What sort of model do you want to use to fit the main peak?  If you want to fit it with some pre-defined line shapes (arctan, Gaussians, etc), then the pre-peak tab might be able to do that.  As I mentioned earlier, you'll have to start with "fit baseline" to get started even though it is basically inappropriate, and then modify the model.     

I should say that I'm not entirely sure that fitting in this way is really what you want to do, but maybe you have some ideas about that is the approach you want to use.

--Matt