| Popis: |
Publisher Summary This chapter focuses on high-pressure and plunge-freeze fixation. In plunge freezing, liquid nitrogen at its boiling point (–196°C) is used to cool a secondary cryogenic liquid, such as propane. Specimens are frozen by plunging them rapidly and deeply into the secondary cryogen: the flow of cryogen over specimen surfaces ensures rapid dissipation of heat. The low boiling point of liquid nitrogen prevents it from being used directly on specimens in plunge freezing, because it boils on contact with the warm specimen. The resultant insulating layer of gas slows freezing sufficiently to allow large and disruptive ice crystals to form in cells. Freezing specimens under high pressure protects them from ice crystal damage by lowering the freezing point of water and reducing the rate of ice crystal nucleation and growth. The design and function of the HPM 010 has been described by Moor. Specimens are sandwiched between two small metal cups or plates, locked into a special holder, which is inserted into the pressure chamber in the HPM 010 and then frozen in a 0.5-s burst of liquid nitrogen pressurized to 2100 atm. In all rapid freezing methods, the quality of freeze fixation is affected by the water and solute content of the specimen. |
