Biomedical Engineering Reference
In-Depth Information
used [20]; however, in our experience at HidenIsochema, if
hydrogen of sufficient purity is connected directly to an instrument
and steps are taken to avoid regulator contamination during bottle
changes, reversible hydrogen adsorption isotherms apparently
unaffected by impurity contamination can be obtained without
the use of hydrogen gas filtration [37]. However, utmost care must
be taken and the repeatability and reproducibility of adsorption
and desorption isotherms for relatively well-understood materials
should be thoroughly tested to confirm that a sufficiently pure
hydrogen supply configuration has been achieved. Impurity levels in
other gases should also be considered, however, because impurities
can potentially block the adsorption of any adsorptive species. This
is particularly relevant in the case of microporous solids because
the strong adsorption potentials of the small pores are more likely
to attract impurities. A liquid nitrogen trap is a good method for
removing potential contaminants from a gas supply if problems are
suspected, but molecular sieves and active filtration systems can also
be used. Zeolite traps can help ensure that the supply is relatively
dry if moisture contamination is suspected.
1..
Sample Degassing
As mentioned in Section 1.3.1, samples must be degassed under
vacuum at an elevated temperature prior to an adsorption
measurement. If the surface of an adsorbent is not sufficiently
clean, its sorption behavior will be affected by the presence of pre-
adsorbed species. The sorption capacity will be reduced by pre-
adsorbed species that do not interact with the adsorptive or, during
higher temperature measurements, the pre-adsorbed species may
interact with the adsorptive. In both cases, the measurement will be
affected and the accuracy of the measurement compromised. Care
must also be taken not to damage the material by subjecting it to too
high a temperature, although carbon nanomaterials typically exhibit
high thermal stability and so may not be affected as significantly
as some other adsorbents in this respect. As discussed in Section
1.3.1, the degassing process can be monitored directly using the
microbalance reading in gravimetric apparatus and indirectly in
volumetric apparatus by observing the vacuum level above the pump
system. Any doubts should be investigated by determining the effect
of varying the degassing period or conditions on the measured gas
sorption behavior.
