eV to ~ 142 eV. That data is shown in the second image. The increased resolution is immediately apparent. The Pb Mz is very clearly resolved as a separate peak on the left of the main peak, as is the Pb Mγ on the right. The S Kβ and Pb Mβ is apparent as a slight shoulder on the right of the main peak. The S Kα and Pb Mα are still left unresolved.
Again, knowing the detector resolution as well as the energies and relative intensities of the Kα/Kβ and Mα/Mβ doublets one can attempt to deconvolute the Pb and S contributions in this higher resolution peak. The result: Pb:S is 61:39. Better, but still far from being a convincing estimate of the 1:1 stoichiometry of galena. This is not a failure of not using standards or a limitation of the matrix corrections. The EDS spectrometer simply can not resolve the peak shape with sufficient resolution and statistical certainty to allow the deconvolution of S Kα and Pb Mα which are 36 eV apart.
How to do better?-- wavelength dispersive spectroscopy (WDS) where the X-rays are diffracted from crystals or multi-layers.