Some Applications Of FT-IR Photoacoustic Spectroscopy

Abstract

The sensitivity and speed of FT-IR make it possible to obtain the photoacoustic spectra of a variety of samples with little or no sample preparation (1,2). A typical photoacoustic (PA) detector consists of an acoustically sealed cell of small volume in which the sample under study is kept surrounded by an inert gas. The cell also contains a sensitive micro-phone. When the interferogram is focussed on the sample, the sample absorption modifies the interferogram and sets up a pressure wave in the cell. This is detected by the micro-phone and Fourier transformed to yield the PA spectrum. A theory of the photoacoustic effect assuming an one-dimensional model for the PA cell has been given by Rosencwaig and Gersho (3), in terms of the optical and thermal properties of the sample, the backing material, and the gas. The general solution for the one-dimensional model is too complex and cannot be used for as such. However, Rosencwaig (3) has simplified the solution for a number of special cases. He has shown that under certain conditions the PA signal is proportional to the thermal diffusion length. This length varies inversely as the square root of the modulation frequency. Thus, it is possible in principle to record the PA spectra from different depths of a sample by simply varying the modulation frequences. In an FT-IR instrument the modulation frequencies can be varied by changing the velocity of the moving mirror. Thus, FT-IR-PA depth profiling studies can be carried out easily.