The detection of oil in water by near infrared pulsed photoacoustic spectroscopy

Abstract

An experimentally based study to investigate near infrared laser photoacoustic spectroscopy as a technique to measure oil pollution is described. Photoacoustic spectroscopy is shown to be well suited to the application of monitoring oil in water in a pipeline. The role of the physical parameters important to photoacoustic generation is shown to lead to an effective amplification of differences in the optical absorption coefficients for many hydrocarbons and alcohols in water. This amplification is typically an order of magnitude and persists at low concentrations of arialyte, such that a Nd:YAG source (2 mJ pulses) yields a 7.2 % increase in the photoacoustic signal magnitude for a 500 mg/I oil in water sample compared to distilled water, corresponding to a detection limit of 4 mg/I. Using a 904 nm diode laser as a source for photoacoustic generation, a detection limit of 400 mg/1 of oil is demonstrated. In addition there is found to be no discernible effect on the magnitude of the photoacoustic signal caused by the presence of optically scattering particles. The design and development of a photoacoustic instrument to measure oil in water is described, and the performance of such an instrument is compared to that of the competing technology.