Basics

Freeze Drying and Vacuum Concentration (RVC)

Vacuum concentration and freeze drying are related methods used for the gentle drying or preservation of thermally sensitive materials. This often involves the removal of aqueous media. As can be seen from the state diagram of water, which can also serve as an example for other solvents, above a certain process pressure (in this case 6.1 hPa for H2O) drying occurs from the liquid phase. At pressures below this level, the water temperature drops below 0 °C.

Drying occurs directly from the frozen state, bypassing the liquid phase. The triple point (in this case p = 6.1 hPa and T = 0 °C for H2O) can be regarded as a sort of boundary between vacuum concentration (or drying) and freeze drying. Ice, water and vapour can be observed simultaneously in the chamber. The key features of the two methods are listed in the following table.

CriterionVacuum concentrationFreeze drying
Evaporation temperature range (gentle process?)-5 °C to + 20 °C
++
-60 °C to 0 °C
+++
SubstancesUsually dissolved;
residue is a powder or a crystalline substance
Solids
(including ceramics, meat, archeological objects,
bones, plants and books)
SolventsVarious solvent types (some exotic)Aqueous solutions,
only small quantities or
special solvents
Water/solvent contentApprox. 5% final contentLow residual moisture possible (< 1%)
Process timeMinutes to hours0.5 – 3 days;
several weeks in some cases

Christ systems also allow the two methods to be combined in a Rotational Vacuum Concentrator (RVC). Starting at a relatively high pressure (e.g. 20 hPa) and then reducing it to a lower pressure (e.g. 1 hPa) concentrates an aqueous DNA solution to a small residual volume, which then freezes in the device at the lower pressure. Final drying occurs by sublimation.

Product concentration by sedimentation at the bottom of the test tube is a desirable side effect.
This means that Rotational Vacuum Concentration (RVC) not only protects the specimen against superheating but also prevents diffuse product distribution in the test tube, which can lead to low recovery rates.