Two-Step Laser Mass Spectrometry of Asphaltenes

Abstract

Defined by their solubility in toluene and insolubility in n-heptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and most polar fraction of crude oil. Although asphaltenes are critically important to the exploitation of conventional oil and are poised to rise in significance along with the exploitation of heavy oil, even as fundamental a quantity as their molecular weight distribution is unknown to within an order of magnitude. Laser desorption/ionization (LDI) mass spectra vary greatly with experimental parameters so are difficult to interpret: some groups favor high laser pulse energy measurements (yielding heavy molecular weights), arguing that high pulse energy is required to detect the heaviest components of this mixture; other groups favor low pulse energy measurements (yielding light molecular weights), arguing that low pulse energy is required to avoid aggregation in the plasma plume. Here we report asphaltene mass spectra recorded with two-step laser mass spectrometry (L2MS), in which desorption and ionization are decoupled and no plasma is produced. L2MS mass spectra of asphaltenes are insensitive to laser pulse energy and other parameters, demonstrating that the asphaltene molecular weight distribution can be measured without limitation from insufficient laser pulse energy or plasma-phase aggregation. These data resolve the controversy from LDI, showing that the asphaltene molecular weight distribution peaks near 600 Da and previous measurements reporting much heavier species suffered from aggregation effects.

Combining biomarker and bulk compositional gradient analysis to assess reservoir connectivity

“…The soft ionization of petroleum molecules provided by ESI has been corroborated by good agreement of molecular weights measured by ESI with those measured by other soft ionization techniques such as two-step laser mass spectrometry (Hortal et al, 2007;Hortal et al, 2006;Pomerantz et al, 2008;Pomerantz et al, 2009;Rodgers and Marshall, 2007). ESI FT-ICR MS is useful for the analysis of complex mixtures because each elemental composition (C c H h N n O o S s ) has a unique molecular mass.…”

Model molecules mimicking asphaltenes

“…For instance Small-Angle X-ray and Neutron Scattering (SAXS and SANS) experiments were generally performed at concentrations close to 10 g.L -1 for asphaltenes dissolved in model solvents 17,18,[48][49][50][51] . It was not until 1999, using techniques such as fluorescence depolarization technique [52][53][54][55][56] , fluorescence correlation spectroscopy [57][58][59] and mass spectrometry 60,61 that reliable results, accounting for the molecular aggregation at higher concentrations, became available. These results have shown that typical mean molecular weights of asphaltenes were ∼750 g.mol -1 with a factor of 2 in the width of the molecular weight distribution.…”

Petroleomics: Chemistry of the underworld

“…Laser desorption/ionization mass spectrometry, although highly useful for biomolecule analysis, does not provide a reliable representation of petroleum components because of significant aggregation and fragmentation at the laser power required to generate a useful number of gas-phase ions (10). However, recent introduction of a two-color laser method (11), in which the first laser desorbs the neutrals and the second laser ionizes them, appears to overcome the disadvantages of single-color laser desorption/ ionization.…”