Biomedical Engineering Reference
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1000 Å
200
60 Å
120 Å
200 Å
160
-12
120
-8
60 Å
1000 Å
80
-4
2000 Å
40
0
0
0
-10
-20
-30
0
20
40
60
80
(a)
Applied voltage (kV)
(b)
Linear velocity (mm/min)
FIGURE 14.11
Effect of pore diameter of supporting silica particles (a) on EOF velocities,
and (b) on Van Deemter plots for the second eluted enantiomer of
trans
-stilbene oxide. CSPs
prepared by coating of cellulose tris(3,5-dimethylphenylcarbamate) (20%; w/w) on aminopro-
pylated silica of various pore sizes. Capillary, fused silica (24/32.5 cm × 100 μm i.d.); particle
size of silica, 5 μm; applied voltage, −1 kV to −30 kV; separation medium, 2.5 mM ammonium
acetate in methanol (pH 7.7). A pressure of 8 bar was applied on the inlet and the outlet vial.
(Modii ed from Girod, M. et al.,
Electrophoresis
, 22, 1282, 2001. With permission.)
separation factors. One problem, though, constituted the availability of wide-pore
materials with particle diameter smaller than 5
m.
Polysaccharide selectors are commonly coated onto wide-pore silica particles in
a relatively thick i lm at about 20% (w/w). Two problems emerge from such efi cient
coatings. First, the polysaccharide selectors are neutral and do not support EOF gen-
eration. Moreover, the polymer coating shields the silanols which yields weak EOF.
No clear preference for tested silica and aminopropyl-silica supports was claimed
[63]. However, aminopropyl-silica may have some advantages under acidic condi-
tions [20]. When less selector was coated onto the surface, the EOF was stronger, e.g.,
by more than a factor of 2 upon reduction of the loading from 20% to 5% (w/w) of
cellulose tris(3,5-dimethylphenylcarbamate) on 5
μ
m 2000 Å silica and 10 mM ammo-
nium acetate as eluent [64]. Simultaneously, however, retention factors decreased
and enantioselectivities were clearly compromised (see Figure 14.12). On the other
hand, CSPs with a thinner coating of 5% or 10% (w/w) revealed higher efi ciencies
and very l at
H/u
-curves with the optimum typically at the highest achievable l ow
rate. Plate heights
H
of less than 5
μ
m were reported for a CSP with 5% (w/w) selector
being equivalent to a reduced plate height of unity, which is quite remarkable for
enantiomer separations.
Further, two other approaches were considered to overcome the inherent limita-
tion of a weak EOF due to the shielded surface charges by the polymer coating. First,
Zou's group regioselectively immobilized cellulose 2,3-
O
,
O
-bis(phenylcarbamate)
at position 6 of the glucose unit onto diethylenetriaminopropylated silica (DEAPS)
using 2,4-diisocyanato-1-methylbenzene as bifunctional reagent (CSP I of Figure 14.8b).
Its EOF characteristics was compared to a corresponding CSP in which the selec-
tor was immobilized onto 3-aminopropyl-silica (CSP II in Figure 14.8b) [21]. The
DEAPS-based CSP with additional amino groups in the spacer that act as additional
EOF sources showed indeed up to 60% higher EOF. Second, Shamsi and coworkers
μ
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