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Hi everyone

Does anyone happen to know if/how I can tell whether peaks outside the spectral width were either folded or aliased into a (NOESY) spectrum, when all I have is the spectrum file (in .ucsf format, say), and I do not know how the spectrum was recorded? I am not aiming at finding out whether there were any such 'outside peaks', what I would like to know is, in case there were some, where I would have to look for them.

Thanks a lot for your time!

asked Feb 02 '12 at 04:40

canavanin's gravatar image

canavanin
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Normally, you would not expect to see any peaks folded or aliased in the direct dimension, due to the presence of analog filters (unless someone has turned off the Butterworth filter, or whatever is the default analog filter, being used on your system). If the spectral width in the direct dimension is less than the range of frequencies of large peaks then it may lead to baseline distortion in the observed spectrum.

In general, the spectral width in the direct dimension is chosen to be sufficiently large to ensure that there is no folding or aliasing. The following ASSUMES that the spectral width is adequate in the direct dimension:

If the method of STATES et al., is used for obtaining phase-sensitive spectra and if the spectral width is less than the range of chemical shifts in the indirect dimension of the NOESY, then a discontinuity will be observed for the diagonal peaks, due to the aliasing in the indirect dimension. (The diagonal line of autocorrelation peaks will reach the maximum value at the top, and will reappear at the bottom and continue to increase)

If the TPPI method is used for obtaining phase-sensitive spectra and if the spectral width is less than the range of chemical shifts in the indirect dimension of the NOESY, then a set of anti-diagonal peaks is to be expected for the autocorrelation peaks, if data is displayed in the standard format, due to the apparent folding in the indirect dimension. (The diagonal line of autocorrelation peaks will reach the maximum value at the top and then appear to reverse direction in the indirect dimension).

If the selected spectral width is short in both dimensions, then the situation is more complicated than that mentioned above.

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answered Feb 11 '12 at 21:09

sekhar%20Talluri's gravatar image

sekhar Talluri
601

Thanks for the reply! What you have written confirms my initial impression that the way the spectrum was recorded is indeed relevant in this context. - canavanin (Feb 13 '12 at 01:04)

I would just like to add that modern spectrometers no longer use Butterworth or Bessel analog filters, but use digital signal processing instead. The rolloff of these digital filters is so steep that aliasing (folding) is, for all practical purposes, eliminated. - Kirk Marat (Feb 14 '12 at 08:08)

This only applies to the acquisition dimension, of course :-) - Kirk Marat (Feb 14 '12 at 13:38)

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