Photoacoustic Spectroscopy
The Photoacoustic Spectroscopyis a method used to attain spectra of gases or powdered samples inside a gas phase. The principle of PAS technique is the following: Radiation entering the cell heats and expands the gas inside when absorbed by the sample. This expansion, and subsequent contraction, of the gas makes an audible sound whenever absorption takes place and can be detected by means of a microphone.
PAS is also a nondestructive sampling technique but unlike the other methods its recorded spectra is not dependent on the size and shape of the sample. Band shifts, optical artifacts and scattering and annoying reststrahlen bands or inverse peaks are absent. Nevertheless, PAS is particularly sensitive to interfering spectra from evolved gasses and extreme caution must be exercised to minimize vibration of the apparatus.
The signal to noise ratio may increase with the sample surface area and, although loading and unloading can be tedious, no further preparation or conditioning of the sample is required.

Since the spectrum obtained by the PAS technique is a direct measure of the absorption of radiation by the sample, optical problems and interference fringes are absent. On the other hand, pressure waves related to thermal diffusivity may affect the detection of the spectra. This effect varies significantly from one sample to another making comparison of spectra between different kinds of samples more difficult than in other techniques. In the case of depth profiling this problem is increased as thermal diffusivity is not constant with sample depth.
Without need of a preparation process and not depending on the size and morphology of the sample, Photoacoustic Spectroscopy method is particularly interesting for powdered samples with rough surfaced whose nature may be altered if exposed to a preparation procedure. In this way it PAS can be sued to study surface adsorbates and lattice vibration modes in the same sample. This cannot be accomplished by any of the other techniques and with the advantage that no dilution is required.

