Optimisation of molecular mass range estimates of complex hydrocarbon mixtures through the use of planar chromatography combined with laser desorption-MS and size exclusion chromatography

摘要

Recent advances in the application of characterization techniques to mass ranges of complex materials will be described. The selected samples were coal or petroleum derived soft carbon precursors with molecular masses beyond the range of common techniques such as gas chromatography or GC-MS. A more robust procedure for estimating mass ranges and number average molecular weights (Mn) for complex hydrocarbon mixtures will be outlined. The use of planar chromatography (PC) in fractionating samples prior to evaluation with laser-desorption mass spectrometry (LD-MS) will be described. The fractionation by PC greatly reduced interference from high mass artifacts (multimer and cluster ions) and improved the repeatability and reproducibility of LD-MS. This improvement is related to the narrower bands of material obtained from PC fractionation, which reduces the possibility of multimer ion formation, facilitates multimer identification, and improves mass discrimination. In addition, performing laser desorption direct from the silica surface of the PC plate acts to reduce the amount of material desorbed by each laser shot. The method was found to improve applications of LD-MS to a range of complex hydrocarbon mixtures, ranging from creosote oils to pitches. The use of the PC method uncovered clear evidence of the presence of high mass material not apparent when the bulk sample is analyzed. The findings of the PC/LD-MS examination have been cross referenced against observations from SEC based mass estimates and the relative sizes of aromatic cores from UV-fluorescence spectroscopy.