TRACE GAS-DETECTION WITH CAVITY RING DOWN SPECTROSCOPY

Trace gas detection of small molecules has been performed with cavity ring down (CRD) absorption spectroscopy in the near UV part of the spectrum. The absolute concentration of the OH radical present in trace amounts in heated plain air due to thermal dissociation of H2O has been calibrated as a function of temperature in the 720-1125 °C range. Detection of NH 3 at the 10 ppb level is demonstrated in calibrated NH 3/air flows. Detection of the background Hg concentration in plain air is performed with a current detection limit below 1 ppt. The effect of the laser linewidth in relation to the width of the absorption line is discussed in detail. Basic considerations regarding the use of CRD for trace gas detection are given and it is concluded that CRD spectroscopy holds great promise for sensitive [(sub)-ppb] and fast (kHz) detection of many small molecules. © 1995 American Institute of Physics.