Baseline Correction of a FID
Both real and imaginary parts of the FID should decay to zero by the end of the acquisition, but sometimes that "zero" level is not truly zero, and the offset may not be the same for both channels. This mis-match creates a glitch in the center of the resulting spectrum.
In the figure below, the red line is zero. Note that the imaginary half of the FID (in green) decays to zero, but real half (blue) decays to a value that is slightly above zero.
This "DC offset" appears to be small, but is large enough to create a significant "center glitch" after FT, as shown below.
A BC command, issued before the FT, is used to adjust the DC level of each channel to real zero, and will eliminate the center glitch. Below is the same data, processed with BC before FT.
There is an exception to this – if the FID has NOT decayed to zero (e.g., truncated 2D data), applying a BC may create a DC offset, and cause a glitch to appear. A dramatic example is shown for a 3D data set.
Last update: 3/11/04