Technique for the detection of chemicals by ionization, followed by the use of a magnetic or electric field to separate the ions according to their mass-to-charge ratio. The ionization can be achieved by bombardment with a stream of electrons from a heated filament (electron impact) or by the transfer of charge from a prepared ion to the species of interest (chemical ionization). Highly sensitive mass spectrometers have been used to measure small differences in the ratio of isotopes in certain molecules to ascertain, for example, the age of the sample or to assess the nature of the source of the sample.