Absorption coefficient, 793 pressure sensitive

In modern devices the flexible membrane is part of a capacitor with the other plate fixed. The pressure rise causes a corresponding change of the capacitance, which can be converted to an AC voltage (Fig. 4.76b). This sensitive detector, which is essentially a capacitor microphone, is now widely used in photoacoustic spectroscopy (Sect. 6.3) to detect the absorption spectrum of molecular gases by the pressure rise proportional to the absorption coefficient. 

When the laser beam is chopped at frequencies below 10 KHz, periodical pressure variations appear in the absorption cell which can be detected with a sensitive microphone placed at the inner side of the cell. The output signal from the microphone is proportional to the absorbed laser energy and therefore allows determination of the absorption coefficient. Because this method uses the conversion of photon energy into periodical pressure variations it is called photoacoustio spectroscopy and the device is named spectra-phone. 

Atomic absorption spectroscopy (AAS) is a technique for quantitative determination of metals and metalloids by conversion of a sample to atomic vapour and measurement of absorption at a wavelength specific to the element of interest. Owing to high sensitivity and selectivity, the technique is widely used for fundamental studies in physics and physical chemistry measurements of oscillator strengths, diffusion coefficients of gas phase species, partial pressure of vapours, rate constants of homogeneous and heterogeneous reactions, etc. The widest application of AAS, however, is in analytical chemistry. Nowadays it is one of the most popular techniques for trace analysis of over 65 elements in practically all types of samples (environmental, biological, industrial, etc.). 

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