Page 396 - 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

Photoacoustic Spectroscopy 3

Another example is photothermal radiometry, where the
modulated heat is monitored by the corresponding mod-
ulation in the amount of infrared radiation emitted from
the sample, sensed by an infrared detector.
The PA effect was discovered in solid samples at the
end of the nineteenth century. This was first reported over
one hundred years ago by Alexander Graham Bell in his
attempt to produce a light-activated telephone. In Bell’s
original observation, chopped sunlight was focused onto a
selenium photocell, and Bell’s main interest was to detect
the electrical changes in the selenium cell. Incidentally
he found that sound at this chopping frequency can heard
by a nearby observer. The modern photoacoustic experi-
ments show striking similarities to this effect, and this ef-
fect has proven to be an important and sensitive technique
for studying the interaction between matter and light. FIGURE 2 Photoacoustic signal generation depends on the ther-
During the past two decades there has been revival of mal properties of the backing material, sample, and the gas inside
interest in photoacoustic phenomena, and intense efforts the cell.
have been made to revive and develop suitable instrumen-
tation. With the advent of modern electronics, a laboratory (3) generation of acoustic pressure oscillations due to the
instrument suitable for investigating the spectral proper- thermal expansion.
ties of optically thick samples was designed during 1970s. If a sample placed in a hermetically closed cell absorbs
Soon a theoretical treatment for understanding the basic the modulated light of wavelength λ, the electrons will be
principles of the PA effect was given in many reviews. excited to higher states. After a brief time, the electrons
In principle, the incident light beam transforms the sam- decay back to the ground state by radiative or nonradia-
ple into an excited state. The thermal deactivation process tive processes. The PA effect is solely due to nonradiative
from the sample induces the changes in the temperature decay processes. A periodic heat distribution is created in
of the thin layer of the coupling medium surrounding the the sample at the point of absorption. This thermal dis-
sample. The coupling medium could be atmospheric air tribution then diffuses to the sample surface and gives up
or any desired gas. These heat-induced changes in the its thermal energy to the surrounding air (Fig. 2). A thin
coupling medium can be detected by various means. The boundary layer of air responds to this heat, and it period-
signal origin and propagation in different kinds of sam- ically expands and contracts as it is heated and cooled at
ples such as solids, liquids, and gases are better explained the modulation frequency. This thin layer of air acts as an
in several review papers. In biological samples such as acoustic piston on the remaining column of air, creating
leaves and human skin, the signal generation is a com- pressure fluctuations which are detected by a microphone,
plex phenomenon. The excited chromophore molecules and the resulting electronic signal is processed by a lock-in
of the cell deactivate thermally inside the cell, and the amplifier as the PA signal. Depending on the nature of the
resulting heat wave propagates through the cytoplasm to sample and experimental conditions, the acoustic signal
the cell surface and contributes to the signal generation. generation may be within the sample or on the surface of
The medium at the cell surface could be intercellular air the sample.
in the case leaves, or an aqueous medium in the case of
animal or human tissues. This technique has been applied
to different fields of research. This review mainly focuses III. ADVANTAGES
on the instrumentation and its potential applications.
The PA process has several important unique features
which make PAS advantageous over conventional optical
II. PHOTOACOUSTIC EFFECT spectroscopy.
AND SIGNAL GENERATION
1. The PA signal is insensitive to the light-scattering
In general terms, the PA effect can be considered as a effects of the sample. Unlike conventional optical spec-
three-step sequence involving (1) intermittent absorption troscopy, where scattered light gives rise to unwanted
of light in a sample from a periodically modulated beam, and misleading signals, in PAS only modulated absorbed
(2) heat production by the absorbed energy, and finally light leads to a PA signal. Besides this, as the response is