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Handbook of Size Exclusion Chromatography and Related ...

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largely used, being complemented by only a few other, mainly hydrophilic<br />

materials. The chemical structure <strong>of</strong> the organic column fillings is <strong>of</strong>ten nondisclosed<br />

by column producers. Quite popular are fillings based on hydroxyethyl<br />

methacrylate <strong>and</strong> (hydrolysed) glycidyl methacrylate resins.<br />

Rapid progress in SiO2 binding chemistries (42) <strong>and</strong> in synthetic polymer<br />

column filling materials including monoliths (43,44) raises the hope that the<br />

situation regarding column fillings for polymer HPLC will soon improve. Also,<br />

further developments in polymer HPLC <strong>and</strong> 2D-HPLC may eventually attract the<br />

attention <strong>of</strong> column producers.<br />

Silica gel is a very complex material with complicated physical <strong>and</strong><br />

chemical structures (45,46). Many parameters <strong>of</strong> silica gels have been studied in<br />

detail, <strong>and</strong> technology for the production <strong>of</strong> HPLC silica-based column fillings<br />

has substantially improved in recent years. This is manifested, for example, in<br />

better batch-to-batch reproducibility <strong>of</strong> silica gel HPLC column packings from<br />

most producers. Still, numerous questions concerning this material remain so far<br />

unanswered <strong>and</strong> further progress in controlling silica gel properties is needed. As<br />

mentioned, the structure <strong>of</strong> silica gel pores is advantageous for fast mass transfer<br />

<strong>of</strong> samples, which results in increased column efficiency.Silica gels with various<br />

pore diameters D(up to several hundreds <strong>and</strong> even thous<strong>and</strong>s <strong>of</strong> nm) <strong>and</strong> pore<br />

volumes (up to 2mL g 1 )have been synthesized; however, the market <strong>of</strong> HPLC<br />

columns for separation <strong>of</strong> low molar mass substances dictates parameters <strong>of</strong> most<br />

commercial silica gels. Mean values <strong>of</strong> Dfor most silica gels in the range<br />

6–12nm (60–120A ˚ ).Their relatively low pore volumes in the range <strong>of</strong> 1mL g 1<br />

allow high pressure resistance. Silica gel based bonded phases with diameters <strong>of</strong><br />

30nm <strong>and</strong> sometimes 50nm are applied to HPLC <strong>of</strong> proteins. Silica gels with<br />

pore sizes up to 400nm that were on the market afew years ago are hardly<br />

available any more. As aresult, most HPLC separations <strong>of</strong> polymers are carried<br />

out with mesoporous column packings. Macromolecules with molar masses<br />

above 50–100kgmol 1 are excluded from pores <strong>of</strong> such packings under weak<br />

interaction regimes. Evidently,the outer surface <strong>of</strong> 5or 10mm particles, which<br />

lies in the range <strong>of</strong> hundreds <strong>of</strong> cm 2 g 1 ,is too small to allow selective retention<br />

<strong>of</strong> macromolecules. This supports the hypothesis on the barrier mechanism <strong>of</strong><br />

polymer retention in many isocratic <strong>and</strong> gradient procedures <strong>of</strong> polymer HPLC<br />

(Sec. 5.2). It is widely accepted that polar interactivity <strong>of</strong> silica gel is caused<br />

mainly by the presence <strong>of</strong> surface silanol groups. Siloxane moieties are less<br />

interactive. The adsorption activity <strong>of</strong> silanols depends on their topology <strong>and</strong><br />

concentration. As was shown for low molar mass analytes, the most active are<br />

isolated silanols, followed by geminal <strong>and</strong> vicinal silanols. The adsorptive<br />

activity <strong>of</strong> latter types <strong>of</strong> silanols is reduced by their mutual hydrogen bonding.<br />

Therefore the activity <strong>of</strong> silica gel in terms <strong>of</strong> polymer adsorption seems to reach<br />

its maximum at the intermediate silanol concentration at which the highest<br />

population <strong>of</strong> isolated silanols is anticipated (47).<br />

© 2004 by Marcel Dekker, Inc.

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