Page 516 - Tandem Techniques

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but is also provides plug flow through the column, as opposed to the parabolic velocity profile normally
associated with fluid-flow through tubes. The lack of a parabolic velocity profile can have a dramatic
effect on the column efficiency, as shown in Chapter 1. One of the early reports on the successful use of
electrosmotic flow in LC, was that of Schmeer et al. [12], and the layout of their apparatus is depicted
in Figure 13.14.

Figure 13.14
The Capillary Electrochromatography Apparatus

Electro-osmotic flow is obtained by applying a strong electric field across the length of the column. The
velocity of the flow is independent of the particle size of the packing, and so columns 0.5 mm in
diameter, packed with particles 1.2 µm in diameter, can readily be used if so desired. Employing
columns of this diameter, packed with such small particles, efficiencies of 200,000 theoretical plates
can easily be obtained. In practice, to minimize resistance heating, columns of 50 to 100 µm I.D. are
usually employed, with flow rates up to 2 µl/min. This magnitude of the flow rate is ideal for
electrospray nebulizing and consequently, no sheath flow is necessary. However, in order to assure
stable flow conditions a supplementary flow of mobile phase was provided by a mechanical pump as
shown in Figure 13.13. The fused capillary column 20 cm long, 100 µm I.D., and 360 µm O.D., was
fitted with a terminal fused silica gel frit