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Handbook of Size Exclusion Chromatography and Related ...

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The Separon HEMA <strong>and</strong> Separon HEMA BIO column packing materials,<br />

based on crosslinked polymethylmethacrylate, are suitable for SEC separations <strong>of</strong><br />

lignins over a wide range <strong>of</strong> MMs (70). A minimum concentration <strong>of</strong> 0.005 M LiBr<br />

was enough to suppress the polyelectrolytic effects regardless <strong>of</strong> the sample<br />

concentration <strong>of</strong> the lignin samples that were analysed. The column packings <strong>and</strong><br />

applied analytical conditions <strong>of</strong> SEC for lignin preparations allow fast analyses<br />

with good reproducibility; however, alkaline conditions may not be used in this<br />

system.<br />

The organic material in effluent samples from a TCF (totally chlorine free)<br />

full-scale bleaching <strong>of</strong> kraft pulps has been characterized. The average MM <strong>of</strong><br />

lignin <strong>and</strong> carbohydrates dissolved during the different stages <strong>of</strong> this bleaching<br />

sequence were characterized by SEC (29,71,72).<br />

Six alkali soluble lignin fractions were extracted from the cell wall material<br />

<strong>of</strong> oil palm trunk <strong>and</strong> empty fruit bunch (EFB) fibers with 5% NaOH, 10% NaOH,<br />

<strong>and</strong> 24% KOH/2% H3BO3 (73). The lignin fractions contained low amounts <strong>of</strong><br />

associated neutral sugars (0.8–1.2%) <strong>and</strong> uronic acids (1.1–2.0%). The lignin<br />

fractions isolated with 5% NaOH from the lignified palm trunk <strong>and</strong> EFB fibers<br />

gave a relatively higher DP shown by weight-average MMs ranging between 2620<br />

<strong>and</strong> 2840, whereas the lignin fractions isolated with 10% NaOH <strong>and</strong> 24% KOH/<br />

2% H3BO3 from the partially delignified palm trunk <strong>and</strong> EFB fibers showed a<br />

relatively lower DP, as shown by weight-average MMs ranging between 1750 <strong>and</strong><br />

1980. In another investigation Abaca fiber was treated with 1, 2.5, <strong>and</strong> 5% sodium<br />

hydroxide at 25 <strong>and</strong> 508C for 0.5–5 h (74). The dissolved alkali lignins were<br />

separated from the solubilized polysaccharides using a two-step precipitation<br />

method. All the lignin fractions were free <strong>of</strong> associated polysaccharides. Their<br />

weight-average MMs, ranging from 1960 to 2640, were determined by SEC using<br />

alkaline eluents. The use <strong>of</strong> size exclusion chromatography <strong>of</strong> lignin as ion-pair<br />

complexes has also been investigated (75).<br />

3.3.2 MMD Determinations <strong>of</strong> Lignins During<br />

Enzymatic Treatments<br />

Enzymatic treatments <strong>of</strong> pulps in order to improve bleachability <strong>and</strong> pulp<br />

properties is today widely investigated. One important parameter relates to the<br />

changes that occur in the MMDs <strong>of</strong> lignin during treatment. The use <strong>of</strong> alkaline<br />

eluents in the investigation <strong>of</strong> enzymatic treatments has several benefits due to<br />

the good solubility <strong>of</strong> the lignin samples.<br />

The chemical <strong>and</strong> structural composition <strong>of</strong> native lignins from trunks <strong>of</strong> oil<br />

palm was isolated by ball milling <strong>and</strong> enzymatic hydrolysis <strong>and</strong> subsequent<br />

extraction (76). These lignins have been characterized <strong>and</strong> their MMDs have been<br />

determined.<br />

© 2004 by Marcel Dekker, Inc.

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