Photoacoustic detectors utilize the principles of infrared absorption and the piezoelectric effect to detect gas phase organic molecules. Gases are irradiated with infrared energy at a frequency corresponding to a resonant vibration frequency of the gases, which causes the gas molecules to be excited to a higher vibrational energy state. When the molecules subsequently relax back to the initial vibrational state, the excess energy is released in the form of heat which 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. The amplitude of the acoustic wave depends upon such factors as the geometry of the gas cell, the incident light intensity, the absorbing gas concentration, the absorption coefficient, and the background gas. 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..