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You can use cubic spline to subtract baseline on Fit Plot. To create spline curve click on
Add button in
Fit Curves tab of Fit Plot and select
Spline menu item.
Do not use splines to subtract baselines which can be fit well enough with
Line curve (line or constant baseline). You may mistakenly subtract wide peaks using spline. In some cases
Parabola curve may be more suitable.
Created spline has three anchor points by default. You can move, add and remove anchor points:
Baseline checkbox in spline row in fit curves table to subtract spline from data.
Spline anchor point (x, y) coordinates are treated as fit parameters so you can perform fitting with spline although we don't recommend this technique. Fitting the baseline with some adequate model function is preferred.
It is recommended to set appropriate fit intervals which contain only baseline without peaks. In such case
Fit One Curve button is more acceptable than
Fit by Sum button, because the individual interval for current curve will be used and the interval from
Fit Interval tab (which is used to fit by sum of curves) is ignored. Select spline curve and set
Edit Interval checkbox in the bottom of the panel to edit spline interval, then click on
Fit One Curve button. The anchor point coordinates will be varied but the number of points will remain. You also can lock some parameters (usually x coordinates) by setting
Lock checkboxes in parameters table.