Handbook of Size Exclusion Chromatography and Related ...

polydispersity of asample comprised as 50% by weight of two monodisperse
polymers, one of molar mass Mand the other of molar mass 2M. The result is
simply P¼1:125, which is generally believed to be a“small” polydispersity
despite the 100% range of molecules present. Results of polydispersity
calculations performed by MALS have been criticized repeatedly in the literature
as being too small as the sensitivity of MALS measurements decreases with
decreasing molar mass. Certainly Mw is best measured by MALS and Mn
by membrane osmometry,yet this idea has been debunked recently by Podzimek
(29)whomademeticulousMALSmeasurementsaswellasmembraneosmometry
determinations for several polymers. His conclusion was that MALS yields
the most plausible values of polydispersity despite its decreasing sensitivity
with decreasing molar mass. A powerful example of a truly (confirmed by
MALS) monodisperse samplewas analysedbyShortt (27). Hisstudy showedthat
certain polystyrene standards are actually far narrower than their manufacturers
believe.
10 THE PERFECT COMPANION FOR SEPARATION
SCIENCES: MALS
Therearevirtuallynoliquid chromatographytechniquesfor which theaddition of
sequential classical light scattering measurements through MALS cannot benefit
inordinately.Not only are all results previously based on calibration methods and
related empirical methods rendered absolute, but information regarding the
separation processes themselves is often revealed. An extensive bibliography of
well over 1,500 peer-reviewed papers describing the ever-increasing range of
results and applications may be found at www.wyatt.com.
Poorchromatography(thatis,apossiblewrongchoiceofcolumnsormobile
phase, or both) is often seen immediately by examining the MALS output. Thus
Fig. 16, for example, shows the MALS r.m.s. radius vs. SEC elutionvolumefor a
high molar masspolysaccharide. Notethat theelution isnotcharacteristic ofSEC
wherewe expect radius to decreasewith elutionvolume. The separation indicates
that the method was flawed because of poor chromatography.
The presence of long-chain branching is detected quite easily through
MALSwhenaso-calledconformationplot(14)ismadefollowingsampleelution.
Figure 17 shows such aplot (25) of the log(r.m.s. radius) vs. log(mass) for linear
and randomly branched polystyrene. The linear molecules exhibit a slope
consistent with a random coil (0.5–0.6) whereas the branched molecules produce
a conformation whose compactness increases with molar mass (slope decreasing
from that of the linear polymer).
Many powerful examples of MALS may be seen in reversed phase
chromatography where the elution depends on the molecular/column affinity
© 2004 by Marcel Dekker, Inc.