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., Ss and SsOo), 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.