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 similar to 100 Da to similar to 10 000 Da. For asphaltenes, the number-average molecular weight (M-w) and weight-average molecular weight (M-n) afforded values in the 1900 +/- 200 and 3200 +/- 400 intervals, respectively. Consistently heavier values were observed for the OA derivatives (M-n (approximate to) 2300 +/- 200 and M-w approximate to 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 M-n values was observed for OA materials (calculated from asphaltene M-n and n, the number of octyl groups introduced, as determined from elemental analysis); second, M-n values, as measured by VPO and MS, were determined to be equal, within an average standard deviation of +/- 27.0%. These results and calculations strongly suggest that the MWM, the molecular weight range, and the molecular weight averages determined using the present MS technique are reasonable estimates of the molecular weight properties of asphaltenes and not the result of artifacts such as fragmentation, polymerization, incomplete volatilization, etc., which may be occurring during the MS experiment.