On the limitations of UV-fluorescence spectroscopy in the detection of high-mass hydrocarbon molecules

This work compares UV-fluorescence (UV-F) and UV-absorption (UV-A) as detection methods in the analysis of coal and petroleum-derived materials, using size exclusion chromatography (SEC). A UV-F spectrometer that was equipped with a flow cell was connected in series to an SEC chromatograph with a conventional UV-A detector. Samples were examined via SEC, using both UV-F and UV-A detectors that were operating in tandem. They included asphaltenes from heavy petroleum residues and three fractions of a coal tar pitch obtained by solvent solubility separation. The chromatogram of the lightest fraction of the coal tar pitch (the acetone solubles) showed a single peak, with close agreement between both detection systems. The rest of the samples showed an early-eluting peak that corresponded to material excluded from the column porosity, in addition to a retained peak. UV-F showed little sensitivity to material eluting under the excluded peak in any of the samples and also was less effective than UV-A in detecting the material eluting at shorter times under the retained peak, only responding to the smallest molecules. Number and weight averages of the molecular mass distributions calculated for the retained material from UV-A were significantly higher than those calculated from UV-F data. UV-F fails to detect the entire range of compounds present in these complex samples, and it is particularly insensitive to the heavier ends. It seems that detection by UV-F is more dependent on structural features than UV-A.