Handbook of Size Exclusion Chromatography and Related ...

Table 1 Errors (in %) Characteristics of Good and Poor
Chromatography
Angles Molar mass Radius
Good chromatography
Many 0.07 0.6
Low þ 908 þ High 0.2 2
Very low þ 908 0.6 10
Poor chromatography
Many 0.5 3.0
Low þ 908 þ High 1 5
Very low þ 908 14 80
of good and poor chromatography. Naturally, single 908 detectors cannot be
expected to yield any reliable results for molecules whose size may be measured
from MALS.
7 COPOLYMERS
Copolymers may be formed of monomers A and B in a random manner, that is,
of a structure such as AABABBABABAABBABA, or with monomer blocks
such as AAA and BBB to form a block copolymer such as AAABBBBBBAA-
ABBBAAAAAA, and so on. These copolymers, in turn, may be homogeneous
(that is, with relative contribution of each homopolymer independent of molar
mass) or heterogeneous (that is, with varying relative compositions that vary
with molar mass). Since each homopolymer has its corresponding dn/dc, it
becomes very difficult to characterize the scattered light from chromatically
separated species of such copolymers in terms of a specific molar mass at each
elution. However, if the copolymer is homogeneous, we may then take a
weighted average of the two dn/dc values dnA=dc and dnB=dc. Heterogeneous
copolymers present a challenge for MALS because there are many combinations
that could produce copolymers having identical hydrodynamic sizes, that is, that
therefore would co-elute if separated by SEC columns, for example. For such
separations, each slice would be expected to contain a variety of molecular
structures and molar masses.
The reader should be aware that the light scattering characteristics of
polymers (and copolymers) are assumed to be described by the Rayleigh–
Gans–Debye (RGD) approximation discussed, for example, in Refs 5 and 14. In
this approximation, each constituent of a molecule scatters light independently
© 2004 by Marcel Dekker, Inc.