OPTIMIZATION AND APPLICATION OF JET-FREEZING

Cryofixation is considered to be the best method for immobilizing biol ogical material in its natural state. In jet-freezing, the specimen ty pically is sandwiched between two carriers and kept in place while a c oolant is moved very rapidly against the opposite surfaces. The JFD 03 0 jet-freezing device has been used to optimize the operating paramete rs. The course of the temperature in place of a specimen was measured with thermocouples and recorded by an IBM-compatible personal computer using a specifically developed software program. Mean cooling rates, over the temperature range of 273K to 173K, achievable with different cryogens, including the non-flammable HCFC 124 (SUVA 124-CHC1FCF(3)), were measured under a variety of conditions. The freezing capability o f the JFD 030 was evaluated by analyzing transmission electron microsc opic results obtained from freeze-substituted plant cells and freeze-f ractured cosmetic emulsions. Jet-freezing, despite its limits in freez ing thick specimens, can be applied to cell suspensions as well as to semi-thin sections by the use of thin Ti supports, a higher pressure o f the nitrogen gas or by a brief pretreatment with a cryoprotectant (e .g., sucrose). In addition, with the non-flammable HCFC 124 in combina tion with the thin Ti supports, freezing rates similar to those achiev able with propane and standard copper supports can be reached giving r esearchers the chance to use jet-freezing without the dangerous propan e.