Instant Notes: Analytical Chemistry

their distributions between the running buffer and the micelles. The general

order of elution is:

(i) cationic micelles and cationic solutes;

(ii) neutral solutes partitioning into the cationic micelles;

(iii)EOF;

(iv) neutral solutes partitioning into anionic micelles;

(v) anionic micelles and anionic solutes.

Sodium dodecyl sulfate, SDS, is a frequently used anionic surfactantfor

MEKC. The most hydrophobic neutral solutes migrate at the same velocity

as the SDS, which is slower than the EOF, and the least hydrophobic neutral

solutes migrate at the same velocity as the EOF(Fig. 2a).

An example of an MEKC electropherogram, the separation of some compo-

nents of cold relief-products, is shown in Figure 2b. The addition of chiral selec-

tors such as the cyclodextrins (Topic D6), in place of a surfactant, enables

mixtures of enantiomers to be separated more cheaply than by HPLC(Fig. 2c).

184 Section D – Separation techniques

Fig. 1. Capillary zone electrophoresis (CZE). (a) EOF and order of solute migration. (b) Separation of some artificial
sweeteners and preservatives by CZE capillary, 65 cm, 50 mm i.d.; buffer, 0.02 M borate, pH 9.4; temperature, 25°C;
voltage, 30 kV, injection, hydrodynamic 50 mbar sec; detection, UV absorbance at 192 nm. Reproduced from D.N. Heijer,
High Performance Capillary Electrophoresis, 1992, with permission from Agilent Technologies UK Ltd and D.N. Heijer.

(a)

80

60

40

20

0

1

2

3

4

5

6

7

8

9

10

11

12

3.8 4.8 5.8 6.8 7.8

Time (min)

1 Phenylalanine

2 Aspartame

3 p-propyl hydroxybenzoate

4 p-ethyl hydroxybenzoate

5 p-methyl hydroxybenzoate

6 Dihydroxyacetic acid

7 Cyclamate

8 Sorbic acid

9 Benzoic acid

10 Aspartic acid

11 Saccharin

12 Acesulfame

(b)