Molecular characterization of sulfur-containing compounds in petroleum

Sulfur-containing compounds (SCCs) are the most abundant heteroatom compounds in petroleum. The characterization of SCCs in crude oils and petroleum products has great importance not only for the downstream refining process control/optimization and environmental compliance but also for upstream geochemical studies for exploration and production. In recent years, a variety of methods to analyze SCCs have been developed and practiced where chromatography and mass spectrometry have played major roles. This review summarizes the analytical strategy and some of the most important and promising technologies, from sample pretreatment to molecular level characterization. For the convenience of discussions and practical applications, we divide the petroleum mixtures containing sulfur components into gas chromatography (GC)-amenable and nonvolatile fractions. For the former, gas chromatography (GC) and comprehensive two-dimensional gas chromatography (GCxGC) utilizing sulfur-selective detectors or mass spectrometry (GC-MS) are predominant analytical techniques. For the latter, molecular characterization is much more difficult and challenging. High performance liquid chromatography (HPLC) or direct liquid infusion and its combination with mass spectrometry (LC-MS), especially ultra-high resolution mass spectrometry, such as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Orbitrap MS, associated with various atmospheric ionization techniques are the most practical approaches. Some ionization techniques are underutilized, such as atmospheric pressure chemical ionization (APCI). The use of surface desorption ionization techniques, direct electrospray ionization (DSEI) and desorption atmospheric chemical ionization (DAPCI), for the molecular characterization of insoluble kerogens that has been characterized by X-ray near edge structure spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) remain to be explored.