Probing Aggregation Tendencies in Asphaltenes by Gel Permeation Chromatography. Part 1: Online Inductively Coupled Plasma Mass Spectrometry and Offline Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

This study probes the nanoaggregation behavior of asphaltenes by gel permeation chromatography (GPC). Compounds containing sulfur, vanadium, and nickel were monitored online with elemental detection by inductively coupled plasma mass spectrometry (ICP-MS), and four fractions that vary in nanoaggregation state were analyzed by positive atmospheric pressure photoionization 9.4 T Fourier transform ion cyclotron resonance mass spectrometry ((+)APPI FT-ICR MS). We also highlight some of the challenges associated with the analysis of asphaltene fractions by direct infusion. Nanoaggregate size and monomer ion yield were inversely correlated. The extremely low ionization efficiency for the largest aggregate GPC fractions collected from the asphaltenes limited their characterization to only a few of the most abundant heteroatom classes. However, for all of the characterizable heteroatom classes, aggregation closely correlated with increased relative abundance of larger, more aliphatic compounds. These observations agree with results from the parent whole crude oil, suggesting that the interactions among the more alkylated compounds in asphaltenes may be a major contributor to asphaltene nanoaggregation.

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

“…As discussed above, "tailing" species may also experience adsorption on the stationary phase and therefore elute after the full permeation volume. The global compositional trends are consistent with the results reported by Putman et al, 63,69 who concluded that the most abundant species in larger aggregates (e.g., MMW) contain more carbon atoms in alkyl-side chains, suggesting that London forces may be fundamental in asphaltene selfassociation. Several reports 22,86−89 suggest extensive trapping/occlusion of porphyrins within asphaltene aggregates, which is consistent with the abundant detection of vanadiumcontaining compounds in large/sizable aggregates in this work.…”

“…Conversely, analysis of the later-eluting fractions (less aggregated asphaltenes/free molecules) revealed abundant alkyl-depleted/low-molecular-weight PAHs. In another work, Putman et al 63 performed off-line FT-ICR MS characterization of GPC asphaltene fractions and observed that earlier-eluting compounds were more aliphatic, with MSderived H/C ratios above 1.2. In contrast, later-eluting fractions revealed abundant highly-aromatic/alkyl-depleted species (H/C < 1.0).…”

Understanding Asphaltene Fraction Behavior through Combined Quartz Crystal Resonator Sensor, FT-ICR MS, GPC ICP HR-MS, and AFM Characterization. Part I: Extrography Fractionations

“…However, all MS-based techniques are limited because of selective ionization, ion suppression, and matrix effects. − Currently, there is not a “universal” ion source that allows for ionization of complex mixtures without any discrimination toward specific compound classes and structures. Therefore, prior sample fractionation is critical to achieving more complete molecular characterization of complex samples such as asphaltenes. − …”

Speciation of Metals in Asphaltenes by High-Performance Thin-Layer Chromatography and Solid–Liquid Extraction Hyphenated with Elemental and Molecular Identification

“…These results are consistent with the UV spectroscopy behavior. Putman et al [33] has reported limitations in MS characterization of asphaltenes due to low ionization efficiency of specific chromatography fractions. In this work, according to Table 3 and molecular formulas; such compounds could be completely "free" or present on the aggregate surface and therefore available for MS analyses.…”