Handbook of Size Exclusion Chromatography and Related ...

RSR arrangement with one or several full retention–elution column(s)
allowssamplestoringandreconcentration/focusing,aswellaspartialexchangeof
sample solvent in column #2 (Fig. 20).
12 APPLICATIONS OF 2D-HPLC TO COMPLEX
POLYMER SYSTEMS
In this section, we shall briefly discuss application strategies of two-dimensional
polymer HPLC to the most important groups of complex polymer systems. Several
practical applications of 2D-HPLC to particular complex polymers are reviewed,
for example, in recent monographs (3–5). To avoid unnecessary disappointment,
the readers are advised to evaluate and optimize each procedure published and
carefully check its applicability to the system of interest. 2D-HPLC of complex
polymer systems possesses many pitfalls, for example due to:
. differences in exclusion and interaction retention properties of
commercial columns from different producers,
. often rather limited reproducibilities of HPLC columns from the same
producer,
. large effects of minute variations in mixed eluents composition on the
sample retention, and problems with repeatability of mixed eluents
preparation,
. important influence of small amounts of admixtures/impurities present
in many solvents. Content of solvent admixtures may change from batch
to batch and also with time, for example, due to preferential evaporation,
moisture absorption, and oxidization reactions, and
. possible pressure dependence of retention volumes, in particular with
mixed mobile phases. Pressure may vary in the course of the experiment
due to partial column exit blockage.
All above effects may negatively affect retention volumes in regard to both
repeatability and reproducibility.
An important negative aspect of 2D-HPLC method design is also
“optimism” of some authors who tend to overlook even well-known general
shortcomings of procedures they apply (4,5).
As mentioned, researchers dealing with synthesis of complex polymers very
often characterize their products using conventional SEC. Average values of molar
masses and molar mass distributions of synthesis products are calculated directly
from polystyrene calibrations. The resulting data should be designated as
“polystyrene equivalent values.” They can give valuable preliminary, semiquantitative
information on synthesized polymers, but in some cases they may also be quite
misleading, for example, when a product is de facto a polymer mixture in which
© 2004 by Marcel Dekker, Inc.