The most abundant and problematic metal compounds in crude oil exist as organic complexes of vanadium and nickel in porphyrin structures derived from biological molecules (chlorophyll and heme), the first petroleum biomarkers discovered by Alfred Treibs in the early 1930s. 1 Detailed characterization of the type and structure of porphyrins is critical for development of petroleum upgrading processes, but also to link crude oil to source rock conditions. Because petroporphyrins concentrate in heavy oils, direct characterization challenges routine analytical techniques due to the increased complexity associated with heavy crudes. Atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provides ultrahigh resolving power (m/Δm 50% > 1 000 000 at m/z 500) and subppm mass error (<50 ppb) to identify nickel porphyrin isotopes for unambiguous elemental composition assignment. We also report the first simultaneous identification and categorization of both vanadyl and nickel porphyrins in the same sample, without prior sample preparation. More than 85 000 mass spectral peaks are resolved and identified in a single mass spectrum, and represent the most extensive molecular deconvolution of an organic mixture characterized to date.

We report a novel chromatographic method to enrich and separate nickel and vanadyl porphyrins from a natural seep sample and combine molecular level characterization by positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Vanadyl and nickel porphyrin model compound elution from primary secondary amine (PSA) stationary phase combined with UV-vis spectroscopy confirms enrichment and subsequent fractionation of nickel and vanadyl porphyrins into polarity-based subfractions. A more than 100-fold increase in signal-to-noise ratio for nickel porphyrins enables unequivocal elemental composition assignment confirmed by isotopic fine structure for all isotopes >1% relative abundance, and the first mass spectral identification of (61)Ni porphyrin isotopologues derived from natural seeps. Oxygen-containing vanadyl porphyrins and sulfur-containing vanadyl porphyrins are isolated in the same fraction simultaneously from the same sample. We provide the first chromatographic evidence of carboxylic acid functionalities peripheral to the porphyrin core, in agreement with previous studies.

We have examined the aggregation behavior of a typical atmospheric residue feedstock by gel permeation chromatography (GPC). The size profiles for compounds containing sulfur, vanadium, and nickel were determined online from elemental detection by inductively coupled plasma (ICP) mass spectrometry. Four fractions that vary in aggregation state were analyzed by positive atmospheric pressure photoionization (APPI) 9.4 T Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS). Results showed an inverse relationship between fraction aggregate size and monomer ion yield and revealed that aggregation tendency did not correlate with higher polar or aromatic species abundance. Aggregation in the atmospheric residue more closely correlated with increased relative abundance of larger and more aliphatic compounds. The molecular composition of the GPC aggregate fractions suggests that nonpolar intermolecular forces between saturated, long-chain alkyl substituents contribute more to aggregation than pi–pi interactions.