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Handbook of Size Exclusion Chromatography and Related ...

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provided the FAD column is packed with nonporous particles (see Sec. 4.1.2 <strong>and</strong><br />

the role <strong>of</strong> polymer adsorption with narrow pores).<br />

Thefullentrapment<strong>of</strong>nonpolarmacromoleculeswithinthesilicaC 18phase<br />

fromapolareluent(reversedphase)utilizingpartitionmechanisminFREismore<br />

problematic. In this case, the extent <strong>of</strong> retention within the column packing may<br />

depend on the degree <strong>of</strong> silica coverage with C18 groups <strong>and</strong> on polymer molar<br />

mass. For example, full retention <strong>of</strong> narrow molar mass polystyrenes from<br />

dimethylformamideonsilicanonporousC18wasattainedonlyaboveamolarmass<br />

<strong>of</strong> 90kmolg 1 (28). FRE procedures based on phase separations may be even<br />

more difficult because they are affected by (slow) dynamics <strong>of</strong> phase separation<br />

processes (Secs 3.2.3 <strong>and</strong> 5.2).<br />

FAD allows extensive compression <strong>of</strong> chromatographic b<strong>and</strong>s, that is, the<br />

reconcentration <strong>of</strong> diluted polymer solutions. For example, areconcentration<br />

factor <strong>of</strong> 600 was easily achieved for poly(methyl methacrylate) using bare,<br />

nonporous silica-based FAD column packing (91). FRE procedures can also be<br />

used for sample storing <strong>and</strong> sample matrixexchange(Sec. 9). FRE column(s) can<br />

be directly connected with an SEC system. In thisway we arriveat the FRE/SEC<br />

quasi two-dimensional HPLC system. An optimized FAD/SEC method was used<br />

for separation <strong>and</strong> molecular characterization <strong>of</strong> multicomponent polymer blends<br />

(up to six components) (92) <strong>and</strong> also for determination <strong>and</strong> characterization <strong>of</strong><br />

minor macromolecular admixtures ( 1%) in polymer blends (93). Using FAD,<br />

Lazzari et al. (94) also successfully separated four-arm highly syndiotactic star<br />

poly(methyl methacrylate)s from their linear PMMA pendant.<br />

Many different arrangements are possible for pursuing full retention–<br />

elution separations. Atypical FAD/SEC system is shown in Fig. 16. The system<br />

can be altered to meet particular needs. For example, instead <strong>of</strong> amixing device<br />

(an HPLC gradient maker) aseries <strong>of</strong> displacing liquids with precisely adjusted<br />

compositions can be stored in separate containers. Several FAD columns can be<br />

arranged in parallel or in series. Fillings used in these columns may be identical<br />

or have different interaction activities. Further, sample injection valve (V2 in<br />

Fig. 16) can be substituted by an independent HPLC system <strong>and</strong> in this way we<br />

arrive at the real 2D-HPLC system enforced with aFAD system (Sec. 9). In the<br />

latter case, the FAD column may act also as an additional separation unit to form<br />

a quasi three-dimensional HPLC system. Further, an efficient retention<br />

promoting liquid, for example, an adsorli, can be continuously added to the<br />

sample (that is to the HPLC column effluent) to assure retention <strong>of</strong> analytes<br />

leaving (95). Afull retention approach can also be used for sample purification<br />

(Sec. 8).<br />

FRE procedures may also assist sample detection in the second dimension<br />

column effluent, for example, by additional reconcentration <strong>of</strong> fractions leaving<br />

column #2, or by eluent exchange for NMR <strong>and</strong> infrared spectroscopic<br />

measurements.<br />

© 2004 by Marcel Dekker, Inc.

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