A spectrum y includes a waveform-of-interest component and a baseline component serving as a wide-band component. An optimum solution of a signal model x is determined according to a first condition to fit a corresponding portion SIFx of a baseline model Fx with respect to a representative portion yI of the baseline component, and a second condition to minimize an Lp norm (wherein p≤1) of the signal model x. An estimated baseline component determined from the optimum solution of the signal model x is subtracted from the spectrum y.