Pulsed photoacoustic laser spectroscopy was used for depth-resolved analysis of artificial tissue models. The technique was applied to investigate the spatial resolution capabilities of a fiber-optical-coupled photoacoustic sensor head. The time-resolved measurements confirmed the theoretical predictions of a depth resolution of 0.1 mm. In an adapted skin model, a strongly absorbing target could be detected up to a layer depth of 14 mm. At a layer depth of 5 mm, a lateral resolution of 3.5 mm was achieved. Because of the depth-resolving capability, this method is well suited as a complementary approach for two-dimensional imaging techniques.

Developments towards a portable sensor system for the detection of benzene, toluene and xylene for screening are described. The monitoring system is based on the resonant photoacoustic technology. As light sources, NIR laser diodes at wavelengths of overtone vibrations of the alkyl CH bonds are used. The resonator is of cylindrical shape with diameter 2R = 5 cm and total length L = 10 cm and is operated in its first azimuthal mode. The acoustic wave is detected by an electret microphone. Test measurements of water and toluene vapor are presented. For the detection of water vapor a 1.31 mm NIR laser diode was used. Water concentrations down to 0.5 mg l 21 in air could be measured. For the detection of toluene vapor a wavelength of 910 nm exciting the third overtone vibration of the alkyl CH bonds was used. The sensitivity of 3.2 mg l 21 in air may be increased by several orders of magnitude as soon as suitable laser diodes with wavelengths fitting the absorption lines of the first overtone become available.

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.

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