Francis William Aston

Francis William Aston (1 September 1877 – 20 November 1945) was a British chemist and physicist who invented the mass spectrometer. He won the 1922 Nobel Prize in Chemistry for his work with isotopes. 
Aston built the first mass spectrometer in 1919. This tool measures the mass-to-charge ratio of particles. By this time, Frederick Soddy had discovered different forms of some radioactive elements. These were called isotopes. However, Soddy could not prove that isotopes had different mass.
Using his mass spectrometer, Aston proved this. He first found that neon had isotopes with different masses. He discovered many isotopes for non-radioactive elements. In total, he discovered 212 isotopes.
Aston developed the whole number rule, which says that the mass of atoms have whole-number ratios to each other.
Aston was a fellow of the Royal Society and Fellow of Trinity College, Cambridge.