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Handbook of Size Exclusion Chromatography and Related ...

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fully bleached pulps. Although isolated lignin samples are easily dissolved in<br />

LiCl/DMAc, unbleached samples containing high amounts <strong>of</strong> lignin cannot be<br />

completely dissolved. Hitherto, no systematic study concerning the limiting<br />

amount <strong>of</strong> lignin content has been reported, but unbleached hardwood kraft pulp<br />

samples can, in general, easily be dissolved. Irrespective <strong>of</strong> applied activation–<br />

dissolution procedure, I or II, s<strong>of</strong>twood kraft pulps cannot be completely<br />

dissolved in LiCl/DMAc (67,200,219,220) <strong>and</strong> a gel-like residue can be<br />

isolated by ultracentrifugation (67). In addition, the chromatography <strong>of</strong> s<strong>of</strong>twood<br />

kraft pulp samples has apoor reproducibility.Although the solution from this<br />

type <strong>of</strong> samples appears clear, the solution is difficult to filter <strong>and</strong> an increasing<br />

pressure during SEC is commonly observed also for ultracentrifuged sample<br />

solutions, indicating adsorption onto the stationary phase. At our laboratory we<br />

have found that the column material can be regenerated by increasing the LiCl<br />

concentration <strong>of</strong> the mobile phase to 8% LiCl, <strong>and</strong> continuing the washing at<br />

this high concentration over night. The limited solubility <strong>of</strong> kraft pulp samples<br />

in LiCl/DMAc, <strong>and</strong> the problems arising during chromatography have been<br />

attributed to glucomannan (67), a hemicellulose that is typical for s<strong>of</strong>twood<br />

samples. The presence <strong>of</strong> glucomannan may also explain why not even fully<br />

bleached s<strong>of</strong>twood kraft pulps can be completely dissolved. Since the chemical<br />

composition <strong>of</strong> the initial fibers <strong>and</strong> the residue differs, care must be taken when<br />

s<strong>of</strong>twood kraft pulps dissolved <strong>and</strong> chromatographed in LiCl/DMAc are<br />

evaluated.<br />

The shape <strong>of</strong> the MMD also differs between hardwood <strong>and</strong> s<strong>of</strong>twood kraft<br />

pulp samples (209). This is true for the carbohydrate polymers <strong>of</strong> the pulps, as<br />

detectedbydifferentialrefractiveindex(DRI)detectorbutals<strong>of</strong>orthepulplignin,<br />

as visualized by using an UV detector. Irrespective <strong>of</strong> detector used, hardwood<br />

kraft pulp samples always give abimodal MMD (Fig. 2), representing cellulose<br />

<strong>and</strong> xylan, respectively,although the pulp lignin also contributes to the xylan<br />

distributioninthelowerMrange(209,212).Dissolveds<strong>of</strong>twoodkraftpulpshavea<br />

more complex elution pr<strong>of</strong>ile as compared to hardwood kraft pulps when using a<br />

DRIdetector(209).Fromcarbohydrateanalysis<strong>of</strong>afullybleacheds<strong>of</strong>twoodkraft<br />

pulp sample (92% dissolved) it was found that the hemicellulose portion elutes<br />

over the entire Mrange (Fig. 3), although it is known that hemicelluloses (just as<br />

lignin) have a much lower M than cellulose. The MMD <strong>of</strong> lignin, as obtained by<br />

UV detection, differs between hardwood kraft pulps <strong>and</strong> s<strong>of</strong>twood kraft pulps. The<br />

elution behavior <strong>of</strong> lignin has been proposed to be due to covalent linkage between<br />

lignin <strong>and</strong> cellulose (204,207) possibly through reaction between lignin <strong>and</strong><br />

glucomannan during kraft cooking <strong>of</strong> s<strong>of</strong>twood (221), even though no conclusive<br />

evidence has been found. In these studies about 80% <strong>of</strong> the untreated s<strong>of</strong>twood<br />

kraft pulp was dissolved, whereas s<strong>of</strong>twood pulps produced by bisulfite <strong>and</strong> acid<br />

sulfite are almost completely dissolved in LiCl/DMAc (204,207). The obtained<br />

MMD pr<strong>of</strong>iles <strong>of</strong> s<strong>of</strong>twood pulps produced by these acid processes resemble those<br />

© 2004 by Marcel Dekker, Inc.

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