UV spectrophotometry for monitoring toxic gases

The need for gas compound measurement concerns overall three domains: environmental monitoring, emission mea- surement and risk assessment. These fields are different because of concentration range (from 10 -3 to thousands mg.m-3). A fast technique has been developed based on UV spectrophotometry. Simple robust optics and absence of interference from water vapour and carbon dioxide are two of the main benefits of this method. All measurements are performed with a quartz flow cell of 10 cm pathlength. In this condition, the detection limits of various compounds (ammonia, hydrogen sulphide, sulphur dioxide, benzene, toluene, ethylbenzene and p-xylene) vary between 30 and 100 mg.m-3. This UV spectrometry system has been tested with success in two applications. The first one is during gaseous ammonia dispersion, simulating a chemical accident. The sec- ond one is BTEX monitoring measurement in a process control of soil remediation. In this case, UV is associated with spectral data treatment software. All results are compared with reference methods (Nessler reagent for ammonia, gas chromatography for BTEX). An acceptable agreement was found.