Photoacoustic Detection of CO2

In 1880 Alexander G. Bell reported work on the development of a photophone, an apparatus aimed at transmitting voice with the help of sunlight. The sound wave of the speech modulated the position of the membrane mirror and, hence, the intensity received by the selenium element. This led to a modulation of the temperature and the resistance of the selenium element, and sound was audible from the speaker. Photoacoustic (PA) gas analysis is used for different spectroscopic detection schemes. To determine the sensitivity limit for the system, several contributions have to be considered like noise of the detector/preamplifier system, background signals originating from the walls and windows of the cell, acoustic noise, and mechanical vibrations of the cell. The first widely used lasers for mid‐infrared PA trace gas detection were carbon dioxide (CO2) and carbon monoxide gas lasers. In PA trace gas detection, the established quantity for system evaluation is the normalized noise‐equivalent absorption.