Monday, October 29, 2007
Quadrature images are caused by an imbalance in the magnitude of signals in the x and y channels of the receiver. Such an imbalance will create a quadrature spike in the Fourier transformed spectrum (see entry for October 11, 2007) as well as quadrature images. An example of quadrature images is shown below for the 1 scan proton spectrum of a 10% ethylbenzene in CDCl3. Quadrature images are small reflections of large signals in the spectrum about the center. They are much less of a problem with newer instruments than they were with older ones. They become smaller as more scans are collected due to receiver phase cycling. You should be aware of quadrature images if you are searching for very small signals in the presence of very large ones.