High performance capillary electrophoresis - T.E.A.M.

Modes
Absorbance
0 10 20 30 40
Retention time [min]
Retention time [min]
40
30
R
R
20
1.2
0.8
0.4
1.2
0.8
0.4
0 0.01 0.02 0.03
Concentration [M]
Phosphate conc. [M]
Figure 23
Chiral analysis using b-cyclo-dextrin 9
a) Separation of Quinagolide enantiomers
b) Effect of CD concentration on
migration time
c) Effect of buffer concentration on
resolution
d) Effect of temperature on resolution.
Conditions: 30 mM b-CD in 50 mM
phosphate pH 2.5
a)
b)
c)
0
0 0.02 0.04 0.06 0.08 0.10
d)
0
15 20 25 30 35 40 45
Temperature [˚ C]
Selectivity can be tuned by adjusting the type and concentration
of the chiral additive, and also by the addition of
modifiers such as alcohols, surfactants, urea, and metal
ions. CZE has been used successfully for the separation of
D,L amino acids and chiral drugs, as well as positional
isomers, among others. CDs have also been used to separate
hydrophobic, but non-chiral specifiers such as polycyclic
aromatic hydrocarbons.
Cyclodextrins (CDs) are the most widely used chiral
selectors. CDs are nonionic cyclic oligosaccharides consisting
of six, seven, or eight glucose units, and are called a, b,
and g, CDs, respectively. CDs have the shape of a hollow
truncated cone with a cavity diameter determined by the
number of glucose units. The cavity is relatively hydrophobic
while the external surface is hydrophilic. The circumference
contains chiral secondary hydroxyl groups. Chiral
selectivity results from inclusion of a hydrophobic portion
of the solute in the cavity and also from the hydrogen
bonding to the chiral hydroxyl moieties. Various CD derivatives,
such as carboxyalkylated, succinylated, and soluble
ionized polymers, can be used to both alter selectivity and
improve detection properties.
The use of b-CDs for the separation of racemic mixtures of
Quinagolide, a dopamine agonist, is shown in figure 23. As
is typical, resolution is influenced by the CD concentration,
buffer concentration, and temperature, as shown in the
figure. From the migration time versus CD concentration
plot, the complex equilibrium constant, K, can be determined.
In this example, the migration time increases with increasing
buffer concentration due to both the decreased EOF and
the increased hydrophobic interactions between the solute
and the CD. At high buffer concentration, the slope of the
migration time versus buffer concentration curve levels out
due to excessive Joule heating.
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