A series of chemically modified porous silicas, which can be used
in high-performance liquid chromatography
(HPLC), were prepared by a two-step modification process.
Aminopropyl groups were first attached to the
silica surface and subsequently converted to amides via the reaction
with appropriate acid chlorides. Changes
in the surface properties of the silica caused by the chemical
modification were monitored using elemental
analysis and high-resolution thermogravimetry. In addition,
complete nitrogen adsorption isotherms were
measured at 77 K in the relative pressure range from
10-6 to 0.99. Standard adsorption
characterization of
the materials included the evaluation of the specific surface area,
total pore volume, and pore size distribution.
Moreover, adsorption energy distribution functions were calculated
from submonolayer adsorption data using
the regularization method. It was estimated that about 40% of the
available surface silanols were converted
to amine groups and about 60% of amines were converted to amides.
It was shown that although the chemical
modification of the surface does not significantly alter the porous
properties of the starting silica, it significantly
affects the interaction of nitrogen probe molecules with the surface.
Moreover, it was demonstrated that
nitrogen adsorption is quite sensitive to the presence or absence of
various functional groups on the silica
surface and that the information obtained from the adsorption
measurements can be utilized for the
characterization of silica-based materials used in chromatographic
applications.
A series of physically coated and/or chemically bonded silica gel
samples was characterized by means
of nitrogen adsorption at 77 K in a relative pressure range from
10-6 to 0.99. The LiChrospher
Si-100
chromatographic silica gel was modified by using 4‘-cyano-4-biphenyl
[4-(4-pentenyloxy)]benzoate and
4-biphenyl [4-(4-pentenyloxy)]benzoate, whose contents in the
samples varied from 8 to 46 wt %. Standard
characterization methods were used to obtain the specific surface area
(BET method), the micropore volume
(t-plot), the total pore volume, and the pore size
distribution (BJH method). Moreover, the surface
properties
of the samples were assessed from low-pressure adsorption data and the
adsorption energy distributions
were calculated using a procedure based on the regularization method.
It was shown that nitrogen adsorption
allows to changes to be monitored in the specific surface area, porous
structure, and surface properties
of the materials under study. Low-pressure measurements revealed
that the coating procedure provided
a quite uniform coverage of the organic molecules on the silica
surface, but the uniformity of the coverage
was inferior to that obtained by the chemical bonding. Moreover,
the chemical bonding procedure did not
lead to blocking of smaller pores as was the case in the coating
procedure. The obtained results proved
the usefulness of nitrogen adsorption measurements in characterization
of new and/or modified porous
materials.
The practical application of Immobilized bovine serum albumin (BSA) as a chromatographic packing for the resolution of enantiomers has been demonstrated by a number of Investigators. In most cases minor changes In eluent conditions lead to very large variations In chromatographic resolution. Because of this, a better understanding of basic separation mechanisms and the Influence of solution equilibria and other pertubatlng conditions such as temperature are necessary In order to assure chromatographic reproducibility as well as to design novel applications. The current work examines the effect of temperature and eluent pH on the chromatographic performance and binding characteristics of sIHca-lmmoblllzed BSA. Measurements of both the background and the sitespecific binding are made using d-and L-tryptophan. In the case of the latter Isomer, maxima In plots of the natural logarithm of the capacity factor vs reciprocal temperature are observed and may be due to a phenomenological change In the bound protein.
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