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3. Critical Point Drying (C.P.D.)

Table of Contents:

1. Introduction
2. The Critical Phenomena
3. Critical Point Drying
4. Intermediate Stage
5. Transitional Stage

It may now be apparent that we can utilise the Critical Phenomena as a drying technique, as it achieves a phase change from liquid to dry gas without the effects of surface tension and is therefore suitable for delicate biological specimens.

However, it is not surprising that the initial investigations were CO2 as will be apparent from Figure 2, showing a table of Critical Constants for some common substances. Even the practical achievement of the critical conditions would not assist the Biologist, as the specimens would suffer significant thermal damage if we attempted to apply the technique direct in the removal of water from the specimens.


Figure 2.

Therefore, CO2 remains the most common medium for which to apply the C.P.D. procedure, and is termed the 'Transitional Fluid', however, it is not miscible with water and we have to replace the water in the specimen with another fluid, which is miscible with CO2, this is termed the 'Intermediate Fluid'. Ideally and often it can also replace the water in the specimen, also serving as the 'Dehydration Fluid'. This is not exclusively the case, and additional steps may be used for particular circumstances.

However, where it is being utilised for both processes, texts may refer to it under the different headings, Dehydration and Intermediate depending at what stage it is being used in the specimen preparation schedule. Prior to any of these stages would be fixation of the specimen; this is typically a Glutaraldehyde -Osmium procedure.

NOTE:- The whole discipline of specimen preparation prior to the Transitional Stage is only mentioned in its most basic terms, procedures vary according to specimen and further references should be obtained.

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