28.02.2013 Views

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

<strong>and</strong><br />

e¼<br />

2a 1<br />

3<br />

(18)<br />

The e parameter [Eq. (18)] is used to take into account deviations from u<br />

conditions (22). This approach has been evaluated with good success using<br />

polystyrene samples (20,23). If aviscosity detector is used in series with arightangle<br />

light-scattering detector, Eq. (16) can be used in an iterative procedure to<br />

correct for angular asymmetry (see Sec. 3.2.6).<br />

3.2 Light Scattering<br />

3.2.1 Determination <strong>of</strong> the Specific Refractive Index Increment <strong>and</strong><br />

Solvent Refractive Index<br />

The accuracy <strong>of</strong> the light-scattering measurement depends on prior determinations<br />

<strong>of</strong> the solvent refractive index <strong>and</strong> <strong>of</strong> the specific refractive index increment dn=dc<br />

<strong>of</strong> the sample in the solvent [Eq. (7)]. The solvent refractive index can be measured<br />

with a conventional refractometer or values found in the literature. The dn=dc<br />

value can be measured using either a differential refractometer or, less frequently,<br />

an interferometer. Measurements should be made at the same temperature as the<br />

light-scattering measurement <strong>and</strong> ideally at the same wavelength. Because <strong>of</strong><br />

the dependence <strong>of</strong> the optical constant on the square <strong>of</strong> dn=dc, extreme care must<br />

be taken with the measurement because any error is doubled in the calculated<br />

molecular weight. Detailed discussions <strong>of</strong> the measurement principles <strong>and</strong><br />

methods can be found in Refs. 2 <strong>and</strong> 24.<br />

A comprehensive tabulation <strong>of</strong> experimental values for dn=dc has been<br />

published (25). Many <strong>of</strong> these values are at different wavelengths, <strong>and</strong> the value at<br />

the desired wavelength can be obtained by extrapolation <strong>of</strong> a plot <strong>of</strong> dn=dc against<br />

the inverse <strong>of</strong> the wavelength squared using the relationship<br />

dn<br />

dc ¼ k0 þ k00<br />

l 2<br />

(19)<br />

where k 0 <strong>and</strong> k 00 are the intercept <strong>and</strong> slope, respectively.<br />

Values <strong>of</strong> dn=dc have a nearly linear dependence on solvent refractive index,<br />

so that if values are not available in the solvent to be used it can also be determined<br />

by extrapolation from other solvent systems. If the polymer refractive index np <strong>and</strong><br />

the partial specific volume <strong>of</strong> the polymer in the solvent np are known, then dn=dc<br />

can be estimated by the Gladstone–Dale rule (2),<br />

© 2004 by Marcel Dekker, Inc.<br />

dn<br />

dc ¼ np(np n0) (20)

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!