Photoacoustic spectroscopy utilizes the principles of radiation absorption and the photoacoustics for molecular detection. Samples, which can be solids, semi-solids, liquids, or gases, are placed into a closed cell containing a non-absorbing gas.  The sample is then irradiated and energy absorbed by the solid is released when the surrounding gas molecules, which are heated by the solid, subsequently relax back to the initial vibrational state. The excess heat energy causes a change in the pressure of the gas cell. The pressure change results in an acoustic wave, which can be detected with a sound measuring device, such as a microphone or a piezoelectric sensor.  Visible and ultraviolet light may be used as radiation sources in PAS, but the most modern instrumentation relies on infrared radiation. Advantages of PA detectors include high sensitivity and stability, wide dynamic range, and quick response. The main disadvantage of PA detectors is poor specificity and susceptibility to interference. As a result, detectors using this technology are not recommended for screening of unknown samples.  PAS has been used in studies of biological systems, pharmaceutical analysis, nanoparticulate materials, and materials science.  PAS instruments are available in both benchtop and portable models.  The technique is mature, but seeing increased use with the inclusion of infrared radiation sources.  Commercial instruments are readily available.