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Handbook of Size Exclusion Chromatography and Related ...

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4 CALIBRATION AND CALCULATION<br />

The most commonly used polymer MW st<strong>and</strong>ard is probably the polystyrene (PS)<br />

st<strong>and</strong>ard,because <strong>of</strong> availability <strong>of</strong> narrow distribution st<strong>and</strong>ards over awide MW<br />

range, from close to 10 10 6 tounimer, <strong>and</strong> because <strong>of</strong> its solubility in various<br />

commonorganicsolventsforSECstudies.Inmanyapplications,whentheabsolute<br />

MWis not necessary,the MW results calculated using PS st<strong>and</strong>ards are acceptable<br />

forrelativecomparison.Itishighlyrecommendedtoaddtheunimer,hexylbenzene,<br />

MW ¼162, in calibration, especially when stabilized THF is used as the mobile<br />

phase. The BHT stabilizer peak <strong>of</strong>ten shows up between the trimer, 370MW,<strong>and</strong><br />

theunimerpeaks.Itisimportanttonotethattherefractiveindex<strong>of</strong>trichlorobenzene<br />

(TCB)happenstobeveryclosetothetrimer.Therefore,thetrimerbecomesinvisible,<br />

while the dimer <strong>and</strong> the unimer become negative peaks (Fig. 10).<br />

Poly(ethylene glycol) (PEG) is another useful MW st<strong>and</strong>ard for SEC in THF<br />

<strong>and</strong>morepolarsolvents.ThehigherMWst<strong>and</strong>ards(.1,000)aredifficulttodissolve<br />

inTHFatroomtemperature.Theycanhedissolvedatelevatedtemperature<strong>and</strong>will<br />

stay in solution when the solution is cooled. It is also important to note that the<br />

retentiontimes<strong>of</strong>verylowMWoligomers<strong>of</strong>PEG(,200)arenotlinear relativeto<br />

higher MW PEG st<strong>and</strong>ards for an unknown reason (10).<br />

It is difficult to use an on-line MW detector, such as light-scattering<br />

photometer or viscometer, for absolute MW analysis <strong>of</strong> low MW oligomers by<br />

SEC because <strong>of</strong> lack <strong>of</strong> sensitivity.However, an absolute MW calibration curve<br />

may be created if the low oligomer peaks can be resolved <strong>and</strong> the MWs can be<br />

assigned.Figure 11isanexamplefor polyols,wherethelowMWoligomer peaks<br />

Figure 10 FSECchromatograms<strong>of</strong>styreneoligomersinTHF<strong>and</strong>inTCB.(A)Column:<br />

PLgel MIXED-E; mobile phase: TCB with 250ppm BHT, 1.0mL/min. (B) Same<br />

conditions as in Fig. 5.<br />

© 2004 by Marcel Dekker, Inc.

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