EQU - Introduction to Critical Point Drying

Critical Point Drying is so named as it includes, as part of its process, the occurrence known as the continuity of state for which there is no apparent difference between the liquid and gas state of a medium, the surface tension between this interface reducing to zero. This occurs at a specific temperature and pressure with resulting density, and is known as the Critical Point. This condition of zero surface tension can be used to dry Biological Specimens, avoiding the damaging effects of surface tension.

In biological specimens we are mainly concerned with the removal of water. The critical point of water of +347°C and 3212 p.s.i. is inconvenient, and would cause heat damage to the specimen. The most common and convenient transitional medium for critical point drying is Carbon Dioxide CO² which has its critical point at 31°C and 1072 p.s.i. However, it is not miscible with water, and therefore we have to involve a third medium, commonly Ethanol and Acetone, which is termed the intermediate fluid. We can now convert our transitional fluid, typically CO² from liquid to gas without surface tension at the critical point. The Critical Point Drier is designed for use with CO², having first replaced any water in the specimen by a series of dehydration steps, often in a fluid such as Acetone, which will also be the intermediate fluid.

Typical Dehydration Steps

1. (Wet Specimen) 
2. Fixation
3. Ethanol, Acetone - 10% - 100%
4. CO² C.P.D. 
5. Dry Specimen ready for coating for SEM. 

The specimens for critical point drying are located in the pressure chamber of the Critical Point Drier. The chamber is pre-cooled to allow it to be readily filled with liquid CO² from a gas cylinder. The chamber is then heated to just above the critical temperature with subsequent critical pressure being achieved. The CO² gas is vented through a needle valve, to avoid specimen distortion.