Compositional Trends for Total Vanadium Content and Vanadyl Porphyrins in Gel Permeation Chromatography Fractions Reveal Correlations between Asphaltene Aggregation and Ion Production Efficiency in Atmospheric Pressure Photoionization

Abstract

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has exposed the ultracomplexity of fossil fuels, thereby validating the compositional trends that rule petroleum distillation, known as the Boduszynski Continuum. Routine FT-ICR MS analysis of a single crude oil sample can reveal tens-of-thousands of unique molecular formulas; however, currently available ionization methods suffer from limitations for such complex mixtures that are not yet completely understood. Simply put, MS detects ions, and thus, it depends heavily on the ability of ion sources to indiscriminately volatilize and subsequently ionize samples of interest. Despite advances in soft ionization methods, the characterization of complex matrices remains a challenge due to the lack of an ion source, commercial or custom-built, that can vaporize and ionize all compounds without bias, save analyte concentration. However, atmospheric pressure photoionization (APPI) has been shown to provide the most uniform ion production for mixtures of petroleum model compounds and real samples, with little to no fragmentation. In this work, we investigated the molecular composition of PetroPhase 2017 asphaltenes and its extrography fractions, with a focus on the total vanadium content and molecular composition of vanadyl porphyrins as a function of aggregate size distribution, accessed through separate experiments: online gel permeation chromatography (GPC) inductively coupled plasma−MS (ICP−MS) and online GPC APPI FT-ICR MS (at 21 T). The results reveal that the extrography separation provides asphaltene fractions (i.e., acetone, Hep/Tol, and Tol/THF/MeOH) enriched in 51 V-containing compounds with distinctive aggregate size distributions. The acetone fraction features smaller aggregate sizes, as it elutes later in the GPC chromatogram than Hep/Tol and Tol/THF/MeOH fractions, and overall, presents up to ∼14-fold higher ionization efficiency in APPI. Such behavior suggests a correlation between aggregate size and production efficiency of monomeric ions in APPI. Bulk compositional trends accessed by GPC separation and highlighted by ICP−MS detection indicate that despite multiple separation steps (i.e. extrography followed by GPC), APPI FT-ICR MS can only access ∼37% of the total V-containing compounds. Although the more stable/larger aggregates dominate the size distributions of all asphaltene samples studied, it is the weakly aggregated/monomeric species that are preferentially observed by APPI-MS. Tendencies in the molecular composition of vanadyl porphyrins and S/O-containing compounds strongly suggest that London forces might be central in the self-assembly process of asphaltene nanoaggregates to produce more massive clusters. The results demonstrate that the observed compositional trends (albeit limited) can be accessed when coupling advanced chromatographic separations with online high-field FT-ICR MS detection.

SiO2 Biogenic Nanoparticles and Asphaltenes: Interactions and Their Consequences Investigated by QCR and GPC-ICP-HR-MS

“…Our results confirm that nanoparticles not only reduce the amount of asphaltenes in solution but also decrease the size of the asphaltene aggregates in the remaining solution by adsorbing the most unstable and larger nanoaggregates. Interestingly, the GPC-ICP MS Vanadium chromatogram of the samples G and H, with a signal starting from 1260s is very similar with the results reported in our previous works 37,43 , for acetone fraction, with a GPC-ICP MS chromatogram starting at 22.5min (1272s) and presenting 3 maximums (23.2 min =1400s, 25.5min=1530s and 28.5=1710s). This acetone fraction also showed a very low tendency to form deposit on the quartz surface, and the FT-ICR MS analysis proved that it presents a higher aromaticity and more island-like structure than the other fractions (Hep/Tol and Tol/THF/MeOH).…”

“…The column calibration was only used to verify the performance of the columns and not to determine the MW of the asphaltenes, due to the difference in complexity between the polymer standards and asphaltene's matrix. However, in our previous publications we have shown that HMW size distribution are primarily aggregate compounds, while LMW are "free" compounds, and MMW a mixture of "free" and aggregate compounds 34,36,37 .…”

“…The Gaussian fitting treatment shows that the sHMW and the main part of the HMW compounds, the HMW1 family, are completely adsorbed while only a small fraction of the MMW and LMW compounds are adsorbed. It has been recently 37 reported that "free" porphyrins eluted in the LMW range, thus the low reduction of this size distribution observed in Figure 6.b suggests that's these compounds do not interact strongly with the nanoparticles.…”

Advances and Challenges in the Molecular Characterization of Petroporphyrins

“…112 Qian et al 38 further reported a wide diversity of petroporphyrin structures that contain multiple sulfur and oxygen atoms (CcH2c+zN4VOoSs) in vacuum residue using APPI FT-ICR MS. 38 The first comprehensive characterization of vanadyl porphyrins in unfractionated heavy petroleum and virgin asphaltenes using APPI FT-ICR MS was performed by McKenna et al 31 The authors highlight the need for FT-ICR MS to achieve unambiguous molecular formula assignment (˂300 ppb) and document the likelihood of misassigning vanadyl porphyrin peaks (N4O1 51 Vanadyl porphyrins have been detected in asphaltenes isolated from vacuum gas oil distillate cuts and in pentane, in-between C5C7 asphaltenes, and heptane asphaltenes using APPI FT-ICR MS. 113,114 Additional studies have applied APPI FT-ICR MS to asphaltene extrography fractions and highlight structural characterization using infrared multiphoton dissociation (IRMPD), confirming the dominance of single core vanadyl porphyrin structures. 84 Supercritical fluid extraction and fractionation (SFEF) of Venezuelan Orinoco extra heavy crude vacuum residue concentrates vanadyl porphyrins in high boiling end cuts detected using APPI FT-ICR MS. 66 Furthermore, Chacón-Patiño et al 115 investigated the molecular composition of vanadyl porphyrins in PetroPhase 2017 asphaltenes and extrography fractions, with a specific focus on the molecular composition of vanadyl porphyrins as a function of the aggregate size distribution, using GPC ICP-MS and online GPC APPI FT-ICR MS at 21 tesla (Figure 5). The authors performed quantitative detection of vanadium using GPC ICP-MS and concluded that the majority of the vanadyl porphyrin ions are produced from later eluting GPC fractions (low aggregation LMW part) and that highly aggregated fractions contain abundant porphyrins with much longer homologous series than those species from the less aggregated GPC fractions.…”

Review of Asphaltenes in an Electric Field

“…Heavy oils have a considerable amount of asphaltene-resinous components, heteroatoms (S, N, and O), and metals (vanadium and nickel) . The asphaltenes in heavy oils contain iron, vanadium, and nickel (up to 1%), of which the proportion of vanadium is more than 80%. , Vanadylporphyrins, one of the most common forms of crude oil metalloporphyrins, play an important role in asphaltene aggregation, fouling, and deposition. − …”