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Handbook of Size Exclusion Chromatography and Related ...

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eluentcomponents,aswellaswithoptimizedeluentgradientshape,EGHPLCcan<br />

giveveryhighseparationselectivities.Polymers<strong>of</strong>differentcompositions(79–83)<br />

<strong>and</strong> architectures (84) have been successfully discriminated applying adsorption<br />

<strong>and</strong> enthalpic partition retention mechanisms. The application <strong>of</strong> a phase<br />

separation retention mechanism (3–5,39,40) (eluent gradient phase separation<br />

liquid chromatography,high-performance polymer precipitation chromatography,<br />

precipitation–redissolution liquid chromatography) seems to be more complicated.Solubility<strong>of</strong>polymers<strong>of</strong>tentoostronglydependsontheirmolarmasses<strong>and</strong><br />

their redissolution may be slow. The ultimately dissolved highest molar mass<br />

speciesmaybetoomuchdelayedtocatchlowermolar massfractionsbeforethese<br />

leave the column. This is especially important with crystalline polymers where<br />

dissolution kinetics are <strong>of</strong>ten rather slow (40). Creation <strong>of</strong> two phases during the<br />

precipitation/redissolution processesmayleadtozonesplitting, especiallyif both<br />

phases contain macromolecules that inevitably differ in their molar masses (Sec.<br />

3.2.3).Ontheotherh<strong>and</strong>,thephaseseparationbasedEGHPLCexhibitsveryhigh<br />

separation selectivities <strong>and</strong> may give valuable information about many practically<br />

important complex systems, in particular if quantitative interpretation <strong>of</strong><br />

chromatograms is possible, for example, if HPLC is hyphenated with mass<br />

spectrometry.<br />

Eluent gradient HPLC <strong>of</strong> polymers is at present the most important group <strong>of</strong><br />

separation methods for a variety <strong>of</strong> complex polymer systems. Abovementioned<br />

chance to attain molar mass independ retention at high selectivity with compressed<br />

chromatographic zones <strong>and</strong> at high column loadability predestine EG HPLC to be<br />

applied as the first dimension separation system. The second dimension separation<br />

SEC column can be <strong>of</strong>ten directly attached.<br />

Barrier-coupled liquid chromatographic procedures have undergone long<br />

periods <strong>of</strong> development, with a distinct acceleration during the 1990s. Precipitation–<br />

redissolution liquid chromatographic separations have been proposed by the father<br />

<strong>of</strong> size exclusion chromatography, Porath (85), <strong>and</strong> its high-performance<br />

arrangement was pioneered by Glöckner (3). Adsorption/partition-based eluent<br />

gradient procedures were initiated by Belenkii et al. (86) <strong>and</strong> Inagaki et al. (87) in<br />

the TLC arrangement, <strong>and</strong> column EG HPLC separation <strong>of</strong> complex polymers was<br />

proposed by Teramachi et al. (88). Local gradient approaches have been suggested<br />

in this laboratory. Additional literature sources on barrier methods <strong>of</strong> polymer HPLC<br />

can be found in a number <strong>of</strong> reviews (3–5,56,78).<br />

Some practical hints for HPLC barrier methods users include the following:<br />

1. Apply the possibly mildest conditions for retention <strong>of</strong> separated<br />

macromolecules. Using narrow pore column fillings (narrow pore<br />

particulate column packings <strong>and</strong> monoliths with narrow separation<br />

pores) is advantageous for suppression <strong>of</strong> molar mass retention volume<br />

dependence <strong>and</strong> for enhancement <strong>of</strong> peak compression (focusing).<br />

© 2004 by Marcel Dekker, Inc.

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