Hi all, Michel Schelgel said:
I think an evaluation of the spectrum noise may be obtained from the FT contributions peaking at high 'distances' (e.g.,
10Å).
Although I think the scheme outlined by Michel is basically a good one, I would not advise using the high r FT as a measure of the spectrum's overall noise, at least on unfamiliar beamlines. It is my experience that on some beamlines there are spurious effects, such as those related to monochromator lock-in, pumps, etc. that introduce high-frequency (i. e. high r) oscillations without affecting oscillations in the 1-10 Angstrom range. The method IFEFFIT uses for estimating errors in fitted parameters is fortunately not dependent on this error estimate, although some statistical information, such as the value of chi-squared, is. But using high-r noise to weight spectra during merges may rely more about the behavior of a pump or a feedback mechanism than on the actual noise within the FT's region of interest. Unless I have a better way of estimating noise, I prefer to throw out obviously screwy scans and count the rest equally. --Scott Calvin