Biomedical Engineering Reference
In-Depth Information
complete when the weight of the sample is no longer decreasing
appreciably. In volumetric instrumentation the process cannot be
observed in the same way, but the quality of the vacuum measured
above the pump system provides a convenient method to determine
whether any pre-adsorbed contaminants are still evolving from the
sample. The International Union of Pure and Applied Chemistry
(IUPAC) guidelines on the measurement of gas adsorption for
surface area and porosity determination [2] advise that a pressure
of ≈ 10 mPa (10
-4
mbar) is “satisfactory.” This seems a reasonable
minimum threshold, bearing in mind that the pressure at the sample
in sorption apparatus will not be the same as the pressure directly
above the vacuum pump. In practice, this means that the use of a UHV-
compatible unit, such as a turbomolecular pump, is a prerequisite. It is
important that contamination levels in a system used for adsorption
measurement are minimized, and it is therefore advisable to use a
dry backing pump, such as a membrane (or diaphragm) pump. This
will avoid the possibility of the back-streaming of oil vapor, which
can occur when using an oil rotary pump.
1..
Thermal Equilibration
Once the sample has been degassed sufficiently, the sample must
be brought to the measurement temperature. In practice, this is
difficult due to the lack of heat transfer when the system is held at
vacuum. However, this is dependent on the choice of measurement
temperature. At low temperatures, which are typically required to
achieve significant hydrogen adsorption capacities for carbonaceous
materials, it is often necessary to provide an initial low pressure dose
of hydrogen before thermal equilibrium can be achieved. This is less
of an issue for gas sorption measurements performed at higher, near
ambient, temperatures.
1..
Gas Dosing
During the determination of an adsorption isotherm, gas is dosed
to the sample at increasing pressures. In volumetric measurement,
the pressure is monitored in a closed system at each step. In
gravimetric measurement, the weight is measured either in a closed
system or, in the isobaric case, while the gas pressure is maintained
