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Handbook of Size Exclusion Chromatography and Related ...

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nonporous particles. The quasi stationary phase volume in HDC is much smaller<br />

than in SEC <strong>and</strong>, correspondingly,selectivity <strong>of</strong> separation is lower in the former<br />

case. SEC column packings will be further discussed in Sec. 4.1.1.<br />

In entropy-driven separations such as SEC <strong>and</strong> HDC, retention volumes<br />

increase with decreasing sizes <strong>of</strong> macromolecules, that is, with decreasing molar<br />

masses <strong>of</strong> linear homopolymer samples. Atypical dependence <strong>of</strong> SEC retention<br />

volumes on polymer molar mass is schematically depicted in Fig. 4, curves 1–5.<br />

Dependences<strong>of</strong>thistypeareusedfordetermination<strong>of</strong>localmolarmasses<strong>of</strong><br />

polymericanalytesfromretentionvolumes.Theyareobtainedbyelution<strong>of</strong>aseries<br />

<strong>of</strong> polymer samples with known molar masses <strong>and</strong> are called SEC calibration<br />

dependences.Weshallexplainsomegeneralfeatures<strong>of</strong>polymerretentioninHPLC<br />

columns by means <strong>of</strong> this kind <strong>of</strong> diagram applying the term “calibration<br />

dependences,” though these plots are <strong>of</strong>ten constructed for other than calibration<br />

reasons.<br />

3.2 Enthalpic Retention Mechanisms<br />

Total change <strong>of</strong> enthalpy connected with the transfer <strong>of</strong> macromolecules from<br />

mobile into (quasi) stationary phase [DH in Eq. (3)] is composed <strong>of</strong> enthalpic<br />

interactions<strong>of</strong>polymersegmentswiththecolumnfilling.ThereforeDH islargein<br />

polymer HPLC provided attractive interactions <strong>of</strong> macromolecules with column<br />

filling exceed attractiveinteractions between eluent molecules <strong>and</strong> column filling,<br />

Figure 4 Schematics <strong>of</strong> calibration dependences in polymer HPLC for a constant<br />

exclusion contribution <strong>and</strong> different enthalpic interaction contributions expressed with<br />

segmental interaction energies, 1. 1: 1,0; 2: 1 0; 3,4: 1.0; 5: 1 0; 6: 1¼1cr (full<br />

entropy 2enthalpy compensation); 7: 1.1cr; 8: 1 1cr; 9: 1 1cr (nearly complete<br />

entropy 2enthalpy compensation, 1slightly depends on polymer molar mass); 10: 1<br />

strongly depends on polymer molar mass. For further explanation see Sec. 3.2.1.<br />

© 2004 by Marcel Dekker, Inc.

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