Handbook of Size Exclusion Chromatography and Related ...

stirring at room temperature for an extended time. Dissolution has also been
achieved by maintaining the temperature at 1008C for a period of time before
lowering the temperature to 508C for an additional period of time (186,195). The
total dissolution time is, as always for cellulosic samples, dependent on type,
molecular mass, and degree of crystallinity of the cellulose.
Irrespective of the activation–dissolution procedure, the dissolved sample is
diluted with DMAc prior to chromatography, and the final concentration of sample
and LiCl is commonly 0.05–0.1% and 0.5–1.0%, respectively. Owing to the high
viscosity of the final sample solutions, SEC is commonly performed at 808C.
Since water has a deleterious effect on the dissolution, efforts to use dry salt
and solvent are crucial. To completely avoid the presence of water is a difficult
task, since LiCl as well as DMAc are highly hygroscopic. Thus, for practical
applications, the solvent system should be regarded as a ternary solvent system,
consisting of LiCl, DMAc, and water (215). Since the maximum solubility of LiCl
in dry DMAc is 8.46%, reported concentrations above this value may be due to the
presence of water. In order to obtain comparable SEC or light scattering results
from cellulose–LiCl/DMAc solutions, Potthast and co-workers (215) recommend
that the water content of the solvent system should be specified, and also describe a
method by which this could be done. Another complication to consider is that
heating/refluxing DMAc or LiCl/DMAc generates a number of chromophores in
the solvent (216). One of these, N,N-dimethylacetoamide, is able to react with
glucose and form a furan structure, which also was reported found in heated
solutions of different pulps in DMAc or LiCl/DMAc. According to these findings,
the dissolution procedure that includes heating, for example, procedure II, should
be avoided. In a recent review on the characterization of cellulose by LiCl/DMAc-
SEC, it was concluded that further improvements with respect to ionic strength and
pH of the mobile phase are needed (217).
The formed solution is stable (218), although a slight decrease in the
viscosity of solutions stored at 308C for 30 days has been reported (64). Strlič et al.
(190) found that oxidized cellulose samples are stable when the sample is
dissolved at room temperature, that is, according to procedure I. Recently, Jerosch
and co-workers (201) compared the stability of 8% LiCl/DMAc solutions of
untreated and differently aged cellulose samples dissolved by procedure I. After
1–23 days storage at 35–408C, the solutions were diluted and characterized by
SEC. The solutions of untreated bleached sulfite softwood pulps and bleached
cotton linters were found to be stable for 12 days and 6 days, respectively. The
corresponding aged samples were more susceptible to degradation, the more
initially degraded the faster was the solvent-initiated degradation. The authors
recommend that the temperature as well as dissolution time should be lowered to
avoid degradation.
Characterization of wood pulps is more complex since these types of
samples also contain hemicellulose and lignin, the latter being absent only in
© 2004 by Marcel Dekker, Inc.