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 not 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. Unfortunately, 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 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 K850 Critical Point Drier is designed for use with CO², having first replaced any water in the specimen by a series of dehydration, often in the same fluid such as Acetone, which will also be the intermediate fluid.
(Wet Specimen) - Water - Acetone - 30% - 100%-CO² - C.P.D. - (Dry Specimen)
The specimens for critical point drying are located in the pressure chamber of the K850. 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.
The K850 is fitted with thermoelectronic heating and adiabatic cooling and temperature control of +5°C cooling and +35°C on heating. This ensures the critical point is accurately obtained, avoiding excess pressures or temperatures, or the need to rely on pressure relief valves to control pressure during the heating cycle. The chamber is vertical, with top loading, to ensure specimens do not become uncovered during the drying process, with a side viewing port to locate the meniscus for the correct level when initially filling the chamber.
See also CPD Technical Briefing