I am working on Zn binding to Fe and Al oxides and I am trying to fit the second and third shells. I have three spectra from three samples that differ in a particular treatment. Visually speaking, the Fourier transforms for all these spectra show second and third shells at 2.6 and 3.2A (uncorrected for phase shift), respectively. I fit the second shell with Fe and the third shell with Al. From the fitting results I obtained the Zn-Fe and Zn-Al radial distances for the second and third shells. However, these Zn-Fe and Zn-Al radial distances were different among the samples. For example, the Zn-Fe radial distance was 3.18A for sample 1, 3.44A for sample 2 and 3.51A for sample 3. My question is: if the peaks in the Fourier transforms are at the same position for all samples, why is it that the fitting shows that the radial distances are actually different? NOTE: I fit the 1st,2nd and 3rd shells(R range:1.0-3.2A) together.