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Handbook of Size Exclusion Chromatography and Related ...

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packing materials for SEC <strong>of</strong> synthetic lipophilic polymers. Different mean pore<br />

diameters are applied in SEC. They range from about 4to over 400nm. PS/DVB<br />

SEC column packings are considered noninteractive, yet important enthalpic<br />

interaction <strong>and</strong>, consequently,retention volume increase, has been observed for<br />

polymers <strong>of</strong> medium <strong>and</strong> high polarity with some PS/DVB packings (16,18,19).<br />

The extent <strong>of</strong> polar interactivity <strong>of</strong> PS/DVB column packings varies from<br />

producer to producer (18,19). It is anticipated that polar interactivity <strong>of</strong> PS/DVB<br />

HPLC column packings is caused by various polar groups present on the<br />

packing surface (Secs 3.2.1 <strong>and</strong> 4.1.2). Highly polar substances such as formic<br />

acid, trifluoroacetic acid, dimethylformamide, <strong>and</strong> so on, are sometimes added to<br />

eluents in order to prevent adsorption <strong>of</strong> analytes within SEC column packings.<br />

Alternatively, polar eluents may be applied. If, however, the eluent appears to be<br />

a poor solvent for the polymeric analyte (for example dimethyl formamide for<br />

polystyrene samples), enthalpic partition phenomena may appear <strong>and</strong> VR again<br />

increase (41). The general condition for the ideal SEC (1 0) is a certain degree<br />

<strong>of</strong> symmetricity in the system. In other words, interactions between all three<br />

essential constituents <strong>of</strong> the HPLC systems should be similarly positive with the<br />

slightly prevailing role <strong>of</strong> eluent. This means that eluent must be strong toward<br />

column packing <strong>and</strong> rather good for the polymer sample while polymer <strong>and</strong><br />

column packing should not exhibit strong attractive or repulsive interactions. It is<br />

evident that new, tailored SEC column packings should be synthesized to cover<br />

the broad range <strong>of</strong> polarities <strong>of</strong> the polymer analytes.<br />

4.1.2 Column Fillings for Enthalpic Interaction HPLC<br />

Numerous particulate column packings <strong>and</strong> several monolithic HPLC materials<br />

have been synthesized in various research laboratories. Practically all <strong>of</strong> them were<br />

designed for HPLC <strong>of</strong> low molar mass substances. Consequently, the choice <strong>of</strong><br />

interactive column fillings for polymer HPLC is limited to a few materials widely<br />

applied in HPLC <strong>of</strong> low molar mass substances. Tailored column fillings that<br />

would allow the fine tuning <strong>of</strong> retention <strong>of</strong> macromolecular analytes are practically<br />

nonavailable. The HPLC column packing market is dominated with porous <strong>and</strong><br />

nonporous particulate, as well as monolithic SiO2 materials both bare <strong>and</strong> bonded<br />

with various groups. Other inorganic bonded phase carriers such as zirconia,<br />

titania, <strong>and</strong> alumina so far have not found wide application. Of the many different<br />

bonded groups prepared on a laboratory scale, only a few are suitable for polymer<br />

separation <strong>and</strong> are commercially available. Unfortunately, composite stationary<br />

phases comprising a mechanically stable (inorganic) carrier, pores <strong>of</strong> which are<br />

filled with a homogeneously crosslinked network <strong>of</strong> organic macromolecules (26),<br />

are missing from the market.<br />

Alternative column filling materials are those based on heterogeneously<br />

crosslinked synthetic <strong>and</strong> natural polymers. Styrene–divinyl benzene resins are<br />

© 2004 by Marcel Dekker, Inc.

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