Photoacoustic Spectroscopy: An introduction and applications

Photoacoustic (PAS) or optoacoustic spectroscopyis a unique form of spectroscopy that uses sound and light to examine the behaviour of material. This technique is used tostudy thermal emission developing from non-radiative de-excitation after absorbing radiation from the incoming light. Alexander Graham Bell was the first scientist to detect the photoacoustic effect in 1880. He discovered that thin disc when exposed to an interrupted beam of sunlight produces an audible sound in UV and IR electromagnetic spectrum of the sun. After the invention of laser in late 1960s Atwood and Kerr used laser as a source in photoacoustic spectroscopy for trace gas analysis because of its high spectral brightness. More briefly in this technique a thermal state of the sample changes after absorbing a periodically modulated light beam.If the sample is excited it emits heat pulses of same frequency as that of incident light. These thermal emissions produce pressure changes in the gas phase present above the sample that creates acoustic signal.

Like other spectroscopic techniques, PAS also uses a conventional light source e.g.lamp (incandescentarc lamps) or a laser (He-Ne laser and balloon-borne spin-flip Raman laser). A monochromator is utilized for the selection or scanning of excitation wavelength. The light beam is periodically interrupted by a chopper. For the detection ofan acoustic signal microphone is used in gas phase whereas, in condensed phases piezoelectric transducer can be applied as a detector. The transducer is physically connected to the condensed phase both in case of liquid or solid sample. Lock-in amplifier is used in PAS to electrically amplify an acoustic signal and thus the magnitude of that signal is fed on a recorder. In case of double-beam PAS system carbon black is used as a reference sample.

This technique provides some advantages over other spectroscopic techniques. It permits theanalysis and characterization of opaque and high light-scattering materials particularly powders (metals, drugs and insulators), suspensions (algae, bacteria and cell organelles), gels (films), amorphous solids (glasses), and tissues (skinand leaf). It can be employedfor depth-profile analysis of a substance as well as the thermal and optical properties of the sample. It is also helpful to investigate the de-excitation states of molecules and the life expectancy of the intermediates during chemical reactions.

It is also worthy to mention that no complex preparation, purification process and treatment is required before measurements. PAS detects the minimum absorption coefficients of 10-6 cm-1 and absorbance as low as 10-7 and can be measured. Beside advantages there are some limitations of PASthat the laser light used as source does not possess a broad bandwidth; for identification the molecules of analyte also absorb some light from the source (lamp or laser). Saturation effect is also troublesome. Simplicity, small size and robustness are the exceptional features of PA cell but they cannot be fully utilized if a systemis not connected with a competent laser source. Recent advances are made to use efficient lasers for the sensitive trace gas analysis or to increase the absorption range for detection.

Real applications of PAS include the analysis of stack gas emission, car exhaust emission and in ambient air monitoring. In medical sciences, PA cells are allowed to investigate living skin, blood, eye lenses, drug-laced tissues and tumors. The determination of pollutants in liquids, pesticides in aqueous solution and the behaviour of nitrogen fixation have been studied. This is also applicable to analyze the rate of penetration and time of habitation of UV-light in different layers of skin. The photosynthetic activity of plants and effect of water stress, temperature deviations, gaseous pollutants and changes in light flux has been investigated.PAS has been developed to directly measure the oxygen evolution rate even at a single leaf.PA apparatus is a valuable tool in the field of food sciences and used to determine iron content in milk powder, fatty acid content of margarine, contamination in ground pepper or to examine the adulterated powdered coffee.

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