EQUIPOS DE PREPARACIÓN DE MUESTRAS: Freeze
Driers 
FEDELCO,
S.L.
FREEZE DRIERS:
The
technique of Freeze Drying as a method of specimen dehydration and its
subsequent preparation for viewing in both S.E.M. and T.E.M. has been
well established for some time and the detailed procedures available are
outside the range of this article. In any case they vary considerably in
their application and users have a wide choice of what parameters to
apply for their particular application.
In its simplest terms we freeze the water in the wet specimen and then
remove it by process of sublimation, from solid to vapour phase,
maintaining both its structure and chemical composition as realistically
as possible.
This avoids the use of chemical dehydration and can be conveniently used
to prepare bulk specimens or embedded sections for subsequent S.E.M. or
T.E.M. studies, in particular utilising X-Ray microanalysis for the
study of diffusible substances.
The application of freeze drying for specimen preparation for TEM and
SEM is a well established practice.
Its application is to reduce the distortion which occurs when a wet
specimen dries by normal evaporation. This distortion is due to the
forces of surface tension going from a liquid to vapor phase such as
water to water vapor, commonly the situation in a Biological Specimen.
However, if we freeze the specimen and maintain it frozen, then by
applying a vacuum, we can obtain a situation where we can remove the
frozen water by sublimation, avoiding the liquid phase, and reducing the
distortion. This rate of sublimation is very much a function of
temperature and vacuum, and associated drying time which is in the order
of several hours or longer. Ideally, freeze drying could be carried out
at temperatures below the recrystallization of ice, which will require
an inordinately long drying period. In practice, temperatures of -60°C
have been found to give reasonable results under vacuums achievable with
two stage rotary pumps, having ensured that good, fast freezing of the
specimen has been carried out initially.
