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Physical Principles
Physical principles
In transmission mode we are measuring the attenuation of the X-ray beam as it traverses a sample of thickness x:
I(E) = I0(E)e−μ(E)·x
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where I0 and I are the intensity of the incident and transmitted X-ray beam, respectively, and μ is the absorption coefficient. In turn, μ depends on the atomic concentration (ρ) and absorption cross-section (σ):
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For a sample consisting of different atoms, the total absorption will be given by the sum of the product for each chemical element present in the sample:
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The cross section is a physical property of the element, i.e. a constant (you can find the tabulated values at http://csrri.iit.edu/periodic-table.html ). As such, the way you prepare your sample would affect the absorption coefficient through the atomic concentration (ρi).
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The total absorption of the sample should be such that you have enough transmission through the sample to get a decent signal for the X-ray beam after the sample (i.e. in the second ion chamber, I).
A correct edge jump (, where E2 and E1are the energies above and under the edge, respectively) of approximately 1 assures that you can acquire a XAS signal good enough to be analyzed. This is achieved only if the element of interest is a major component because the difference of the absorption coefficient above and under the edge is mainly due to the change in the cross section of the investigated element. In the event the sample is too dilute (by element of interest) you will not be able to acquire a good spectrum because your edge jump will not be big enough even if you obtain the correct value for the total absorption. Adding more product will not help to solve the problem in this case, rather you will need to explore doing the measurement in fluorescence mode.
Which kind of sample do I need?
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The state of the sample is not particularly important for XAS measurements (it can be a powder, foil, film, amorphous) since this technique probes the local structure around the absorber (no need for a long range ordering). However, certain homogeneity is required.
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