Validation of convection-limited cooling for Electron Microscopy

Abstract

Rapid freezing is a universal method of sample fixation for electron microscopy. Freeze-fracture replication, low temperature embedment and cryo-sectioning, and cryo-microscopy all depend on freezing specimens rapidly enough so that structure can be preserved unaltered. The most common method of rapid freezing is plunging specimens directly into liquid cryogens. In an earlier study, convection from the cryogen to the specimen was predicted to be the limiting step in rapid freezing of small samples for electron microscopy when the Biot modulus, (h V /kA) which determines the ratio of convective to conductive heat transfer, is less than 1. For convection-limited cooling, cooling rates are proportional to the surface area/volume ratio (A/V), independent of the sample thermal conductivity (k), linearly dependent on the heat transfer coefficient between the liquid cryogen and the specimen (h), and inversely proportional to the product of sample density and heat capacity (pCp):Ts and Tc are the sample temperature and the cryogen temperature, respectively.