Critical Point Drying
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Critical Point Drying is an established method of dehydrating biological
tissue prior to examination in the Scanning Electron Microscope. The
technique was first introduced commercially for SEM specimen preparation
by Polaron in 1970. The original design concepts, which included a horizontal
chamber, are still embodied in the design of the E3000 and E3100 CPD
systems.
Both E3000 and E3100 have found general acceptance in many laboratories throughout
the world. Together these critical point dryers offer the user a choice most
suited to the particular specimen preparation requirements.
The Critical Point Drying Method
The phase diagram shows the pressure to temperature ranges where solid, liquid
and vapour exist. The boundaries between the phases meet at a point on
the phase diagram called the triple point. Along the boundary between the
liquid and vapour phases it is possible to choose a particular temperature
and corresponding pressure, where liquid and vapour can co-exist and hence
have the same density. This is the critical temperature and pressure.
Critical point drying relies on this physical principle. The water in biological tissue is replaced with a suitable inert fluid whose critical temperature for a realisable pressure is just above ambient. The choice of fluids is severely limited and Carbon Dioxide is universally used today, despite early work with Freon 13 and Nitrous Oxide. With CO2 a critical point of approximately 35oC can be achieved at a pressure of around 1200 psi. Therefore if the water is replaced with liquid CO2 and the temperature then raised to above the Critical Temperature, the liquid CO2 changes to vapour without change of density and therefore without surface tension effects which distort morphology and ultrastructure. Since liquid CO2 is not sufficiently miscible with water it is necessary to use an intermediate fluid which is miscible with both water and liquid CO2. In practice intermediate fluids commonly used are methanol, ethanol, amyl acetate and acetone.
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