アスファルテンの分子量をどう測るか(<総説特集>アスファルテンの凝集)

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Molecular weight (MW) is one of the most important properties of asphaltene, but there have not been infallible measurement methods yet. In order to establish proper MW measurement method, LDI-MS, GPC and GPC-MS were applied. By LDI-MS analysis, AS-MY showed bimodal spectrum at the peak tops of 500 and 1800amu. The use of a matrix to determine asphaltene MW was not useful as it enhanced the ionization of some of asphaltene fractions. There are optimum laser power for significant ionization of high-MW molecules and for minimum polymerization and fragmentation. By GPC and GPC-MS analyses, there was a significant gap between molecular weights calculated from retention time and measured by APPI-MS. Molecular weight measured by MS was similar to that from retention time about 900, but was significantly higher in the lower molecular weight range, with a linear correlation between them. Three new chemicals were selected for new calibration standards to convert molecular weight from retention time to that by MS. Three asphaltenes were fractionated into five fractions and a residue using a preparative GPC, followed by an average structural analysis to estimate the structural parameter distribution. For high-MW fractions having average MW higher than 1700 by GPC showed only 1700 by LDI-MS. It is thought that LDI-MS measured the MW for single molecules while GPC did the aggregates judging from the number of units estimated by an average structural analysis. The H/C atomic ratio increased and carbon aromaticity (fa) decreased as the molecular weight increased up to 1700 and was nearly constant above it, while H/C + fa was constant throughout the range of MW. The parameters on the numbers of total and aromatic rings were nearly constant. The estimated formula weight of a unit was between 400 and 1200. The results suggest that high molecular weight molecules consist of several units which form aggregates, and no further enlargement occur on a unit size.
2007-10-20

アスファルテンの分子量をどう測るか(<総説特集>アスファルテンの凝集)

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