The relative efficiency of cryogenic fluids used in the rapid quench cooling of biological samples

Abstract

SUMMARY The heat transfer characteristics of various cryogenic fluids used in the rapid quench cooling of biological samples are examined. It is concluded that cryofixation should be achieved during the initial plunge stage of the cooling. Liquid nitrogen maintained near its melting point of 63·1 K at a pressure in excess of the critical value of 33·5 atm will produce the quickest cooling. Nitrogen could also provide the best ultrastructure at atmospheric pressure if the minimum plunge depth and velocity criteria are satisfied. Plunging bare thermocouples into cryogenic fluids will lead to erroneous conclusions about the relative cooling efficiencies of various liquids for cryofixation. A qualitative explanation for the results obtained by Silvester et al. (1982) during the rapid cooling of water drops in various fluids is proposed.