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Handbook of Size Exclusion Chromatography and Related ...

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lignocellulosic materials. The lignins from the spent liquors are mostly isolated by<br />

lowering the pH <strong>and</strong> collecting the precipitated lignin. The structures <strong>and</strong><br />

molecular size <strong>of</strong> the lignins depend on the cooking <strong>and</strong> bleaching processes. The<br />

residual lignins left in the pulps after cooking <strong>and</strong> bleaching processes can be<br />

isolated either by dissolution in acidic dioxane/water or by enzymatic hydrolysis<br />

<strong>of</strong> the polysaccharides in pulps or other lignocellulosic materials or by a<br />

combination <strong>of</strong> the methods. Depending on which method is used, different<br />

fractions, <strong>and</strong> thus different MMDs, are obtained (4).<br />

Although it should be advantageous to use one SEC method for all kinds <strong>of</strong><br />

lignin, the different structures <strong>and</strong> properties <strong>of</strong> lignins imply that different<br />

methods have to be used depending on the origin <strong>of</strong> the lignins.<br />

2 SIZE EXCLUSION CHROMATOGRAPHY (SEC) OF LIGNINS<br />

2.1 General Background<br />

In several recent investigations it has been emphasized that during SEC<br />

measurements <strong>of</strong> polymers in general (5) <strong>and</strong> especially <strong>of</strong> lignins there occur<br />

interactions between lignin molecules (association), lignin <strong>and</strong> solvent<br />

(solvatation), <strong>and</strong> lignin column packing material (adsorption), which should all<br />

be minimized in order to obtain absolute MMDs (6–9). However, it should be<br />

emphasized that in many practical applications the knowledge <strong>of</strong> relative molar<br />

masses gives important information about changes in the size <strong>of</strong> lignin molecules<br />

during different processes <strong>and</strong> reactions.<br />

The molar masses (MM) calculated from the MMD are number-, weight-,<br />

<strong>and</strong> z-average values (Mn, Mw, <strong>and</strong> Mz) which are obtained according to what kind<br />

<strong>of</strong> detector is in use. Calculation <strong>and</strong> data collection programs are continuously<br />

developed but the results are essentially dependent on the performance <strong>of</strong> the SEC<br />

system during MMD determinations. The resolution <strong>of</strong> the columns, the eluent,<br />

the type <strong>of</strong> detectors, the st<strong>and</strong>ards used in the calibration <strong>of</strong> the columns <strong>and</strong><br />

interactions between lignin, eluent, <strong>and</strong> column packing material determines the<br />

overall reliability <strong>of</strong> the results.<br />

2.2 Determination <strong>of</strong> Molar Masses <strong>and</strong> Calibration<br />

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

2.2.1 Determination <strong>of</strong> Molar Masses<br />

Theoretically, absolute molar mass values for lignin <strong>and</strong> polymers can be obtained<br />

from viscosity measurements, light-scattering measurements using multi-angle or<br />

low-angle laser light scattering (LALLS, MALLS), sedimentation equilibria<br />

© 2004 by Marcel Dekker, Inc.

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