Biomedical Engineering Reference
In-Depth Information
also important. The measurement of sample sizes lower than 50 mg,
for example, on a typical laboratory balance may contain a significant
error. In gravimetric systems, this is less of an issue for small samples
because the instrument typically incorporates a highly sensitive and
accurate microbalance that can measure accurate sample weights
of less than a milligram. An appropriate sample size choice in each
technique is also highly dependent on the adsorption capabilities of
the material in question. The sample size of a high-capacity material
can be significantly smaller than that of a low-capacity material to
achieve equivalent accuracy. Consideration of the number of moles
of adsorbate that is likely to be adsorbed through the course of a
measurement is crucial. The molar quantity is calculated directly in
volumetric measurement and can be readily converted to a weight
for consideration in the context of the anticipated gravimetric
capacity of a material. One that adsorbs only 0.6 wt.% hydrogen, for
example, will need to be measured with a sample size an order of
magnitude greater than a material that adsorbs 6.0 wt.%, in order to
achieve equivalent accuracy, assuming that the other measurement
parameters have equivalent uncertainties and that the sample
size does not significantly affect the error in the dead volume, the
buoyancy effect corrections, and so forth. This may seem an obvious
point but one result of this dependency is that specific statements
regarding required sample sizes for each technique cannot be
made.
A further consideration in gravimetric measurement is the
buoyancy contribution from each of the other components in the
microbalance chamber, including the sample pan, hangdowns, and
the counterweight. If these are significant, then the sample should not
be small enough for the uptake due to adsorption to be overwhelmed
by the buoyancy correction. Therefore, for small sample sizes it is
particularly desirable to minimize the buoyancy contributions of
these additional components by using high-density materials, and
minimizing their volumes. The buoyancy effect corrections are also
dependent on the molar mass of the adsorptive and are therefore
greater for heavier species.
1..
Sample Purity
An important consideration when characterizing the adsorption
properties of a material is the purity of the sample. This has
