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Handbook of Size Exclusion Chromatography and Related ...

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Because DMAC/LiCl is also agood solvent for cellulose <strong>and</strong> pulps (see<br />

Chapter 12, This volume), it is possible to monitor molar mass distributions <strong>of</strong> the<br />

fiber lignin (residual lignin) left in the pulps after delignification. This kind <strong>of</strong><br />

analysis should be possible for different kinds <strong>of</strong> cellulosics soluble in DMAC/LiCl.<br />

In investigations by Westermarck <strong>and</strong> Gustafsson (37) unbleached birch pulps were<br />

dissolved in DMAC/LiCl <strong>and</strong> the MMDs monitored using both RI <strong>and</strong> UV detectors<br />

so that both polysaccharides <strong>and</strong> lignin could be detected simultaneously. The<br />

molecular size pr<strong>of</strong>ile <strong>of</strong> the unbleached pulp showed a cellulose peak with a low<br />

polydispersity <strong>and</strong> by the UV detector it was seen that the lignin in the pulp mainly<br />

eluted together with the low MM hemicellulose, indicating a possible chemical<br />

linkage. A similar system was applied to investigations <strong>of</strong> unbleached s<strong>of</strong>twood pulp<br />

<strong>and</strong> the corresponding isolated residual lignins. Both universal <strong>and</strong> conventional<br />

calibrations were used (38). The results indicated that the isolated residual lignin had<br />

a lower molar mass than the same residual lignin in situ in the pulp, suggesting that<br />

there may also be linkages between lignin <strong>and</strong> cellulose in the pulp. The low intrinsic<br />

viscosity for the isolated residual lignins suggested a ball shape <strong>of</strong> the molecules,<br />

which is a generally approved result. Berthold et al. (39) have developed a system<br />

using ethylcarbanilation for the complete dissolution <strong>of</strong> s<strong>of</strong>twood kraft pulps in<br />

DMAC/LiCl; this also made it possible to monitor the MMD <strong>of</strong> the total residual<br />

lignin in the pulp.<br />

In investigations <strong>of</strong> properties <strong>of</strong> residual lignins isolated from kraft pulps <strong>of</strong><br />

Eucalyptus globulus MMDs were determined using a HPSEC system with DMF/<br />

LiCl as eluent, a UV detector, <strong>and</strong> conventional calibration (40). St<strong>and</strong>ards for<br />

HPSEC analysis <strong>of</strong> lignins in order to obtain better <strong>and</strong> precise results were<br />

prepared by preparative SEC from lignin fractions obtained during the acetosolv<br />

process <strong>of</strong> sugar cane bagasse (41).<br />

A detailed study <strong>of</strong> the elution behavior <strong>of</strong> various preparations <strong>of</strong> lignins<br />

<strong>and</strong> lignocarbohydrate complexes by SEC have also been performed using pure<br />

dimethylformamide <strong>and</strong> dimethylsulfoxide as eluents <strong>and</strong> column packing<br />

materials such as porous silica <strong>and</strong> glycomethacrylate gels (42). The use <strong>of</strong><br />

Spheron P-1000 <strong>and</strong> Sephadex G-50 columns have shown that lignins have<br />

polyelectrolytic properties <strong>and</strong> that their elution behavior is conditioned by the<br />

summed-up polyelectrolytic effects.<br />

An HPSEC system based on the use <strong>of</strong> DMSO:water (90:10) as eluent <strong>and</strong><br />

equipped with UV <strong>and</strong> RI detectors has been applied in the determination <strong>of</strong><br />

MMDs, mainly for xylan, but also for lignin impurities (43).<br />

3.3 SEC Systems Using Aqueous Eluents, Buffers,<br />

Salt, <strong>and</strong> Alkaline Solutions<br />

3.3.1 MMD Determination for Native <strong>and</strong> Technical Lignins<br />

SEC <strong>of</strong> lignins using aqueous eluents have been applied in several investigations<br />

together with structural characterization <strong>of</strong> lignins <strong>and</strong> lignin–carbohydrate<br />

© 2004 by Marcel Dekker, Inc.

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