The mass spectrometer sorts ionized atoms emitted by specific heated substances according to their individual masses. The analysis is usually performed by passing these ions through a vacuum chamber placed in a strong magnetic field. The path of individual ions is a function of their mass, velocity, and charge.
The foundation of the technique was the investigation of the ions produced by cathode ray tubes, research that led to the discovery of isotopes by J. J. Thomson in 1913. An effective mass spectrometer was produced after World War I by Arthur J. Dempster (1886-1950), who used an electrical collector in 1918 at the University of Chicago. One year later Francis W. Aston (1877-1945) used a photographic plate as a detector at the University of Cambridge. Aston discovered 212 naturally occurring isotopes, and was awarded the Nobel Prize for Chemistry in 1922.
By the 1940s mass spectrometers were being commercially manufactured and were being used by many experimenters—geologists, to measure the age of rocks by looking at their radioactive decay products; chemists, to analyze complex organic molecules; and space scientists, to analyze the composition of the upper atmosphere. Electron beams and lasers were used to excite the ions.
Miniature mass spectrometers were produced for portable medical applications, and versions were placed on spacecraft to analyze the atmospheres of Venus and Mars and the gases emitted from Halley's comet. Not only could the instruments measure minute samples, they could also measure the mass of an atom to the accuracy of one part in a billion.
