Handbook of Size Exclusion Chromatography and Related ...

and Mzþ1) and polydispersity of the original sample and its fractions were
determined by high-performance aqueous SEC using Ultrahydrogel columns.
Preparative SEC using a Superdex column with 0.1 M NaOH as eluent has
been used in investigations of MMDs of native lignins obtained by ball milling and
enzymatic hydrolysis (63). The preparatively obtained fractions were precipitated
and characterized. The results were compared to those obtained earlier for kraft
pulps and it was suggested that at least a part of the high molar mass fraction in
pulp residual lignin originates from native lignin galactan complexes.
The MMD and structures of lignins dissolved during organosolv
delignification of eucalyptus batch and successive processes were investigated at
various reaction times (64) and the possible occurrence of topochemical effects
was considered.
The MMD of kraft lignin in alkaline solution (65) has also been investigated
by calibrated ultrafiltration membranes. The membranes were first tested with
probe macromolecules to obtain sieving curves at the same conditions as the lignin
analyses. It was seen that the results were different from the nominal cut-off values
when the MMD of a lignin sample was fractionated into five different fractions at
pH 13.0. The SEC results confirmed the calibrated cut-off values for the MMD.
The effect of pulping variables on the MM and MMD of dissolved kraft lignin
prepared by cooking slash pine (Pinus caribaea) wood chips in a pilot-scale batch
circulation digester was investigated (66). The effect of four pulping parameters on
the MM of dissolved lignin was examined. Generally, the MM of dissolved lignin
increased in both bulk and final phases as the delignification proceeded. Prolonged
cooking at the end of the final phase delignification caused degradation of lignin in
the liquor and decreased its MM. The MMD of lignin- and xylan-containing
macromolecules that were isolated from kraft pulps derived from aspen and spruce
have been determined using SEC under highly alkaline conditions (67). The
changes in MMD as a result of treatment with xylanase and under acid conditions
were evaluated in order to examine the role of lignin–carbohydrate complexes in
enzyme prebleaching of kraft pulp. The MM and MMD of kenaf bast and core
lignin during kraft pulping were determined together with the polydispersity and
intrinsic viscosity, which increased with increasing cooking time (68). The MMD
indicated the presence of only one component of high or low MM in the enzyme
lignin of kenaf bast and core. Both the MM and polydispersity index of lignin in
kenaf core were higher than those in kenaf bast under the same cooking condition,
so the kenaf bast was easier to delignify than kenaf core.
In investigations of the delignification processes of Eucalyptus grandis
wood the MMD of isolated native lignin, organosolv lignin, and kraft lignin were
determined by an HP/SEC system (69). The weight-average MM (Mw) ofthe
lignins decreased in the order MWL . organosolv lignin . kraft lignin. The
weight-average MM decreased with increasing degree of delignification measured
as the amount of extracted lignin.
© 2004 by Marcel Dekker, Inc.