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Monday, November 19, 2007

Increasing the Signal-to-Noise Ratio in Solids MAS Spectra

Often MAS or CPMAS NMR spectra of solids will have several spinning sidebands. In such spectra, where there are no other complications, one can increase the signal-to-noise ratio for the isotropic signals by simply adding the sideband intensity to the isotropic spectrum. Below is a 50.68 MHz, room temperature, 15N CPMAS spectrum of a clay sample which absorbed some 15N labelled pyridine and then was heated to 400 degrees. The spinning speed was set to 2.5 kHz. The full spectrum is shown in the bottom panel. The isotropic region is shown on the top left panel. The spectrum in the top right panel was obtained by shifting the spectrum by multiples of the spinning speed and adding the shifted spectra to the original spectrum. Note that the isotropic region of the spectrum has a much improved signal-to-noise ratio.


2 comments:

Anonymous said...

Wouldn't that distort (or correct for) the relative ratios of isotropic signals for different species? How do you compare it with increasing the spinning speed?

Thanks.

Glenn Facey said...

Anonymous,

Thank you for the question. Processing the data in this way would make the isotropic speectrum more representative of the relative intensities of different resonances as the total intensity of each resonance is determined by the sum of intensities of the centerband and spinning sidebands. The result of processing the data in this way is similar to what one would expect by increasing the spinning speed. Sometines, increasing the spinning speed is not an option.

Glenn