Sulfur-containing compounds are the most abundant of the heteroatom species found in petroleum. They must be removed in refining operations to produce clean transportation fuels. Hence, detailed characterization of sulfur compounds is needed to develop hydrodesulfurization catalysts and optimize refining processes. Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a powerful tool to distinguish polar compound classes in complex mixture. Sulfoxides are prone to form Na1O2S2 dimers with high relative abundance in positive–ion ESI, which would seriously affect the composition analysis of sulfoxides in petroleum. Fig.1 showed serious molecular aggregation of sulfoxides when using positive ion ESI FT-ICR MS to analyze four crude oils. The aggregation phenomenon of sulfoxides can be explained as: the electronegativity of the oxygen atom in the sulfoxide molecule is greater than that of the sulfur atom, causing the sulfur atom to have a positive charge and the oxygen atom to have a negative charge. In the +ESI ionization process, due to the low concentration of Na+, most of the sulfoxide ions are ionized in the form of S+-O--Na+-O--S+, that is, Na1O2S2 ions are formed. In this paper, the aggregation property and ESI ionization mechanism of Na1O2S2 dimer were studied by collision-isolation dissociation (CID) experiments. The Na1O2S2 dimers can be significantly eliminate when a small amount of NH4OH is added into the sample solution, achieving selective ionization of the sulfoxides in positive–ion ESI (as shown in Fig 2).

Characterization of sulfoxides in petroleum by positive mode ESI FT-ICR MS using NH4OH to eliminate molecular aggregation

ACS Fall 2021

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