Comparison of optical spectroscopy techniques for monitoring the stages of thermoxidation of soybean biodiesel

The thermoxidation of biodiesel was monitored using different spectroscopic techniques: UV–Vis and MIR absorption, Raman spectroscopy, and visible fluorescence. As the oxidation progressed, the UV–Vis absorption spectra showed an increase in the spectral range between 34,000 and 26,000 cm−1, while two main fluorescence bands (under ultraviolet excitation) were observed at around 21,000 and 15,000 cm−1. The UV–Vis absorption and fluorescence intensities presented opposite behavior during the processes, irrespective of the temperature used (90, 140, or 190 °C). These effects could be explained by the consumption of natural antioxidants, followed by the formation of primary oxidation compounds. The Raman and MIR absorption results indicated the existence of a cis-trans isomerization effect, followed by the formation of hydroperoxides during the oxidation, with the behavior being the same as that revealed by the UV–Vis absorption and fluorescence analyses. The comparison, under the same conditions, of different spectroscopy techniques that can be used to monitor the stages of thermoxidation of soybean biofuel provides important information for selection of an appropriate technique for evaluating biodiesel integrity.