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2(b). Environmental

Table of Contents:

1. Introduction
2. Physical Consideration
(a) Cryogenic
(b) Environmental
3. Cryo SEM Preparation System
(a) Specimen Stub & Freezing Chamber
(b) Transfer Device & Preparation Chamber
(c) Microscope Stage (SEM) & Applications

Having frozen a specimen, it is necessary to carry out preparation and viewing activities, to attempt to achieve at least equivalent validity to that of more conventional techniques, which together with the perceived advantages of Frozen Hydrated Specimens, justifies the consideration of such systems as a routine as well as research facility. The ‘unnatural environment’ in which the specimen has to be contained, has to be considered, this may not necessarily mean the ultimate achievement of any one condition, but rather a calculated compromise to achieve optimum conditions.

It is necessary to prevent contamination of the specimen with condensing water vapour or other contaminants, while inhibiting sublimation of water from the specimen, (other than where this may be deliberate as part of the specimen preparation procedure, such as etching). Referring to Figure 1, at temperatures lower than -120oC, sublimation is relatively slow.

Sublimation Rate

Figure 1.

However, it can also be seen that unless we have very good vacuums, even at these temperatures the specimen will be subject to contamination by condensing molecules. At even lower temperatures these requirements are even more stringent and could significantly enhance the possibility of contamination.

We can see the differing conditions for condensation/sublimation at the specimen surface,

Sublimation Conditions

Figure 2.

As we may well wish to operate at a temperature of the order of -140oC to satisfy minimal sublimation and avoid ice re-crystallisation at -130oC, at somewhat average vacuums which are readily achievable, then we may have to expect to utilise some form of cold shields and shroud arrangements, at temperatures somewhat lower than the specimen itself.

It has been indicated the condition of vacuum will serve to reduce the contamination and it may be expedient to ensure that the specimen is subject to vacuum condition, with cold shield and shroud protection, on all occasions, as is practically achievable, from its initial cryogenic preparation, thereby satisfying the environmental requirements.

While it is appreciated that this may be considered a rudimentary explanation of the conditions, which prevail, it is trusted, that it will at least account for the approach in the development of a very viable and fully comprehensive system.

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