Sulfur Speciation in Petroleum:  Atmospheric Pressure Photoionization or Chemical Derivatization and Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Molecular characterization of sulfur-containing species in petroleum is important because sulfur-containing compounds are detrimental to the environment and the refining processes. In a recent report, the sulfur-containing compounds in a vacuum bottom residue (VBR) were methylated to enhance their detectability by electrospray ionization (ESI) mass analysis. The most abundant sulfur compounds exhibited relatively low double bond equivalents (4 < DBE < 12). Alternatively, atmospheric pressure photoionization (APPI) mass analysis can provide molecular characterization without chemical derivatization. Here, we compare the sulfur speciation of a petroleum vacuum bottom residue by ESI and APPI with a 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Even after methylation, ions produced by APPI extend to much higher DBE than by ESI. Moreover, analysis of the saturates and aromatics fractions of underivatized VBR by APPI shows comparable ionization efficiency across a broad DBE range. We conclude that methylation is hindered for high-DBE species (DBE > 20), so that methylation followed by ESI MS is not suitable for sulfur speciation of higher-boiling fractions from petroleum crude oil.

Evaluation of Quantitative Sulfur Speciation in Gas Oils by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Validation by Comprehensive Two-Dimensional Gas Chromatography

“…Furthermore, in case of MeESI, a slow decrease of response factors towards higher DBE values can be projected. Extrapolation of the MeESI response factor versus DBE values (equation in Figure 4c) predicts a response factor close to zero (no methylation occurring) for DBE=24, which is consistent with previous observations [18].…”

“…For the same purpose, a selective derivatization of the nonpolar sulfur species to methylsulfonium salts prior to their analysis was reported [11] but found to exclude molecules with more than six or seven aromatic rings [18]. In this context, APPI was proposed more suitable for the analysis of compounds with higher aromaticity.…”

Molecular characterization of sulfur compounds in some specieal sulfur-rich Chinese crude oils by FT-ICR MS

“…Previous studies showed that through chemical methylation treatment, thioether, and thiophenic etc. OSCs in crude oil can be ionized for selective oxidation in the Electrospray Ionization (ESI) mode (Müller et al, 2005;Shi et al, 2010aShi et al, , 2010bPan et al, 2010;Liu et al, , 2010bLiu et al, , 2011, or in the Atmospheric Pressure Photo-Ionization (APPI) mode (Purcell et al, 2007;Liu et al, , 2010b in order to conduct FT-ICR MS analysis to characterize their distributions.…”

“…Purcell et al (2007) further compared the results in the APPI and ESI mode (Purcell et al, 2007) and showed that methylation pretreatment preformed a relatively stronger selectivity on those OSCs with low DBE values and proved incapable on those OSCs with high DBE values. In fact, different ionization method on OSCs analysis cannot have the same results (Panda et al, 2009).…”

Fourier transform ion cyclotron resonance mass spectrometry in the speciation of high molecular weight sulfur heterocycles in vacuum gas oils of different boiling ranges

“…Recently, continuous-flow field desorption [14] and lowvoltage electron ionization [15] coupled with FT-ICR MS were reported for the study of VGOs of different origin and of different boiling ranges, respectively. Other reports [16] compared the use of the derivatization method using ESI detection with atmospheric pressure photoionization (APPI) and indicated that the range in the first case mentioned was limited to double bond equivalent (DBE) values below 15 and therefore the APPI approach was favored. It has to be noted here that the chemical derivatization process generates salts which are in fact solution-phase ions that are useful for ESI without further solvent modifiers.…”