Photoacoustic detection unit

Figure 10.18—Photoacoustic detection device. The spectrum is obtained in the photoacoustic units PAS = 100/s//rcf (where 7S is the sample signal intensity and /ref is the reference intensity). The reference consists of activated charcoal. The spectrum resembles an absorption spectrum.

Gas-phase photoacoustic detection of propane in N2 was attempted on a Si chip. In this method, a modulated light beam is incident on the sample. If the wavelength of the modulated light couples to an energy transition in a gas, the gas absorbs the modulated light resulting in periodic gas expansions and contractions, which are manifested as an acoustic wave. This wave can be detected by a microphone. In contrast to conventional absorption spectroscopy, the sensitivity of photoacoustic spectroscopy scales inversely with dimension, and hence this method is favored in the microscale. This is because photoacoustic spectroscopy is a differential technique in which the absorption is measured as the intensity per unit surface area [796],... 

Since 1975 [63, 115], PZT transducers have often been used for the detection of photoacoustic signals from solutions. Many different piezoelectric detectors have been developed for these experiments [67, 70, 116-121]. Piezoelectric ceramics in tubular form are frequently used. Then the detectors serve simultaneouly as a container for the sample solution. Despite many proposals for the constructicm of a piezoelectric detector, it is necessary to (fcwlop a sf al cell and detector unit to perform photoae>ustk experiments on highly radic ti cotrc ive dilutions. The following requirements had to be taken into special consideration in the d gn ... 

An improvement introduced in the LPAS operation is a simultaneous calibration of the detection signal to increase the precision of tte measurement [40]. The photoacoustic signal of the sample is normalu by the signal of a reference standard solution of known absorbance. Both sample and reference solutions are measured simultaneously by the dual channel LPAS set-up (see Fig. 5). By this method, the influence of long term instability of laser energy and laser beam profile on the photoacoustic signal are eliminated and at the same time the spectrum is converted to absolute absorbance units. 

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