Mass spectrometry (MS) is nowadays a widespread instrumental technique used for both the qualitative and quantitative analysis of small molecules and their complex mixtures. In 1897, the seminal experiments of Thomson with cathode rays established MS and contributed to the development of modern chemistry by discovering the isotopic effect and the electron. The separation of neon isotopes by their masses was performed by Thomson, and this technique was subsequently improved by F.W. Aston and A.J. Dempster and was later developed into the analytical method called MS. In fact, Dempster established the fundamental theory of MS and created the basic design of mass spectrometers that has been used ever since. The experiments of Thomson and Dempster significantly contributed to the amazing development of MS, from the first sector instruments to the nowadays used hybrid Fourier-transform (FT) FTMS instruments. The combination of mass spectrometers with separation methods (GC-MS, HPLC-MS, CE-MS, and ICP-MS) offers a complete analytical arsenal today, which can be used for the analysis of complex mixtures with relatively short analysis times (typically 20-30 min) and also for samples of very low quantities (10− 15-10− 21 g).