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

Abstract

Two-step laser mass spectrometry (L2MS) 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, L2MS involves two laser pulses and spatially and temporally separates the desorption and ionization events. This process allows L2MS 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 L2MS 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 L2MS does not suffer from significant mass discrimination in the relevant mass range and that L2MS provides a sl...