Handbook of Size Exclusion Chromatography and Related ...

een extensively studied by methods such as viscometry, osmometry,
ultracentrifugation, and precipitation–fractionation and the SEC characterizations
could thus be compared with known methods. The preparation procedure of the
derivative was considered mild. Nitric acid together with either phosphoric acid
and phosphoric pentoxide (76) or acetic acid and acetic anhydride (77) has been
used for derivatization. The phosphoric acid or acid anhydride binds the water that
is split off in the ester formation, and is thus of importance in the reaction to
achieve a complete trisubstitution of cellulose, that is, a nitrogen content of 14.1%,
corresponding to a repeating unit molecular mass of 297.
The SEC characterizations of cellulose trinitrate have preferentially been
performed using series of columns packed with porous polystyrene particles of
different exclusion limits (78–83). Besides THF, which was used in the given
examples, ethyl acetate has also been used as mobile phase with polystyrenepacked
columns (84). Silica particles, both underivatized and derivatized (85–87),
have also been utilized as packing materials.
Today, cellulose trinitrates are rarely used to investigate the molecular mass
of cellulose. During the past decade there have only been a few reports concerning
cellulose trinitrate (88,89) or nitrocellulose (90,91). The decreased interest in
using SEC of cellulose trinitrate as a means to characterize cellulose reflects the
uncertainty of the method. The main doubt concerns possible acid hydrolysis of
the cellulose chain during derivatization and instability of the derivative. Presence
of microgels and the chromatographic behavior are also drawbacks to consider. For
a more detailed review of cellulose nitrates, the reader is referred to the first edition
of this handbook (10).
4.2 Cellulose Trimethylsilylates
Silylation is a well-known method for solubilizing and quantifying monosaccharides
by gas chromatography. Silylation of cellulose has been investigated with
a number of silylation agents and solvents. N,O-bis(trimethylsilyl)acetamide (92),
chlorotrimethylsilane (93–95), hexamethyldisilazane (HMDS) (96–98) have been
used for silylation of cellulose although HMDS requires addition of a catalyst. A
drawback is the tedious purifications of the derivative, which are required to
remove excess reagent, solvents, and salts. This is probably the main reason for the
limited number of reports on SEC studies of cellulose trimethylsilylates.
Trimethylsilylcellulose (TMSC) can be obtained with high DS (2.5–3); the
reactivity depends on the solubility of the cellulose sample during derivatization.
Pyridine (94), LiCl/DMAc (97,99), or formamide (96) are commonly used as
solvent. The solubility of TMSC in THF, the preferred solvent for SEC of
derivatives, depends on the DS and DP of the cellulose. Mormann and Demeter
(100) studied cotton linter (DP 1100), microcrystalline cellulose (DP 220 and
DP 290), and hydrocellulose (DP 40) and found that trimethylsilylates of
© 2004 by Marcel Dekker, Inc.