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Handbook of Size Exclusion Chromatography and Related ...

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historymayalsoplayanimportantroleinconventionalSECmeasurements.Itcan<br />

substantially reduce the precision <strong>and</strong> accuracy <strong>of</strong> results. The successive<br />

deposition<strong>of</strong>sampleconstituent(s)withinpackingis<strong>of</strong>tenresponsibleforlimited<br />

life-time <strong>of</strong> HPLC columns, which is evidenced by irrepeatability <strong>of</strong> retention<br />

volumes <strong>and</strong> by large b<strong>and</strong> broadening effects.<br />

Adsorption<strong>of</strong>analytesdependsalsoontemperature<strong>and</strong>pressurewithinHPLC<br />

columns. Temperature can be used as an important parameter to affect retention <strong>of</strong><br />

macromolecular analytes (22). Correspondingly, temperature must be carefully<br />

controlled in enthalpy-driven HPLC <strong>of</strong> polymers. This may be difficult due to heat<br />

evolved in acolumn by the friction <strong>of</strong> the flowing mobile phase. As aresult, both<br />

axial<strong>and</strong>radialtemperaturegradientsmaybecreatedinHPLCcolumns(23).Direct<br />

pressure effects are anticipated only at very high pressures <strong>of</strong> hundreds <strong>of</strong> MPa<br />

(thous<strong>and</strong>s<strong>of</strong>bars).Ontheotherh<strong>and</strong>,importantVRvariationsmayalreadyappearat<br />

muchlowerpressurechangeswhenworkingwithmixedmobilephases.Preferential<br />

sorption <strong>of</strong> the mixed eluent components within the column packing (Sec. 4.1) is<br />

<strong>of</strong>ten strongly affected by pressure variations as low as a few MPa <strong>and</strong>, consequently,<br />

analyte retention can be altered (24,25). This may happen, for example, due to flow<br />

rate adjustment or due to partial blocking <strong>of</strong> the exit column filter.<br />

3.2.2 Enthalpic Partition<br />

Enthalpic partition <strong>of</strong> analyte molecules between (at least) two chemically<br />

different liquid phases gives rise to the second important retention mechanism in<br />

polymer HPLC. The stationary liquid phase can be created, for example, by<br />

adsorption or absorption <strong>of</strong> a liquid immiscible with eluent on the inner <strong>and</strong> outer<br />

surface or within the pore volume <strong>of</strong> a particulate or monolithic column filling.<br />

Alternatively, a dynamic (quasi) stationary phase can be formed as a result <strong>of</strong><br />

preferential adsorption <strong>of</strong> a mixed eluent component on the filling surface. The<br />

HPLC approaches, which are based on the above phenomena are accompanied<br />

with important experimental problems connected with stationary phase relating to<br />

both “bleeding” <strong>and</strong> composition changes. Therefore, chemically bonded<br />

stationary phases are preferred in modern HPLC. In spite <strong>of</strong> numerous attempts<br />

to modify surfaces <strong>of</strong> various carriers based on inorganic oxides with polymers<br />

(for review see Ref. 26) the HPLC field is presently dominated by materials<br />

prepared by bonding short aliphatic groups C4,C8,C14, C22, C30, <strong>and</strong> mainly C18 onto porous <strong>and</strong> nonporous SiO2 particles (Sec. 4.1). Recently, monolithic silica<br />

C18 also became available (27). Other important HPLC column fillings represent<br />

heterogeneously crosslinked porous, nonporous, <strong>and</strong> monolithic systems based on<br />

natural or synthetic polymers (Sec. 4.1).<br />

At present, silica C18 materials are almost exclusively used in enthalpic<br />

partition HPLC <strong>of</strong> polymers. Consequently, the enthalpic partition retention<br />

mechanism seems to be limited to weak London nonpolar interactions. In fact,<br />

© 2004 by Marcel Dekker, Inc.

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