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Chapter 1: Introduction
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 a 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 vacuum in the system may recover
before adding another sample to the system. If the
vacuum does not recover to less than 133 x 10
-3
mBar,
the capacity of the freeze dry system has been exceeded
and the sample should be removed.
If there is a problem with a particular type of sample
melting when placed on the freeze dry system, dilution
of the sample with more water or providing some
insulation around the flask to decrease the rate of heat
absorption by the sample may help. If the eutectic
temperature of the sample is –40 to –60°C, the freeze
dry system selected for use must be equipped with
