The freeze drying method

Freeze drying specimen preparation reduces the distortion and shrinkage effects that occur when a wet specimen dries by normal evaporation. Distortion is due to the forces of surface tension that occur when going from a liquid to a vapour phase, such as from water to water vapour - the normal situation with biological specimens.

The freeze drying method overcomes this problem by careful sublimation of frozen specimens under vacuum - a process that avoids the liquid phase and thereby reduces distortion effects. The rate of sublimation is a function of temperature and vacuum, with typical drying times being several hours or longer.

Achieving results

Ideally, freeze drying should be carried out at temperatures below the recrystallisation point of ice, but this would require very long drying times. In practice temperatures of -60°C (if back-up water cooling at 15°C is used – see E4860 option) have been found to give reasonable results under vacuums achievable with two-stage rotary vacuum pumps.

For certain applications, however, it is necessary to dry at temperatures below -80°C with lower sublimation rates for delicate specimens. This requires better vacuum than can be obtained using a rotary vacuum pump alone and the lower temperatures associated with liquid nitrogen. For such applications, the K775X is recommended.

Technique

Both the temperature and process time can be pre-selected and the drying cycle completed automatically. Provision is made at the end of the drying cycle to allow specimens to be warmed prior to embedding.

Disposable desiccant containers are positioned in the preparation chamber to enhance water vapour removal. Additionally, with a suitable container (eg a polystyrene pot), the vacuum chamber can be used to prepare ‘slushy’ liquid nitrogen for fast freezing specimens prior to freeze drying.