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usually absorbed on one side of the frozen sample and must travel through the frozen
layer to vaporize water at the other surface. In addition, as the sample is freeze dried,
the water vapor must travel through the layer of dried material. The thicker the sample,
the greater the chance that the dried layer may collapse which would cause an
additional decrease in the rate of freeze drying.
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.
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.
Freeze Dry Capacity
The volume of 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. 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 tray at a
time so that the vacuum in the system may recover before adding another sample to the
system. If the vacuum does not recover, the capacity of the freeze dry system has been
exceeded and the sample tray should be removed.
Samples Containing Volatile Substances
In certain cases, the solvent in a sample to be freeze dried may contain volatile
components such as methanol. In addition to this substance having an effect on the
eutectic temperature, it may increase the vapor pressure at the surface of the sample.
Also, compared to water, it will require the absorption of less heat for sublimation to
occur. Hence, samples that contain volatile substances will have a greater tendency to
melt. If a sample containing a volatile substance tends to melt when placed on a freeze
dry system, dilution of the sample with more water will help keep the sample frozen.
