On Mon, Nov 22, 2010 at 4:55 PM, Scott Calvin
Some follow-up. This, for example, is from an excellent workshop presentation by Rob Scarrow:
Errors from large particles are independent of thickness
Yes... one can have a sample that is uniform, or made of small particles, and still too thick. In that sense, having large particles or sample with widely varying thickness is a separate issue from having a sample that is too thick.
The relative (%) variation in thickness depends on the ratio (particle diameter / avg. thickness), so it is tempting to increase the avg. thickness (i.e. increase μx) as an alternative to reducing the particle diameter.
However, simulations of MnO2 spectra for average Δμ0x = 1, 2 or 3 show that the errors in derived pre-edge peak heights and EXAFS amplitude factors are significant when diameter > 0.2 / Δμ0, but that they are not affected by the average sample thickness. (Δμ0 refers to the edge jump)
The equation at right is given by Heald (quoting earlier work by Stern and Lu). D is particle diameter, μ1 is for just below the edge, and Δμ =μ(above edge) - μ1.
I've seen similar claims elsewhere, although Scarrow's is particularly clear and unambiguous.
OK. Are you saying there something wrong with this? Did he say that spheres were stacked directly on top of one another? I'm not seeing that assumption in what you quote. I read it as saying that you can't have spheres that are too thick. --Matt