In some cases the automatic baseline correction command abs does not generate a desired baseline. This often happens when some of your peaks are broad and the correction algorithm has a hard time decide whether some data points in a certain spectral area is signal or baseline. In such cases, you can try an advanced baseline correction method: bas. Upon giving the command bas, you are offered a number of choices. I use the first and the third choices most often.

Choice 1, manual baseline correction. The correction is done by manual fitting of the baseline by a fifth order polynomial: y = A + Bx + Cx2 + Dx3 +Ex4 + Fx5. First, you display the range of the spectrum that you want to correct (note: you can baseline correct a small range of the spectrum instead of the entire spectrum). Second, You drag each of the five buttons A, B, C, D, and E to adjust the red baseline until it passes through the spectral baseline. I found that adjusting A, B, and C is often good enough. Finally, you click the “Return, Save regions” button, which will perform the correction. This option is very useful if you want to integrate a small peak riding on the shoulder of a big peak.

Choice 3, “Auto Correct Spectral Range…. only”. You need to decide two factors for this correction method: (1) a chemical shift range of spectrum for which you want baseline correction (ABSF1 and ABSF2). Since it is often difficult to perfectly correct baseline across the entire spectrum, only correcting a smaller range often produces better result. For this, you will need to properly choose the left and right limit of the range that you want the baseline correction be done. Choose the limits such that there are at least 0.5 ppm wide of signal-free regions at both ends of your spectral window. This will help the correction algorithm to recognize the baseline. (2) Shape of the polynomial (ABSG). I often find that lower orders of polynomial (0, which means that I only correct a constant offset A; 1, which means I only use the shape A + Bx; or 2, which means I only use the shape A + Bx + Cx2) do a better job than the ones that use all five orders, which often overdo the job.