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Handbook of Size Exclusion Chromatography and Related ...

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<strong>and</strong> especially in the area <strong>of</strong> excluded molar masses (Sec. 3.2.1). Full retention<br />

is preceded with decreased sample recovery. Evidently, the highest molar<br />

masses are fully retained while smaller macromolecules are still eluted. If<br />

overlooked, this phenomenon may cause large errors in determined polymer<br />

characteristics. This is one <strong>of</strong> the important limitations <strong>of</strong> many enthalpyaffected<br />

HPLC polymer separations, especially <strong>of</strong> liquid chromatography at the<br />

critical adsorption point (Sec. 5.1). Curve 9in Fig. 4corresponds to the system<br />

where entropic <strong>and</strong> enthalpic contributions to VR fully compensate only in a<br />

limited area <strong>of</strong> molar masses. The back-turn shaped calibration dependence 10<br />

will be discussed in Sec. 3.2.1.<br />

Four principal retention mechanisms in HPLC <strong>of</strong> macromolecules are<br />

adsorption, enthalpic partition, phase separation, <strong>and</strong> ion interactions. Ion<br />

interactions comprise ion exchange, ion inclusion, ion exclusion, <strong>and</strong><br />

polyelectrolyte expansion. The latter phenomena plays an important role in<br />

HPLC <strong>of</strong> macromolecules bearing charges on their main chains, or branches<br />

(polyelectrolytes), or on their functional groups, especially when working with<br />

charged column fillings. Ion interactions may, however, appear also if low<br />

molecular ions (eluent additives <strong>and</strong>/or sample impurities) are adsorbed on<br />

macromolecules (pseudo-polyelectrolytes) <strong>and</strong>/or on the column filling surface.<br />

Most analysts involved in characterization <strong>of</strong> synthetic macromolecules try to<br />

systematically suppress possible ion interactions in HPLC systems rather than<br />

to utilize them in separation. Therefore, we shall deal only with the remaining<br />

three enthalpic retention mechanisms, namely adsorption, enthalpic partition,<br />

<strong>and</strong> phase separation. While the former two retention mechanisms play a<br />

decisive role in many HPLC separations <strong>of</strong> small molecules, the phase<br />

separation retention mechanism is an almost exclusive domain for<br />

macromolecules. Surprisingly, the differences between the three principal<br />

retention mechanisms in HPLC <strong>of</strong> macromolecules are not recognized in most<br />

papers <strong>and</strong> even in monographs (4,5) dealing with polymer separation. This<br />

results in confusing statements such as “polymer adsorption was promoted by<br />

adding anonsolvent to eluent.” The following sections should help orientation<br />

<strong>of</strong> readers, in particular those just entering the field <strong>of</strong> polymer HPLC.<br />

3.2.1 Adsorption<br />

Adsorption <strong>of</strong> macromolecular substances on solid surfaces <strong>and</strong> on liquid<br />

interfaces plays an important role in many systems containing biopolymers <strong>and</strong><br />

synthetic polymers <strong>and</strong> also in numerous areas <strong>of</strong> technology. Therefore, the<br />

science <strong>of</strong> polymer adsorption keeps developing rather intensively, as it has over<br />

100 years (see, for example, Refs 13 <strong>and</strong> 14). The adsorption retention mechanism<br />

<strong>of</strong> low molecular substances has already been studied in the initial stages <strong>of</strong><br />

development <strong>of</strong> various chromatographic techniques. However, adsorption <strong>of</strong><br />

© 2004 by Marcel Dekker, Inc.

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