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Monday, October 22, 2007

Eliminating t1 Noise in 2D-Homonuclear Data

2D experiments often have t1 noise (not to be confused with T1 relaxation). This noise is evident as "stripes" perpendicular to the directly detected domain (see the left panel of the figure below). Several factors such as instrumental instability and temperature instability can lead to t1 noise. In homonuclear experiments one can remove the t1 noise by symmetrization of the data about the diagonal. In this process all regions symmetric about the diagonal are compared. The region having the largest signal is thrown away and the region with the smallest signal is put on both sides of the diagonal. This procedure will retain all symmetric signals (i.e. cross peaks) and eliminate t1 noise. Below is an example of the 1H COSY spectrum of ethylbenzene.

One should use this method with care as artificial cross peaks will appear for uncoupled signals with excessive t1 noise. Before symmetrization, one should look for the smallest real off-diagonal signal. Make a mental note of the signal. Symmetrize the spectrum and then scale it such that the smallest real off-diagonal signal noted above is the smallest signal in the symmetrized spectrum.

On a Bruker spectrometer one can symmetrize the data with the commands "sym" and "syma" for magnitude and phase sensitive data, respectively. On a Varian spectrometer the command is "foldt".

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