Instrument used to measure the masses of molecules and atoms by volatilizing and then ionizing them. The ions are then separated magnetically according to their 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 ion or fragment. Since the charge is often +1, the mass and the mass-to-charge ratio are often the same. in order to determine the chemical or isotopic composition of a substance.