Abstract

When chemical bonds are broken by absorption of short-wavelength radiation, the newly formed radicals can undergo subsequent chemical reaction. Since an acoustic signal is produced as a consequence of heat release, it follows that the ensuing chemical reactions can dominate the production heat and that the role of the radiation is merely to trigger an exothermic reaction. Such is the case when the H₂ + Cl₂ reaction, for example, is initiated by photodissociation of Cl₂: the energy released by the two center chain reactions that produce HCl far exceeds the energy absorbed from the radiation beam. Here this chemical amplification of the optoacoustic effect is used for trace detection of hydrocarbons. The effluent from the gas chromatogrpah is flowed along with Cl₂ into a small volume spectrophone cell that is irradiated with modulated laser radiation. The output of an electret microphone placed in the cell is detected by a lock-in amplifier and displayed on a strip chart recorder. Several hydrocarbons have been detected using this method including CH₄, C₂, H₆, C₃H₈, C₂H₄, and C₂H₂. Detection limits are a few hundred picomole for these species.

© 1985 Optical Society of America