A process used to identify chemicals in a substance by their mass and charge. Mass spectrometers are instruments that measure mass and charge of molecules. A mass spectrometer also can determine how much of a compound is present in a mixture. Also known as mass spectroscopy. Mass spectrometry is an analytical technique that uses an instrument called a mass spectrometer to measure the mass-to-charge ratios of molecular ions. Molecules fragment within the mass spectrometer to produce a mass spectrum, which can be interpreted to determine the identity of the molecules in the sample.