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Handbook of Size Exclusion Chromatography and Related ...

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temperature. The efficacy <strong>of</strong> an adsorli depends on the length <strong>of</strong> adsorbed<br />

macromolecular trains it allows. For an onset <strong>of</strong> full polymer retention already<br />

relativelyshorttrainsmaybesufficient.Similarly,efficacy<strong>of</strong>adesorlidependson<br />

its ability to block active sites on both packing surface <strong>and</strong> macromolecules. In<br />

HPLCsystems,animportantrolemayalsobeplayedbykineticparameters<strong>of</strong>both<br />

train formation <strong>and</strong> destruction. So far, little is known about these features <strong>of</strong><br />

polymeradsorption.Ourmeasurementshave,however,revealedthatattachment<strong>of</strong><br />

macromolecules onto nonoccupied adsorbent surface, which causes their full<br />

retention, is a fast process. Similarly, detachment <strong>of</strong> polymer chains from a<br />

nonporous adsorbent surface, which results from a desorli action, is quick<br />

provided the system is well mixed (20,21). It seems that an instantaneous contact<br />

between strongly interacting polymer <strong>and</strong> adsorbent pair or atwinling desorli<br />

action are sufficient for the full adsorption or desorption <strong>of</strong> macromolecules,<br />

respectively. On the other h<strong>and</strong>, conformational changes <strong>of</strong> adsorbing<br />

macromolecules <strong>and</strong> the diffusion-controlled mutual displacements <strong>of</strong> polymer<br />

speciesfromthenearlysaturatedadsorbentsurfacemaybemuchmoredilatory.As<br />

aresult, the equilibrium adsorption <strong>of</strong> polymers is considered aslow process,<br />

which needs hours or even days to fully develop (13). In any case, kinetic effects<br />

must be considered when evaluating adsorptive retention <strong>of</strong> macromolecules<br />

within porous HPLC column fillings.<br />

Asingle liquid or amixture <strong>of</strong> adsorli <strong>and</strong> desorli that is strong enough to<br />

allowelution <strong>of</strong> at least afraction <strong>of</strong> polymer sample from acolumn packing at a<br />

temperatureisdenotedadisplacer(0 1,1cr).Somebasicparameters<strong>of</strong>HPLC<br />

eluents will be discussed in Sec. 4.2.<br />

In conclusion, adsorption within an HPLC column causes achange in<br />

analyteretentioninadditiontoretentionduetotheeverpresententropicexclusion.<br />

In the weak adsorption regime (low 1values) the exclusion mechanism prevails<br />

<strong>and</strong> retention volumes <strong>of</strong> macromolecules increase with decreasing molar mass.<br />

On the contrary, retention volumes increase or even strongly rise with analyte<br />

molar mass in the strong adsorption regime (high 1values). Strong adsorption <strong>of</strong><br />

analyte molecules can lead to their full retention (Fig. 4<strong>and</strong> Sec. 7). Moreover,<br />

adsorption <strong>of</strong> analytes can bring about chromatographic b<strong>and</strong> broadening <strong>and</strong><br />

splitting. These two phenomena frequently appear with narrow pore HPLC<br />

column packings <strong>and</strong>, in particular for high molar mass (excluded) analytes. Often,<br />

it is difficult to remove large macromolecules adsorbed within narrow pores <strong>of</strong> the<br />

column packing. Very effective desorli must be applied <strong>and</strong> the desorbing process<br />

may be rather slow (15).<br />

A phenomenon that is termed “column history” can complicate<br />

experimental work with the adsorbing solutes. (Macro)molecules that were fully<br />

retained within packing <strong>and</strong> were not entirely removed by a careful column<br />

flushing procedure may promote adsorption <strong>of</strong> subsequent analytes. As a result,<br />

retention volumes may change in the course <strong>of</strong> a series <strong>of</strong> experiments. Column<br />

© 2004 by Marcel Dekker, Inc.

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