Environmental SEM ESEM

As a special development in recent years, SEMs have been designed which no longer necessitate high vacuum (environmental SEM, ESEM variable pressure SEM, VPSEM). This development is important for the imaging of samples with a residual vapour pressure, such as aqueous biological or medical samples, but also samples in materials science (wet rock) or organic chemistry (polymers). 

A recent addition is the environmental SEM (ESEM) which operates with the sample chamber at greater than 20 torn. Water vapour can be maintained in the chamber by stabilizing the liquid water at pressures equal to the vapour pressure of water. At a given vapour pressure, the sample temperature can be adjusted to maintain, evaporate or condense water. The polymers can now be imaged wet and real-time studies can be carried out. 

The specimen for SEM has to fulfill certain requirements. It needs to withhold vacuum and electron bombardment, it has to be electrically conductive and furthermore, and it needs to be clean and dry. A special version of an SEM is the environmental SEM (ESEM) which uses a different vacuum system and other types of detectors. The ESEM can be operated in higher, more natural pressures. 

In addition to the numerous elemental characterisations of ambient particles, we have recently seen more applications in fundamental physico-chemistry. This includes laboratory studies on gas-particle reactions, e.g. sea salt or soil dust with nitric acid. Environmental SEM (ESEM) or environmental TEM (ETEM) does offer excellent future prospects in this field. In ESEM and ETEM, it is possible to obtain high quality images and to do some chemical analysis while the gaseous environment around the sample is controlled i.e. vacuum in the neighbourhood of the sample is not necessary, the relative humidity can be varied and a temperature around — 30 °C can be maintained). Ice nucleation properties of individual atmospheric particles have also been studied recently. Both ESEM and ETEM are relatively new techniques, of which the potential has not fully been explored. 

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