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Correcting the Baseline Manually

You can fit the baseline of a 1-D spectrum visually and subtract it from the spectrum. The model function is a polynomial, up to the 7th degree. This is a time-consuming operation that requires some skill, therefore it's not recommended for routine spectra. It allows, however, to do exactly what you want. For particular difficult cases, each region of the spectrum can be corrected with its own polynomial function.

To Correct the Baseline with the Manual Method :

Step 1

If your baseline shows a minimum or maximum, ⌘-click to create a mark in that point.

The mark will be the axis of symmetry of even-order corrections and the axis of anti-symmetry for odd-order corrections. It is therefore useful if the mark coincides with the geometrical center of your baseline.

It is not necessary to put the mark at the transmitter offset, because that is already the default origin (x = 0) of the polynomial.

Step 2

Choose Process > Baseline Correction.

iNMR will automatically call View > Full for you. The dialog that appears has 8 sliders, one for each term of the polynomial. Use the little arrows for fine tuning. The horizontal line into the main window is the current correction.

Step 3

Move the slider no. 0 until the current correction intersects the spectrum at the level of the vertical mark.

Now you have already found the optimal zero-order correction.

Step 4

Move the slider no. 1. Try to fit the baseline around the vertical mark (only a narrow region needs to be fitted).

Step 5

Adjust, sequentially, the sliders in increasing order. Each time, try to fit a wider region.

It may be necessary to return to the lower order corrections at the end.

Step 6

Click Subtract.

After the correction, amplify the spectrum with the plus key and decide if further correction is required. In this case return to step 3; otherwise, close the dialog.

iNMR stores separately the spectrum and the polynomial correction. There's no difference if you have constructed the polynomial manually or if you have used the other dialog for Automatic Baseline correction. The correction is performed on the fly, whenever the program needs to plot the spectrum. This is convenient, because you can undo the correction at any time, with the command Process > Clear Correction. It also allows you to mix freely Phase and Baseline correction. The console command perm() subtracts the current polynomial permanently, from the whole spectrum or from a selected region only. The second case is useful when you want to subtract different polynomials from different regions of the spectrum.

Related Topics

Automatic Baseline Correction

Piecewise Baseline Correction

Baseline Correction in 2-D and 3-D

Manual Phase Correction