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Handbook of Size Exclusion Chromatography and Related ...

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4 COLUMNS FOR HOPC<br />

DetailsaregiveninChapter23<strong>of</strong>Ref.6.However,inshort,poroussilicaparticles<br />

with anarrow pore size distribution are preferred to the polymeric gel beads<br />

commonlyusedinSEC.Specifically,controlledporeglasses(CPG)(17)available<br />

from CPG, Inc. (http:==www.cpg-biotech.com=) <strong>and</strong> Prime Synthesis<br />

(http:==www.primesynthesis.com=) <strong>of</strong>fer excellent separation. CPG is available<br />

inaverageporediametersthatrangefrom 80A ˚ to 3000A ˚ .Thesurface<strong>of</strong>CPG<br />

needs to be modified to prevent adsorption <strong>of</strong> polymer. Adsorption may lead to<br />

clogging <strong>of</strong> the column. Various silanization agents are available from Gelest<br />

(http:==www.gelest.com=)<strong>and</strong>Fluka<strong>of</strong>Aldrich(http:==www.sigmaaldrich.com=).<br />

Surface modification methods are described in the literature (1–10).<br />

The mean pore diameter should be sufficiently small to exclude most <strong>of</strong> the<br />

injected polymer at low concentrations but admit its low-MW components at high<br />

concentrations. Specifically, the ratio <strong>of</strong> the mean pore diameter to the radius <strong>of</strong><br />

gyration Rg <strong>of</strong> the polymer at its average MW should be between 1 <strong>and</strong> 2 (2,3,6).<br />

This size criterion is equivalent to 1=4 <strong>of</strong> the pore size in the columns used in<br />

SEC to analyze the same polymer. If the polymer can be analyzed by nonaqueous<br />

SEC, its chromatogram will give an estimate <strong>of</strong> Rg <strong>of</strong> the polymer. The following<br />

approximate formula (18) is convenient:<br />

Rg (nm) ¼ 0:0125 [M(g=mol)] 0:595<br />

where M is the polystyrene-equivalent average MW (weight-average or peak MW).<br />

Analytical-size columns can be used in HOPC for preparative purposes. Past<br />

studies indicate that columns <strong>of</strong> dimension 3:9 300 mm <strong>and</strong> 7:8 300 mm give<br />

a better resolution than thinner columns (6). Using a longer column or cascading a<br />

few columns, a practise that improves resolution in SEC, does not necessarily<br />

improve the resolution in HOPC.<br />

5 OPERATION OF HOPC<br />

Once a column (or a bank <strong>of</strong> columns) is selected according to the criteria<br />

described above, there are still several parameters a user can choose or must decide<br />

upon. They include the solvent, the concentration, the injection volume, the flow<br />

rate, <strong>and</strong> the column temperature.<br />

The solvent must dissolve the polymer at high concentrations. A good<br />

solvent, a theta solvent, <strong>and</strong> any solvent between them can be used.<br />

The concentration should be as high as possible unless the solution is too<br />

viscous for injection. Typically, the viscosity <strong>of</strong> honey at room temperature is<br />

adequate.<br />

© 2004 by Marcel Dekker, Inc.

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