Velocity flow profile capillary columns

The "plug-like velocity flow profile for electrokinetically pumped capillary columns (see Figure 1) is important in minimizing resistance to mass transfer within the mobile phase (4). Hydrostatically-pumped capillaries, have parabolic flow profiles which tend to severely disperse solute bands unless extreme narrow-bore (i.d.s less than 10 pm) capillaries are employed (12). Fortunately, larger capillaries, with less stringent detector volume requirements, can be efficiently used in MECC. 

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 different velocities across a laminar profile, band broadening results as the analyte zone is transferred through the conduit. The flat electroosmotic flow profile created in electrodriven separations is a principal advantage of capillary electrophoretic techniques and results in extremely efficient separations. 

Despite the improved mass transfer characteristics of the "plug-like" flow profiles observed in MECC, "intra-column" resistance to mass transfer is significant at higher flow velocities (i.e., at high applied voltages). Although not as dramatic as in our work with hydrostatically-pumped open capillary LC, we have observed improvements in efficiency with the MECC technique when column diameter is reduced. This is illustrated in Figure 6. 

Figure 1 Frontal zone profile of electroosmotic flow in open-tubular capillary column. A rectangular capillary (1 mm x 50 pm and 16.4 cm long) was used. Colored sample methanol solution of 0.1 mM Rohdamine 6G. Frontal zone profiles of 0, (white) and 02 (black) were successively taken. The period between two zones was 11.44 s. The distance between two frontal zones was 6.52 mm. Flow velocity of the center was 0.57 mm/s. The ratio of the flow velocities given by (flow velocity at half-radius)/(flow velocity at center) was 1.0027. The retarded speed of the flow velocity at the center compared to that at the corner was only 0.4%. Although the same scale was used for the X and Y axes, there are time intervals between 0, and 02 in (A). Figure (B) was obtained by the combination of 0, and 02 (overlapping two frontal zone profiles at the corner). Therefore the right and left profiles correspond to 0, and 02, respectively. Applied voltage, 1.59 kV current, 120 nA.

Fig.l Schematic representation of the flow profiles obtained with the same capillary column connected to an electric-driven system (a) and to a pressure-driven system (b). Arrows indicate flow velocity vectors. 

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... 

Table 2.2 Order of magnitude comparison of the contribution to band broadening by non-uniform local mobile phase velocities, the flow profile (i /Dm) in capillary columns (the Cq term in Golay s equation)...

In the case of laminar flow through packed columns, because of radial and axial directions changes, the velocity profile is no longer parabolic, as in capillary columns, but flattens at the center. 

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