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AQUALAB VSA
humidifiers and determining equilibrium moisture content at each water activity level.
Equilibrium is assumed when the weight of the sample stops changing. This process is often
accomplished using sealed chambers such as desiccators and the equilibration process
can take weeks. There are several additional challenges to executing this manual method.
Tracking the weight of the samples can be difficult. Also when removing the samples for
weighing, you potentially expose them to ambient humidity, and at high humidities there is
the possibility of mold growth. The METER VSA Isotherm Generator uses the DVS method
described above to speed up and automate the construction of equilibrium isotherms,
eliminating the challenges of manual determination. In additional to equilibrium moisture
contents at a given water activity, equilibrium isotherms provide information about kinetics
of sorption and water vapor diffusion properties.
Moisture sorption isotherms can also be determined using dynamic methods such as
the DDI method used by the VSA. The DDI method directly measures water activity while
gravimetrically tracking weight, so there is no dependence on equilibration to known water
activity levels to determine water activity. Adsorption occurs as saturated wet air passes
over the sample. Desorption happens as desiccated air passes over the sample. After a
short period of time, the VSA halts airflow and takes a snapshot of the sorption process by
directly measuring the water activity and weight. The advantages of this method include:
increased analysis speed as the sample does not have to wait for equilibration to a known
water activity and an unmatched level of resolution. This makes it possible for dynamic
isotherms to produce high resolution isotherms in a matter of days instead of the weeks it
would take to make a comparable isotherm using equilibrium techniques. The high resolution
of dynamic isotherms makes them valuable for observing sudden changes in sorption
properties associated with matrix changes such as glass transition.
The dynamic nature of the DDI method means that moisture contents may or may not be at
equilibrium and it is possible for there to be differences between dynamic and equilibrium
isotherms. For samples with fast vapor diffusion, penetration by water vapor into the
whole sample is rapid and isotherms using the DDI method for these types of products
are comparable to equilibrium methods. However, for samples with slow diffusion rates,
moisture movement through the sample is slow and complete diffusion of moisture into
and out of the sample may be slow enough to give the appearance of vapor equilibrium in
the headspace during water activity analysis. In reality, the moisture has not had time to be
completely absorbed by the sample. Isotherms for these types of samples developed using
the DDI method may have lower moisture contents during adsorption and higher moisture
contents during desorption than equilibrium isotherms, resulting in higher levels of apparent
hysteresis.
3.1 COMBINING DVS AND DDI IN ONE INSTRUMENT
A unique and important feature of the AQUALAB VSA is that it can generate both dynamic
and equilibrium isotherms. As explained above, both DVS and DDI methods have advantages
and disadvantages. While the data they generate agrees in some cases, it is the uniqueness
of the results from each method that gives them value. There is information that can only
be obtained from dynamic isotherms such as critical water activities for glass transition.
Similarly, there is information that can only be obtained by equilibrium isotherms such
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