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Handbook of Size Exclusion Chromatography and Related ...

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Apolymer produced by UV irradiation <strong>of</strong> aconiferyl alcohol solution was<br />

studied at the molecular levels using scanning tunneling microscopy (STM) (30).<br />

The molecular structure <strong>of</strong> the polymer was compared with the structure <strong>of</strong> a<br />

polymerobtainedbytheperoxidase-catalyzedpolymerization<strong>of</strong>coniferylalcohol.<br />

The results obtained by STM were in agreement with the MMD <strong>of</strong> the two<br />

polymers.<br />

3.2 SEC Systems Using Mainly Aprotic Eluents,<br />

Alone <strong>and</strong> With Salts<br />

ThemostcommonaproticeluentsforHPSECareDMF<strong>and</strong>DMACalone,orwith<br />

the addition <strong>of</strong> salts such as LiCl <strong>and</strong> LiBr in order to decrease the association<br />

between lignin molecules. Both eluents are good solvents for lignins. DMF has<br />

alsobeenusedbothwiths<strong>of</strong>tgels<strong>of</strong>theSephadextype(31)<strong>and</strong>recentlyalsowith<br />

rigid crosslinked polystyrene packing materials.<br />

SEC using DMF <strong>and</strong> DMAC alone <strong>and</strong> with salts as eluent <strong>and</strong><br />

crosslinked polystyrene columns has been applied for acetylated, methylated,<br />

<strong>and</strong> underivatized kraft lignin fractions (32,33). Absolute molar mass<br />

determinations were performed using both universal calibration <strong>and</strong> analytical<br />

ultracentrifuge. The sets obtained from sedimentation equilibrium data for the<br />

pauscidisperse acetylated, methylated, <strong>and</strong> underivatized kraft lignin fractions<br />

isolated by preparative GPC could be curve fit to functions representing the<br />

sums <strong>of</strong> separate terms.<br />

MMDs <strong>of</strong> soluble residual pine kraft lignin samples isolated during<br />

different stages <strong>of</strong> kraft flow-through cooking processes (34) have been measured<br />

in DMAC/LiCl, DMF/LiCl, <strong>and</strong> THF.It was seen that the relative MMs <strong>of</strong> the<br />

lignin samples changed in asimilar way irrespective <strong>of</strong> the mobile phase used.<br />

The MM <strong>of</strong> the dissolved lignin increased during the cooking process. In<br />

contrast, the change in molar mass <strong>of</strong> the residual lignin samples did not show a<br />

clear trend with respect to cooking time. One explanation for this irregular<br />

change may be the low efficiency <strong>of</strong> the acid dioxane extraction <strong>of</strong> the pine kraft<br />

pulp obtained early in the cook. This is an example <strong>of</strong> the importance <strong>of</strong> knowing<br />

the origin <strong>of</strong> the samples when interpretating SEC results. For all samples, higher<br />

MMs <strong>of</strong> the MMD were seen when DMAC/LiCl was used as the mobile phase<br />

instead <strong>of</strong> THF (Fig. 2).<br />

The explanation for this behavior is that the polystyrene st<strong>and</strong>ard elutes later<br />

from crosslinked polystyrene-based columns compared to the lignin samples when<br />

a mobile phase <strong>of</strong> higher polarity is used. The shapes <strong>of</strong> the distributions<br />

were different in LiCl/DMAc <strong>and</strong> THF, whereas LiCl/DMAc <strong>and</strong> LiCl/DMF<br />

gave similar distribution pr<strong>of</strong>iles. These results indicate the importance <strong>of</strong> using<br />

the same mobile phase <strong>and</strong> column packing material when comparing MMD <strong>and</strong><br />

molecular size <strong>of</strong> different lignin samples. A similar investigation performed for<br />

© 2004 by Marcel Dekker, Inc.

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