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electron-probe microanalyser An instrument used to determine the chemical composition of a 1 μm diameter specimen of mineral or glass at the surface of a polished rock or mineral slice. A narrow beam of electrons is focused on to the polished surface of the specimen to cover a 1 μm diameter spot. The electrons excite the atoms in the specimen to emit X-rays, whose wavelength is characteristic of the elements present and whose intensity at a given wavelength is proportional to the relative concentration of the element corresponding to that wavelength. By comparing the X-ray intensity at any one wavelength with that in a standard sample of known composition, the absolute concentration of the element in the sample can be deduced.