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Handbook of Size Exclusion Chromatography and Related ...

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differential refractometer, a viscometer, <strong>and</strong> a UV/visible spectrophotometer.<br />

Interdetector dead volumes are critical in SEC applications (26) because<br />

fractionated material elutes in peaks that pass fairly quickly through the detectors.<br />

Most time-dependent reactions that have been the subject <strong>of</strong> ACM techniques,<br />

however, occur on a scale <strong>of</strong> minutes or hours, so interdetector dead volume is not<br />

a critical issue.<br />

A specific configuration involving a home-built multi-angle light-scattering<br />

instrument, Shimadzu SPD-10AV UV/visible detector, Waters 410 RI, <strong>and</strong> a<br />

home-built viscometer has been treated in several references.<br />

Light scattering data is normally analyzed according to the well-known<br />

Zimm approximation (27)<br />

Kc 1<br />

¼<br />

I(q, c) MP(q) þ 2A2c þ [3A3Q(q) 4A 2 2MP(q)(1 P(q))]c2 þ O(c 3 ) (1)<br />

where c is the polymer concentration (g/cm 3 ), P(q) the particle form factor, q is<br />

the amplitude <strong>of</strong> the scattering wave-vector q ¼ (4pn= ) sin(u=2), where u is the<br />

scattering angle, <strong>and</strong> K is an optical constant, given for vertically polarized<br />

incident light by<br />

K ¼ 4p 2 n 2 (@n=@c) 2<br />

NA 4 (2)<br />

where n is the solvent index <strong>of</strong> refraction, is the vacuum wavelength <strong>of</strong> the<br />

incident light, <strong>and</strong> @n=@c is the differential refractive index for the polymer in the<br />

solvent. Q(q) involves a sum <strong>of</strong> complicated Fourier transforms <strong>of</strong> the segment<br />

interactions that define A2. In the limit <strong>of</strong> q ¼ 0, P(0) ¼ Q(0) ¼ 1, so that for a<br />

polydisperse polymer population, this becomes<br />

Kc 1<br />

¼ þ 2A2c þ 3A3c<br />

I(0, c) Mw<br />

2 þ O(c 3 ) (3)<br />

For low enough concentrations that the c 2 term in Eq. (1) is negligible, <strong>and</strong> for<br />

q 2 kS 2 l z , 1, another, frequently used form <strong>of</strong> the Zimm equation becomes<br />

Kc 1<br />

¼<br />

I(q, c) Mw<br />

1 þ q2 kS 2 l z<br />

3<br />

þ 2A2c (4)<br />

where kS 2 l z is the z-average mean square radius <strong>of</strong> gyration. As pointed out in<br />

other polyelectrolyte studies (28,29), in this limit kS 2 l z can be determined at low<br />

concentrations if Mw is known, without a full extrapolation to c ¼ 0.<br />

The voltage V(t) <strong>of</strong> the single capillary viscometer is directly proportional to<br />

the total viscosity <strong>of</strong> the solution flowing through the capillary. This allows the<br />

© 2004 by Marcel Dekker, Inc.

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