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Handbook of Size Exclusion Chromatography and Related ...

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However, strong adsorption <strong>of</strong> macromolecules in very narrow filling<br />

pores (15,17) may deteriorate results due to b<strong>and</strong> broadening <strong>and</strong><br />

splitting, as well as due to decreased sample recovery. Presence <strong>of</strong><br />

macromolecules trapped in the narrow pores can <strong>of</strong>ten be revealedby a<br />

blank experiment performed after the actual separation experiment <strong>and</strong><br />

under identical experimental conditions but without polymer sample.<br />

Trapped macromolecules are successively eluted in the course <strong>of</strong> blank<br />

experiments with retention volumes (practically) identical to the<br />

originally eluted sample (17). Similar to LC CC, enthalpic partition is<br />

usually easier to fine tune than adsorption. Interfacial adsorption on<br />

polar bonded groups (amino-, nitril-, nitro-, glyceryl-, propyl-, <strong>and</strong> so<br />

on) is preferred over that on abare silica gel surface. Strong desorli<br />

eluentcomponentsshouldbeappliedifcolumnfillingmaterialcontains<br />

surface silanols accessible to macromolecules (this includes many C18<br />

silica bonded phases, see Sec. 4.1.2) <strong>and</strong> analytes are highly polar.<br />

2. Application <strong>of</strong> a phase separation mechanism should be carefully<br />

reconsidered <strong>and</strong> tested. The danger <strong>of</strong> b<strong>and</strong> broadening <strong>and</strong> splitting<br />

increases with both polymer molar mass <strong>and</strong> crystallization tendency.<br />

However, EG HPLC in the phase separation mode is very useful for<br />

separation <strong>of</strong> many nonpolar polymers <strong>and</strong> oligomers (3).<br />

3. Avoidhybridseparationmechanisms<strong>and</strong>,inparticular,thecombinations<br />

<strong>of</strong> phase separation with adsorption or with enthalpic partition. Use<br />

thermodynamicallygoodsolventsforanalyzedsamplesasmobilephase<br />

components, <strong>and</strong> beware <strong>of</strong> the co-nonsolvencyphenomenon.<br />

6 SECOND DIMENSION HPLC SEPARATION SYSTEMS<br />

Onceacomplexpolymerhasbeenseparatedprimarilyorexclusivelyaccordingto<br />

one single characteristic, the second dimension separation <strong>of</strong> fractions is<br />

substantially simplified. Fractions leaving the first dimension HPLC system are<br />

usually forwarded into a “regular” SEC system. If the molar mass effect is<br />

suppressed/deleted in the first dimension HPLC system the SEC data for each<br />

fraction leaving the first separation column <strong>and</strong> possessing narrow distribution in<br />

chemicalstructure orarchitecture directly reflect its molar mass distribution. Still,<br />

determination <strong>of</strong> true molar masses from retention volumes may be complicated.<br />

Forexample,sequencelengthdistributionwillbepresentinstatisticalcopolymers.<br />

Even if this third property distribution is neglected, we encounter problems<br />

connected with selective detection (Sec. 10) <strong>and</strong> with the complicated relation<br />

between retention volume <strong>and</strong> molar mass <strong>of</strong> fractions. To apply Benoit’s universal<br />

calibration dependence (51), functional dependence <strong>of</strong> viscosity law constants<br />

© 2004 by Marcel Dekker, Inc.

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