Innate Sulfur Compounds as Internal Standard for Determining Vacuum Gas Oil Compositions by APPI FT-ICR MS

Innate sulfur components in high boiling petroleum samples were used as an internal standard for the estimation of the sample's composition through atmospheric pressure photo ionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The first step was recording a representative mass spectrum for the aromatic components through the use of a time-of-flight mass spectrometer (TOF MS) to tune the FT-ICR MS parameters. The molecular weight and number of sulfur atoms per species identified in the high-resolution measurement, their mass spectrometric abundance, and the total sulfur content were combined to calculate the mass fraction of each (and by sum all) aromatic sulfur species present. Aromatic hydrocarbon (HC) compound quantification was based on an equimolar response compared to the aromatic sulfur species. Nitrogen species were treated analogously to the sulfur compounds by distributing the total nitrogen content. The mass balance of all aromatic compounds yields the mass fraction of saturated compounds (saturates), which is not directly accessible through the APPI process. The validity and biases of this approach were evaluated on a series of vacuum distilled fractions with narrow boiling ranges using comprehensive two-dimensional gas chromatography (GCxGC) as a reference technique. There are clear biases of the presented mass spectrometric approach compared to the GCxGC analysis, especially the underestimation of monoaromatic compounds and the overestimation of diaromatic sulfur compounds (benzothiophenes); however, the results agree surprisingly well for saturated compounds and, overall, for the aromatic hydrocarbon- and sulfur-containing compound families. The saturates fraction results also matched reasonably with gravimetrically determined saturates contents, and the isolated fractions were further characterized using field desorption/field ionization TOF MS. The inclusion of saturated compound carbon number distribution, in combination with FT-ICR MS data, yields a more complete compositional description of the studied vacuum gas oil samples. The method was also applied to two typical boiling range vacuum gas oil samples (derived from a Norwegian and an Arabian crude oil) to provide an insight into the usefulness and limitations of high-resolution mass spectrometry in a semiquantitative context, which is deemed important for many oil and gas industry applications.