Optical Absorbance Measurements of Opaque Liquids by Pulsed Laser Photoacoustic Spectroscopy

摘要

In many relevant industrial applications, UV-vis online process monitoring is hampered by light scattering and opacity of the samples, whereas diluted and filtered samples are rarely available. Pulsed laser photoacoustic (PA) spectroscopy allows the measurement of both high and low absorptions without any need for sample preparation. An optimized detection geometry for absorption measurements in opaque liquids is described. The proposed PA sensor was realized by using two orthogonal detectors based on piezoelectric poly(vinylidene fluoride) (PVDF). Laser-induced pressure waves were sensed perpendicularly to (side-on mode) and along the axis of the laser beam (forward mode). Pressure waves generated by a single laser pulse, optical transmission and absorption, as well as the speed of sound in liquid samples were determined simultaneously using time-resolved detection. Evaluation of the PA signal permits the determination of absorption coefficients ranging from 0.1 to 1000 cm~(-1). The influence of absorbing or scattering compounds on the signal was investigated in dye solutions and suspensions of TiO_(2) particles.