EQUIPMENT & ACCESSORIES CATALOG EDITION IV CRITICAL POINT DRYERS


Techniques and Applications


Critical Point Drying Principles (continued) Therefore CO2 remains the most common medium for the CPD procedure and is termed the 'Transitional Fluid'. However, CO2 is not miscible with water and therefore water has to be replaced in the specimen with another fluid which is miscible with CO2, this is termed the 'Intermediate Fluid'.


Ideally it will be able to replace the water in the specimen, and also serve as the 'Dehydration Fluid'. This is not exclusively the case, and additional steps may be used for particular circumstances.


However, where it is being utilized for both processes, texts may refer to it under the different headings, dehydration and intermediate, depending at what stage it is being used in the specimen preparation schedule. Prior to any of these stages chemical fixation of the specimen must be carried out (normally using glutaraldehyde -osmium procedures).


NOTE


The whole discipline of specimen preparation (chemical or vapor fixation) prior to the transitional stage is only mentioned in its most basic terms, procedures vary according to the type and nature of the specimens. Further references should be obtained.


a) Intermediate Stage


As mentioned previously this involves dehydration and intermediate fluids, the following is a possible schedule.


(Wet Specimen) H2O · Acetone · CO2 · C.P.D. (Dry Specimen)


The specimen is usually processed through varying concentrations of dehydration fluid, culminating in complete replacement of the water with this intermediate fluid. Because it has a low surface tension the specimen is less likely to experience damage due to evaporation while transferring to the chamber, also being miscible with CO2 (the Transitional Fluid) ensures satisfactory conditions after flushing (purging) for the CPD process to commence.


(Wet Specimen) H2O · Acetone Note:


*50/60/70/80/90 typically 10 minutes each ** Flush Typically 3 times


The table (Figure 3) gives an indication of some intermediate fluids. (Water is 73 Dynes/cm.) · 30%* 100% · CO2** · CPD (Dry Specimen) Critical point drying stages


SUBSTANCE ETHANOL ACETONE


FREON (113)


Figure 3: Dehydration Intermediate Fluids for CPD SURFACE TENSION (DYNES/CM)


23 24 19


Having transferred the specimen to the chamber in the Intermediate Fluid, the chamber is flushed several times to replace it with the Transitional Fluid. The process from which the complete techniques derives its name CPD can now be initiated.


CO2 grades required for critical point drying


Generally speaking, the grade we recommend is ‘normal’ grade - that is the one most commonly offered by industrial gas suppliers.


In most parts of the world, ‘normal’ grade of CO2, from the suppliers, is specified as ‘N4.5’ or 99.995% minimum purity with a maximum level of 50ppm of impurities.


There are, however, two other grades available by special order from most gas suppliers. One is ‘N4.0’ or 99.99% (less pure than ‘normal’ grade). The other is ‘N5.5’ or 99.9995% minimum purity. The N5.5 purity is easier to find in those parts of the world where there is high level of activity in electronics, since these customers often demand gases with higher purities.


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We are not aware of anyone who has ever reported either superior results using N5.5 purity vs N4.5, or inferior results using N4.5 vs N5.5. However, we do want our customers to have the benefit of such detail about liquid carbon CO2 procurement in the event they should ever find reason to believe that their particular specimens might benefit from the higher purity product.


Remember, the requirement is for LIQUID carbon dioxide and NOT gaseous carbon dioxide. For this reason a cylinder with an internal ‘siphon’ must be specified. A siphon cylinder is normally denoted by a white stripe painted along its length. No pressure regular is needed.


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