Handbook of Size Exclusion Chromatography and Related ...

volume (or the elution time) scale of the chromatogram has to be transferred to a
logarithmic M scale. For this purpose, three methods for evaluation have been used
for dissolved cellulose or cellulose derivatives: (a) the direct standard calibration
method, (b) the universal calibration method by using differential viscometry (DV)
detector, or (c) by using a light scattering (LS) detector. These methods are
described in detail elsewhere in this volume. The detectors used for SEC
of different cellulose samples are exemplified in the previous sections. Multiple
detectors (LS/DV/DRI) have been used for characterization of cellulose ethers,
that is, by aqueous SEC (223).
The direct standard method requires the use of a set of monodisperse,
(narrow) standards of known M, or polydisperse (broad) standards with known M n,
and either Mw or Mv. In either case the standards should preferably cover the entire
elution range of the sample in hand. Unfortunately, there are no commercially
available cellulose standards. During the last decade, monodisperse pullulan
standards have frequently been used to obtain the MMD of cellulosic samples.
Pullulan consists of polymaltotriose units linked together by a-(1!6) linkages.
Because of its linearity and similar Mark–Houwink constants (192) it is
commonly assumed to have about the same relation between molecular mass and
hydrodynamic volume as cellulose. However, this is not true for all cellulose
samples. The pullulan equivalent molar mass averages for cellulose ethers have
been reported to overestimate the values determined by light scattering by a factor
of 3.2 (224). Recently, an overestimation of the pullulan equivalent molar mass of
cellulose in birch kraft pulp has been reported by using a multi-angle laser light
scattering (MALLS) detector together with the DRI (225). Two ways of correlating
the pullulan equivalent M to the absolute M as determined by MALLS were
presented. One of the methods can be used to obtain reliable average molecular
masses of the cellulose and the other method to obtain the MMD of the cellulose.
A drawback with commercial pullulan standards is that the highest available
standard has an M of around 1.6 10 6 . For cellulosic samples having an M above
this value, for example, the cellulose fraction of wood pulp samples, extrapolation
of the calibration curve becomes necessary. Another obvious drawback in using
pullulan for the evaluation of wood pulps is that these samples also contain other
polymers than cellulose, such as hemicellulose and lignin. There are also reports of
aggregation of pullulan dissolved in LiCl/DMAc (194). In spite of having a
completely different structure, polystyrene standards have also been used to obtain
an M value of cellulose samples (for example, 196). The advantage is that narrow
polystyrene standards are available in a broader M range than the pullulan
standards. Examples of other standards used for calibration are dextrans for
evaluation of CMC (154), and polyethylene oxide/glycols for cellulose acetates
(105). Thus, the reported molecular mass obtained from SEC in these cases is
relative to the molecular mass of the used standards having the same hydrodynamic
volume, that is, elution volume, as the sample, in the used solvent system. This is
© 2004 by Marcel Dekker, Inc.