Capillary electrochromatography with segmented capillaries for controlling electroosmotic flow.
Capillaries consisting of two segments each packed with a different stationary phase were introduced for the control and manipulation of the electroosmotic flow (EOF) in capillary electrochromatography (CEC). This kind of column configuration was called segmented capillary where one segment was packed with octadecyl silica (ODS) and served as the separation segment while the other segment was packed with bare silica and functioned as the EOF accelerator segment. The average flow in the segmented capillary increased linearly with increasing fractional length of the EOF accelerator segment, and consequently the analysis time was reduced. Under a given set of conditions, the average flow can be varied over a certain range that extends from the EOF in the individual ODS capillary at the lower end to the EOF in the individual bare silica capillary at the higher end. The pore size of the bare silica in the EOF accelerator segment influenced the average flow in the segmented capillary. Because of the difference in the EOF of the individual segments, the average flow across the segmented capillary is partially degenerated from EOF to viscous flow. Furthermore, the retaining frits in CEC columns are restrictive points which slow down the average flow, thus furthering the degeneration of the flow from EOF to viscous flow. In other words, in CEC columns containing retaining frits, the flow of the mobile phase is not only based on electroosmosis but is contaminated by a viscous component.