A molecular ion is formed by the removal (positive ions) or  addition (negative ions) of one or more electrons from a  molecule M to form M+ or M-. The mass of the molecular ion corresponds to the nominal or monoisotopic mass of the molecule, with the mass of the electron added or lost usually consequential. Of course, the mass of such a molecular ion reflects the isotopic composition of the ion, rather than the average molecular mass of the molecule. Thus the molecular  ion mass is the sum of the relative masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule.