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 (M
n
) and weight-average molecular weight (M
w
) afforded values in the 1900 ± 200 and
3200 ± 400 intervals, respectively. Consistently heavier values were observed for the OA
derivatives (M
n
≈ 2300 ± 200 and M
w
≈ 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 MWD, 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.