The normal incidence electron gun of the IMS4f is unique in design, allowing self‐compensation of electric charges when negative secondary ions are analysed. This is made possible owing to the formation of a cloud containing an excess of very‐low‐energy electrons above the sample surface. When positive charges appear at the sample surface, the sample will take out of the colud the exact amount of electrons required for compensation. The effectiveness of this self‐compensation is demonstrated by the analysis of bulk insulators, such as alumina and silicon nitride, as well as multilayered specimens. In the positive mode, exact adjustment of the electron intensity monitored by the energy distribution of secondary ions allows one to reach a state where the elecric field inside the sample is small enough to make it possible to analyse very mobile elements, such as Li and Na, without migration. Misadjustment of the electron intensity will create an electric field at the surface and an equilibrium is observed owing to conduction currents. Copyright © 1990 John Wiley & Sons Ltd.
