Dear All, Could anybody help me with a question I have related to preferred orientation?
I have collected some spectra of a sample, on a D4 using flat plate geometry, that I suspect is showing preferred orientation effects. I was under the impression that a way of testing for this is to re-pack the sample, re-run and compare the spectra, as the level of preferred orientation should change between packings and so the spectra should differ slightly. Despite re-packing and comparing spectra from up to 8 scans the spectra overlay perfectly (within noise). This suggests to me that the refinement residuals are probably not due to preferred orientation. I am using Topas to refine the data and despite the comment above, the residuals seem best fitted using a spherical harmonic-prefered orientation model. I have also tried using other parameters to fit the residuals such as the occupancy of some of the sites, using anisotropic displacement parameters or anisotropic broadening effects, but the SH-PO fits best and seems the most physically reasonable treatment. As a test, I have also run the sample in capillary transmission geometry on another instrument, and the refined value of the SH-PO parameter is reduced. This seems to confirm that PO is the problem, but the question I have is how effective the re-packing comparison method is for confirming PO? If I see no difference in the spectra does it confirm that PO is NOT the problem or can I still have PO effects that are very repeatable upon repeat packing? If PO is not the problem in this case, are there any other structural or instrument effects that could lead to refinement residuals that would be well treated with a SH-PO model? Thank you all very much for any information or light you can shed on the subject. Ross Colman
