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Handbook of Size Exclusion Chromatography and Related ...

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largerispressuredrop<strong>and</strong>resultingexperimental problems.Particles intheupper<br />

size range are used mainly for preparative work <strong>and</strong> for separation <strong>of</strong> ultra-high<br />

molarmassmacromoleculesinordertoreducemechanicaldegradation<strong>of</strong>analytes<br />

by shearing.<br />

Pore sizes in column packings <strong>and</strong> sizes <strong>of</strong> separation pores in monoliths<br />

should match the sizes <strong>of</strong> macromolecules. This is especially important for<br />

exclusion-based separations (Sec. 4.1.1).<br />

In most cases, separation efficiencies <strong>of</strong> modern liquid chromatographic<br />

columns are high enough so that a general term high-performance liquid<br />

chromatography (HPLC) can be applied. Monolithic column fillings typically<br />

exhibit much lower flow resistance than packed columns <strong>and</strong>, they are therefore<br />

more suitable for high-speed separations. The sizes <strong>and</strong> volumes <strong>of</strong> separation<br />

poress<strong>of</strong>aravailableinmonolithsare,however,lessfavorablefor polymerHPLC<br />

than those in packed columns.<br />

Separation pores that are suitable for macromolecules range from afew<br />

nanometers up to several hundreds <strong>of</strong> nanometers in size (Sec. 4.1) depending on<br />

preferred retention mechanisms (Sec. 3). Pore volume should be as large as<br />

possible. Unfortunately,increased pore volume is connected with alowering in<br />

mechanicalstability<strong>of</strong>theporousgelmatrix,whichmustwithst<strong>and</strong>highpressures<br />

<strong>of</strong> up to several tens <strong>of</strong> megapascals (hundreds <strong>of</strong> bars) <strong>and</strong> frequent pressure<br />

strokes. Therefore, acompromise must be sought <strong>and</strong> pore volumes <strong>of</strong> modern<br />

HPLC column packings assume barely more than 60 to 70% <strong>of</strong> total particle<br />

volume.<br />

The chemical nature <strong>of</strong> HPLC column packings strongly affects analyte<br />

retention. This feature will be discussed in more detail in Secs. 3<strong>and</strong> 4.<br />

At present, the most popular method for molecular characterization <strong>of</strong><br />

synthetic polymers is size exclusion chromatography (SEC), whichis also termed<br />

gel permeation chromatography (GPC) in the case <strong>of</strong> lipophilic macromolecules<br />

<strong>and</strong> gel filtration chromatography (GFC) in the case <strong>of</strong> hydrophilic<br />

macromolecules. Modern SEC belongs to the family <strong>of</strong> high-performance liquid<br />

chromatographic methods <strong>and</strong>, consequently,it is improper to speak about HPLC<br />

<strong>and</strong> SEC. SEC separates macromolecules according to their size in solution. This<br />

means that macromolecules with particular size will be eluted from acolumn<br />

withinaspecificvolume<strong>of</strong>mobilephasethatiswithinaspecificretentionvolume,<br />

VR. As aresult, molar masses <strong>of</strong> macromolecules leaving the SEC column can be<br />

easily evaluated on the base <strong>of</strong> their retention volumes using appropriate<br />

calibration. Alternatively, the molar mass <strong>of</strong> macromolecules in the column<br />

effluent can be continuously monitored applying on-line light-scattering<br />

measurement or viscometry.Concentration <strong>of</strong> macromolecules leaving the SEC<br />

column is measured by appropriate flow-through HPLC detectors, for example,<br />

by differential refractometers, photometers, evaporative light-scattering detectors,<br />

<strong>and</strong> so on (Sec. 10). Knowing both concentration <strong>and</strong> molar mass <strong>of</strong><br />

© 2004 by Marcel Dekker, Inc.

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