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Handbook of Size Exclusion Chromatography and Related ...

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detector is based on similar principles as the MALDI-TOF-MS detector.<br />

Preliminary results on lignin molar mass determinations using the ESI/MS<br />

technique (11) have been presented. The ESI/MS spectra showed the molar mass<br />

distribution <strong>of</strong> lignin as well as structural features <strong>of</strong> oligomers with molar masses<br />

between 500 <strong>and</strong> 2000 g/mol. The possibility <strong>of</strong> using ESI/MS analysis for<br />

monitoring lignin reactions in solution has also been demonstrated.<br />

2.2.2 Calibration <strong>of</strong> SEC Systems<br />

Conventional calibration <strong>of</strong> SEC systems results in absolute molar masses when<br />

the monodisperse calibration st<strong>and</strong>ards have the same chemical structure as the<br />

polymer under investigation. In the case <strong>of</strong> lignin this is not generally the case <strong>and</strong><br />

the MMDs obtained for the lignins by conventional calibration are relative with<br />

respect to the calibration compounds <strong>and</strong> the type <strong>of</strong> eluent. However, if it should<br />

be possible to manufacture monodisperse lignin samples, with structures close to<br />

those <strong>of</strong> lignins, absolute molar masses could be determined.<br />

Universal calibration is based on the Einstein viscosity law (6):<br />

[h] ¼ const. Vh (2)<br />

which relates the hydrodynamic volume Vh <strong>of</strong> a macromolecule to the intrinsic<br />

viscosity [h] incm 3 /g.<br />

The sphere equivalent to Vh for a flexible polymer has a radius Re in which<br />

Rg is the radius <strong>of</strong> gyration:<br />

Re ¼ C Rg (3)<br />

By development <strong>of</strong> Eqs (1) <strong>and</strong> (2) the well-known Mark–Houwink equation is<br />

obtained:<br />

[h] ¼ K M a<br />

In SEC it is assumed that the penetration <strong>of</strong> the solutes into the pores <strong>of</strong> the column<br />

packing material determines the elution volumes <strong>of</strong> the solute. SEC separates<br />

molecules according to some function <strong>of</strong> size, <strong>of</strong> which V h is the most commonly<br />

used. The radius <strong>of</strong> gyration (R g) <strong>and</strong> the mean end-to-end distance (h 1/2) <strong>of</strong>a<br />

r<strong>and</strong>om coil polymer could also be used.<br />

The universal calibration method is based on measuring simultaneously the<br />

response for the concentration with an RI detector <strong>and</strong> the viscosity <strong>of</strong> the sample<br />

on-line as a function <strong>of</strong> elution volume. By combining these two responses it is<br />

possible to calculate the intrinsic viscosity, which is proportional to Vh.<br />

Commercial programs are available by which it is possible to calculate different<br />

parameters related to the molecular size <strong>of</strong> polymers (6,9).<br />

© 2004 by Marcel Dekker, Inc.<br />

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