Benzene monitoring using laser difference frequency spectroscopy

Summary form only given. The feasibility of benzene (C/sub 6/H/sub 6/) concentration measurements by using mid-infrared laser absorption spectroscopy is reported. The spectrometer is based on laser difference-frequency generation by mixing of two Ti:sapphire lasers in a nonlinear crystal. The infrared radiation, with output power in the range of some tens nW, was continuously tunable from 8 to 19 /spl mu/m. A glass flask was used for the preparation of benzene trace gas samples. The flask was filled with ambient air at a pressure of /spl sim/250 mbar, and an accurately weighed sample of the liquid benzene contained in a microsyringe was injected into the flask. The benzene gas mixture was then introduced into the absorption cell at a reduced pressure in the range of some tens mbar. C/sub 6/H/sub 6/ absorption lines of the /spl nu//sub 4/ R(6) and /spl nu//sub 4/+/spl nu//sub 20/-/spl nu//sub 20/ R(9) near 676.62 cm/sup -1/ were selected for the concentration measurements. The signal-to-noise ratio of /spl sim/20 deduced from the spectroscopic measurement yields a minimum detectable path-integrated concentration of /spl sim/1 ppm-m.