Labconco FreeZone Triad Freeze Dry System 74000 Series - Manual (Page 7)

Chapter 1: Introduction
Product Service 816-333-8811 or 1-800-522-7658 3
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
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 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.
Samples Containing Volatile Substances
In certain cases the solvent in a sample to be freeze dried may contain
volatile components such as acetonitrile, methanol, acetic acid, formic acid or
pyridine.