Handbook of Size Exclusion Chromatography and Related ...

Oxidation of the rubber and asphalt can be used to speed up the dissolution
process(96). Starting withaless viscous base, asphaltmaterialwhichis hardened
as the rubber disintegrates yields amaterialwith excellentPG characteristics with
the 21-minute peak shown in Fig. 18 almost completely degraded.
6 DETECTORS AND “MASS DETECTION”
Researchers have used awide variety of detectors to analyze asphalts in SEC
studies. Generally, the aim is to characterize rapidly the molecular or, more
correctly,the apparent size distributions. This implies the need to determine the
concentration of asphalt in the eluant, which in turn requires adetector having
uniform sensitivity to mass at all retention times and for all types of asphalts,
regardlessofdifferencesinthematerials’ functionalitiesanddegreesofmolecular
association. Such an ideal detector would be atrue mass detector. Because of
asphalt’scomplicated structure and composition, all detectors used to analyze
asphalts by SEC fall short of being true mass detectors (68,151,152). Consequently,no
single detector has gained universal appeal.
Byfar,themostpopularon-linedetectorsforasphaltSECarethedifferential
refractiveindex(RI)andtheultravioletabsorption(UV)detectors.TheRIdetector
measures differences in refractive index between the pure carrier solvent and the
SEC eluant. These differences are related to the amount of solute in the eluant.
The UV detector measures the eluant’sabsorbance of UV light at aselected
wavelength. Here also, the response is related to sample concentration for agiven
solute.
Asphaltcontainsmanydifferentcompoundsthatvarynotonlyinmolecular,
orparticle,sizebutalsoinUVabsorptivityorrefractiveindex.Figure19showsthe
relation between detector response per unit mass and apparent molecular size for
some asphalts (12). Neither detector is uniform, as a mass detector would be. The
UV detector is much less uniform than the RI detector. This is mainly because
paraffinic hydrocarbons, known as saturates, which comprise roughly 10–20%
of a typical asphalt, are very weak absorbers of UV light, and the aromatic
components in the asphalt are strong UVabsorbers. Consequently, a UV detector’s
response to a saturate is much less than to an aromatic compound (151,153). The
effect of molecular association (which occurs in the large molecular size region)
on detector sensitivity is probably significant but is not well understood
(68,85,87).
The RI and UV detectors are popular because they are commonly used in
other high-performance liquid chromatography applications, relatively inexpensive,
reliable, and easy to operate. The UV detector is preferred by some because
it has much lower detection limits, whereas others prefer the RI detector because
it has more uniform response across the entire range of asphalt constituents.
© 2004 by Marcel Dekker, Inc.