The effect of the excitation light intensity during on-line monitoring of biodiesel by fluorescence spectroscopy

•Fluorescence spectroscopy was applied for biodiesel analysis.•Excitation light effect during on-line monitoring was evaluated.•Biodiesel photodegradation may be induced by the excitation light.•On-line monitoring can be successfully applied adjusting the experimental setup.•Biodiesel thermodegradation was correctly monitored by fluorescence spectroscopy.

In the last years, several methods based on fluorescence spectroscopy have been proposed to investigate different aspects related to biodiesel industry for presenting some advantages such as simplicity, reliability, low-cost and real-time analysis. Nevertheless, the excitation light, depending on its beam intensity, may induce by itself sample degradation during the monitoring due to the low oxidative stability of the biodiesel, providing misleading results. The present study shows for the first time a detailed analysis about how excitation light intensity may distort biodiesel analysis based on fluorescence measurements. The results revealed that fluorescence may be successfully used for on-line and in situ monitoring of biodiesel conditions as long as low excitation intensity interacts with a small sample volume in order to avoid biodiesel photodegradation.

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