Biomedical Engineering Reference
In-Depth Information
the surface of 50
m i.d. capillaries. Its feasibility could be demonstrated by the
OT-EC enantiomer separation of a variety of chiral pharmaceuticals [83]. Largely
in agreement with the trends found by Mayer and Schurig, a i lm thickness of only
0.025
μ
m furnished a high efi ciency at low retention and separation factors while
thicker i lms of 0.1
μ
m afforded much better enantioselectivities at unacceptable
peak performance unfortunately. As another drawback, they reported on strongly
reduced EOF by the cellulose polymer coating.
In another approach, thin porous i lms of highly cross-linked molecularly imprinted
polymers (MIP) anchored to the inner walls of 25
μ
m i.d. fused silica capillaries were
prepared by an in situ polymerization process [85]. The MIPs were prepared from
methacrylic acid or 2-vinyl pyridine as functional monomers and ethylene dime-
thacrylate or trimethylolpropane trimethacrylate as cross-linkers in the presence of
toluene and acetonitrile as porogenic solvents.
N
-dansyl (
S
)-phenylalanine served
as the template molecule. This column type resembles a porous-layer open tubular
(PLOT) column in which through the creation of a porous polymer layer the surface
area can be signii cantly enhanced. Although some success could be achieved, the
column suffered from the limited kinetic performance for the peak corresponding
to the templated enantiomer, which is a common problem for separation materials
whose selectivity originates from MIP technology.
Although the feasibility of enantiomer separation by electrochromatography with
enantioselective open-tubular columns has been convincingly demonstrated, the
window of experimental conditions that give highly efi cient separations seems to
be narrow. Especially the problem of easy overloading is hard to overcome and very
seldom separations other than that of standard solutions have been shown. More
applications of the OT-EC approach can be found in Chapter 15.
μ
14.8.2 P
ACKED
C
APILLARY
C
OLUMNS
Packed capillary columns are derived from the typical HPLC column packing tech-
nologies and often the same (commercial) porous particles (3-5
m diameter) as
employed for HLPC are slurry packed by pressure, supercritical-l uid chromatogra-
phy, or electrokinetically into fused silica tubes of 75-100
μ
m i.d. The packed bed
is stabilized with retaining frits on both ends to keep the packing in the capillary
during the CEC runs. Packed chiral CEC capillary columns are nowadays not com-
mercially available. Thus, research groups being active in the i eld have established
their own dedicated packing protocols. Most follow a procedure similar to the one
outlined in Figure 14.7. For example, the chirally modii ed particles are slurried in
suitable media such as acetone or ethanol and then pumped with high pressures into
the empty fused silica capillary tube which has a temporary retaining frit attached at
one end [101]. After exchange of the organic solvent by an aqueous solution contain-
ing sodium ions, retaining frits are sintered on the silica-supported column packing
at about 500°C thereby achieving partial fusion with grain boundary formation
between the particles. The sodium ions, without which no sufi ciently stable frits
can be obtained, support this process of joining the particles to one another by grain
boundaries because they help to overcome the repulsive electrostatic interaction
between the silica beads and promote the sodium silicate matrix formation at the
μ
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