Introduction

One disadvantage of the scanning electron microscope (SEM) is that it is normally not possible to examine samples that produce any significant amount of vapor when placed in a vacuum. Because of this limitation, biological samples must be dried, and many samples like grease, adhesives, liquids, foods, gels, and others semisolids cannot be examined. Some vapor-producing samples can be examined using cryogenic SEM.

However, even cryogenic SEM cannot be used to observed the drying process of adhesives, the curing of cement, the melting of alloys, or the crystallization of materials. In attempt to overcome these disadvantages, progress has been made in recent years in perfecting the environmental scanning electron microscope (ESEM).

ESEM is a new innovation in scanning microscopes specifically designed to study wet, oil bearing, or insulating materials. Polymers, biological cells, plants, soil bacteria, concrete, wood, asphalt and liquid suspensions have been observed in the ESEM without prior specimen preparation or gold coating. Samples may be examined in water vapor or other gasses such as CO₂ or N₂ at near atmospheric pressures due to the unique vacuum system of the ESEM.