Cyclodextrin-Mediated Chiral Separations - Chiral Separations by Capillary Electrophoresis - page 67

5

3

2 4

3

1

6

2

1

4

(a) (b)

4

5

1

6

2

3

(c)

FIGURE 3.15 Schematics of the microl uidic chip channel structures: (a) mono-channel
chip, (b) two-channel chip, and (c) four-channel chip. 1-6: sequence number of electrodes.
(From Gao, Y. et al., Electrophoresis , 26, 4774, 2005. With permission.)

analytes. Recently, Belder et al. [167] presented a combined microl uid reactor/separa-
tion unit in one UV-transparent fused-silica microchip. The authors applied this system
for the online study of the cleavage rate of the substrate glycidyl phenyl ether by differ-
ent enzymes. Using heptakis-6-sulfato-

-CD as a chiral selector, the enantiomers of the
substrate and the reaction product were separated online in the separation channel.
Ölvecka et al. [168] i rst applied the principle of ITP on a polymethylmethacrylate
chip in combination with conductivity detection. Using dl-tryptophan as a model
compound, the authors developed three different modes: single channel ITP, tandem-
coupled channel ITP, and concentration cascade tandem-coupled ITP. In the i rst
approach only one separation channel is used, in the second approach there are two
separation channels used in series, and in the third approach a second leading elec-
trolyte with a lower concentration than the i rst leading electrolyte is used in the sec-
ond separation channel resulting in a concentration cascade. However, best results
were obtained with the latter approach.
An electrochromatographic approach was developed by Zeng et al. [169] using a
monolithic phase. The authors synthesized a polyacrylamide gel in situ in the micro-
channel using allyl-

β

-CD acting both as a cross linker and as a chiral selector. The
wall of the channel was pretreated with 3-(trimethoxysilyl) propyl methacrylate for
i xing the polymer in the channel. The authors applied this system to the chiral sepa-
ration of FITC amino acids.
For more detailed information, the reader is referred to specialized review articles
[170,171] and Chapter 16.

β

3.8 NONAQUEOUS MEDIUM
Comprehensive review articles focus on the use of nonaqueous solvents for chiral
separation in electromigration techniques [172,173]. The reasons for using non-
aqueous solvents are the insolubility of chiral selectors or analytes in water, for