Labconco FreeZone Bulk Tray Dryer 78060 Series - Manual (Page 7)

Chapter 1: Introduction
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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. If the eutectic temperature of the sample is –40 to –60°C, the freeze
dry system selected for use must be equipped with cascade type refrigeration so
that the collector temperature can be cooled to below –75°C, or a dry ice/solvent
trap may be used between the collector and the vacuum pump.
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.
In addition to these substances having an effect on the eutectic temperature, they
may increase the vapor pressure at the surface of the sample. Also, compared to
water, they will require the absorption of less heat for sublimation to occur.
Hence, samples that contain volatile substances will have a greater tendency to
melt, particularly when placed in flasks or exposed to room temperature. 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.
For example, a 0.2M solution of acetic acid is much easier to freeze dry than a
0.5M solution.