Quantification of Absorber through a Scattering Medium of Different Thickness Using Evanescent Light Piping

Abstract

A non-invasive method has been developed for analyte quantification in fluids surrounded by optically-scattering, opaque walls. This method is based on steady state, visible wavelength reflectance measurements made simultaneously at multiple positions on the surface of a sample. Previous work has shown that reflectance measurements contain information about underlying scattering layers in layered scattering samples. We hypothesise that similar information about an absorbing layer below a scattering layer can be obtained from evanescent wave effects. Principal component analysis showed the data to be composed of three components, which were refined by a multivariate curve resolution alternating least squares (MCR-ALS) approach with non-negativity constraints. The first component is related to the scattering layer thickness, the second is associated with analyte concentration and the third is due to a minor back reflection within the sample cell. Both MCR and stagewise multi-linear regression (SMLR) approaches were taken to estimate analyte concentration and scattering layer thickness, for samples having thicknesses between 1 mm and 8 mm. Results demonstrate that a simple experimental configuration can easily predict optical properties of unknown samples. With the adoption of a multi-wavelength approach to this method, it is expected that improved absorption coefficient (μa) estimation accuracy can be realised in a variety of application areas such as in analysis through opaque containers, in vivo measurements and in-line monitoring of reactions.