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Handbook of Size Exclusion Chromatography and Related ...

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displacing effectivities could allow fractionation <strong>of</strong> multicomponent polymers<br />

according to their chemical structure or architecture irrespectively <strong>of</strong> their molar<br />

masses.<br />

The corresponding methods are termed liquid chromatography under<br />

limiting conditions (LC LC) <strong>of</strong> adsorption, enthalpic partition, or solubility,<br />

respectively.Thesamplesolvent<strong>and</strong>displacingzonescanbeformed,forexample,<br />

by mixtures <strong>of</strong> eluent components with increasing displacing efficacies.<br />

Figure 12 depicts areversed situation when mobile phase is adisplacer but<br />

sample solvent promotes polymer retention (74). Alternatively,azone <strong>of</strong> liquid<br />

that forms a barrier can be injected into the column just before sample<br />

introduction, applying, for example, aten-port two-way valve provided with two<br />

loops (Fig. 13) or an autosampler. Macromolecules <strong>of</strong> Atype that exhibit low<br />

retentivitycansurmountthesolventbarrier<strong>and</strong>areeluted,forexample,intheSEC<br />

mode. Polymer B, however, is decelerated by the barrier <strong>and</strong> elutes irrespective<strong>of</strong><br />

its molar mass just behind the barrier. Again, aseries <strong>of</strong> barriers with increasing<br />

efficacies can be created to selectively block fast progression <strong>of</strong> macromolecules<br />

with different retentivities due to differencies in their chemical structure or<br />

architecture. Adsorption, enthalpic partition, or phase separation retention<br />

mechanismscanbeapplied.Independenceontheprevailingretentionmechanism<br />

taking part in the barrier action, the corresponding methods are termed liquid<br />

chromatography at limiting conditions <strong>of</strong> desorption, repartition, or insolubility.<br />

ThesixpossibleLCLCproceduresforseparation<strong>of</strong>complexpolymersystemsare<br />

at present only in the first stage <strong>of</strong> their development.<br />

The principle <strong>of</strong> polymer fractionation applying the continuous eluent<br />

gradientbarrierisshowninFig.14.Theaction<strong>of</strong>astepwisebarrierisinprinciple<br />

similar, although the latter may exhibit some advantages <strong>and</strong> also drawbacks.<br />

In the gradient elution approach, the polymer is injected into a mobile phase,<br />

which brings about its effective full retention within the column due to adsorption,<br />

enthalpic partition, or phase separation. It is preferable also if the sample solvent<br />

strongly promotes adsorption or enthalpic partition in favor <strong>of</strong> the stationary<br />

phase. Eluent must be either an adsorli, a poor solvent promoting enthalpic<br />

partition, or a nonsolvent. Next, the displacing efficacy <strong>of</strong> the eluent starts to<br />

increase continuously or stepwise. Macromolecules <strong>of</strong> similar retentivities are<br />

successively displaced; they move along the column with different velocities <strong>and</strong><br />

undergo fractionation. A very important feature <strong>of</strong> the described eluent gradient<br />

HPLC (EG HPLC) is the possibility <strong>of</strong> identifying experimental conditions under<br />

which macromolecules <strong>of</strong> different retentivities elute independently <strong>of</strong> their molar<br />

masses. This situation is typical for many high polymer systems (usually with<br />

molar masses above 50kgmol 1 ) using narrow pore columns so that<br />

macromolecules can be fully excluded from the pores in the weak interaction<br />

regime. The simplified explanation <strong>of</strong> molar mass independent retention in EG<br />

HPLC considers the barrier effect <strong>of</strong> eluent gradient, similar, for example, to<br />

© 2004 by Marcel Dekker, Inc.

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