Determination of the Content of C80 Tetraacids in Petroleum
2014
Sutton, Paul A. | Rowland, Steven J.
In order to formulate mitigation strategies before the onset of oil pipeline and topside deposition problems caused by so-called calcium naphthenates or chelates of “ARN” acids, it is necessary to develop an analytical technique suitable for the quantification of the constituent C₈₀ tetraacids in small samples (∼1 g) of crude oils at low parts per million (ppm) concentrations. Here we report a method for the (semi)quantitative determination of tetraacids in crude oils using a relevant C₈₀ 8-ring acid as an internal standard. After addition of the internal standard, an “acid” fraction is isolated from the oil using sequential ion exchange solid-phase extraction (SPE) and the acids converted to their per-methylated esters before further SPE refinement to obtain a “tetraacid” fraction. Tetraacids are analyzed as the ammoniated adducts of their per-methyl esters using liquid chromatography coupled with electrospray ionization-mass spectrometry. This method has a limit of quantitation (LOQ) of approximately 0.1 ppm for individual tetraacids in crude oils. As an example, the use of this method for the determination of the individual and total tetraacid contents of five crude oils is shown.
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