Application of separation science and mass spectrometry to the analysis of fuels

Abstract

Biodiesel has been used as alternative energy because of the shortage of petroleum fuel resources. Biodiesel is a mixture of fatty acid methyl esters (FAMEs) having different molecular structures with varying chain lengths, and levels of unsaturation. Electrochemistry-mass spectrometry (EC-MS) was used for investigation and monitoring oxidation products of FAMEs. Through direct coupling of this device to the electrospray ion source of the mass spectrometer typical oxidation products were observed in minutes compared to months/years for the auto-oxidation process. The efficiency of glassy carbon (GC) and magic diamond (MD) working electrodes was compared. Oxygenated species for methyl oleate (C18:1), methyl linoleate (C18:2) and methyl linolenate (C18:3) with up to +2(O), + 6(O), and + 9(O), (2.5 V, GC electrode) and +6(O), +8(O), and + 12(O) (3.0 V, MD electrode) respectively were observed. The potentials of oxidation increased in the order methyl esters of C18:3 < C18:2 < C18:1 for both MD and GC electrodes. The primary oxidation products, hydroperoxides-FAME, in the auto-oxidation of unsaturated FAMEs (C18:1, C18:2, C18:3) can undergo further oxidation to produces a numerous of volatile and non-volatile secondary oxidation products. The volatile oxidation products, e.g. 2,4-decadienal and methyl 9-oxo-nonanoate were observed in the gas chromatography-electron ionisation mass spectrometry (GC-EI MS) analyses whereas the less volatile species require electrospray ionisation (ESI) and chromatographic introduction of the samples.

High performance liquid chromatography-electrospray ionisation mass spectrometry (HPLC-ESI MS), ultra-high performance liquid chromatography-electrospray ionisation mass spectrometry (UHPLC-ESI MS) and ultra-high performance supercritical fluid chromatography-electrospray ionisation mass spectrometry (UHPSFC-ESI MS) can be used for the separation of non-volatile species of the oxidation products. The elemental formula for these oxidised FAMEs were determined by ultra-high performance liquid chromatography-quadrupole-time of flight mass spectrometry (UHPLC-Q-TOF MS) and infusion Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). The presence of oxygenated species up to 6 oxygen atoms of the ion at m/z 411.1999 [C19H32O8 + Na]+ with a 2.6 ppm error observed in positive ion ESI-FT-ICR MS. It expected as the hydroperoxy bis-cyclic peroxides from auto-oxidised methyl linolenate.

Revision of the Energy Institute IP/585 reference method for quantitation of rapeseed methyl esters (RME) by GC-MS was required to afford quantitation of the short chain FAMEs in aviation turbine fuel (AVTUR) now appearing in certain geographic regions of the world. The revised method has delivered showed partial success and an improvement in qualitatively detecting and a positive move towards quantitation of the major components of coconut methyl ester (CME). HPLC and UHPLC coupled to MS offer alternative orthogonal approaches for analysis of these materials. The selective ionisation afforded by ESI affords ready detection of the FAMEs though the reversed-phase chromatography does not fully separate the FAME from the AVTUR fuel matrix, this would lead to matrix effects and ion suppression issues which could compromise any quantitative analysis. UHPSFC-MS provides a solution where the coconut methyl ester (CME) and rapeseed methyl ester (RME) are completely separated from the AVTUR whilst still delivering the benefits of the selective ionisation provided by the electrospray interface. Further supercritical fluid carbon dioxide (scCO2) is readily compatible with direct injection of the AVTUR fuel with the analytical benefits of supercritical fluid chromatography delivering base-line resolved peaks for all the FAMEs of interest in under 3 min.

More information

Identifiers

ORCID for John Langley: orcid.org/0000-0002-8323-7235

Catalogue record

Export record