Indexed on: 15 Feb '17Published on: 15 Feb '17Published in: Analytical Chemistry
Identification of the heteroatom (nitrogen, sulphur, and oxygen)- containing compounds of petroleum is of key importance when considering industrial and environmental issues associated with crude oil production. The more commonly performed methods of crude oil fractionation are often insufficient in the extent to which they separate oils, not allowing defined 'molecular' fractions to be obtained. Methods capable of performing a class type separation are uncommon and are often extensive, and resource and time intensive. Here we report a method for the separation of crude oils into discrete compound classes. The method utilizes both ion exchange and normal phase chromatography to generate fractions of saturate hydro-carbons, aromatic hydrocarbons, basic compounds, naphthenic acids and other oxygen containing species, carbazoles, sulfones, and thiophenes from small crude oil samples (~0.5g). Method validation with a suite of model compounds has shown the fractions to be well-defined; with classes of model compounds isolated within discrete fractions. Application of the method to five crude oils of varying API gravity (12.1-38.3°) demonstrates a potential for wide-ranging use. Sample recoveries were high (93-103%) once corrected for evaporative losses, as was repeatability, demonstrated by triplicate analyses of model compound mixtures, oils spiked with model compounds and oils alone. Separation selectivity was further demonstrated by application of the scheme to the Alaska North Slope (ANS) crude oil and analysis of fractions by comprehensive two dimensional gas-chromatography mass-spectrometry (GC×GC-MS) and/or liquid-chromatography high-resolution accurate-mass mass-spectrometry methods (LC-HRAM-MS). Isolation of discrete fractions then allowed excellent separation (by LC and GC methods) of carbazole, dibenzothiophene, fluorenones, xanthones, and quinoline fractions. Individual parent and C1-5 alkyl homologs were easily separated (GC×GC-MS) allowing high quality mass spectra (EI) to be obtained for the individual compounds in many cases. Analysis of fractions by GC×GC-MS also allowed a series of thioxanones to be identified.