Really bad baselines  

Some spectra have baselines that cannot be fit with a polynomial.  The options for working with such a spectrum are:

  1. Backwards linear prediction to correct the early points in the FID which are causing the distortion.  As many as 12 points may need to be corrected.  Experiment with parameters to obtain the best results.
  2. Divide the spectrum into multiple sections, and apply polynomial correction in the FB routine to each section separately.  The baseline curvature within each section may then be of sufficiently low order to be corrected.  The best procedure is to select each section of the spectrum using the Zoom subcommand F, specifying the sections by points, not by frequency limits.  For example, the first section would be point 1 to point n, and the second section would start at point n+1.  This should avoid discontinuities at the edges of each section.
  3. Humps can sometimes be removed by subtracting a "deduced" baseline.  See example.
  4. Within FB, the F subcommand will remove DC and tilt for each selected region separately, forcing each region to be flat.   Note that this correction cannot be un-done.  This is illustrated below.

Not all spectra can be corrected with a polynomial, such as the spectrum below.  The FB routine includes a "fudge" operation which can be used to improve the situation.

There is a "real" peak at -200ppm. 

 

Enter FB, use S to select all regions, then un-select the regions around -200ppm. 

 

The F ("fudge") subcommand applies a DC-and-tilt correction to each region separately, forcing each to be flat.  The un-selected regions are corrected based on adjacent regions, to avoid discontinuities.  Once F is applied, it cannot be un-done.  It can be applied to part of a spectrum, but this will create discontinuities at the edges.  Note that the results of this type of correction are very sensitive to where the boundaries between selected and un-selected regions fall.  Integration of such a corrected spectrum should be viewed with skepticism! 

 

Above is a before-and-after comparison, showing the results of the "fudge" operation.