Organizations & People
Title: Data treatment in size
exclusion chromatography of polymers
Chang, Klaus Lederer,
Jorge R. Vega, and
Wallace W. Yau
Completion Date: 2007 - project completed
In spite of the ample use of Size Exclusion Chromatography (SEC)
for measuring the molar mass distribution (MMD) of polymers, there
are huge variations in the results obtained by different laboratories,
when analyzing the same polymer sample. One of the possible reasons
for such variations may be the applied data treatment. Thus, it
seems indispensable to standardize the SEC data treatment.
The IUPAC Mission is "to provide
leadership, facilitation, and encouragement of chemistry, promoting
the norms, values, standards, and ethics of science and the free
exchange of scientific information". The standardization of
the SEC data treatment through a prestigious and non-profit organization
such as IUPAC seems the ideal solution for producing more uniform
and accurate estimates of the MMD among the different chemistry
laboratories of the world.
Develop some novel, more
practical, and more efficient procedures for the correction of
BB and other sources of error; in order to improve the accuracy
of the measured MMD and other polymer molecular characteristics.
Compare and critically
evaluate the different classical and novel data treatment methods,
with the final aim of recommending a standard procedure that may
be useful for general SEC user and for the instrument manufacturer.
SEC is the main analytical technique for determining the MMD of
synthetic polymers. In spite of its ample use, the accuracy of the
technique is relatively low, as it is verified by the dispersion
of results obtained in round-robin tests. For good quantitative
results, not only the equipment must be in good physical shape.
Also required are a careful experimental work and a judicious data
Due to BB, the polydispersity is overestimated
when calculated from a mass signal and a calibration curve, but
is generally underestimated when obtained from the combined mass
and "molar mass" signals. This bias can be important if
the sample MMD is narrow or multimodal. So far, it is rather surprising
that no instrument or standards manufacturer offers any practical
BB correction software. Possible reasons for this are: a) the BB
function is difficult to estimate; and b) the theoretically exact
solution, requires an ill-posed deconvolution operation. Many approximate
solutions have been proposed to overcome the classical deconvolution
approach, e.g.: a) to assume simple analytical functions for the
kernel and the mass chromatogram; b) to propose semi-empirical corrections
based on the chromatogram shapes; and c) to modify the molar mass
calibration rather than correcting the raw chromatograms. All of
these approximations offer different limitations, advantages and
drawbacks. The final project aim
is to standardize a method that is simple to apply, while still
providing reasonable estimates. Some of the items to be analyzed
in relation to the BB correction problem are: a) kernel determination;
b) robust deconvolution; c) non-deconvolution methods; d) oligomer
analysis; e) use of light scattering and viscosity detectors; and
f) solution via more fundamental physico-chemical models (rather
than on phenomenological models).
Apart from the BB problem, other sources
of error can be significantly more serious for their effect on the
MMD estimates. Examples of these are: a) polymer degradation; b)
column saturation, especially at high molar masses; c) insufficient
sensitivity of the light scattering (LS) and intrinsic viscometer
sensors to low molar mass material; d) chromatogram baseline selection;
e) detector nonlinearities; and f) propagation of errors when signals
ratio are required (e.g.: to calculate the instantaneous composition
or molar mass). The first aim here is to carry out a propagation
of error studies that will highlight the significance of these effects
on quantifying the MMD or other characteristics such as degrees
of branching or copolymer composition. This will determine the main
items to be cared for when estimating such properties.
The present project is a continuation of an earlier
project titled: "Band Broadening Correction in SEC" (Coordinator:
Jörg Baumgarten); the main outcome of that inital project was
a publication by J.L. Baumgarten, J.-P. Busnel, and G.R. Meira (2002),
"Band Broadening in Size Exclusion Chromatography of Polymers.
State of the Art and Some Novel Solutions", 25th. Anniversary
Issue, J. Liq. Chrom. & Rel. Technol., Vol. 25, N°
13-15, pp. 1967-2001. That project was part of the activities of
the former IUPAC Working Party IV.2.2: "Molecular Characterization
of Commercial Polymers" (Coordinator: Duan Berek).
Dec 2007 - project completed
The final report was presented at Polychar-15 World Forum on Advanced
Materials, in Rio de Janeiro, Brazil, 16-20 April 2007, and
later published in Macromolecular
Vol 258 covering the event. The report is "Band Broadening
Function in Size Exclusion Chromatography of Polymers: Review of
Some Recent Developments", Macromol. Symp. 2007, 258,
186197 [doi: 10.1002/masy.200751221]
text pdf -176KB
Last update: 10 December 2007
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Page last modified 10 December 2007.
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