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Friday, November 9, 2007

Increasing the Signal-to-Noise Ratio in Solid State Wideline 2H Spectra

At low magnetic field strengths where chemical shielding anisotropy is not a problem, the wide line 2H spectra of solids produce symmetrical spectra. One can increase the signal-to-noise ratio in these spectra by a factor of the square root of two by reversing the spectrum and adding it to itself as illustrated below for the 2H spectrum of perdeuterated poly-methyl methacrylate.

2 comments:

Reza Siavashi said...

Hi Glen,

First of all, many thanks for this beautiful blog.
Just have a question regarding adding the reverse spectrum and adding it to the original spectrum, is this the same as zeroing the imaginary FID? and if it is , wouldn't we lose the information about which spins precess faster or slower than the Larmor frequency.

I have the problem of an asymmetric spectrum using solid (quad) echo with a scout spectrometer. I have found that the signal related to the FID of the imaginary channel does not happen at the same time as the real channel and that causes the spectrum to be asymmetric. I know I should get the imaginary channel echo as flat as possible, but with a lot of tries it still has some signal in it. Do you have any idea how can this be fixed?
Thanks

Glenn Facey said...

Reza,

Thank you for the questions. Adding the reversed spectrum to the observed spectrum is equivalent to zeroing the imaginary channel since the FT of a real signal (with no imaginary signal) always gives a spectrum symmetric about the carrier frequency. Since the spectrum is symmetric about the center, it is irrelevant to consider spins that precess slower or faster than the carrier frequency.

If your 2H NMR spectra are not symmetric (i.e. the imaginary channel is not fully nulled), you might try using higher power to achieve shorter pulses and adjusting the probe tuning slightly to achieve more uniform excitation across your spectrum. If, after this, your spectra are still slightly asymmetric you can symmetrize the data as described in the BLOG post.

Glenn