Diffuse-light absorption spectroscopy by means of optical fiber technology for quality assessment of liquids

Abstract:

Absorption spectroscopy of liquids is influenced by dispersed scattering particles that cause turbidity. Filtering is needed prior to measurements, with a consequent loss of time and increased costs. An innovative fiber optic setup for the scattering-free absorption spectroscopy of liquids in the wide 400-1700 nm spectral range is proposed. It makes use of an integrating sphere that contains the sample under test, coupled to a fiber optic supercontinuum source and to a fiber optic spectrometer. The bright broadband light illuminating the sphere’s surface is diffusely reflected and collected by the fiber-coupled spectrometric detector. When an absorbing medium is placed in the cavity, a reduction occurs in the radiance of the sphere. The reduction is related to the absorption and volume of the medium, and is independent of non-absorbing objects within it, such as scattering particles. The wide scattering-free spectrum is a unique fingerprint of the sample. A comparison and classification of liquids with different qualities can be achieved by a multivariate processing of the spectroscopic data, which also can predict quality indicators. Two test case studies have been successfully conducted by measuring the wear of lubricant oils from different turbines, as well as by spotting out the adulteration of extra virgin olive oils. The proposed setup is precursor of implementing a probe to be inserted online in any industrial process where a scattering-free analysis is needed.