Characterization of Heteroatom Compounds in a Crude Oil and Its Saturates, Aromatics, Resins, and Asphaltenes (SARA) and Non-basic Nitrogen Fractions Analyzed by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

A Liaohe crude oil was separated as saturates, aromatics, resins, and asphaltenes (SARA) and neutral nitrogen fractions. The crude oil and its subfractions were analyzed by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that neutral nitrogen and acidic heteroatom compounds in the crude oil contain 15−55 carbon atoms with double-bond equivalent (DBE) values of 1−27, containing N1, N2, N1O1, N1O2, N1O3, N1O4, O1, and O2 heteroatom classes. No molecules in the saturate fraction can be ionized by ESI. The aromatic fraction contains N1 and N1Ox compounds with high molecular weights but low DBE values. The resin and asphaltene fractions contain highly aromatic and acidic class species, which are enriched in oxygen- and nitrogen-containing compounds with lower molecular weights than those found in the aromatic fraction. The distribution patterns of N1, N1O1, and O1 class species in the resins and asphaltenes are similar. The mass spectrum of the neutral nitrogen fraction differs from those for the bulk crude oil and its SARA fractions; the neutral nitrogen fraction is enriched with N1 and N1O1 class species. Neutral nitrogen compounds with molecular weights lower than 200 were discriminated in the FT-ICR MS spectrum under the chosen operating conditions. However, the nitrogen species detected by gas chromatography only accounted for a small amount of that found in the neutral nitrogen fractions. Some of the neutral nitrogen species were entrained in asphaltenes during the deasphalting step of sample fractionation.