This website includes a simple description of how a mass spectrometer works. Mass spectrometry (MS) is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass-to-charge ratios. Keywords: mass spectrometer, mass spectrometry, mass spectrum, mass spectra, spectrum, spectra, mass, ionisation, acceleration, deflection, detection