Handbook of Size Exclusion Chromatography and Related ...

compoundsandwasexplainedbyadsorptioneffectsofthepackingmaterial.Itwas
suggested that with 0.5 MNaOH as eluent the most reliable results were obtained
due to agood solubility of different lignins and ahigh ionic strength, which
decreases association between lignin molecules.
ThedependenceonthestrengthofNaOHfortheshapeoftheelutioncurves
was described in detail in Ref. 6and in references therein. Oneway of decreasing
the association between residual lignins, which are isolated by enzymatic
hydrolysisfromthepulpistodeterminethemolarmassdistributionsdirectlyfrom
thehydrolysateusing0.5 MNaOHaseluentandaUVdetector(4).Inthiswaythe
formation of associates between lignin molecules during precipitation, which is
needed for structural characterization, is avoided.
Aqueous eluents (45) have been applied in an SEC method for
commmercial lignosulfonates, hard- and softwood kraft lignin, and birchwood
dioxane lignin. In another investigation 0.5 MNaOH as eluent and Superdex
gels of different pore size were used. Na-polystyrene sulfonates were used in
the conventional calibration of the columns.
The relative molar mass distributions of enzymatically isolated residual
lignins from pulps and spent liquor lignins were investigated. The possible
changesintheperformanceofthegelfiltrationmediumasafunctionof itsagehas
to be considered, as seen in Fig. 2. This system was also used for the preparative
fractionation of lignins for further characterization. The results were also
compared with those obtained with an HPSEC system using DMAC/LiCl as
eluent and pullulans as standards. Different shapes of the elution curves were
obtained due to different void volumes of the column, interactions between lignin
andeluent,and differences incolumnpacking material. Theordersofmolar mass
using the two systems were the same although the values differed.
Alkaline eluents were also used in SEC investigations of carbohydrate- and
lignin-containing samples prepared from wood and pulp samples (47) (Fig. 3).
The elution of carbohydrates and lignin macromolecules was monitored by a
pulsed amperometric detector resp. UV detector. In the investigation the
importance of careful consideration of the stability of signals from samples stored
in alkaline solutions was emphasized. Interactions between lignins and the column
packing materials depend on the concentration of alkali in the eluent. The method
can be used to follow the effect of chemical and enzymatic treatments on the MMs
of lignins and polysaccharides, because no adsorptive interactions between
carbohydrate and lignin macromolecules were seen. The SEC method has also
been used to clarify the contribution of lignin–carbohydrate complexes to the
mechanism occurring when xylanase enhances the bleaching of kraft pulp. In this
work the MMD distribution of lignin and carbohydrate molecules were
investigated using Toyopearl HW-50S and HW-55S resins (48) and 1 M NaOH
as eluent. The elution curves were monitored using UV and differential
refractometer detectors and dextrans were used as MM standards, which means
© 2004 by Marcel Dekker, Inc.