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photoacoustic detectors, has been a major area of
success for the PAS technique. These
spectral regions are rich in chemical information, and modern search and
chemometric software allow qualitative and quantitative results to be readily
obtained from PAS spectra in the presence of the more modest saturation effects
found in these spectral regions. At
this time, FT-IR PAS is a broad field of research that continues to develop in
the areas of instrumentation, applications, and data analysis.
This chapter will be restricted to primarily FT-IR PAS of solid samples
and to a much lesser degree of liquids.
The article Transient Infrared Spectroscopy for on-line
analyses
may also be of interest to readers because transient infrared spectroscopy
(TIRS) is also a
thermal-wave based technique that has similar capabilities to PAS but operates
on moving samples.
Photoacoustic
Signal Generation, Processing and Interpretation
The
photoacoustic signal contains information on the sample’s absorption spectrum
and on the depth below the sample’s surface from which the signal evolves,
allowing materials with layered or gradient compositions to be studied. Photoacoustic signal generation can be modeled5,6
using the heat equation7 and assuming a one-dimensional heat flow
within the sample and adjacent gas atmosphere that is in the direction opposite
to that of the light beam. The most instructive model for general purposes
also assumes an optically and thermally homogeneous slab sample geometry which
is thick on the scale of the thermal-wave decay length with the rear sample face
thermally grounded and optically nonreflective. The model is shown
schematically in Figure 1.
