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Handbook of Size Exclusion Chromatography and Related ...

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Both strength <strong>and</strong> thermodynamic quality <strong>of</strong> eluents can be adjusted by<br />

temperature variation <strong>and</strong>, especially, by mixing two or several substances.<br />

Unfortunately,mixed eluents possess several complicating features. In order to<br />

efficiently control the resulting strength <strong>and</strong> quality <strong>of</strong> a mixed eluent, its<br />

components must exhibit rather different polarities, that is, different strengths<br />

toward the column packing or different quality toward macromolecular analyte.<br />

Thisresultsinpreferentialsorption<strong>of</strong>eluentcomponentsinthedomain<strong>of</strong>column<br />

packing <strong>and</strong>/or in preferential solvation <strong>of</strong> analyte macromolecules. Preferential<br />

sorption causes an increase <strong>of</strong> aparticular eluent component concentration near<br />

the packing surface or within the bonded stationary phase. The term preferential<br />

solvation st<strong>and</strong>s for increased concentration <strong>of</strong> one eluent component in the<br />

domain <strong>of</strong> sample macromolecules. The extent <strong>of</strong> both preferential sorption <strong>and</strong><br />

preferential solvation depends on temperature <strong>and</strong> pressure. Variations in<br />

preferential sorption resulting, for example, from temperature or pressure (19)<br />

changes may complicate retention control. Preferential sorption is co-responsible<br />

for theappearance<strong>of</strong>systempeaksonHPLCchromatogramsduetodisplacement<br />

effects (53). System peaks appearing on chromatograms obtained with mixed<br />

eluents arealsoduetopreferentialsolvation<strong>of</strong>thesample(54).Bothphenomena,<br />

preferential sorption <strong>and</strong> preferential solvation, complicate sample detection <strong>and</strong><br />

appropriate corrections are necessary (4,55).<br />

Adsorption <strong>of</strong> analytes within polar HPLC column fillings is efficiently<br />

controlled by adding polar, strong modifiers into anonpolar, weak mobile phase.<br />

Historically,thisapproachiscalledHPLCwithnormal(straight)mobilephase(NPLC<br />

or NP HPLC) in liquid chromatography <strong>of</strong> low molar mass substances. Extent <strong>of</strong><br />

retentionduetoenthalpicpartitioninfavor<strong>of</strong>nonpolar(bonded)phasesiscontrolled<br />

by adding less polar modifiers into amore polar major eluent constituent (usually<br />

water). This approach iswidely known as reversed mobile phase (high-performance)<br />

liquidchromatography(RPLCorRPHPLC)<strong>of</strong>smallmolecules.ThetermsNPHPLC<br />

<strong>and</strong> RP HPLC are sometimes also used in polymer liquid chromatography.<br />

Numerous other parameters are important for the eluent component choice<br />

(50). They include, for example, transparency in the ultraviolet <strong>and</strong> sometimes in<br />

the infrared wavelength range, high boiling point, as well as viscosity,corrosive<br />

properties, toxicity, <strong>and</strong> price. In 2D-HPLC systems, further important eluent<br />

parametersmustbeconsideredsuchasmutualmiscibility<strong>of</strong>mobilephasesinboth<br />

separation systems <strong>and</strong> overall compatibility <strong>of</strong> the column #1 eluent with the<br />

column #2 packing. Also for this reason, it is useful to apply SEC eluent (column<br />

#2) as one <strong>of</strong> the column #1 eluent components.<br />

4.3 Polymer Reference Materials (St<strong>and</strong>ards) for HPLC<br />

Dependences <strong>of</strong> polymer retention volumes on molar mass (Fig. 4) are<br />

constructed by eluting a series <strong>of</strong> homopolymer probes with different molar<br />

© 2004 by Marcel Dekker, Inc.

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