Unit used by mass spectrometers to separate ions. The mass-to-charge ratio is simply a ratio of the mass to the size of the electronic charge for any given molecular Atom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. Click on Term to Read More or fragment. Since the charge is often +1, the mass and the mass-to-charge ratio are often the same.