Archive image from page 33 of Cytochemistry a critical approach (1953). Cytochemistry: a critical approach cytochemistrycri00dani Year: 1953 20 FIXATION PROCEDURES such as isopentane, rather than plunge the specimen directly into liquid air or liquid nitrogen, since, if the specimen is di- rectly in contact with liquid air or nitrogen, gas bubbles form -Vacuum flask â Bath at - 78°C Water-trapping surface Speciman at controlled temperature P,0, Fig. 1. Illustrating principle of new-t}pe freeze-dryer. Former freeze- dryers pumped the water off into a P2O5 trap. In the present type the wat
Archive image from page 33 of Cytochemistry a critical approach (1953). Cytochemistry: a critical approach cytochemistrycri00dani Year: 1953 20 FIXATION PROCEDURES such as isopentane, rather than plunge the specimen directly into liquid air or liquid nitrogen, since, if the specimen is di- rectly in contact with liquid air or nitrogen, gas bubbles form -Vacuum flask â Bath at - 78°C Water-trapping surface Speciman at controlled temperature P,0, Fig. 1. Illustrating principle of new-t}pe freeze-dryer. Former freeze- dryers pumped the water off into a P2O5 trap. In the present type the water is trapped on the walls of the evacuated tube, which are maintained at â78° C. until the specimen is diy. Then on warming the tube the water is returned by the P2O5 at the tube base. During the drying process the specimen is warmed by a thermostatically controlled heater to a de- fined temperature, usually â40° C. When the vacuum is maintained by a two-stage rotary pump, drying requires about 2 days. If a diffusion pump is used, backed by a rotary pump, the drying time can be reduced to 6-10 hours for a specimen not more than 1 mm. thick. But there is usually no advantage in reducing the drying time to this extent. around the specimen, due to vaporization of the liquid. As a consequence, the specimen is insulated from the liquid air and cools very much more slowly than when good thermal contact is maintained between the cooling agent and the specimen. Isopentane freezes at approximately â160°. When an optimal
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