Surfaces The majority of the SEM secondary electron signal (SE) comes from the first few (<2nm) nanometers of the surface. The quality of the outermost few atomic layers of an exposed and reactive sample can therefore have a profound effect on image contrast. With light elements and low voltages the SE signal is dominated by scattering at the point of incidence of the primary probe, but more generally there is a significant contribution to surface SE generation by primary electrons backscattered from deeper in the sample. With the advent of very low voltage EDS, and the application of more conventional surface science methods, we learn that even without a coating the surface of a sample may be atypical of the bulk (Fig. 1). Contamination of an otherwise clean surface occurs by transport of the sample through the air, leading to oxidation, hydrocarbon accumulation, and occasionally to rather more dramatic pyrophoric effects.