The principle of the microprobe analysis of chemical elements is illustrated in Fig. i. Some kind of radiation is directed on to the specimen, generating signals characteristic of the elements present. Local analysis in situ is achieved in one of two ways. Most often the impinging beam is finely focused so that the signal at any moment comes only from a selected microregion. Alternatively, in some instruments, the impinging beam floods a larger region but the emergent signals characteristic of a particular element may be selected and focused to give an elemental ‘map’ or ‘image’.