Fluorescence lifetime of endogenous fluorophores for assessing the thermal degradation of biodiesel

Abstract

This work reports the potential of fluorescence lifetime data to evaluate the oxidative stability of soybean biodiesel under heating and airflow as a function of the heating time. The fluorescence decay at 420 nm for unheated soybean biodiesel showed a 3-exponential decay with lifetimes of 0.65 ± 0.04 ns (24.4% ± 0.9); 2.45 ± 0.05 ns (55.5% ± 0.9), and 8.10 ± 0.02 ns (20.1% ± 0.8). The lifetime components, as well as the mean lifetime, were influenced by heat and airflow. The data of steady-state fluorescence, UV–vis absorption, and gas chromatography-mass spectrometry (GC-MS) of biodiesel heated at 110 °C indicated a strong pairwise Pearson correlation with the mean fluorescence lifetime at 420 nm, suggesting a relationship between the concentration of major esters and the degradation of minor endogenous fluorophores (conjugated tetraenes) in biodiesel. Induction time measured by the Rancimat test corroborates the absorption, fluorescence, and chromatographic results. Therefore, our findings suggest that the formation and degradation of conjugated tetraenes could be used as fluorescence probes in steady-state and lifetime measurements to assess the thermal degradation of biodiesel.