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Handbook of Size Exclusion Chromatography and Related ...

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where Nis the column efficiency expressed as theoretical plate number, VR,n is<br />

retentionvolume<strong>of</strong>thenthcomponent,<strong>and</strong>Vm isthetotalvolume<strong>of</strong>theliquidin<br />

the column (void volume).<br />

The peak capacity for atwo-dimensional chromatographic system n2D is<br />

n2D ¼n1n2sinu (2)<br />

wheren1 <strong>and</strong>n2 arepeakcapacities<strong>of</strong>one-dimensionalchromatographicsystems,<br />

whichareincludedintothetwo-dimensionalsystem,<strong>and</strong>uiscalledtheseparation<br />

angle between particular dimensions. u¼90 W<br />

holds for two procedures that<br />

separateanalytesexclusivelyaccordingtoone,different,property.Itisevidentthat<br />

the peak capacity <strong>of</strong> two-dimensional chromatographic separations largely<br />

exceeds selectivity <strong>of</strong> one-dimensional procedures.<br />

Theaboveconsiderationscanbeextendedalsotopolymericanalyteswhich,<br />

<strong>of</strong> course, can be separated into chemical individuals only in the range <strong>of</strong><br />

oligomers with rather low molar masses.<br />

Fractions leaving the first separation system C#1 are <strong>of</strong>f-line or on-line<br />

transferred into the second separation system C#2 (Fig. 3). The <strong>of</strong>f-line<br />

arrangements are generally more flexible but also time, sample, <strong>and</strong> labor<br />

intensive. Therefore, we shall deal in this chapter mainly with the on-line<br />

2D-HPLC systems.<br />

The two HPLC systems are to be selective to particular molecular<br />

characteristics <strong>of</strong> polymer sample (u between 60 <strong>and</strong> 908), that is, each system must<br />

separate macromolecules preferably or exclusively according to one molecular<br />

characteristic. The option u ¼ 90 W<br />

strongly simplifies data processing. If the first<br />

separation system C#1 discriminates macromolecules exclusively according to one<br />

single characteristic, the second HPLC system C#2 <strong>of</strong>ten does not at all need to be<br />

selective only to the second characteristic. For example, if C#1 separates molecules<br />

<strong>of</strong> copolymers exclusively according to their chemical composition <strong>and</strong> molar<br />

mass does not affect retention volumes, the second separation system may be a<br />

normal SEC column, because each fraction from the first separation system<br />

contains only species within a narrow composition range (sequence length<br />

distribution is neglected). On the contrary, if both separation systems discriminate<br />

macromolecules according to both characteristics with similar selectivities, the<br />

quantitative data evaluation is practically impossible. Therefore, the sequence <strong>of</strong><br />

particular separation systems is very important. One <strong>of</strong> the first attempts for twodimensional<br />

separation <strong>of</strong> statistical copolymers was published by Balke <strong>and</strong> Patel<br />

(2). These authors combined two liquid chromatographic systems, both <strong>of</strong> which<br />

separated macromolecules mainly according to their size. The first dimension<br />

separation system was an SEC column. Fractions from the SEC column, each<br />

containing species <strong>of</strong> different molar masses <strong>and</strong> compositions, were forwarded<br />

into the second dimension separation column, which combined entropic<br />

© 2004 by Marcel Dekker, Inc.

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