28.02.2013 Views

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

narrowmolecular weight range, individual pore size packings aresuitable.This is<br />

particularlythecaseforsmallmoleculeseparationsasshowninFig.5.Forpolymer<br />

analyses, where resolution is required covering several decades <strong>of</strong> molecular<br />

weight, mixed gel or linear columns arewidely applicable. Figure 6illustrates the<br />

application<strong>of</strong>mixedgelscolumnstotheanalysis<strong>of</strong>polyethylene,whichtypically<br />

has ahigh polydispersity.<br />

Resolution in SEC is dependent on:<br />

1. the slope <strong>of</strong> the calibration plot dlogM=dv, <strong>and</strong><br />

2. efficiency.<br />

These two parameters should be manipulated in order to optimize resolution (20).<br />

Calibration slope can be decreased by the addition <strong>of</strong> more columns in series <strong>and</strong><br />

the effect on resolution is illustrated in Fig. 7. Efficiency is dependent on particle<br />

size <strong>and</strong> smaller particle size, higher efficiency columns are generally preferred.<br />

The effect <strong>of</strong> particle size on the separation <strong>of</strong> polystyrene oligomers is shown<br />

inFig.8.Columnsetsshouldcomprisepackingmaterials<strong>of</strong>thesameparticlesize<br />

as the full potential efficiency <strong>of</strong> the system will never be achieved if large <strong>and</strong><br />

small particle size columns are combined.<br />

In achromatographic bed the largest tangential shear stresses in themoving<br />

eluentstreamwouldbeexpectedtobeinthemostopenareassubjecttothehighest<br />

flows, that is, in the spaces between the particles. It has been estimated (21) that<br />

Figure 5 Separation <strong>of</strong> dialkylphthalates, two columns PLgel 3mm, 100 A˚,<br />

300 7.5 mm, eluant tetrahydr<strong>of</strong>uran, flow rate 1 mL/min, detector refractive index<br />

(RI); (1) dioctyl phthalate, (2) dibutyl phthalate, (3) diethyl phthalate, (4) dimethyl<br />

phthalate, (5) toluene.<br />

© 2004 by Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!