Asphaltene Molecular-Mass Distribution Determined by Two-Step Laser Mass Spectrometry

Abstract

Two-step laser mass spectrometry (L 2 MS) is explored as a technique to measure the molecular-mass distribution of asphaltenes. Unlike widely used laser desorption/ionization (LDI) mass spectrometry, in which a single laser pulse is used to desorb and ionize asphaltenes, L 2 MS involves two laser pulses and spatially and temporally separates the desorption and ionization events. This process allows L 2 MS to measure the asphaltene molecular-mass distribution free from artifacts resulting from aggregation and insufficient laser power, as occur in LDI. Studies of relevant model compounds show that L 2 MS detects these molecules without aggregation (unlike LDI), with only a minimum of fragmentation/multiple charging, and with relatively uniform sensitivity across the relevant mass range; however, the efficiency of the resonant ionization process is sensitive to molecular structure. These data suggest that L 2 MS does not suffer from significant mass discrimination in the relevant mass range and that L 2 MS provides a slight underestimate of the asphaltene molecular-mass distribution. Petroleum asphaltenes from different geographical origins are found to have similar mass spectra, all showing a peak at every nominal mass under an envelope beginning at 200 units, peaking at 500-600 units, and extending to 1000-1500 units. Coal asphaltenes are found to be considerably lighter and less complex, showing pronounced clusters of peaks separated by 14 units under an envelope beginning at 200 units, peaking at 300 units, and extending to 500 units. These results bring the molecular-mass distribution of asphaltenes as measured by laser desorption mass spectrometry in accordance with many other mass spectrometry and diffusion measurements.

Interpretation of DFA Color Gradients in Oil Columns Using the Flory-Huggins Solubility Model

“…The modified Yen model provides us with a framework/foundation for understanding the dispersion of asphaltenes in crude oil and has found application and success in oilfield case studies (Mullins 2010). Pomerantz et al (2009) confirmed the molar mass distributions of asphaltene monomers using two-step laser mass spectrometry. The results show that petroleum asphaltenes have many components under a mass envelope beginning at 200 g/mol, peaking at ~600 g/mol and extending to 1000-1500 g/mol.…”

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.…”