Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is consistently used to access molecular information of ultracomplex mixtures such as heavy fuels. For instance, routine direct-infusion FT-ICR MS of heavy petroleum reveals tens-of-thousands of unique molecular formulas, usually with high heteroatom content (N, O, S, V, and Ni). However, the analysis is heavily reliant on the ability to indiscriminately volatilize and ionize thousands of molecules present in complex mixtures. Atmospheric pressure photoionization (APPI) seems promising to advance the characterization of heavy fuels, because it has been shown to achieve the most uniform and thoroughly soft ionization for mixtures of model compounds and samples.

In this work, we investigated the molecular composition and the ionization of an ultracomplex fossil fuel fraction, asphaltenes, by gel permeation chromatography coupled to (+) APPI 21-tesla FT-ICR MS. GPC has been used to separate samples based on nanoaggregation trends. Importantly, high magnetic field (21 T) enables larger ion populations for each mass spectrum, which improves dynamic range, resolution, and mass accuracy, which facilitates molecular formula assignment for single scan data, and allows for chromatographic resolution. Quantitation for ⁵¹V-containing species, obtained by GPC coupled to inductively coupled plasma (ICP) MS, revealed that APPI-MS analysis of fossil fuels is hindered by nanoaggregation. ICP-MS data indicates that samples with stronger aggregation elute much earlier in GPC, but most of the ions produced by APPI arise from later eluting, non-aggregated compounds. Despite multiple separation steps (i.e., solvent extractions followed by GPC), APPI FT-ICR MS observes only 40% of the total V-containing compounds. Besides the limitations, online FT-ICR MS analysis of the GPC-eluting fractions makes possible the characterization of tens-of-thousands of molecules that remain undetected by direct infusion MS analyses of fractions from multiple separation steps. The results highlight GPC 21 T FT-ICR MS potential for a comprehensive understanding of complex mixtures whose characterization is likely hampered by nanoaggregation, e.g., dissolved organic matter.

Chad Weisbrod

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