5. Fluorescence Spectroscopy as an Alternative Analytical Tool for Monitoring Biodiesel Oxidative Stability: Thermal Oxidation Effect on the Endogenous Chromophores and Fluorophores in Biodiesel

Thermal stability of biodiesel is an important quality factor and it must be precisely evaluated. Despite the existence of conventional methods, fast, accurate, and robust analytical procedures are needed and still being developed. In this study, soybean and canola biodiesels underwent degradation by heating the samples and a systematic dependence on the concentration of endogenous chromophores (conjugated dienes, trienes, and tetraenes) and fluorophores (conjugated tetraenes) was optically monitored. UV–Vis absorption and fluorescence spectra of biodiesel allowed to identify the molecules formed in the initial thermal degradation stage. Absorbance and fluorescence intensities systematically changed at thermal treatment over 100 °C. Therefore, monitoring of the degradation compounds allows to evaluate the biodiesel degradation and the evolution of the degradation compounds content. The results indicate that analytical methods based on UV–Vis absorption and fluorescence spectroscopy may be able to monitor the biodiesel degradation, providing basis for the development of simple, portable, and low-cost devices.