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Handbook of Size Exclusion Chromatography and Related ...

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that is, log[h] vs. logM,generated using universal calibration. The interdetector<br />

volume is adjusted until the expected Mark–Houwink exponent is obtained (34).<br />

Anotherapproachtodeterminingtheinterdetectorvolume<strong>of</strong>aviscometeris<br />

first to establish an [h] vs. elution volume calibration curve using aseries <strong>of</strong><br />

narrow polymer st<strong>and</strong>ards <strong>of</strong> known intrinsic viscosities. A broad molecular<br />

weight st<strong>and</strong>ard is then injected <strong>and</strong> the interdetector volumeis adjusted to obtain<br />

superimposition <strong>of</strong> the intrinsic viscosity calibration curve (35).<br />

Withalight-scatteringdetector,alogMvs.elutionvolumecalibrationcurve<br />

is constructed from aseries <strong>of</strong> narrow molecular weight distribution polymer<br />

st<strong>and</strong>ards. Abroad molecular weight distribution st<strong>and</strong>ard is then injected, <strong>and</strong> an<br />

iterative procedure finds the interdetector volume that superimposes the broad<br />

MWD st<strong>and</strong>ard calibration curve onto the one established by the narrow<br />

st<strong>and</strong>ards (36).<br />

Finally,aspectrophotometric method has been proposed in which alowangle<br />

laser light-scattering (LALLS) detector isused as an absorption photometer<br />

(33). Interdetector volume is then determined by injecting asolute that absorbs<br />

radiation from the LALLS detector. Mourey <strong>and</strong> Miller (33) used copper<br />

cyclohexanebutyrate as the solute <strong>and</strong> determined interdetector volume using the<br />

peak onsets <strong>of</strong> the LALLS detector <strong>and</strong> refractometer.<br />

3.5 B<strong>and</strong> Broadening<br />

SEC does not provide infinite resolution <strong>of</strong> species with different hydrodynamic<br />

volumes; as a result each slice has some residual polydispersity. This is<br />

primarily the result <strong>of</strong> the finite time required for agiven polymer to diffuse<br />

into <strong>and</strong> out <strong>of</strong> the stationary phase. The effect can be compounded by extra<br />

dead volume in the detectors or connecting tubing. In conventional SEC, b<strong>and</strong><br />

broadening leads to an overestimate <strong>of</strong> sample polydispersity. This is because<br />

the eluting peak is broadened so that it appears to cover awide molecular<br />

weight range.<br />

If alight-scattering detector is used as adetector, then the true molecular<br />

weight at each elution volume can be directly measured. If there is any b<strong>and</strong><br />

broadening, each elution volume is polydisperse in molecular weight, <strong>and</strong> the<br />

measured quantity is aweight average. The slope <strong>of</strong> the measured Mw against<br />

elution volume is flatter than the calibration curve for the molecular weight<br />

because <strong>of</strong> b<strong>and</strong> broadening, <strong>and</strong> the sample appears less polydisperse. Although<br />

the weight-average molecular weight for the sample can still be measured<br />

correctly,the number-average molecular weight is overestimated because <strong>of</strong> the<br />

lack <strong>of</strong> resolution. As a result, polydispersity is underestimated. The error<br />

introduced to molecular weight parameters as afunction <strong>of</strong> b<strong>and</strong> broadening is<br />

given in Fig. 7. These results are based on computer simulation studies (37).<br />

© 2004 by Marcel Dekker, Inc.

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