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Handbook of Size Exclusion Chromatography and Related ...

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masses <strong>and</strong> narrow molar mass distributions. Such polymer st<strong>and</strong>ards are used<br />

also in optimization <strong>of</strong> HPLC column/eluent/detector systems <strong>and</strong> for<br />

evaluation <strong>of</strong> b<strong>and</strong> broadening/splitting phenomena. Several—but not too<br />

many—series <strong>of</strong> homopolymers with different MMM <strong>and</strong> narrow MMD are<br />

available from a h<strong>and</strong>ful <strong>of</strong> producers. SEC calibration dependences can be<br />

constructed also by applying well-characterized broad molar mass polymers.<br />

The latter are especially suitable for periodic checking <strong>of</strong> SEC instrument<br />

performance. Unfortunately,only few well-defined complex model polymers are<br />

commercially available, such as block-, graft-, <strong>and</strong> statistical-copolymers,<br />

further star-, cyclic-, branched-, stereoregular-, <strong>and</strong> so on, species. These<br />

models are very much needed for HPLC method development, otherwise many<br />

separations will be evaluated using poorly defined polymer species. Many<br />

polymer synthesists are reluctant to make their samples available to<br />

chromatographers, probably in order to prevent bad disclosures, or because<br />

they try (or hope to be able to in the future) to characterize their samples<br />

themselves. In any case, the lack <strong>of</strong> well-defined polymer models hampers<br />

progress in HPLC <strong>of</strong> complex polymers including 2D-HPLC method<br />

development.<br />

5 FIRST-DIMENSION SEPARATION SYSTEMS<br />

(COUPLED HPLC PROCEDURES)<br />

As mentioned in Sec. 2, an important part <strong>of</strong> each polymer 2D-HPLC strategy is<br />

either to suppress or to strongly increase separation selectivity according to one<br />

molecular characteristic in the first separation column while separation according<br />

to the second molecular characteristic may remain essentially unchanged.<br />

Selectivity <strong>of</strong> SEC separation is limited by column packing pore volume.<br />

Augmentation <strong>of</strong> entropy-dominated separation selectivity according to polymer<br />

molar mass canbeachievedbyadding anenthalpic retentionmechanism(1 .0).<br />

The resulting benefit is, however, rather limited except in the case <strong>of</strong> some<br />

oligomers (Sec. 3.2 <strong>and</strong> Fig. 4).<br />

Much more promising seems to be the suppression <strong>of</strong> HPLC separation<br />

selectivity according to one molecular characteristic, especially according to<br />

polymer molar mass. One can speak about one-parameter HPLC separation <strong>of</strong><br />

macromolecules. An inspection <strong>of</strong> Eq. (3) <strong>and</strong> Fig. 4 reveals one such<br />

possibility. If DS <strong>and</strong> DH contributions to DG are equal, K in Eq. (3) is 1 <strong>and</strong><br />

VR is a constant, independent <strong>of</strong> molar mass <strong>of</strong> polymer analyte. The linear<br />

part <strong>of</strong> SEC calibration dependence can be expressed with the following<br />

equation:<br />

© 2004 by Marcel Dekker, Inc.<br />

VR ¼ C D log M (7)

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