Chapter 1: Introduction
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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.
Safety Symbols
Your freeze dry system was designed with safety in mind, however
conditions may exist that could be hazardous.
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