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Handbook of Size Exclusion Chromatography and Related ...

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however, aliphatic bonded groups may exhibit a rather “polar” character if<br />

solvated with eluent molecules that contain both nonpolar <strong>and</strong> polar moieties.<br />

The advantage <strong>of</strong> working with weaker enthalpic interactions is the “s<strong>of</strong>tness” <strong>of</strong><br />

the corresponding HPLC systems. In contrast to the adsorption-based retention<br />

mechanism, fine control <strong>of</strong> retention volumes is easier in enthalpic partition<br />

HPLC <strong>of</strong> macromolecules. On the other h<strong>and</strong>, silica-based C18 column packings<br />

contain many remnant free silanols which, for steric reasons, cannot be bonded<br />

with C18 groups. In spite <strong>of</strong> numerous attempts to block these free silanols with<br />

low molar mass silanes (“end capping”), even the best “deactivated” silica gel<br />

C 18 materials still contain some 50% <strong>of</strong> initial silanols. The latter are,<br />

surprisingly,accessible for macromolecular analytes <strong>and</strong> under certain conditions<br />

may be responsible for their extensive adsorption (16,17). As result, enthalpic<br />

partition <strong>of</strong> macromolecular analytes in silica C18 phases is <strong>of</strong>ten accompanied<br />

with their adsorption. This may hold even for less polar polymers such as<br />

poly(methyl methacrylate), <strong>and</strong> so on, in nonpolar mobile phases. For the sake <strong>of</strong><br />

clarity,the adsorption effects will be neglected in the following discussion.<br />

Theenthalpicpartition<strong>of</strong>macromoleculesinfavor<strong>of</strong>thebonded(e.g.,C18)<br />

phase takes place if solvated bondedgroups representathermodynamicallybetter<br />

solvent than the mobile phase for analyte macromolecules. This <strong>of</strong>ten happens if<br />

mobile phase is apoor tovery poor solvent for polymer sample (Secs 1<strong>and</strong> 4.2).<br />

When eluent quality for a polymer sample decreases, the phase separation<br />

threshold may be attained. As arule, phase separation starts with the largest<br />

macromolecules. Wearrive at ahybrid separation mechanism, enthalpic partition<br />

plus phase separation. In some systems, even all three enthalpic retention<br />

mechanisms, that is, enthalpic partition, adsorption, <strong>and</strong> phase separation, can be<br />

present simultaneously <strong>and</strong> act antagonistically.Evidently,it may be difficult to<br />

underst<strong>and</strong> <strong>and</strong> to control elution <strong>of</strong> analytes under hybrid retention mechanism<br />

conditions. The strength <strong>of</strong> interaction between the mobile phase <strong>and</strong> the bonded<br />

phasehassomewhatdifferentmeaningbetweenadsorption<strong>and</strong>enthalpicpartition<br />

HPLC mechanisms. At least one strong eluent component that is able to solvate<br />

the bonded phase must be present to prevent the stationary phase (C18 groups)<br />

from acollapse <strong>and</strong> to allow efficient enthalpic partition <strong>of</strong> analytes. At the same<br />

time, the mobile phase (usually one <strong>of</strong> the eluent components) must efficiently<br />

repel macromolecules <strong>and</strong> push them into the stationary phase.<br />

The enthalpic partition <strong>of</strong> amacromolecule between the C 18phase <strong>and</strong><br />

eluent is schematically depicted in Fig. 8. It is evident that the enthalpic partition<br />

process is also accompanied with de-coiling <strong>of</strong> macromolecules <strong>and</strong> with large<br />

entropic effects.<br />

Similar to adsorption, enthalpic partition phenomena directly affect<br />

retention volumes <strong>of</strong> analytes. Owing to a relatively low strength <strong>of</strong> interactions<br />

between the C18 phase <strong>and</strong> macromolecules, the enthalpic partition effects are<br />

generally less pronounced when compared with adsorption. Therefore,<br />

© 2004 by Marcel Dekker, Inc.

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