Page 399 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN012C-562 July 26, 2001 15:30

6 Photoacoustic Spectroscopy

FIGURE 5 Schematic presentation of optical microphone.

5. Layered Prism Cell ration, depth-profiling capability, and high signal satu-
ration limit. It can be used for analyzing any kind of
Layered prism cell was assembled using a pair of dove-
sample in any state. Thermophysical properties such as
tail prisms and a pair of piezoelectric transducers. The
thermal diffusivity and conductivity of certain samples
prisms are clamped together and separated by a shim. A
such as specialty papers have been investigated using
1-cm hole in the shim forms the sample compartment, and
Fourier transform-infrared–PAS (FTIR–PAS). This tech-
has the geometry of a thin disk. The inlet and outlet holes
nique has been used to improve our understanding of the
through one of the prisms permit the flow of the solution
chemical origin of depth inhomogeneities.
through the sample compartment. Light passes through the
FTIR–PAS can also be used as a rapid and nondestruc-
first prism, into the sample compartment, and out through
tive technique for quantitative characterization of the sur-
the second prism. This cell uses the transmission of light
face degradation of aged polymers and other compounds.
through the prisms, and it combines the enhanced time
Using FTIR–PAS spectroscopy, oxidation profiles of the
resolution capabilities of “layered” front face irradiation
various photooxidized polyisoprene–polystyrene copoly-
geometry with zero-background and broadband flexibility
mers have been investigated.
of classical cuvette geometry. As described, different labs
have developed different types of cells depending on the
B. Biophysics of Proteins
needs of their work.
and Organic Molecules
V. APPLICATIONS The respiratory proteins hemoglobin and myoglobin have
been extensively investigated by PAS for their structures in
solution and in the crystalline state and for the mechanism
PAS proved to be a very useful and sensitive tool for pho-
of their interaction with binding substrates. In general it
tophysical and biophysical investigations in a number of
may be stated that PA calorimetry is a very important tool
fields. Since the early 1970s, several interesting observa-
to investigate the changes in the extent of ligand formation,
tions have been made on organic molecules, gases, solids,
enthalpy, and volume characterizing the specific interac-
isolated biomolecules, organelles, membranes, cell sus-
tions of chromoproteins, including hydration and solvent
pensions, microbial organisms, and intact leaves, among
interaction phenomena. Similarly, a series of proteins have
others. Various types of instruments have been used in col-
been examined for their light-induced nonthermal volume
lecting data on a variety of samples ranging from solids
changes by measuring heat release.
and suspensions to gels, liquids, and gases. Several in vivo
PAS has been of specific interest for investigating tryp-
studies have been made on plants, human objects, and
tophanbecauseofitsrelevanceasanintrinsicopticalprobe
microorganisms.
for proteins. Using PAS, intramolecular energy transfer
processes have been investigated in crystal complexes of
A. Chemical Analysis
tryptophan with Sm(III), Tb(III), and Dy(III).
PAS applications in chemical analysis have been widely PA calorimetry has been successfully employed to
reported, because of its unique features such as nonde- understand the reactive intermediates of vitamin B 12
structive, noncontact measurement, simple sample prepa- compounds.Itprovedtobeadirectmethodfortheaccurate