Handbook of Size Exclusion Chromatography and Related ...

adequate when evaluating the influence of different treatments on a cellulosic
sample or to follow changes during a reaction, but should not be confused with the
true M of the cellulose.
Calibration curves have also been constructed employing celluloses from
different sources (79), celluloses obtained by acid hydrolysation of high M
cellulose (83) or by fractional precipitation of cellulose derivative (111). The
required characteristics of the homemade standards are then determined off-line by
osmometry (Mn), viscometry (Mv), or light scattering (Mw) before use. Even if
these latter methods give a better value of the M of cellulose than noncellulose
standards they are rarely used today, primarily because they are much more timeconsuming
than using commercially available standards such as pullulan and
polystyrene. Ultrasonic degradation has also been used to produce homologous
series with respect to M of sulfoethyl celluloses (226). The degraded samples were
evaluated with on-line MALLS/DRI.
To bypass the need for cellulose standards dual detectors have been used;
one concentration detector, commonly DRI, and either a DV detector or a lightscattering
detector. The use of a DV detector provides the intrinsic viscosity, which
makes it conveniently possible to apply the universal calibration method
(186,192). The universal calibration is based on the observation that the product of
intrinsic viscosity and molecular mass ([h]M), that is, hydrodynamic volume is
independent of polymer type (227). To determine the M of the cellulose sample at
a given elution volume the column is calibrated with standards of known M,
commonly polystyrene. Other molecular characteristics than M such as the Mark–
Houwink coefficients for the cellulose under the chromatographic conditions
employed can also be obtained. For the solvent LiCl/DMAc a number of different
values of the constants for cellulose and polystyrene have been reported and
reviewed recently (217). As mentioned before, the presence of water and variations
of ionic strength in LiCl/DMAc also affect the conformation of the polymer in
solution, and thereby the obtained constants. The root-mean-square radii of gravity
(Rg) have also been studied as a parameter for universal calibration employing
pullulan and dextran standards in aqueous SEC (228).
During the past decade there have been an increasing number of reports
where LS detectors have been used for evaluation of the MMD of cellulose
samples. The advantage in using LS detectors is that the absolute M can be
obtained in the whole MMD range without using any standards. Low-angle laser
light scattering (LALLS) and multi-angle laser light scattering (MALLS) detectors
have been used both for aqueous and organic SEC. Besides giving the molecular
mass of the eluting polymer, they also offer the possibility of detecting the
occurrence of aggregates. When evaluating wood pulps, the presence of lignin
has to be taken into account, especially when an argon laser (488 nm) is used,
because the fluorescence of lignin adds to the scattered light (225,229,230). To
avoid interference of any fluorescence, narrow band-pass filters should be used.
© 2004 by Marcel Dekker, Inc.