All methods of elemental identification depend on a determination of either the mass or the charge of the atomic nucleus. The nuclear charge can be inferred from the energy required to create a vacancy in an inner electron shell or from the energies of the electronic transitions involved in the decay of the vacancy. The masses are determined from the recoil momentum imparted to a noble gas ion or by removing atoms from the surface, by sputtering or high electric fields, for analysis in a mass spectrometer. Quantitative analysis, which is not yet possible, will depend on an understanding of structural information contained in secondary features of the spectrum such as line shapes or ion yields.