The most common cause of composition modification to a specimen during electron probe microanalysis is the field induced migration of light elements. This is an indirect effect which occurs in response to the long range electric fields that form when dielectric specimens suffer charge imbalance. The result is that the ions are redistributed within the sample according to their respective mobilities and the affect is enhanced rather than eliminated when the sample is coated. The ions typically move radially outward in thin samples because of the excess production of secondary electrons from the specimen surfaces, Cazaux(1986)and downwards in conventional SEM samples when the field is due primarily to the deposition of electrons within the bulk of the specimen

Field induced migration is responsible for most of the elemental signal variations observed during the microanalysis of silicate glasses containing sodium or potassium ions.