Direct Injection Electrospray Mass Spectrometry (DI-ESI-MS) and Liquid Chromatography Electrospray Mass Spectrometry (LC-ESI-MS) method development for characterisation of organic acids in petroleum and bio-oil samples

Abstract

Analysis and characterisation of complex mixtures such as petroleum and bio-oils are unarguably challenging. Finding suitable analytical method therefore needs to be considered carefully by analytical chemists. Electrospray Ionisation (ESI) can be used to give a profile of the organic molecules in the mixture. A liquid chromatography electrospray ionisation mass spectrometry method for the separation and profiling of organic compounds in crude oil and lignin derived bio-oil mixtures has been developed. The analytical method was developed in a reverse phase chromatography with gradient elution program using water and acetonitrile plus 0.1% acetic acid. The method was developed using standard compounds which include, naphthoic acid, naphthol, methyl benzoic acid, guaiacol, guaiacol glyceryl ether, catecol, dihydroxybenzene, dihydroxybenzoic acid, methyl catecol, ethyl catecol, quinolinol, NH2-benzamide, and carbazole. The separation is reproducible over repeated injection of the standard compounds with an acceptable variation in the retention times. A solid phase extraction protocol using a normal phase cyano-bonded stationary phase was modified to fractionate the petroleum and bio-oils into polar and non-polar components. Hexane/dichloromethane was used to elute the non-polar and hydrocarbon components of the bio-oil and petroleum respectively, while acetonitrile and methanol was used to elute the polar fractions which were submitted to the reverse phase column for separation and profiling. The application of the DI-ESI and LC-ESI-MS method both in direct fingerprinting and polar fractions of the both oils showed that molecular masses of interest were detected. These include, m/z 121 (benzoic acid), m/z 143 (naphthol) and m/z 171 (naphthoic acid) for crude oils. In bio-oils, m/z 109 (catecol or dihydroxybenzene), m/z 137 (ethyl catecol), m/z 143 (naphthol), m/z 123 (methyl catecol or guaiacol), m/z 151 (propylbenzene-1,3-diol), m/z 165 (propyl guaiacol), m/z 177 (methoxy eugenol), m/z 179 (guaiacol acetone) and 175, 191, 193, 199, 243, 285 etc., were detected.