Grieb, Sally Jane (1995) Investigations of polysaccharide carbamate-coated chiral stationary phases. PhD thesis, University of Warwick.

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Abstract

This thesis describes work carried out in order to increase knowledge
concerning the influence of support structure and support surface chemistry on
the enantioselectivity of cellulose tris(phenylcarbamate)-coated phases using
high-performance liquid chromatography and flash chromatography.
In the past cellulose carbamates have been coated (25% w/w) onto large
particle (7 to 10 gm), macroporous (4000A) aminopropylated silica (APS) and
packed into long (250 mm) HPLC columns. Therefore, the aim of the first part
of this project was to use a small particle support, coated with a cellulose
carbamate that could be packed into a short HPLC column in order to achieve
rapid, efficient chiral separations.
Hypersil APS with a particle size of 2.5 gm and a mean pore diameter of
120A was chosen as a potentially suitable support. As a result of using a small
pore diameter support, the cellulose carbamate coating was not easily able to
gain access to the pore volume and a 15% w/w cellulose carbamate loading was
found to be optimum. As anticipated, 2.5 gm Hypersil APS coated with 15%
w/w cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) was significantly
more efficient than similarly coated 5 and 10 μm Hypersil APS phases and high
eluent flow rates (≥ 1 ml/min) could be used without significant loss in
efficiency. A 30 mm column packed with 15% w/w CDMPC-coated 2.5 μm
Hypersil APS permitted the baseline separation of a range of chiral analytes in
less than three minutes.
In the next phase of the work, the influence of the support surface
chemistry on enantioselectivity was investigated. Hypersil supports which were
both more polar (underivatised silica) and less polar (octadecylated silica) than
APS, and Hypercarb, a very non-polar porous graphitic carbon (PGC) support,
were chosen in order to span a wide polarity range.
Underivatised silica, due its small polar surface group, was able to accept
a 20% w/w CDMPC loading and for many analytes this phase was found to be
more enantioselective than a 15% w/w CDMPC-coated APS phase. In contrast,
the large non-polar surface groups on octadecylated silica (ODS) appeared to
significantly exclude CDMPC from the pore volume and a 15% w/w loading
was found to be too high. However, because the octadecyl groups were able to
shield acidic silanol sites to some extent, CDMPC-coated ODS showed potential
for the separation of basic analytes. Hypercarb, which has a larger pore volume
(250A) than Hypersil supports and has virtually no surface functional groups,
accepted a 25% w/w CDMPC loading. This phase showed superior
enantioselectivity for basic and acidic chiral analytes over CDMPC-coated APS.
However, since PGC interacts strongly with flat molecules, badly tailing peak
shapes were observed for a few bi- and polyaromatic chiral analytes.
The high efficiency of the CDMPC-coated underivatised silica in HPLC
columns led us to consider whether a modestly efficient, inexpensive flash
chromatography silica coated with CDMPC could be used for preparative scale
separations.
Initially, a standard flash chromatography column packed with 20% w/w
CDMPC-coated Davisil irregular silica (40-63 gm, pore size 150A) was used to
investigate the separation of a range of chiral analytes. Resolutions were
monitored by fraction collection with subsequent HPLC analysis. The method
was found to be extremely easy, rapid and sample loadings of tens to hundreds
of milligrams were achieved. Later two modifications were made; (i) in order to
reduce the tedious collection of fractions, the flash column was modified to
allow on-line UV detection and (ii) a 20% w/w CDMPC-coated Bondapak ODS
flash column was prepared and was shown to dramatically improve the
preparative resolution of basic analytes.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Polysaccharides, Carbamates, Stationary phase (Chromatography), Enantioselective catalysis, Catalyst supports
Official Date:	September 1995
Dates:	Date Event September 1995 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Matlin, S. A.
Sponsors:	Science and Engineering Research Council (Great Britain) (SERC) ; SmithKline Beecham Pharmaceuticals
Extent:	vii, 183 leaves
Language:	eng

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