Handbook of Size Exclusion Chromatography and Related ...

Table 6 (Continued)
Packing
material
Exclusion
limits of
each
column (A ˚ ) a
Temperature
(8C)
Detector(s)
and
wavelength
(nm)
Flow
rate
(mL/min) Reference
Waters — RI/ 0.735 122
HT6 2 10 5 to 10 6
MALLS (488)
HT5 5 10 3 to 6 10 5
HT4 5 10 2 to 3 10 4
PL gel 80 UV (295)/DRI 1.0 134 c
4 mixed A 40 10 6
a A ˚ ¼ 10 2 10 m, generally defined as the exclusion limit of polystyrene dissolved in THF.
b CTCs having different DS of substituents on the phenyl group.
c Phenyl, ethyl, and propyl carbanilate, LiCl/DMAc as mobile phase.
Unbleached wood pulp samples need to be (chlorite) delignified prior to
derivatization (115,116), but for cellulose fibers having a lignin content below
approximately 2.5%, delignification is not necessary (117). The general procedure
for derivatization of cellulose includes several steps: (a) activation, (b) reaction of
cellulose with phenyl isocyanate (OCNC 6H 5), (c) addition of methanol to react
with the excess reagent, (d) precipitation in a nonsolvent, (e) repeated washing of
the precipitated derivative, (f) freeze-drying, and (g) dissolution in THF. The long
preparation time and the risk of losing low molecular mass constituents of the
sample in the precipitation step are some disadvantages of this procedure.
Activation to open up the structure of the sample prior to derivatization has
been pointed out as necessary for some sample types such as regenerated cellulose
samples and high molecular mass cellulose samples. The activation has been
carried out in water (116,118), liquid ammonia:pyridine (119,120), ammonia
(121), pyridine (122), ammonia:DMSO (119), DMSO (123), DMSO:pyridine
(124), and LiCl/DMAc (125).
The heterogeneous carbanilation reaction is commonly performed in
dimethylsulfoxide (DMSO) or pyridine. Since the reaction proceeds faster in
DMSO (120), the reaction temperature is kept lower than when pyridine is used,
typically 708C for DMSO and 808C for pyridine. It has however been reported that
DMSO degrades high molecular mass samples when the reaction time is longer
than 32 hours, although a CTC prepared from the same source, that is, bleached
cotton linters, did not suffer appreciable loss of the molecular mass (M) after
treatment in phenylisocyanate in DMSO at 708C for 72 hours (120). Using an online
MALLS detector during the chromatography, LaPierre and Bouchard
(117,126) found that the DP of CTCs prepared from softwood kraft pulps was
© 2004 by Marcel Dekker, Inc.