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Handbook of Size Exclusion Chromatography and Related ...

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It should be noted that dn=dc also varies with molecular weight. Typically,<br />

the dn=dc value increases with increasing molecular weight <strong>and</strong> reaches an<br />

asymptotic limit for molecular weights greater than approximately 20,000g/mol.<br />

For polymers with fractions in this low-molecular-weight regime, this effect<br />

should be taken into consideration because it generally leads to an error in the<br />

measurement <strong>of</strong> the low-molecular-weight region <strong>of</strong> the distribution; that is, the<br />

number-averge molecular weight is most affected. For example, if dn=dc<br />

decreases with molecular weight, then the molecular weight at each elution volume<br />

is overestimated, especially Mn. If the entire polymer MWD is below 20,000, then<br />

dn=dc values should be determined separately for the required molecular weight<br />

range.<br />

One other consideration is the effect <strong>of</strong> ionic groups on synthetic<br />

polyelectrolytes <strong>and</strong> biopolymers. To measure a reliable value for dn=dc, the<br />

polymer solution, containing electrolyte, must be dialysed against the solvent<br />

system until a constant chemical potential is obtained. Details on the determination<br />

<strong>of</strong> dn=dc <strong>of</strong> polyelectrolytes can be found in Refs. 2, 24, <strong>and</strong> 26.<br />

3.2.2 Instrument Calibration<br />

Determination <strong>of</strong> the Rayleigh ratio from the scattered light intensity requires that<br />

the light-scattering detector be calibrated to account for detector sensitivity, cell<br />

geometry, <strong>and</strong> so on. Utiyama (27) discusses calibration procedures <strong>and</strong> st<strong>and</strong>ards<br />

for light-scattering measurements. Because procedures vary depending upon<br />

instrument <strong>and</strong> cell design, discussion <strong>of</strong> instrument calibration is not presented<br />

here <strong>and</strong> the reader is advised to consult manufacturers’ instruction manuals.<br />

3.2.3 Measurement <strong>of</strong> Molecular Weight Distribution<br />

When dn=dc <strong>and</strong> n0 have been determined, <strong>and</strong> the instrument calibrated, the<br />

molecular weight can be calculated from the light-scattering intensity <strong>and</strong> the<br />

concentration at each elution volume [Eq. (9)]. These values can then be used to<br />

determine the molecular weight distribution. If there is any polydispersity at a<br />

given elution volume caused by heterogeneity <strong>of</strong> composition or structure, the<br />

calculated value is a weight-average molecular weight.<br />

3.2.4 Measurement <strong>of</strong> Sample Mw<br />

It can be shown that the weight-average molecular weight can be determined from<br />

the ratio <strong>of</strong> the area <strong>of</strong> the light-scattering intensity measured at low angle, ,108,<br />

<strong>and</strong> the concentration chromatograms, corrected for their respective calibration<br />

constants (28):<br />

P P<br />

Mici Ru<br />

Mw ¼ P ¼ P<br />

i=K*<br />

(21)<br />

ci ci<br />

© 2004 by Marcel Dekker, Inc.

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