SIZE
EXCLUSION CHROMATOGRAPHY
Size
exclusion chromatography (SEC), also known as gel permeation chromatography
(GPC), is the preferred technique for characterizing polymers. It does not rely
on chemical differences to effect the separation but separates polymer molecules
on the basis of their size relative to the pores in the packing material. Its
primary use is in measuring molecular weight and molecular weight distributions.
However, closer consideration reveals that the size of the polymer molecule also
depends on primary and secondary structural factors. The major limitation of
conventional SEC using only a concentration detector, such as a refractometer,
is that it is unable to distinguish between the effects of molecular weight and
structural differences on molecular size.
During the first two decades of SEC acceptance the emphasis was on improving the
fundamental aspects of chromatography, such as column technology, optimizing
solvents, and improving precision of analysis. But, over the past ten years
there has been an increasing demand for getting more information out of SEC,
driven by the need to characterize more fully an increasingly complex array of
new polymers.
Two specialized detectors have been developed with specific applications for
SEC. The first is the laser light-scattering detector, which has been around for
almost twenty years, but now has new electronics and computer data acquisition
capabilities. Substitution of small, inexpensive diode lasers for the bulky He-Ne
gas lasers has greatly reduced the size and cost of laser light-scattering
detectors and the development of reference flow viscometers has provided similar
size and cost advantages for viscometer detectors.
Each detector measures a different and complementary variable. The
light-scattering detector gives a response which is proportional to molecular
weight and concentration. Likewise the viscometer detector response is
proportional to the intrinsic viscosity and concentration. Intrinsic viscosity
is inversely proportional to molecular density.
Because both of these detector responses are proportional to the concentration
of the sample, neither can operate very well without a concentration detector.
Therefore, that nearly universal detector in liquid chromatography, the
refractometer, becomes the unglamorous but necessary third partner in this
enterprise now called SEC3.
Viscotek coined the term SEC3
(read S E C cubed) to describe not merely the three detectors, but the three
dimensions created by the addition of these detectors to the SEC technique. The
first dimension is the chromatographic process that separates polymer molecules
according to molecular size. The second is the light-scattering detector
response that yields molecular weight. And the third dimension comes from the
viscometer detector, which gives a response inversely proportional to molecular
density. Together, these variables present a more complete picture of the
molecular structure.