Handbook of Size Exclusion Chromatography and Related ...

“Universal” detectors, which combine continuous RI and intrinsic viscosity
(IV) detection, propose to remove some of the error caused by chemical
functionality differences within asample. SEC columns separate on the basis of
hydrodynamic volume, or the volume amolecule or association of molecules
occupies in solution. Hydrodynamic volume is converted to molecular weight
using calibrations of standard molecular weight molecules, such as polystyrene.
However, molecules having the same molecular weights can have considerably
different hydrodynamic volumes because of differences in molecular structure.
Linear molecules, such as paraffins, have higher hydrodynamic volumes than
branched molecules, like polar aromatics, of the same weight. Therefore, in SEC
withconventionalconcentrationdetection,thesemoleculeseluteatdifferenttimes
and appear to have different molecular weights. Intrinsic viscosity detection
gathersinformation onmolecular structure(degreeofbranchingorcompactness),
which is used to convert hydrodynamic volumes to molecular weights.
Theonlyuniversaldetectorsensitiveenoughtodetectasphalt(becauseof its
relativelylowmolecularweight)isthedifferentialviscometer(165,166).Itutilizes
aWheatstone bridge flow resistance scheme that measures intrinsic viscosity
differences between the column eluant and the carrier solvent. Other viscosity
detectors measure absolute intrinsic viscosity of the eluant and are not as precise.
In Fig. 21, several supercritically refined asphalt fractions having avariety of
molecular weights (Mw) are seen to have similar RI and IV molecular weights in
the low-molecular-weight regions (19). In high-molecular-weight regions, where
fractions have higher asphaltene contents, viscosity detection results in higher
molecular weights than RI detection. This is because asphaltenes are much more
compact than polystyrene, have lower hydrodynamic volumes relative to
molecular weight, and therefore elute at the same time as a smaller polystyrene
molecule. Maltenes and polystyrene seem to have similar compactness. The
detector still cannot account for errors caused by tailing or molecular associations
in solution. At present, there are no instances of universal detection providing
improved characterization in terms of chemical composition or performance
properties.
Other on-line detectors receive rare mention in the literature and are used
for specialty applications. Nickel and vanadium detectors have been used to detect
the distribution of metal porphyrins in asphalts (167). Fluorescence detectors
have been used to detect cut-points between associated and nonassociated
constituents (36).
While searching for a mass detector, it must be remembered that other
chromatographic problems still prevent the determination of asphalt molecular size
distributions. Large, polar molecules tend to interact with the column packing and
cause adsorption–desorption tailing in the chromatograms. Therefore, material
that appears to have low molecular size may actually be of very large molecular
size. Also, asphalt forms associations of molecules that may individually be of
© 2004 by Marcel Dekker, Inc.