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:

• Move anchor point with mouse
• Double click on spline curve to add new anchor point
• Double click on anchor point to remove it

Set 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.

• Nonlinear Curve Fitting: Fit Plot
• Specifying Custom Fit Equation
• Guessing Peaks
• Predefined Fit Curves Equations