Characterization of Vanadyl and Nickel Porphyrins Enriched from Heavy Residues by Positive-Ion Electrospray Ionization FT-ICR Mass Spectrometry

Abstract

Petroporphyrins were enriched and purified from atmospheric residues of two typical heavy oils, Canadian oil sand bitumen (OSAR: Ni, 80 ppm; V, 190 ppm; S, 3.97 wt %) and Chinese Liaohe heavy oil (LHAR: Ni, 68.7 ppm; V, 1.81 ppm; S, 0.36 wt %) by silica-gel chromatography. The separation and purification were confirmed by atomic absorption spectroscopy (AAS) combined with UV–vis spectroscopy, and the petroporphyrins were characterized by positive-ion electrospray ionization (ESI) Fourier transform–ion cyclotron resonance mass spectrometry (FT-ICR MS). Vanadyl and nickel porphyrins in OSAR are simultaneously identified by mass measurement and isotopic fine structure. Vanadyl porphyrins with structures of N4VO, N4VO2, and N4VOS are all detected as protonated analyte ([M + H]+). Both molecular ion (M+•) and protonated analyte ([M + H]+) as well as their corresponding isotopes are observed for N4Ni porphyrins in OSAR and LHAR with an average mass resolving power of over 400000 (m/Δm50%). This is rarely detected by FT-ICR MS using ESI technique previously. Formation of molecular ion can be attributed to the low oxidation potential of nickel porphyrins, the effect of oil matrix on the solution conductivity, and the relatively low flow rate of solution into the capillary. Three more highly unsaturated types of N4VO porphyrins were identified in addition to the six well-documented structures. Compared to N4VO porphyrins, N4VOS porphyrins present higher DBE ranging from 21 to 27 while N4VO2 porphyrins show lower DBE ranging from 18 to 20 and narrower carbon number distribution, suggesting possible different origins of sulfur (pyrolysis of kerogen) and oxygen (diagenesis of chlorophyills). Ni/V and the ratio of relative abundance of ETIO porphyrins to DPEP porphyrins (∑ETIO/∑DPEP) for nickel porphyrins indicate that Liaohe oil and Canadian oil sand bitumen are continental and marine sediments, respectively, and Liaohe oil has a higher maturity. Enrichment by the simple chromatographic method facilitates the mass spectral identification of nickel porphyrins even for heavy residue with low content of nickel and high content of sulfur.