There is disclosed a method of extracting information about the mass of a substance from a complex mass spectrum containing peaks arising from polyvalent ions. These polyvalent ions possess electric charges which are integral multiples of the elementary electric charge. Ions including these polyvalent ions are detected, and a mass spectrum is obtained. On the mass spectrum, ion intensity I is represented as a function of a variable x corresponding to mass-to-charge ratio. The mass spectrum, given by I(x), is transformed into a mass spectrum I(t) of a variable t, using the relation t=1/(x-H), where H is a unit mass of ions added to the polyvalent ions. Peaks appear on the mass spectrum I(t). Among these peaks, at least a given number of peaks appear at regular intervals. The spacing between these regularly spaced peaks is found.
