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Thursday, October 11, 2007

Quadrature Spikes and How to Get Rid of Them

Have you ever seen a small sharp peak in the exact center of your NMR spectrum? This is called a quadrature spike and is due to a small offset in the two channels used for quadrature detection. It can sometimes be confused with a real NMR peak. You can get rid of this peak very simply by reprocessing your data. In XWINNMR or TOPSPIN do the following:
Type "bc_mod qfil"
Type "bcfw 0.02"
Type "efp" to reprocess your data

This will null a region 0.02 ppm in width at the center of the spectrum. In the unlikely event that you have a real signal at the exact center of the spectrum, you should not use this technique as it will null out the real signal as well. Below is an example of removing such a spike in a 13C NMR spectrum.

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