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Handbook of Size Exclusion Chromatography and Related ...

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complexes. The relative molar mass information is <strong>of</strong>ten very useful as such. In the<br />

use <strong>of</strong> alkaline eluents, association <strong>of</strong> lignin molecules <strong>and</strong> their interactions with<br />

eluent <strong>and</strong> column packing materials should be considered (6). The benefits <strong>of</strong><br />

using aqueous alkaline eluents are the good solubility <strong>of</strong> most lignins, <strong>and</strong> also the<br />

possibility <strong>of</strong> measuring MMDs directly from lignin containing spent liquors<br />

formed during pulping <strong>and</strong> bleaching processes <strong>and</strong> other treatments <strong>of</strong><br />

lignocellulosics. UV detectors measuring at 280 nm are mainly used in aqueous<br />

SEC, but when a diode-array detector, covering a range from 200 to 700 nm is<br />

used, additional information is obtained. When quantitative conclusions are made<br />

one should be aware <strong>of</strong> possible differences in absorptivity <strong>of</strong> lignins <strong>of</strong> different<br />

origin. When RI detectors are added to the system, analyses <strong>of</strong> linkages <strong>and</strong><br />

interactions between lignins <strong>and</strong> polysaccharides may be investigated in detail.<br />

Absolute molar masses can be determined with low-angle laser light scattering<br />

(LALLS) detectors coupled on-line with SEC measurements.<br />

The characterization <strong>of</strong> both molecular size <strong>and</strong> structures <strong>of</strong> technical<br />

lignins is <strong>of</strong> great industrial interest. Aqueous SEC is mainly performed in alkaline<br />

solutions (0.1–0.5 M NaOH) using different s<strong>of</strong>t <strong>and</strong> semirigid crosslinked<br />

agarose- <strong>and</strong> dextran-based packing materials. Semirigid synthetic resins are also<br />

prepared. Common s<strong>of</strong>t packing materials include those under the Superdex,<br />

Sephacryl, <strong>and</strong> Sephadex trademarks. Semirigid hydrophilic synthetic gels such as<br />

Toyopearl HW-resins are also available. Recently, rigid hydrophilic synthetic<br />

packing materials such as Ultrahydrostyrgels have become available. For most<br />

materials the strength towards alkaline eluents is more or less restricted <strong>and</strong> should<br />

be considered when evaluating results.<br />

A kraft lignin (6) isolated from an industrial black liquor was fractionated by<br />

preparative SEC using the Sephadex G-100 column <strong>and</strong> 0.1 M NaOH as eluent in<br />

nine paucidisperse fractions. Absolute molar masses for the kraft lignin fractions<br />

were determined from sedimentation equilibrium using an ultracentrifuge. These<br />

results were used for the calibration <strong>of</strong> the column <strong>and</strong> also in order to obtain<br />

information about the association behavior <strong>of</strong> the kraft lignins.<br />

The MMDs <strong>of</strong> fractionated lignosulfonates (LS) were determined on<br />

Sephadex G-50, G-75, <strong>and</strong> Sephacryl S-300 gels using water as eluent. The molar<br />

masses were determined by light scattering <strong>and</strong> then used in the calibration <strong>of</strong> the<br />

columns (44). By comparing the retention volumes <strong>of</strong> proteins <strong>and</strong> lignosulfonate<br />

fractions with known molar masses, it was shown that several commercially<br />

available proteins can be used for calibration <strong>of</strong> the columns. The polyelectrolytic<br />

behavior <strong>of</strong> lignins (related to different numbers <strong>of</strong> free phenolic <strong>and</strong> carboxylic<br />

acid groups) affects strongly the elution behavior <strong>of</strong> lignins with the same molar<br />

mass but with different numbers <strong>of</strong> ionic groups. The shape <strong>of</strong> the elution curve is<br />

thus affected by ionic strength <strong>and</strong> alkalinity <strong>of</strong> the eluent. The effect <strong>of</strong> chemical<br />

stucture on fractionation according to molecular size was indicated by the<br />

Sephadex G-25 gel using 0.5 M NaOH as eluent for monomeric lignin model<br />

© 2004 by Marcel Dekker, Inc.

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