Molecular Weight of Petroleum Asphaltenes: A Comparison between Mass Spectrometry and Vapor Pressure Osmometry

Seven asphaltene samples and six octylated asphaltene (OA) derivatives were analyzed using laser desorption ionization−time-of-flight (LDI−TOF) mass spectrometry (abbreviated as MS) and vapor pressure osmometry (VPO) techniques. Molecular weight distributions (MWDs) that were determined using MS spanned, for all asphaltenes samples and their octylated counterparts, a similar range, from ∼100 Da to ∼10 000 Da. For asphaltenes, the number-average molecular weight (Mn) and weight-average molecular weight (Mw) afforded values in the 1900 ± 200 and 3200 ± 400 intervals, respectively. Consistently heavier values were observed for the OA derivatives (Mn ≈ 2300 ± 200 and Mw ≈ 3600 ± 200). To select the adequate laser power for these measurements, experiments at different laser powers were performed, to increase volatility and reduce fragmentation to a minimum. Several other experiments were performed to validate these results. First, good agreement between the measured and calculated Mn values was observed for OA...