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Interaction-free
GPC/SEC (Theoretical background )
Gel Filtration, Gel Permeation, Size Exclusion Chromatography
The key to successful
GPC/SEC separation is the correct choice of columns and solvent.
Interaction-free GPC/SEC requires a balanced system of sample, mobile
phase (solvent), and stationary phase (GPC/SEC column material).
The sample is the determinant factor: the polarity
of the sample defines the polarity of the solvent and that of the
stationary phase. Since the polarity of the sample can
be from very
polar (e.g. Poly (acrylic acid) and other polyelectrolytes) to
very
unpolar (e.g. Poly(styrene)) it is inescapable to have polar (e.g.
SUPREMA) and unpolar (e.g. SDV) stationary phases.
For some applications interaction-free
chromatography can be achived by adding a salt or co-solvent to the
mobile phase. E.g. in aqueous GPC/SEC often phosphate buffer solutions
are used to overcome interactions between the column material surface
and the sample. Therefore a careful method development is essential for
GPC/SEC.
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Why is it important that
GPC/SEC is interaction-free?
GPC (Gel permeation
chromatograpy), or SEC (Size exclusion chromatography), is a
chromatographic method that relies on the separation according to the
size of the polymer in solution. The size determines the elution
volume; larger molecules elute first, since they can not penetrate the
pores in the column (stationary phase) and therefore spend less time on
the separation column.
GPC/SEC allows to determine molar mass averages (e.g. Mn, Mw, PDI) and
the molar mass distribution if
- a calibration curve, that describes
the relation between the elution volume and the molar mass, is
available (e.g. established using a calibration kit or PSS ReadyCals)
- molar mass sensitive detectors
(e.g. an on-line MALLS detetor or an on-line viscometer), that measures
the molar mass of the eluting fractions on-line is available
For reliable and precise
results both methods require a perfect separation according to size
only. If any kind or interaction with the column material appears
(leading to a shift in the elution volume), both methods fail. In some
cases bulk values, like the Mw for light scattering or the intrinsic
viscosity for the viscometer, can still be measured. But distribution
information and PDI will be wrong.
For reliable and precise
results both
methods require a perfect separation according to size only. If
any
kind or interaction with the column material appears
(leading to a
shift in the elution volume), both methods fail. In some
cases bulk
values, like the Mw for light scattering or the intrinsic
viscosity for
the viscometer, can still be measured. But
distribution information and
PDI will be wrong.
So the first goal in GPC/SEC, no matter what
detection method is used, is to achieve interaction-free chromatography
without any kind of interactions between column material (stationary
phase) and sample.
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picture for close up view
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