The analytical instrument called a mass spectrometer was developed in 1919 by Francis Aston in Cambridge, England. A spectrometer is a spectrograph with measurement capabilities. The importance of this technology was immediately recognized, and Aston was awarded the Nobel Prize for his work in 1922. Mass spectrometers permit the experimental determination of atomic and molecular masses with great accuracy. Aston's mass spectrometer had a precision of one part in 10,000, which was sufficient for him to discover the isotopes of many elements.
Modern instruments are even more precise.
McREL Mass spectrometers operate under conditions of high vacuum, typically 10 -8 torr. Low-pressure samples in the spectrometer's ionization chamber are exposed to a beam of rapidly moving, energetic electrons shot out of an electron gun. The samples can be a gaseous element such as neon, the vapor of a solid or liquid element such as mercury, or even the vapor of a molecule such as water or methane. With modern technology it is possible to introduce a wide variety of materials, including mixtures, into a mass spectrometer. In the ionization chamber, the atom or molecule hits or is hit by the accelerated electrons.