Quorum K850 Critical Point Dryer (CPD)
Critical point drying is an established method for the controlled dehydration of biological tissue prior to examination in a scanning electron microscope (SEM).
Water in the specimen is replaced with liquid carbon dioxide (CO2) whose critical temperature for a realisable pressure of around 1,200 psi is just above ambient (around 32◦C). Therefore if the water is replaced with liquid CO2 and the temperature raised to above the critical temperature, liquid CO2 changes to vapor without change of density. This avoids the surface tension effects which distort specimen morphology and ultrastructure. Since CO2 is not sufficiently miscible with water it is necessary to use intermediate fluids, such as ethyl alcohol and acetone, which are miscible with both water and CO2.
The instrument is fitted with thermo-electronic heating and adiabatic cooling and temperature control of +5°C cooling and +35°C during heating. This allows pre-cooling of the chamber (vertical with glass viewing port that includes magnetic stirrer located in the base of it) and ensures that the critical point is accurately obtained.
It is fitted with three valves: fluid inlet, flushing and a gas venting system which uses a fine needle valve to give controlled pressure let down at the end of the critical point drying process. A built-in magnetic stirrer ensures thorough mixing of specimens with circulating fluids.