Direct diffusion sampling-based photoacoustic cell for in situ and on-line monitoring of benzene and toluene concentrations in water

Photoacoustic (PA) sensor for in situ and on-line concentration monitoring of toluene and benzene in water is being discussed. Selective detection was accomplished by transferring (by means of a permeable membrane) the analyte of interest into the gas phase. Unlike what is encountered in the conventional approach, where sampling and detection unit are separated, here the membrane is inserted directly into the central part of the photoacoustic resonator, eliminating thereby the need for various gas-handling components as well as the use of a purging gas. In this way, the system becomes simpler, more compact, with a response time of 40 min and a potential for fully automated operation. The use of a near-infrared (1.67 μm) diode laser coupled to the PA cell by an optical fibre resulted in detection limits of 350 μg (for benzene) and 1.1 mg (for toluene) per 1 l of water. When placed directly into water wells, the proposed sensor can serve as a warning system for long-term and automatic observation.