1913

Mass Spectrometry

Joseph John Thomson (1856–1940), Arthur Jeffrey Dempster (1866–1950), Francis William Aston (1877–1945), Ernest O. Lawrence (1901–1958)

Mass spectrometry is a technique originally used by physicists, then moved into chemistry, where it is equally at home. In every case, it depends on generating charged ions from atoms and whole molecules, and letting them fly through a small vacuum chamber in the presence of electric and magnetic fields. This causes the paths of the charged particles to bend, and that deflection depends on how heavy those particles are. What this gives you is a way to separate out the various compounds in a mixture by their molecular weights, and that, as it happens, is an endlessly useful thing to be able to do.

British physicist Joseph John Thomson was the first to notice this, when he deflected a stream of pure ionized neon atoms in 1913 and saw that it separated into two. He was seeing neon-20 and neon-22, the first physical evidence for the existence of stable isotopes (forms of the same element with different numbers of neutrons). His student Francis William Aston, a British physicist and chemist, went on to build a better instrument and found that many other elements were composed of isotopic mixtures. Canadian-American physicist Arthur Jeffrey Dempster’s machine was better yet, and he discovered that an electrical spark could be used to ionize a sample inside the vacuum chamber. By the time of World War II, American physicist Ernest O. Lawrence was developing mass spectrometers that could be used to separate uranium isotopes not just to analyze them, but also to purify larger amounts for actual use—albeit with a huge amount of effort.

But by this time, chemists were finding ways to ionize entire molecules. Bombarding them with streams of electrons (the electron-impact method), or with streams of small ions (the chemical-ionization method), allowed a wide variety of compounds to “fly” under mass spectrometry conditions. Larger and larger molecules became open to analysis, at finer and finer resolution, providing unambiguous compound identification for a huge number of applications. Medical assays, drug research, geology, and forensic science all depend on it.

Mass spectrometers (calutrons) being used to produce enriched uranium for the Manhattan Project at the Y–12 plant in Oak Ridge, Tennessee. The operators, mostly young women with only high school educations, worked in shifts around the clock and out-produced the PhDs they replaced.