Labconco FreeZone Bulk Tray Dryer 78060 Series - Manual (Page 6)

Chapter 1: Introduction
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SAFE TEMPERATURE AND DRYING TIMES FOR SELECTED MATERIALS
Material 10mm Thick Safe Temp °C Collector Temp °C Hours (Approx.)
Milk -5 -40 10
Urea -7 -40 10
Blood Plasma -10 to -25 -40 16
Serum -25 -40 18
Vaccinia -30 to -40 -50 22
Influenza Vaccine -30 -50 24
Human Tissue -30 to -40 -50 48
Vegetable Tissue -50 -80 60
*Total sample quantities are contingent on various freeze dryer capacities.
Up to the point of overloading the system, the greater the surface area of the
sample, the faster the rate of freeze drying. By contrast, for a given surface area,
the thicker the sample the slower the rate of freeze drying. This is based on the
fact that the heat of sublimation is 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.
The surface area and thickness of the sample can usually be ignored when each
sample contains only a few milliliters. However, for larger volumes, the samples
should be shell frozen to maximize the surface area and minimize the thickness of
the sample. The volume of the freeze dry flask should be two to three times the
volume of the sample.
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.