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

Photoacoustic Spectroscopy 5

are optically excited by groups of pulses, and data are limitation, a gas-permeable cell was designed to enrich the
acquired by a computer. carbon dioxide levels inside the closed cell. In addition,
from the time the PA method was initially used, in vivo
measurements were made on detached cut samples (either
C. Photoacoustic Cells
plant samples or animal or human skin samples). Though
During the past decade, PA cell architecture has trans- these measurements reflect the in vivo observations, real-
formed significantly from the basic concept of simple istically they may be called in situ observations. Hence,
closed cell with a microphone to that of a highly evolved an open PA cell was designed that can be attached to the
multichambered open cell capable of recording signals on leaf, and is reported to measure the photosynthesis of a
live objects such as intact plants and human beings. An- leaf still attached to the plant.
other factor that has improved significantly is the signal-
to-noise ratio, where the cells were designed for taking
2. Photoacoustic Open Cell
measurements in the field. In the literature, one can see
mention of a variety of photoacoustic cells. Considering certain limitations with closed cells, inves-
tigators at different labs started working on developing
open cells. Recently in our laboratory, we have designed
1. Basic Photoacoustic Cell
a PA cell that can be attached directly to a live human be-
The basic concept is that the PA cell must have a sensi- ing for measuring a drug or solar cream diffusion rates in
tive microphone, and it should be hermetically air-tight the skin. The cell is designed to have identical cylindrical
to generate an acoustic wave and to avoid external noise cavities fitted with microphones. A light pulse or beam is
(Fig. 4). Usually the cell is fitted with a frontal quartz passed through a fiberoptic light guide. One microphone
window for sending in the modulated excitation light onto measures the background noise from the cardiac pulse,
the sample, and the volume of the cell should be small. and the other detects the acoustic waves generated due to
Since it is air-tight, the gas composition of the cell can vary the excitation of compounds of interest inside the skin at
during the experiment. For example, a green leaf such as the contact site. The cell can be strapped to any part of the
photosynthetically active green leaf, which takes up car- body, and the signals can be recorded.
bon dioxide from and releases oxygen into surrounding
air, can alter the composition of the air in the cell. If one
3. Cell with Optical Microphone
measures the photochemical activity of leaf, there may be
limitation of the carbon dioxide to the photosynthetic phe- As mentioned above, the thermal deactivation-induced
nomenon during the measurements. In order to avoid this acoustic waves were detected by sensitive microphones
in a majority of PA studies. In order to improve the sen-
sitivity of the technique, a new detection system with op-
tical microphone was used to detect the acoustic wave
generated from the sample. In this system, the acoustic
wave-induced change in the position of a laser beam on
a pellicle is used to detect the signal. Instead of a micro-
phone to detect the signal, a laser beam positioned on a
Mylar pellicle (10 µM thickness) monitors the acoustic
waves. The vibrations of the pellicle caused by acoustic
waves deflect the laser beam, which is in turn is detected
by a silicon photodiode (Fig. 5).

4. Cuvette Cell

A standard cuvette with 1-cm path length is used in this
type of detection system. Samples are excited with a pla-
nar beam of light and the resulting pressure wave due to
solvent expansion through the solution is converted by a
transducer into a voltage pulse. As the measurements are
made using a cuvette with an attached transducer, the sys-
tem is easily adjustable to the needs of measurements in
FIGURE 4 Components of a typical photoacoustic cell. solutions.