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Handbook of Size Exclusion Chromatography and Related ...

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Quantitative relations between silanol concentration <strong>and</strong> column filling<br />

interactivitytowardpolymerspeciesare,however,difficulttoestablishbecausethe<br />

affinity <strong>of</strong> macromolecules toward column packing depends also on pore size <strong>and</strong><br />

shape, as well as on polymer nature. One must rely on trial-<strong>and</strong>-error tests <strong>and</strong><br />

optimizations.<br />

The activity <strong>of</strong> silanols is also strongly influenced by the presence <strong>of</strong> metal<br />

impurities in the silica matrix. Modern technology <strong>of</strong> ultra-pure silica production<br />

(silica <strong>of</strong> Btype) allows the more efficient control <strong>of</strong> silica gel interactivity<br />

compared to the Atype <strong>of</strong> silica, which contains metal impurities.<br />

Differences in bonding chemistries, as well as in endcapping <strong>and</strong> polymer<br />

coating procedures, represent another source <strong>of</strong> limited producer-to-producer<br />

silica-based filling reproducibility. Further problems are brought about by<br />

chemical attack <strong>of</strong> eluents <strong>and</strong> sometimes also <strong>of</strong> samples on both the silica gel<br />

matrix <strong>and</strong> the bonded groups. Generally,pH below 2<strong>and</strong> above 8as well as<br />

elevated temperatures above808C should be avoidedto keep high repeatability<strong>of</strong><br />

retention.Theformerconditionsareusuallyunproblematicforlipophilicsynthetic<br />

polymers; however, traces <strong>of</strong> moisture in hygroscopic eluents may affect the<br />

column lifetime.<br />

Macroporous, heterogeneously crosslinked organic resins do not only<br />

interact with polymer analytes by their (smooth) surfaces. It is anticipated that the<br />

free ends <strong>and</strong> loops <strong>of</strong> macromolecular chains protrude over filling surfaces <strong>and</strong><br />

create asort <strong>of</strong> “bonded phase,” which may take part in enthalpic partition<br />

processes.<br />

As mentioned, most commercial nonpolar poly(styrene-co-divinylbenzene)<br />

column packings were found to exhibit surprisingly high polar interactivities,<br />

whichcauseincreasedretention<strong>of</strong>medium<strong>and</strong>polar polymerspecies(Secs3.2.1<br />

<strong>and</strong>4.1.1).Thisphenomenonmaybecausedbypolarsubstancessuchasinitiators,<br />

chain transfer agents, porogenes, <strong>and</strong> protective colloids, which are added to<br />

polymerization systems to control porosity,size, <strong>and</strong> (spherical) shape <strong>of</strong> packing<br />

particles. Another source <strong>of</strong> polar interactivity <strong>of</strong> commercial styrene–<br />

divinylbenzene HPLC column packings may be the additional crosslinking <strong>of</strong><br />

some organic resin-based materials (48).<br />

4.2 Mobile Phases<br />

Properties <strong>of</strong> HPLC mobile phases are discussed in many HPLC textbooks<br />

(49,50). As has been repeatedly stressed in Secs 1 <strong>and</strong> 3, adsorption <strong>of</strong><br />

macromolecules within a column filling largely depends on eluent strength.<br />

Snyder, in his classic monograph (49) proposed solvent strength parameter, 10 , for<br />

characterization <strong>of</strong> potential eluents <strong>and</strong> eluent components. Originally, 10 values<br />

were determined for alumina adsorbents, which possess a more homogeneous <strong>and</strong><br />

less interactive surface than silica gels. Still, the same tabulated 10 values are<br />

© 2004 by Marcel Dekker, Inc.

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