EQUIPMENT & ACCESSORIES CATALOG EDITION IV CRITICAL POINT DRYERS


What is... 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. Unfortunately, the critical point for water of +374°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 a 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.


Techniques and Applications


A summary of the critical point drying method


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 Ltd in 1971. The original design concepts, which included a horizontal chamber, are still embodied in the design of the E3100 CPD model.


In recent years we have introduced two further models: the K850, which features built-in chamber cooling and heating, and the K850WM, which is designed for drying a 100mm/4" silicon wafer.


All three models 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 phase diagram shows the pressure to temperature ranges where solid, liquid and vapor exist. The boundaries between the phases meet at a point on the graph called the triple point. Along the boundary between the liquid and vapor phases it is possible to choose a particular temperature and corresponding pressure, where liquid and vapor 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 realizable pressure is just above ambient. The choice of fluids is severely limited and CO2 is universally used today, despite early work with Freon 13 and nitrous oxide.


Mature Spruce Wood


Critical point dried block of mature spruce wood block, demonstrating transverse, tangential and radial views of tracheids and vessels.


With CO2 a critical point of approximately 35°C can be achieved at a pressure of around 1,200psi. Therefore if the water is replaced with liquid CO2 and the temperature then raised to above the critical temperature, the liquid CO2 changes to vapor without change of density and therefore without surface tension effects which distort morphology and ultra structure.


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.


The advent of Scanning Electron Microscopy (SEM) in the study of surface morphology in


50 Barley Leaf


Trichomes and stomatal pores on the epidermal surface of a barley (Hordeum vulgare) leaf. Some very fine wax crystallites are also just visible on the surface of the leaf.


biological applications made it imperative that the surface detail of a specimen was preserved. Air (evaporative) drying of specimens can cause severe deformation and collapse of structure - the primary cause of such damage being the effects of surface tension. The specimen is subject to considerable forces, which are present at the phase boundary as the liquid evaporates. The most common specimen medium, water, has a high surface tension to air; by comparison that for acetone is considerably lower. The surface tension could be reduced by substitution of a liquid with a lower surface tension with thereby reduced damage during air-drying. However, the occurrence of what is known as 'continuity of state' suggests a drying technique for which the surface tension can be reduced to zero. If the temperature of liquefied gas is increased the meniscus becomes flatter indicating a reduction in the surface tension. If the surface tension becomes very small the liquid surface becomes very unsteady and ultimately disappears.


When this 'critical point' is reached, it is possible to pass from liquid to gas without any abrupt change in state.


Stomatal Pore on Xerophyte Leaf Surface


Critical point dried epidermis of a xerophyte (cactus), demonstrating raised stomatal pores.


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