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I. INTRODUCTION - THE OPACITY PROBLEM AND ITS SOLUTIONS
The main sample handling problem in FTIR
analysis of solid and semi-solid materials is that nearly all
materials are too opaque in their normal forms for direct transmission
analysis in the mid-infrared spectral region. Traditionally,
the opacity problem has been remedied by reducing the optical
density of samples to a suitable level by various methods of
sample preparation.1,2,3 This
approach, however, leaves much to be desired due to the time
and labor involved, the risk of sample alteration and preparation
error, and the destructive nature of the process. Consequently,
various other approaches have been tried to avoid or to minimize
sample preparation.
One such approach is to simply avoid the opacity
of the mid-infrared spectral region by using overtone and combination
absorbance bands for analysis in the near-infrared spectral region
where absorption coefficients are significantly lower and samples
are less opaque.4Near-infrared
spectra combined with factor analysis software are successful
in many analytical applications. Unfortunately, less detailed
information is extractable from near-infrared spectra due to
the broad, overlapping character of near-infrared absorbance
bands. Consequently, the mid-infrared, especially the fingerprint
region, remains as the most information-rich spectral region
for analytical purposes.
Another approach is to select one of the alternative
mid-infrared sampling techniques to transmission such as specular
reflectance, diffuse reflectance (DRIFTS), or attenuated total
internal reflectance (ATR).1,2,3All
of these are, however, limited in their applicability. Specular
reflectance requires mirror-like surfaces. DRIFTS often involves
particle size reduction and dilution in KBr. ATR presents problems
related to the reflection element in terms of clean-up, crystal
damage, and reproducible optical contact.
The most broadly-applicable mid-infrared solution
to the opacity problem is photoacoustic spectroscopy (PAS).5Its signal generation process automatically
and reproducibly isolates a layer extending beneath the sample's
surface which has suitable optical density for analysis without
physically altering the sample. PAS directly measures the absorbance
spectrum of the layer without having to infer the desired absorbance
spectrum based on a reflection or transmission measurement. This
chapter will discuss PAS signal generation, instrumentation,
sample handling and results obtained with various classes of
samples.
II. PHOTOACOUSTIC SIGNAL GENERATION
The photoacoustic signal is generated when infrared
radiation absorbed by the sample converts into heat within the
sample. This heat diffuses to the sample surface and into an
adjacent gas atmosphere. The thermal expansion of this gas produces
the photoacoustic signal.
A. Signal Generation Model
The photoacoustic signal generation sequence
is shown schematically in Figs. 1 and 2. Fig. 1 shows the infrared
beam intensity incident on the sample with intensity  . The |
