Silver Cationization for Rapid Speciation of Sulfur-Containing Species in Crude Oils by Positive Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Silver cationization constitutes a complementary approach for analysis of petroleum components with positive-ion electrospray ionization (ESI) mass spectrometry and accesses species that lack a basic nitrogen atom and, hence, are not observed by conventional positive ESI. Four samples of different origin [Canadian bitumen, Canadian bitumen heavy vacuum gas oil (HVGO; 475–500 °C) and South American and Middle East heavy crude oils, all high in sulfur content] were used to study silver cationization by (+) ESI. Cationization with Ag+ is essentially instantaneous and accesses hydrocarbons and nonpolar sulfur-containing heteroatom classes (e.g., S s and S s O o ), providing an attractive alternative to time-consuming derivatization by S-methylation to ionize sulfur-containing species. For each sample, we compare Ag+ cationization (+) ESI to conventional (+) ESI with formic acid to promote ion formation. Other ionization methods, such as chemical ionization (CI), field desorption (FD), matrix-assisted laser desorption ionization (MALDI) chemical ionization, field desorption ionization, and MALDI, are low in throughput and/or involve thermal processes that may degrade substrate molecules from non-volatile high-boiling petroleum components. Mix-and-spray Ag+ cationization avoids tedious separation and time-consuming derivatization and results in the rapid speciation of sulfur-containing compounds in petroleum and its fractions without the need for thermal desorption.

Separation and Characterization of Reactive and Non-Reactive Sulfur in Petroleum and Its Fractions

“…62 Positive ions were generated by (+) electrospray (ESI) with silver cationization as described elsewhere. 43 Each unfractionated sample and corresponding reactive and nonreactive fractions were diluted to a final concentration of 500 μg mL −1 in a 1:1 (v/v) solution of TOL:MeOH. Silver triflate (AgOTf) was added to the solution at 50 μg mL −1 .…”

“…Thus, the presence of silver isotopes enables distinctive isotopic peak pattern that, in turn, provides additional conformation for assignment of elemental composition. 43 FT-ICR mass spectrometry provides broadband ultrahigh-mass resolving power (m/Δm 50% ) > 500 000) enabling the resolution of the SH 4 vs C 3 mass split (common in petroleum samples) up to m/z 1000. Broadband FT-ICR mass spectra for a vacuum gas oil (VGO, sample 6) and its reactive and nonreactive S fractions are complex, and the spectra contain several thousand peaks corresponding to different elemental compositions ( Figure 3S).…”

Selective Analysis of Sulfur-Containing Species in a Heavy Crude Oil by Deuterium Labeling Reactions and Ultrahigh Resolution Mass Spectrometry

“…It is worth noting that the feeding sequence plays an important role on the selectivity towards sulfur. Here, a certain amount of AgBF 4 was added into the mixture of standards or crude oil, by which Ag + is first allowed to selectively coordinate with sulfur atoms to form complex Ag + adducts over a short time [ 26 ], then the alkylating reagent was added dropwise to the system and the S-C covalent bond was formed by the strong driving force of the precipitation of silver iodide (AgI). This derivatization procedure allows detection of sulfur compounds with very high selectivity, and meanwhile avoids the extra complications arising from silver complexation [ 22 , 27 ] and silver natural isotopes (51.84% 107 Ag and 48.16% 109 Ag) [ 20 ].…”

Regio-isomeric effects in tandem mass spectra of sulfonium cations generated from thiacyclane based sulfonium salts under soft ionization conditions

“…Two of its particularly notable applications were dealt with the analysis of peptides involving the use of methionine S-alkylation or N-acylation by the agent containing ready sulfonium fragment (sequence, identification, quantitation) [9][10][11][12] as well as with the detection of oil sulfides (mainly benzothiophenes) in the form of sulfonium salts. [13][14][15][16][17][18][19] It should be noted that some sulfonium salts have been the objects of mass spectrometric investigations based on such desorption/ionization principles as particle-induced ionization (SIMS, FAB), field desorption, and 252 Cf plasma desorption. [20][21][22][23][24] As it has been expected, all these desorption/ionization methods as well as ESI, MALDI, and LDI readily produced corresponding sulfonium cations whose structures were most likely identical and, hence, further decomposition were similar.…”

“…Two of its particularly notable applications were dealt with the analysis of peptides involving the use of methionine S-alkylation or N-acylation by the agent containing ready sulfonium fragment (sequence, identification, quantitation) 9–12 as well as with the detection of oil sulfides (mainly benzothiophenes) in the form of sulfonium salts. 13–19…”