Formation of medium- and long-chain fatty alcohols in long-term stored oil and biodiesels revealed by chemical isotope labeling-liquid chromatography-high resolution mass spectrometry

The effect of oil source and catalyst on alcohol formation variances in the long-term stored inedible seed oil and biodiesel has not been extensively explored. This study presents the direct evidence of medium- and long-chain fatty alcohol formation in long-term stored biodiesels, prepared by different catalysts. Comprehensive screening and relative quantification of the potential alcohols were performed by a novel non-targeted screening method based on chemical isotope labeling-liquid chromatography-high resolution mass spectrometry (CIL-LC-HRMS). It was detected in total 56, 170, and 205 potential alcohols from long-term stored camellia (Camellia oleifera) seed oil (CSO)-based biodiesels, rubber (Hevea brasiliensis) seed oil (RSO)-based biodiesels, and waste soybean (Glycine max L.) cooking oil (WCO)-based biodiesels. Among these, 13 medium- and long-chain saturated fatty alcohols are confirmed by comparing with alcohol standards, and 44 are putatively identified by the degree of unsaturation, the differences in retention time, exact mass, and tandem MS spectrum annotation. Hierarchical clustering analysis and random forest clarification analysis of those 57 (13 +44) fatty alcohols in different oils and biodiesels showed that the alcohol composition in long-term stored biodiesels was dramatically influenced by the inedible seed oil source and the catalyst in biodiesel preparation. The theoretical calculation further investigated the formation of fatty alcohols for the molecular electrostatic potential of long-chain fatty acid and its derived methyl or ethyl esters at the molecular level.