Biomedical Engineering Reference
In-Depth Information
The resolution (
R
S
) for the neutral enantiomeric pair 1 and 2 in MEKC can be dei ned
by the equation [3,92]:
/
(
)
(
)
(
) (
/
)
12
/
/
RN
=
41
α − α
⎡
k
1
+
k
⎤
⎡
1
−
tt
/
1
+
tkt
/
⎤
(8.5)
⎣
⎦ ⎣
⎦
S
2
2
omc
o1mc
where
N
is the number of theoretical plates. This equation predicts the effects of
N
,
,
k
2
, and
t
o
/
t
mc
on the resolution. The migration time and hence retention factor
can be altered by changing the buffer components, ionic strength, pH, capillary
length, inside diameter, inside surface characteristics, and temperature. The opti-
mum
k
2
corresponding to the highest resolution is given by
k
2
= (
t
mc
/
t
o
)
1/2
. On the
other hand, the ratio,
t
o
/
t
mc
, that is directly related to the migration time window is
inversely proportional to
R
S
. Decrease of the ratio will thus increase the resolution,
which can be achieved by the addition of organic solvents, such as methanol, ace-
tonitrile, 2-propanol, or by lowering the pH to acidic. Selectivity in MEKC is easily
manipulated through the addition of tetraalkylammonium salts, urea, and organic
solvents as well as due to changing surfactants. The details of such manipulations
are described elsewhere [93].
α
8.4 MECHANISM OF CHIRAL SEPARATION
The existence and the extent of chiral recognition in two-dimensional as well as in
three-dimensional systems is not a simple consequence of molecular asymmetry. It
is the result of intermolecular interactions and for separations on surfaces, molecule-
substrate interactions specii c to the chiral compound. It is generally accepted that a
three-point-interaction including a short-range repulsive part that ensures that mole-
cules do not overlap (steric hindrance), an attractive component due to van der Waals
(
, ion-dipole, dipole-dipole, dipole-induced dipole), and a specii c interaction
(electrostatic, hydrogen bonding) is required for chiral discrimination. Local chiral
structure formation occurs through preferential orientation of the molecules accord-
ing to these interactions. For example, combination of steric interactions and disper-
sive forces that describe the topological arrangement of atoms in the molecule may
promote alignment for like molecules but inhibit for unlike molecules. However, the
opposite effect is also possible. On the other hand, hydrogen bonding, which is an
electrostatic interaction, may promote a specii c alignment of a set of molecules.
Surfaces can help in chiral resolution in a number of ways. Chiral resolution
of racemates has been experimentally observed at a surface. Eckhardt et al. [94]
showed that chiral amphiphilic tetracyclics separate in two mirror-image domains
at the aqueous surface. On the other hand, chiral stationary phases have been used
in chiral HPLC and gas chromatography (GC) for many years. In chiral chromatog-
raphy, diastereomeric interactions of one of the enantiomers either with the chiral
stationary phase or the chiral solvent (when a chiral solvent is used in combination
with an achiral column) are generated. Chiral separations in MEKC are based on
stereoselective interactions between the chiral analyte and chiral micelles, which act
as a PSP. It is believed that as in HPLC, chiral separation in MEKC involves a tem-
porary diastereomeric interaction between the enantiomers and the chiral surfactant.
π
-
π
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