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Handbook of Size Exclusion Chromatography and Related ...

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In practise, most high-performance aqueous SEC packings exhibit some<br />

degree<strong>of</strong>hydrophobicity<strong>and</strong>ionicchargeduetothechemistriesinvolvedintheir<br />

manufacture.Becauseavariety<strong>of</strong>chemistriesareavailablecommercially(Table3)<br />

the ionic <strong>and</strong> hydrophobic characteristics <strong>of</strong> packing materials may differ. Often<br />

the chemistry applied is necessary to obtain acompromise between the chemical<br />

<strong>and</strong> physical properties <strong>of</strong> the final packing material. Both ionic <strong>and</strong> hydrophobic<br />

characterareundesirablebecausetheyresultinnonsizeexclusionphenomena<strong>and</strong><br />

although manufacturers <strong>of</strong> packing materials aim to minimize such interactions,<br />

eluent modification to suppress them is routine. This normally involves the use <strong>of</strong><br />

salt/buffer solutions (ionic interaction) <strong>and</strong>/or the addition <strong>of</strong> organic modifiers<br />

(hyrophobic interaction)to the eluent. An advantage <strong>of</strong> using such eluent systems<br />

isthatthepresence<strong>of</strong>saltseffectivelyreducespolyelectrolyteviscosity,whichcan<br />

otherwise be excessive due to intramolecular electrostatic attractions within the<br />

polymer chains giving rise to viscous fingering effects in SEC (28).<br />

Depending on the chemistry adopted by the column manufacturer, eluent<br />

selection may be limited with respectto pH <strong>and</strong> type/level<strong>of</strong>organicsolventthat<br />

can be tolerated. For example, the choice <strong>and</strong> level <strong>of</strong> crosslinking agent in<br />

polyvinyl alcohol based packings influences both the pH stability <strong>and</strong> organic<br />

solvent compatibility. In all cases the manufacturers’ literature should specify<br />

eluent compatibility.<br />

4.4 Eluent Selection<br />

The selection <strong>of</strong> the eluent in aqueous SEC is critical as it is <strong>of</strong>ten the only means<br />

<strong>of</strong>controllingsecondaryinteractionsbetweenthesample<strong>and</strong>thecolumn.Specific<br />

interactions can be exploited if the separation <strong>of</strong> discrete components in asample<br />

is to be achieved,for example, purification <strong>of</strong> biological compounds. However, if<br />

SEC is to be used to derive apolymer molecular weight distribution then nonsize<br />

exclusion behavior is undesirable (29). Although it is sometimes difficult to<br />

eliminate interactions completely,they can <strong>of</strong>ten be suppressed by selection <strong>of</strong> an<br />

appropriateeluent.Theselection<strong>of</strong>eluentwillbedependentonthetype<strong>of</strong>sample<br />

<strong>and</strong> on the surface chemistry <strong>of</strong> the packing material. Although it cannot be<br />

assumed that an eluent used for aseparation on one manufacturer’scolumns will<br />

be suitable for aseparation using adifferent type <strong>of</strong> column, certain general rules<br />

apply as outlined in Table 4.<br />

Adsorption effects can be identified by phenomena such as a sharp leading<br />

edge followed by tailing <strong>of</strong> the peak, small peak area, retardation <strong>of</strong> elution, <strong>and</strong><br />

poor reproducibility. Ion exclusion effects can be seen by early elution close to or<br />

even slightly prior to the void volume. When optimizing eluent composition, the<br />

reproducibility <strong>of</strong> chromatograms resulting from systematic changes in<br />

composition can be used as an indicator to determine the best set <strong>of</strong> conditions.<br />

© 2004 by Marcel Dekker, Inc.

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