Labconco FreeZone Benchtop Freezer - Manual (Page 6)

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Chapter 1: Introduction
In order for lyophilization to occur, ice must be removed from the
frozen sample via sublimation. This is accomplished by the
collector and the vacuum pump. The collector, which should be at
least 15 to 20°C colder than the eutectic temperature (melting
temperature) of the sample, traps vapor as ice. Since the vapor
pressure at the collector is lower than that of the sample, the flow
of water vapor is from the sample to the collector. Since this vapor
diffusion process occurs very slowly under normal atmospheric
conditions, a good vacuum is essential to maintain an efficient rate.
In many applications, the maintenance of a vacuum of 0.133 mBar
or less is recommended.
The rate of freeze drying is directly proportional to the vapor
pressure and the vapor pressure is dependent upon both eutectic
temperature and solute concentration of the sample. For example,
a solution of sodium chloride and water would freeze dry at a
slower rate than pure water. The eutectic temperature of a sodium
chloride solution is about –21°C and at this temperature the vapor
pressure is about 1/16 that of water at 0°C. Although the eutectic
temperature is not dependent upon the concentration of sodium
chloride, the vapor pressure of the water would decrease as the
concentration of sodium chloride increased. This is due to the fact
that as the solute concentration increases, less of the surface area of
the frozen sample is occupied by water. In general, most solutions
or biological samples will have a eutectic temperature of –10° to
–25°C. However, if the sample contains a simple sugar such as
glucose or if the sample is animal or plant tissue, the eutectic
temperature may be as low as –30° to –50°C.
Freeze Dry Capacity
The volume of a sample that can be freeze dried at one time is
related to factors discussed previously and the size and design of
the freeze dry system. With any given instrument, the capacity is
based on the surface area of the sample, the eutectic temperature
and concentration of the sample and the rate and amount of heat
transferred to the frozen sample. Of these factors, the eutectic
temperature is the most important factor in determining the amount
of sample that can be freeze dried at one time, particularly when
flasks are used. This is because as the eutectic temperature
decreases, the vapor pressure decreases but the rate of heat
absorption by the sample does not change. This tends to promote
melting of the sample, which leads to a marked increase in vapor
pressure and ultimately overloads the collector and vacuum pump.
Samples that have eutectic temperatures of –20°C or lower should
be placed on the freeze dry system one flask at a time so that the