Systems with Electroosmotic Flow

Plug flow in CEC results in a smaller contribution to the plate height from flow anisotropy and transaxial diffusion compared with pressure-driven column liquid chromatography, while contributions to the plate height that are flow-profile-independent are the same. The absence of a pressure drop in electroos-motically driven systems provides the necessary conditions to achieve a larger total number of theoretical plates in CEC in a reasonable time through the use of smaller particles and longer columns (see Table 2 and Figure 11). Under normal operating conditions CEC columns have the potential to provide column plate numbers 5-10 times higher than LC columns. Ultimately the performance in CEC is limited by Joule heating, which causes additional zone broadening and restricts applications of CEC to the use of micro-columns, since columns with a small internal diameter (<100 |im) are required for efficient heat dissipation. The dominant cause of zone broadening in MEKC is axial diffusion, with significant contributions from slow sorption-desorption kinetics between the analyte and micelles and elec-trophoretic dispersion arising from the polydispersity

Table 2 Achievable theoretical plate numbers in HPLC and CEC

Particle size

(|im)

HPLC

CEC

Length (cm)

Plates/ column

Length (cm)

Plates/ column

5

5

55 000

50

115 000

3

25

45 000

50

170 000

1.5

10

30 000

50

250 000

Column pressure drop = 400 atm for HPLC and the field strength <30kV in CEC for operation at the minimum point in the van Deemter plot.

Column pressure drop = 400 atm for HPLC and the field strength <30kV in CEC for operation at the minimum point in the van Deemter plot.

Figure 11 Separation of aromatic compounds by CEC on a 50 cm x 50 |im i.d. fused silica capillary column packed with 1.5 |im spherical octadecylsiloxane-bonded silica gel with 70% (v/v) acetonitrile buffer as mobile phase, temperature 25°C, and field strength 30 kV.

of micelle sizes. Resistance to mass transfer in the mobile phase is minimized by the capillary dimensions of the column and the small size and homogeneous distribution of the micelles throughout the mobile phase combined with the near-perfect plug flow of the mobile phase. Thermal dispersion, as described for CEC, is an additional potential source of zone broadening resulting from radial temperature gradients. Separations in MEKC are typically carried out with between 100 000 and 500 000 theoretical plates. Adsorption of solutes on the column wall can greatly reduce the potential column efficiency and experimental conditions should be optimized to minimize these contributions whenever possible.

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