Identification of Artifacts in the Methylation Process of Sulfur Compounds in Petroleum

Abstract

Methylation of sulfides and thiophenes into methyl sulfonium salts for electrospray ionization (ESI) mass spectrometry analysis has been a widely used approach for molecular characterization of sulfur-containing compounds in fossil fuels. In this study, we found a series of artifacts in the methylation products of crude oils, which could lead to incorrect assignment of sulfur compounds, severe ionization suppression of S1 species with a high double bond equivalent value, and O x S x species with low abundance. The terminal products from the Friedel–Craft methylation of toluene and its methyl homologues were identified as 1,1,2,3,4,5,6-heptamethylbenzenonium, which showed high abundance in the positive-ion ESI mass spectrum. These compounds associate with Ag+, leading to the formation of abundant Ag-cationized monomer and homo- and hetero-Ag-cationized dimer complexes. In-source high-energy collisional dissociation (HCD) of the homo- and hetero-Ag-cationized dimer complexes [M1 + Ag + M1]+ and [M1 + Ag + M2]+ yielded the monomer complexes [M1 + Ag]+, which further fragmented to yield the molecular ion [M1]+. The abundant Ag-cationized complexes seriously interfered with the identification of sulfur compounds. Feasible suggestions are made to eliminate the interference by Ag-cationized complexes.

Direct sulfur-containing compound speciation in crude oils and high-boiling fractions by APCI (+) FT-ICR mass spectrometry

“…To provide context, S -methylation analyzed via ESI is known to have compromised repeatability and reproducibility due to the molecular selectivity of methylation, the stability of the sulfonium cation, 40 and potential artifact formation. 41 In contrast, the reproducibility of APPI for detecting sulfur-containing compounds was evaluated by Son et al in 2020. 42 They reported a relative standard deviation (RSD) of less than 1% for S1 class compounds detected at high abundances (up to 45%).…”

Characterization of Sulfur-Containing Compounds in Petroleum Using AgSbF6 as a Methylation Reagent

“…The methylation/demethylation method has been successfully used for the separation of sulfides and thiophenes from the petroleum fraction in total or separately. − The separation performance of the method was excellent; however, a long reaction time of the methylation step has limited it wide use in the laboratory as a routine method. Using AgSbF 6 , the methylation/demethylation method will be more useful for the sulfur compound separation.…”

Molecular Characterization of Fossil and Alternative Fuels Using Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Recent Advances and Perspectives

“…S-Methylation of sulfur compounds has been widely used for molecular characterization of sulfur compounds in fossil fuels by ESI FT-ICR MS. However, Wang et al 173 found a series of artifacts in the methylation products of crude oils, which could lead to incorrect assignment of sulfur compounds. The terminal products from the Friedel-Craft methylation of toluene and its methyl homologues were identified as 1,1,2,3,4,5,6heptamethylbenzenonium, which showed high abundance in the positive-ion ESI mass spectrum.…”