Often, sample pretreatment is only partially effective in preparing the samples for SEM examination. For example, wet samples change character when dried, thus appearing different in the SEM examination than in their native state. For porous samples, it is difficult to get the conductive coating into the subsurface spaces. Charge buildup at the pores continues to interfere with the SEM examination. In addition, coating confuses analysis by the EDS and WDS techniques.
Carbon is the typical material for coating when microanalysis is required.
Early EDS detectors could not detect carbon. Therefore, carbon coating was not a severe limitation for microanalysis. Today's EDS systems detect carbon and lighter elements, as light as beryllium. Carbon applied to sample surfaces obviously will interfere with detection of carbon in the sample and can inhibit detection any lighter elements present. As the SEM is increasingly used for routine evaluations, there is increasing demand for examination without pretreatment.