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 have been found to give reasonable results under vacuums achievable with two-stage rotary vacuum pumps. For these applications see K750X.

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

The K775X Freeze Dryer achieves low temperatures by using a liquid nitrogen cold stage - fed from an integral vacuum dewar which is capable of giving several hours holding time between ‘top ups’. For extra convenience and extended drying periods, an optional ‘auto top-up’ system - consisting of a level sensor and free-standing 30L or 60L pressurised dewar - will allow several days unattended operation.

Pre-frozen specimens are loaded onto the cooled stage of the drying chamber through the lid using a specimen transfer holder (two holders are provided - for TEM grids and for SEM stubs). The K775X has controls for time and temperature and at the end of the drying period the stage may be heated prior to specimen removal.

Ten-segment sequence

A microprocessor-controlled 10-segment sequence allows 10 time periods and 10 temperature settings to be programmed to achieve a range of drying protocols. Up to 10 different protocols can be stored for future use. The system also has facilities for purging with nitrogen gas.