Reproduction of Visible Absorbance Spectra of Highly Scattering Suspensions within an Integrating Sphere by Monte Carlo Simulation

Abstract

It is important to avoid the overestimation of absorption due to scattering when using absorption spectroscopy to measure scattering samples. We approached this issue by placing the sample inside an integrating sphere (IS) to collect the scattered light in all solid angles but encountered difficulty when determining the absorption coefficient from the absorbance because the light took various paths inside the IS and the sample. Therefore, by ray tracing inside the IS and the sample using Monte Carlo simulations (MC), we estimated the relationship between the absorption, scattering, anisotropy coefficients, and the measured absorbance. Scattering sample M, prepared by mixing polystyrene microspheres with trypan blue solution, and pure trypan blue solution for comparison were used as samples at various concentrations. MC reproduced the measurement results for the absorbance spectrum and its concentration dependence at 591 nm up to the measurement limit value. In addition, the saturated absorbance of sample M was lower than that of the trypan blue solution. This is because, from the distribution of distance d, light passed through the sample estimated by the MC, and more light with smaller d was detected due to scattering for higher concentration, resulting in a smaller increase in absorbance with the absorption coefficient.