EQUIPMENT & ACCESSORIES CATALOG EDITION IV CRITICAL POINT DRYERS
Techniques and Applications
Critical Point Drying Principles (continued) (b) Transitional Stage
As discussed previously (see Figure 1) the conditions for which the critical point passage can be obtained for CO2 are 31.1°C and 1072 psi. However, it must be remembered that these isothermals are obtained from a fixed mass of gas and an applied pressure for a series of constant temperatures.
In the laboratory application of CPD we have a fixed volume which is filled with the transitional fluid. Some typical examples of which are given in figure 4.
Transitional Fluids for CPD Substance
CARBON DIOXIDE +31.1 FREON 13 FREON 23 WATER
+28.9 +25.9 +374
Figure 4:
Pressure is obtained by the effect of applying heat and while it can be readily appreciated that we can take a liquid from below its critical temperature and obtain the transition to gas above its critical temperature, an understanding of the relevant 'start' and 'end' points and the cycle involved is required in evaluating the design and performance of CPD equipment. It is still useful however, to utilize these CO2 isothermals as indicated in figure 5 with the Superimposed ‘arrows' showing differing conditions for the CPD device.
Temp. C P.S.I 1072 562 495
3212 Starch Grains in Potato Tuber
Critical point dried fractured cell from the tuber of potato (Solanum tuberosum), demonstrating thin cell walls starch grains (amyloplasts).
Glandular Trichomes on Modified Leaf Surface of Butterwort
An insectivorous plant, the butterwort (Pinguicula vulgaris) has modified leaves which bear tiny granular trichomes which trap insects. The capitate head of the trichome then secretes protease enzymes to digest the insect parts.
It is already acknowledged that these circumstances are not exactly comparable. For example, during operation of the CPD we would fill at CO2 cylinder pressure and at ambient temperature: not at saturated vapor pressure. At a lower temperature decompression is as a result of venting and the subsequent reduction in mass of gas, not reduction in externally applied pressure. The relative volume is determined by the initial level of liquid in relationship to the total free volume available (this being the chamber plus sample “boat” etc.)
If we consider 'X' with the liquid CO2 more than half filling the total available volume and we heat from 10°C to 35°C then we will make the transition from liquid to gas. The pressure rise will be rapid as the liquid will expand and the level will increase before the critical temperature is reached. This is termed 'going around' the critical point. Usually (as in the case of instruments supplied by Electron Microscopy Sciences) a pressure-bursting disc is employed to prevent excessive pressure increase.
For condition 'Y' with approximately a full pressure chamber, the liquid level will remain relatively constant, its density decreasing and that of the vapor increasing, and becoming the same when its critical temperature has been reached, together with the corresponding critical pressure.
Looking at condition 'Z' with the pressure chamber less than half full. The level will fall and vaporization will occur before the critical temperature is reached, also the specimens may be uncovered and subjected to unwanted evaporation.
Ideally, we wish have a situation where the liquid fills the specimen chamber, while still only accounting for approximately 50% of the total volume available. This will ensure that specimens are not uncovered during initial flushing stages and in addition this should enable critical constants of temperature, pressure, and density to be achieved relatively simultaneously without excessive pressure or evaporation conditions occurring.
It is also advisable to maintain a temperature somewhat above the critical temperature during decompression, this will avoid the possibility of gas recondensing. It is also important to control the decompression rate itself as there is evidence that rapid pressure equalization can cause specimen damage.
53
Barley Root Tip
Critical point dried tip of barley (Hordeum vulgare) root, demonstrating root cap cells (calyptra) and slightly deformed (compressed) root hairs (Pili).
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