Molecular\nComposition of Middle Eastern Asphaltenes\nby Mass Spectrometry: Field <i>vs</i> Dead-Oil-Derived Deposits

Asphaltenes are complex mixtures of natural compounds\nthat have\nproven to be notoriously difficult to analyze using routine methods.\nHere, a mechanically isolated asphaltene field deposit from an oil\nwell obtained using a gauge cutter and dead oil-derived asphaltene\nfrom the same well in an offshore Abu Dhabi oilfield were analyzed\nby Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR\nMS). Atmospheric Pressure Photon Ionization (APPI), Laser Desorption\nIonization (LDI), and Electrospray Ionization (ESI) were applied using\na combination of positive (APPI, LDI, and ESI) and negative (ESI)\nion modes, and the results were compared to bulk elemental ratios\nbased on independent elemental analysis. Results indicate that the\ndeposit was significantly enriched in sulfur relative to its dead-oil-derived\ncounterpart. The latter was slightly enriched in nitrogenous species\nand also contained maltenes covering a wider compositional space than\nthat of the field deposit. APPI resulted in marginally better agreement\nwith bulk elemental analysis data for the field deposit and significantly\nbetter agreement for its dead-oil-derived counterpart. Whereas LDI\ngenerally preferentially ionizes organic nitrogen, the molar S/C ratio\nof the field deposit is better matched to the LDI-generated molar\nS/C ratio. We conclude that the optimal ionization method for mass\nspectrometric analysis is sample-dependent. Most importantly, we demonstrate\nthat significant compositional differences exist between the dead-oil-derived\nsample and the deposit, raising concerns as to whether dead-oil-derived\nasphaltenes should be used in asphaltene-inhibitor evaluation studies.