Handbook of Size Exclusion Chromatography and Related ...

Figure 19 Comparison of the response of UVand RI detectors to materials of different
apparent molecular size. (From Ref. 12, p. 239.)
The multiple-wavelength UV detector simultaneously scans several wavelengths
in the UV and visible spectra. Spectra from this detector provide information
about which size of molecules, or particles, contain certain UV-sensitive
functionalities. Vanadyl porphyrins, for instance, have specific UV absorbances
at 410 nm and are suspected of affecting asphalt aging processes. The multiplewavelength
UV detector shows (Fig. 20) that the vanadyl porphyrins are present
at all molecular sizes but are concentrated in the small molecular size region
(17,154,155).
Recently,severalevaporativeon-line detectors havebeen developedand are
reported to be true mass detectors. However, when applied to asphalts and heavy
petroleum fractions, these detectors’ responses show signs of being solute
dependent.
Two types of evaporative flame ionization detectors (FID) are the moving
wire(156,157)andtherotatingdiscdetectors(158–160).Theseconveytheeluant
along awire orquartz disc into anevaporationchamber,wherethevolatilecarrier
solvent is removed. The nonvolatile sample is then passed through an FID. Any
unburned sample is removedin an ashing chamber before thewire or disc returns
to its eluant-collecting position.
The FIDs rely only on the amount of combustible material present, rather
thanlightabsorptionor refractioncharacteristicsofthesolvent.Thisshouldmake
them respond more uniformly to mass over the particle size spectrum than RI or
UV detectors. However, the literature indicates that nonuniformities are still a
problem. Saturates and aromatics gave different response factors, possibly as a
© 2004 by Marcel Dekker, Inc.