Development of a photoacoustic trace gas sensor based on fiber-optically coupled NIR laser diodes

A novel photoacoustic sensor system for on-line monitoring of benzene, toluene, and xylene concentrations is described. The radiation emitted by near-infrared (NIR) laser diodes is coupled into a fiber and guided to the resonant cell. The resonator is operated at its lowest azimuthal resonance while the radiation enters the cell at two different locations. The fiber-optical approach allows for the use of various laser diodes with different wavelengths. This approach enables detection of many analytes in parallel and correction for spectral interferences with other substances. Measurements at the first overtone of the CH stretching vibrations (1.67 μm) are discussed. Detection limits obtained for benzene, toluene, and xylene are 70, 100, and 160 μg/L, respectively. In order to compensate for the spectral interferences of water vapor, the concentration of the latter was determined by using the 1.31 μm NIR laser diode. Water concentrations as low as 0.5 mg/L could be determined. Results of studies performed with two analytes at two different wavelengths simultaneously demonstrate the feasibility of the technique for analysis of mixtures.