Page 206 - Multidimensional Chromatography

Multidimensional Electrodriven Separations 199

electrolyte along with it, resulting in a plug-like flow of eluent through the capillary
when the capillary inner diameter is in the range (50–150 m) typically used in
CZE.
The flow profiles of electrodriven and pressure driven separations are illustrated
in Figure 9.2. Electroosmotic flow, since it originates near the capillary walls, is
characterized by a flat flow profile. A laminar profile is observed in pressure-driven
systems. In pressure-driven flow systems, the highest velocities are reached in the
center of the flow channels, while the lowest velocities are attained near the column
walls. Since a zone of analyte-distributing events across the flow conduit has differ-
ent velocities across a laminar profile, band broadening results as the analyte zone is
transferred through the conduit. The flat electroosmotic flow profile created in elec-
trodriven separations is a principal advantage of capillary electrophoretic techniques
and results in extremely efficient separations.

9.3 COMPREHENSIVE SEPARATIONS

In order to distinguish the multitude of coupled techniques from truly multidimen-
sional separations, a comprehensive separation must be defined. A genuinely com-
prehensive multidimensional separation requires a high degree of orthogonality of
the coupled techniques and retention of resolution in the interface between the two
methods. Orthogonal techniques base their respective separations on sample proper-
ties which are as dissimilar as possible. If two techniques are orthogonal and cou-
pling them together causes no loss of resolution, then the peak capacity of the
multidimensional system can be defined as the product of the peak capacities for
each dimension (5). The high peak capacities that result from multidimensional sep-
arations make them applicable to complex sample mixtures containing thousands of
analytes.

Figure 9.2 Pressure-driven (a) and electrodriven (b) flow profiles. Laminar flow in pressure-
driven systems results in a bullet-shaped profile, while the profile of electroosmotic flow is
plug-shaped, which reduces band broadening.