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Handbook of Size Exclusion Chromatography and Related ...

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endered useful in the separation <strong>of</strong> polymers according to their stereoregularity<br />

(61,62) <strong>and</strong> for discrimination <strong>of</strong> linear <strong>and</strong> cyclic macromolecules with similar<br />

molar masses (63–65). Numerous critical systems are listed in Refs. 4, 5, <strong>and</strong> 66.<br />

LC CC was also included in two-dimensional HPLC (4,5,62,67). During<br />

application <strong>of</strong> LC CC, its numerous important experimental limitations were<br />

revealed (68,69). Most <strong>of</strong> them are mentioned at the end <strong>of</strong> this section.<br />

Unfortunately,the drawbacks <strong>of</strong> LC CC were ignored in some recent books (4,5).<br />

Cifra <strong>and</strong> Bleha (70) also showed by Monte Carlo modelling that the molar mass<br />

region in which perfect entropy–enthalpy compensation takes place may be<br />

limited (Fig. 4, curve 9). Still, LC CC remains attractive for characterization<br />

<strong>of</strong> many complex polymers both in direct (one separation system) <strong>and</strong> twodimensional<br />

arrangements. LC CC proved especially successful in molecular<br />

characterization <strong>of</strong> oligomers (4) where, for example, the problems connected<br />

with strong interaction <strong>of</strong> large polymer species with walls <strong>of</strong> narrow pores<br />

(Sec. 3.2.1) are less important <strong>and</strong> also relatively high concentration <strong>of</strong> analytes<br />

are experimentally feasible. The latter feature <strong>of</strong> oligomer HPLC, including<br />

LC CC, allows application <strong>of</strong> a less sensitive flow-through densitometer as an<br />

additional detection system (4).<br />

A situation similar to entropy/enthalpy compensation may also appear in<br />

systems where two enthalpic retention mechanisms, for example, adsorption <strong>and</strong><br />

enthalpic partition, affect retention volumes in an opposite way (71).<br />

The following practical hints may be useful for LC CC users:<br />

1. Whenever possible, the enthalpic partition retention mechanism is<br />

preferential. It is difficult to maintain the HPLC system at the critical<br />

adsorption point. Minute variation in eluent composition due to<br />

preferential evaporation <strong>and</strong> moisture absorption may strongly affect<br />

adsorption <strong>of</strong> polymer analytes. Consequently, the LC CC system must<br />

be frequently controlled <strong>and</strong> critical conditions adjusted for example by<br />

temperature variations. If, however, the adsorption mechanism has to be<br />

applied, try to identify both adsorli <strong>and</strong> desorli, which are as good<br />

solvents for the analysed polymer as are possible. Critical composition<br />

<strong>of</strong> the eluent, which is situated in the vicinity <strong>of</strong> the theta point or even<br />

near the precipitation threshold, may further increase instability <strong>of</strong> the<br />

critical adsorption point <strong>and</strong> increasingly deteriorate repeatability <strong>of</strong><br />

measurements.<br />

2. Avoid using very narrow pore column packings. The danger <strong>of</strong><br />

backward curvature <strong>of</strong> critical calibration curves, as well as problems<br />

connected with peak broadening <strong>and</strong> decreased sample recovery<br />

decrease with rising pore diameter (47). Unfortunately, the selectivity <strong>of</strong><br />

SEC separation for lower molar mass polymer species is at least<br />

partially sacrificed when narrow pore column packing is removed.<br />

© 2004 by Marcel Dekker, Inc.

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