A pulsed photoacoustic system for the spectroscopy and monitoring of hydrocarbon liquids using stimulated Raman scattering in a silica fibre as a near-infrared source

Abstract

Raman scattering in an optical fibre has been utilized as the near-infrared source for a pulsed photoacoustic spectroscopic study of liquid hydrocarbons. An open-cell system is described and the performance criteria for practical photoacoustic instrumentation are discussed. Results show that a typical signal-to-noise ratio of 50 may be achieved, which makes this system suitable for the detection of small concentrations of hydrocarbons in liquids. The photoacoustic system has been used to measure concentrations both of dissolved and of dispersed hydrocarbons and the results of this study indicate the practical potential for novel open-cell photoacoustic instrumentation for the detection of crude oil in water using near-infrared wavelengths in a spectral region for which suitable diode lasers are available.

Optical Absorbance Measurements of Opaque Liquids by Pulsed Laser Photoacoustic Spectroscopy

“…Rosencwaig did not only lay the theoretical fundamental of modulated PA measurements in liquids and solids − but also applied pulsed PA spectroscopy to determine extremely low absorbances in liquids . Not surprisingly, most research on PA spectroscopy of liquids was focused on measurements of absorption coefficients close to the limit of detection. − Theoretical considerations of the PA effect in highly absorbing samples are given by Terzic et al, while the number of applications described in literature is only sparce. − A similar approach to measure only the absorbed fraction of light for highly absorbing samples is the optothermal window technique. , …”

The effects of optical scattering on pulsed photoacoustic measurement in weakly absorbing liquids

“…The dominant source may be different for different compositions. For example, to measure oil contamination or CuCl 2 in water, the µ a change is dominant [8]; to measure glucose in water, the change of βv n /C p is dominant [9]. In this paper, we study a low concentration and high scattering composition (milk) in water (µ s µ a ), of which the scattering contribution is dominant.…”