Eutectics: a neglected area in clinical chemistry

Abstract

When serum is cooled in the deep freeze, ice crystals of pure water form progressively. The progressive removal of water leads to increasing concentrations of salts and proteins in the residual unfrozen solution. This continues until the eutectic point for serum is reached (−23° C) at which point the remaining water, salts and proteins solidify together as an eutectic mixture. Thus at −20° C small lakes of a liquid concentrate of proteins in hypertonic salt solution exist inside a solid matrix of water ice. Under such circumstances denaturation of proteins may occur especially as the pH may fall to as low as 3. Accordingly −20° C may be about the worst temperature at which to store serum, at least for some substances. It may therefore be advisable to use −70° C refrigerators for storage of sera for susceptible substances such as clotting factor standards and controls.