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

Modes
Figure 33
Ion analysis of fermentation broth using
indirect-UV detection 16
Peaks: 1 = K + , 2 = Na + , 3 = Mg 2+ , 4 = Mn 2+ ,
5 = Zn 2+ .
Conditions: 5 mM Waters UVCat-1, 6.5 mM
a-hydroxyisobutyric acid, pH
4.2, v = 20 kV, i = 33 mA, l = 52 cm,
L = 60 cm, id = 75 mm, l = 214 nm,
siphoning injection = 30 s/10 cm
1 2
3
4
5
3 4 5
Time [min]
3.2 Micellar
electrokinetic
chromatography
Micellar electrokinetic chromatography (MEKC or MECC) is
a hybrid of electrophoresis and chromatography. Introduced
by Terabe in 1984, MEKC is one of the most widely used CE
modes. Its main strength is that it is the only electrophoretic
phoretic technique that can be used for the separation of
neutral solutes as well as charged ones.
The separation of neutral species by MEKC is accomplished
by the use of surfactants in the running buffer. At concentrations
above the critical micelle concentration (8 to 9 mM for
SDS, for example), aggregates of individual surfactant
molecules, micelles, are formed. Micelles are essentially
spherical with the hydrophobic tails of the surfactant
molecules oriented towards the center to avoid interaction
with the hydrophilic buffer, and the charged heads oriented
toward the buffer. A representation of micelles is depicted
in figure 34. It is the differential interaction between the
micelle and the neutral solutes that causes the separation.
Figure 34
Schematics of cationic
and anionic micelles
The surfactant and thus the micelles are usually charged
and migrate either with or against the EOF (depending on
the charge). Anionic surfactants such as SDS migrate
toward the anode, that is, in the opposite direction to the
62