Oxidation products of biodiesel in diesel fuel generated by artificial alteration and identified by mass spectrometry

The aim of the present study is the storage stability evaluation of diesel fuel and various blends of diesel with fatty acid methyl esters (FAME, biodiesel), derived from rapeseed and soybean oil, by means of artificial fuel alteration. In this case, this is a lab-based procedure to effect accelerated changing of fuel properties under defined and reproducible conditions via oxidative stress. All oxidation products - volatile, liquid or solid - were captured thus enabling in-depth characterisation of the oxidised fuels obtained by conventional analytical methods as well as gas chromatography coupled with mass spectrometry (GC-MS). Special focus was laid on the identification of oxidation products in the oxidised fuels at various stages of the experiments. It could be demonstrated that especially neutralisation number and water content behaved proportionally to the oxygen consumption. They showed only marginal changes during the induction period, while increasing dramatically with the rapid onset of oxygen consumption as soon as the induction period was passed. The investigated fuels were ranked according to their storage stability in the order of neat diesel fuel > B7 blends > B20 blends > neat FAME. FAME content decreased rapidly in the stage characterised by rapid oxygen pressure break down, corresponding to 60% FAME loss in B20 RME. As main oxidation products in B20 RME, azelaic acid mono-ME, 9-oxo-nonanoic acid ME and hexanoic acid were identified. Moreover, carboxylic acids with chain lengths from C1 up to C9 as well as hydroxycarboxylic acids, dicarboxylic acids, and carboxylic acids with an epoxy group were formed. In B20 SME, main oxidation products were the same as detected for B20 RME. However, a distinctly broader variety of oxidation products was determined, e.g., carboxylic acid methyl esters from C8 to C18, unsaturated carboxylic acids up to C8, alcohols, and aldehydes. The main oxidation products detected in all samples are identical with those evolving during alteration of model mixtures containing linoleic acid methyl ester and linolenic acid methyl ester, respectively.