Plug pressure-driven

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

An advantage of CEC is that the pressure drop across the column is very low so that small particles and longer columns can be used. Also, the electroosmotic flow results in a plug flow profile as opposed to a parabolic or laminar flow derived from a pressure-driven flow (Figure 1). The combination of these advantages leads to highly efficient columns that can be applied to separate components in a mixture. 

CEC has recently become an alternative to HPLC. A capillary is filled or its internal wall covered with a porous sorbent. The free volume remaining in the capillary is filled with an electrolyte. High voltage (on the order of ten kV) is applied across the length of the capillary. Sample plugs are introduced at one end. Sample components are carried to the other end due to electro-osmosis and - in the case of ions - also electrophoresis. In CEC the more important effect is electro-osmosis, which is essentially a flow mechanism of the electrolyte solution without the need for applied pressure. The separation of the sample components occurs mainly due to phase distribution between the stationary phase and the flowing electrolyte. Thus CEC is very similar to HPLC in a packed capillary except that the flow is not pressure driven and that ionic analytes undergo electrophoresis additionally to phase separation. 

An important advantage of the use of EOF to pump liquids in a micro-channel network is that the velocity over the microchannel cross section is constant, in contrast to pressure-driven (Poisseuille) flow, which exhibits a parabolic velocity profile. EOF-based microreactors therefore are nearly ideal plug-flow reactors, with corresponding narrow residence time distribution, which improves reaction selectivity. 

Fig. 9.7.1 Visualization of (a) pressure-driven Poiseuille flow (parabolic) in a 100 )xm ID fused silica capillary and (b) electrokinetically-driven plug flow in a 75 pm ID capillary. Numbers denote time in milliseconds between micrographs in both sequences. See text for details.

The flow profile of the EOF has the form of a plug (Fig. 3.4). The flow velocity is identical over the whole capillary diameter, except for the slower moving diffuse layer close to the capillary wall. This homogeneous velocity distribution minimises band broadening and, thus, increases separation efficiency. A radically different situation occurs with the pressure driven flow used in liquid chromatography. Here, the flow profile is parabolic the flow velocities have a large distribution over the column diameter. Analytes in the middle flow considerably faster than analytes... 

Fig. 3.4. A parabolic flow profile occurs in pressure driven flow such as in liquid chromatography, whereas the EOF flow profile has the form of a plug.

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