edmund.welter at desy.de
Fri Jan 29 04:09:56 CST 2010
> Thanks for your explain. But I am still puzzled. Following your
> instruction, I performed *Self-Absorption *for the samples, choosing
> *XANES(Fluo) *from *algorithm*, but I did not know the real meaning of
> *Formula*. Does it mean the species formula which I wish to detect? Or
> the whole composition of the sample?
The composition of the whole sample. You do not need to know the
species, you "only" need to know thew elemental composition of your
sample. A hairy task if you don't digest your sample completely and put
it into an ICP-MS or anything like that. I don't know which precision is
necessary to yield usable results. Maybe someone else tested or
In principle the matrix produces just a smooth background absorption and
knowing the composition it is easy to calculate the amount of this
absorption using tabulated scattering factors. But it should also be
possible to determine this background experimentally, for instance
extrapolating the background before the edge. But for this you would
need a blank spectrum to "calibrate" the mued value.
If you follow the link to the "document section" in Athena a very
instructive webpage will open and among other things you will find
citations of the original papers underlying the four different
algorithms used for the self absorption correction.
> I used natural mineral to interact
> with selenite, then I wish to check the speciation of the selenium
> reaction product. For my case, should I fill the blank with Se or FeSe
> or both of them (because currently I do not know the exactly containing
As said before, the "chemical Formula" of the entire sample. "Chemical
formula" here does not mean you have any kind of chemical bond, it just
means molar ratio.
> For the angles, normally are they 45? Becuase I measured
> the samples in fluorescence mode, and didnot measure the angles between
> sample, incoming beam and detector. I tried to input *Se *in the
> *Formula* entry, and select 45 for the angles, then I got a corrected
> spectra which was higher than the original one.
45 degrees is the usually used geometry. However, the program asks for
the geometry which you used not for the usually used geometry! If it was
45 degrees than its fine. Obviously the algorithm does what it is
supposed to do, it amplifies the oscillations. But this does not mean
that the result is correct! It will only be "correct" if the input
parameters were correct.
> But when I using this
> spectra for LCF analysis, I still could not got overlapping line for the
> fitted spectra with the sample spectra...
Using pure Se in the self absorption correction is definitely wrong, it
overestimates the effect dramatically. Actually the effect more or less
disappears for diluted samples. But even if the spectra were properly
corrected I can not see any reason why the used set of reference spectra
MUST fit the spectrum of your sample. There are many possible reasons
for this, the most likely is that you are missing one or more important
reference spectrum (Or you are not using enough references/free
parameters in your fit...;-)
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