Labconco FreeZone Benchtop Freezer - Manual (Page 7)

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Chapter 1: Introduction
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.
Sample Surface Area and
Thickness of the Sample
The volume and configuration of the suspension to be freeze dried
often determines how the material will be freeze dried. For
example, the greater the ratio of the surface area to the volume of
the suspension, the faster drying will occur. This is because a
greater area for the water molecules to leave the product exists
compared to the distance they have to travel to reach the surface of
the frozen matrix. Drying occurs from the top of the product and
initially the removal of water molecules is efficient. However, as
the drying front moves down through the product, drying becomes
more and more difficult. The water molecules must now travel