Tuesday, July 2, 2013

Galena: An Illustration of EDS Spectral Overlaps

With an energy resolution of ~ 143 eV at the Mn Kα line (5.893 keV), the problem of spectral overlaps is a common problem with EDS applications. In the top image a spectrum of the mineral galena, PbS, is taken at 30 kV using a 0.5 µs pulse shaping time constant. The S Kα line is at 2.306 keV and the S Kβ is at 2.464 keV, with the Pb Mα and Mβ in the general vicinity-- at 2.342 keV and 2.442 keV respectively. Because of the high Z of Pb, there are other lines just above and below: the Pb Mz at 1.839 keV, the Mγ at 2.652 keV and the M2N4 at 3.124 keV. The difference between the S Kα, S Kβ, Pb Mα, Pb Mβ and Pb Mγ are all within the resolution of the EDS detector which is ~ 263 eV at this time constant.

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.

If one is to use EDS, the only possibility is to increase the resolution by increasing the pulse-shaping time constant 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.

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