|
|
|
|
PAS
sampling is often used without a second thought because it is simple to quickly
obtain a spectrum of nearly any type of sample.
For many qualitative analyses of homogeneous samples this is appropriate,
but for more complicated samples, there are a number of considerations that are
important to be aware of.
Samples
that are inhomogeneous should always be evaluated to define a suitable protocol,
which assures that the data is representative of the whole sample or of the
region of interest. For whole
sample analysis, a fine or coarse powdering of the sample may be necessary
followed by splitting out and analyzing triplicate samples to check
reproducibility. In other cases, it
may be necessary to average the spectra of a number of randomly selected samples
three times and then check for repeatability until the number of samples is
large enough so that their averaged spectra produces a repeatable result within
the limits required.
When a
small region of the sample is to be analyzed, aluminum foil can be used to mask
the area of the sample that is not to be analyzed, or the region of interest can
be excised for analysis using a scalpel or diamond file.
Extraction is also a useful approach with PAS analysis if soluble
components need to be separated from carbon black, which is useful in tire
analyses. The solvent containing
the dissolved sample can be decanted off after the carbon-black settles out.
The decanted solution is then placed in a disposable thermal analysis cup
and the solvent is evaporated off leaving a residue of the components of
interest for analysis without the carbon-black background.
PAS
analysis of liquid samples primarily involves situations where the sample is
unsuitable for transmission measurements due to high opacity, high viscosity,
suspended components, risk of cross contamination, and/or the desire to avoid
the clean-up requirements of sample cells.
PAS sampling of liquids usually uses disposable cups, which eliminates
cross contamination and clean-up problems.
Special care is needed with PAS analysis of liquids if the liquid has
suspended components or is volatile. Suspended
components often settle with time as, for example, in the case of soot in diesel
oil. Consequently, if these
components are being analyzed, it is important to be sure that they are
homogeneously distributed when the sample is placed in the cup and that they
remain so during the analysis. FT-IR
PAS spectra of liquids should be checked for a vapor phase component by putting
a metal disk over the sample. This
prevents the IR beam from reaching the sample but allows the vapor above the
sample to absorb the beam and produce a vapor spectrum.
This vapor spectrum can be subtracted from the spectrum measured with the
disk removed to produce a spectrum purely of the liquid.
The
use of a disk over the sample is also useful with solids if the analysis of
evolved vapors is required. In this
case, higher sensitivity will be realized with a larger amount of sample and a
longer waiting time for the vapor concentration to increase.
In other instances, it is of interest to prevent the evolution of vapor
from the sample while measuring the spectrum, such as with hydrated tissue
specimens. In this case, it is
possible to seal the sample in a vapor-tight cup with a 10-m