Handbook of Size Exclusion Chromatography and Related ...

In general, band-broadening corrections are still required if amolecular
weight sensitive detector is added to SEC, especially if the molecular weight
distribution or the Mark–Houwink coefficients are being determined. As
mentioned, some average values of the distribution Mw by SEC-LS and [h] by
SEC-viscometryareunaffected.InSEC-LSandSEC-viscometryusedwithMark–
Houwinkcoefficients,theerrorsinthedeterminationoftheMWDarelessthanin
conventional SEC. In SEC-viscometry with universal calibration, these errors are
greater,asshowninFig.8.Adetaileddiscussionontheeffectofbandbroadening
with viscometers and light-scattering detectors can be found in Ref. 38.
Onerelatedproblemisthatof interdetectorbandbroadening.Detectorswith
larger cell volumes, if placed after other detectors in the SEC system, exhibit a
broader peak than other detectors. In SEC-LS, for example, the light-scattering
peak is generally narrower than the concentration-sensitive detector peak because
ofthesmallercellvolume.Thiscanleadtoamismatchofthetwodetectorsignals,
even with correct compensation for the interdetector volume. In the SEC-LS
example, this mismatch leads to an overestimate of the molecular weight in the
center of the peak and an underestimate at the leading and tailing edges. If
molecular weight is plotted as afunction of elution volume for anarrow MWD
sample, it appears as an n-shaped curve rather than anearly flat line. The weightaverage
molecular weight in this example is unaffected, but the number and Z
averages are distorted (37).
This effect can be corrected by injecting a narrow MWD sample and
measuring the variance of the peaks in each detector. Because the peak shape is
nearly Gaussian, it should, ideally,be the same for all detectors. If it is not, the
additional variance can be calculated for one of the detectors. In subsequent data
analysis, the narrower peak can be digitally broadened using Gaussian band
spreading to correct for this mismatch.
4 APPLICATIONS
4.1 Viscometry
4.1.1 Molecular Weight Distribution
SEC-viscometry and universal calibration has been widely used to determine the
MWD of synthetic polymers, and selected applications are listed in Table 1. Online
viscometers have been successfully used at high temperatures: Pang and
Rudin (48) measured the MWD of polyolefins dissolved in 1,2,4-trichlorobenzene
at 1458C, and Stacy (17) measured the MWD of polyphenyl sulfide in
1-chloronaphthalene at 2208C.
SEC-viscomettry has also been applied to natural polymers with more
complex molecular weight distributions. Timpa (59) used universal calibration and
on-line viscometry to measure the MWD of cotton fibers to evaluate different fiber
© 2004 by Marcel Dekker, Inc.