Photoacoustic Cells

During the past decade, PA cell architecture has transformed significantly from the basic concept of simple closed cell with a microphone to that of a highly evolved multichambered open cell capable of recording signals on live objects such as intact plants and human beings. Another factor that has improved significantly is the signal-to-noise ratio, where the cells were designed for taking measurements in the field. In the literature, one can see mention of a variety of photoacoustic cells.

1. Basic Photoacoustic Cell

The basic concept is that the PA cell must have a sensitive microphone, and it should be hermetically air-tight to generate an acoustic wave and to avoid external noise (Fig. 4). Usually the cell is fitted with a frontal quartz window for sending in the modulated excitation light onto the sample, and the volume of the cell should be small. Since it is air-tight, the gas composition of the cell can vary during the experiment. For example, a green leaf such as photosynthetically active green leaf, which takes up carbon dioxide from and releases oxygen into surrounding air, can alter the composition of the air in the cell. If one measures the photochemical activity of leaf, there may be limitation of the carbon dioxide to the photosynthetic phenomenon during the measurements. In order to avoid this

FIGURE 4 Components of a typical photoacoustic cell.

limitation, a gas-permeable cell was designed to enrich the 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 plant samples or animal or human skin samples). Though these measurements reflect the in vivo observations, realistically they may be called in situ observations. Hence, an open PA cell was designed that can be attached to the leaf, and is reported to measure the photosynthesis of a leaf still attached to the plant.

2. Photoacoustic Open Cell

Considering certain limitations with closed cells, investigators at different labs started working on developing open cells. Recently in our laboratory, we have designed a PA cell that can be attached directly to a live human being for measuring a drug or solar cream diffusion rates in the skin. The cell is designed to have identical cylindrical cavities fitted with microphones. A light pulse or beam is passed through a fiberoptic light guide. One microphone measures the background noise from the cardiac pulse, and the other detects the acoustic waves generated due to the excitation of compounds of interest inside the skin at the contact site. The cell can be strapped to any part of the body, and the signals can be recorded.

3. Cell with Optical Microphone

As mentioned above, the thermal deactivation-induced acoustic waves were detected by sensitive microphones in a majority of PA studies. In order to improve the sensitivity of the technique, a new detection system with optical 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 microphone to detect the signal, a laser beam positioned on a Mylar pellicle (10 ¡M 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 planar 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 system is easily adjustable to the needs of measurements in solutions.

FIGURE 5 Schematic presentation of optical microphone.

5. Layered Prism Cell

Layered prism cell was assembled using a pair of dovetail prisms and a pair of piezoelectric transducers. The prisms are clamped together and separated by a shim. A 1-cm hole in the shim forms the sample compartment, and has the geometry of a thin disk. The inlet and outlet holes through one of the prisms permit the flow of the solution through the sample compartment. Light passes through the first prism, into the sample compartment, and out through the second prism. This cell uses the transmission of light through the prisms, and it combines the enhanced time resolution capabilities of "layered" front face irradiation geometry with zero-background and broadband flexibility of classical cuvette geometry. As described, different labs have developed different types of cells depending on the needs of their work.

Relaxation Audio Sounds Relaxation

Relaxation Audio Sounds Relaxation

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds called Relaxation.

Get My Free MP3 Audio

Post a comment