Enantioseparations by Capillary Electrochromatography - Chiral Separations by Capillary Electrophoresis - page 407

the minor enantiomer can be furnished in comparison to elution under nonfocused
conditions.
For open-tubular electrochromatography (OT-EC), the A -term contribution is
absent since there is no packing in the capillary tube and the (extended) Golay equa-
tion is usually invoked to describe peak dispersion (Equation 14.11) [46,58]

2 D r

2

2
3(1

k d

2

(14.11)

m c f

H = + C ⋅ ⋅ + u ⋅ ⋅ u

m

2

u D + k ) D

m s

wherein
C m is the mobile phase mass transfer resistance factor
r c the capillary radius
d f the i lm thickness of the stationary phase (coating) and the other parameters
are as dei ned above

The most signii cant difference for electro-driven and pressure-driven OT separations
are then the mobile phase mass transfer terms C m which are different for the l at l ow
proi le in OT-EC and the parabolic proi le of OT-LC

k

2

C = (14.12)

m,flat

2

4(1 + k )

11 k

2

++

6 k 1

C = (14.13)

m,parabolic

2

24(1 + )

A detailed interpretation of the individual parameters of these equations in terms of
their contribution to plate height can be found in Ref. [58]. Generally, the difference
in C m,l at and C m,parabolic represents the essence of the difference in electro-driven and
pressure-driven OT chromatography modes. Roughly speaking, for k < 1 C m,l at is
about a factor 2 smaller than C m,parabolic [58]. Requirements for efi cient separations in
OT formats are the use of very narrow bore capillaries (20 μ m i.d. or smaller), thin

stationary phase i lms or coatings (e.g., <0.2 m thick i lms), small retention fac-

tors that are frequently contradictive with effective enantiomer separations, and slow
mobile phase velocities. More details and limitations in the context of enantiomer
separations will be discussed later.

μ

14.7 MOBILE PHASE MODES
The necessity to maintain a stable current by ionic conduction through the bulk
mobile phase requires reasonable solubility of electrolytes in the eluent. Hence,
it is evident that reversed-phase conditions, i.e., aqueous or hydroorganic eluents,
have been the i rst choice in CEC enantiomer separation in terms of mobile phase
mode especially if chiral recognition involves hydrophobic interactions which
are strengthened in aqueous media. In order to achieve a high EOF velocity and