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  Automatic Baseline (Baseplane) Correction

It is called “automatic” because iNMR automatically samples the baseline and calculate the best fitting polynomial, yet the user has a number of choices:

You launch this task with the command ‘Process/Automatic’. If you want to narrow the fitted range, you have to select that range with the mouse before opening the dialog. The ppm limits will appear in the dialog. They are stored into the document. The next time you try to correct the baseplane, the limits will be shown again, even if nothing is selected, and EVEN IF ANOTHER REGION IS SELECTED. If you need to correct a second region, click the button ‘Reset Region’.

There is also another reason why you should avoid to restrict the fitted range. Good fitting requires many sampled points. The higher the degree of the polynomial, the MUCH higher the need for sampled points. If the range to fit contains few and small regions of pure baseline, fitting will be quite poor. That's why in the 1d case you have the possibility of fitting the baseline by eye.

1D spectra

With 1D spectra, iNMR shows the correction while the dialog is open. You can change the parameters and monitor how they affect the correction. In practice, you should, in this order:

  1. Uncheck the option “Fit with a Polynomial”
  2. Find the optimal value for the “filter” parameter. This is the most important parameter in this kind of correction.
  3. Refine the degree. A lower degree smoothes the correction, an higher degree makes it more similar to the spectrum.
  4. Use the keyboard keys, as always, to move to different parts of the spectrum and check the result.
  5. Press “OK” when satisfied.

After the first minute of practice, you'll have understood the whole theory behind baseline correction. To learn more, read the article: Cobas, J.C. et al. J. Mag. Res. 183, 2006, 145. It describes the smoothing algorithm applied.

How you Decide the Degree (order) of the Polynomial

In theory you can always specify the highest available degree, because the fitting algorithm can decrease it, but can never increase it. This approach, unfortunately, is not always good. Higher degree polynomials are quite troublesome for a computer. You should only use them if necessary.
Another approach is to figure out, by visual inspection, the order of the baseline shape, and specify the same order for the fitting polynomial. This approach is too individualistic to be publicly suggested.
A generalized approach is to start with a middle-of-the-range value, like order 4. If the baseline, after correction, is already flat, there's no need for a different solution. If, instead, the corrected baseline is worse than the uncorrected one, decrease the order (after reloading and reprocessing the fid, in case of a 2D spectrum). If the corrected baseline in better than the original, but still not satisfactory, you can try with an higher degree. In this case you don't need to reload the FID, you can just add correction on correction.

See also

Baseline Correction: an Introduction

Manual Baseline Correction

Undoing your Operations