Electrophoresis deflection

For many applications, diode array detection has become routine. A photodiode array was used for simultaneous detection of 100 capillaries in zone electrophoresis and micellar electrokinetic chromatography (MEKC).1516 Deflection of a laser beam by acoustic waves was reported as a means to scan six capillary channels on a microchip.17 The design of a low-noise amperometric detector for capillary electrophoresis has been reported.18... 

FIGURE 7.3 The rotary confocal fluorescence scanner used to detect pCAE chip separations. Laser excitation at 488 nm (100 mW) is directed up through the hollow shaft of a computer-controlled stepper motor, deflected 1.0 cm off-axis by a rhomb prism, and focused on the electrophoresis lanes by a microscope objective. The stepper motor rotates the rhomb/objective assembly just under the lower surface of the microchip at five revo-lutions/s. Fluorescence is collected along the same path and spectrally, and is spatially filtered before impinging on the four-color confocal detector [980]. Reprinted with permission from the American Chemical Society. 

An old but still important example of continuous 2D separation is deflection electrophoresis. Here flow carries the sample stream in one direction while electrophoresis at right angles causes the differential deflection and separation of the stream into component filaments (see Chapter 8). 

Figure 7.3. In continuous deflection electrophoresis, flow down a paper sheet or between closely spaced plates carries different solutes, which are gradually separated by electrophoresis along axis x, to different collection ports. However, the flow is classified as passive because it does not enhance the separation occurring along the electrical-field axis. The role of flow is to aid continuous sample collection, not to fundamentally alter the separation process.

Two-dimensional (2D) electrophoretic methods are important variants of electrophoresis. In Section 6.4 we noted that electrophoresis could combine with flow in a 2D system providing continuous (and thus preparative) separation. Fundamentally, this is simply zonal electrophoresis converted into a continuous form by nonselective flow (see Section 7.5). If we observe the separation at different positions along the flow axis (as illustrated for one position in Figure 7.3), we have essentially a series of snapshots of the zones evolving with time. Each component zone is deflected from the flow axis at a unique angle as a consequence of the evolution of the electrophoretic separation. 

Many 2D planar structures have been used to implement deflection (continuous flow) electrophoresis. The primary requirement is that flow and electrophoresis be carried out simultaneously and uniformly. Hanging paper curtains soaked with electrolyte and fed a stream of electrolyte from above served admirably for this purpose when the technique was initiated in the 1950s. In recent years thin flow channels enclosed between flat plates have become important. The process is complicated by parabolic flow, which distorts and effectively broadens the electrophoretic zones. More detail is available in the cited references on electrophoresis [3-5]. 

Z Huang, N Munro, AF Huhmer, JP Landers. Acousto-optical deflection-based laser beam scanning for fluorescence detection on multichannel electrophoresis microchips. Anal Chem 71 5309-5314, 1999. 

A modification to electrophoresis is free-flow electrophoresis, which enables the continuous separation of a mixture according to charge with subsequent collection of the sample band of interest [244]. For this, an transverse electric field is applied in pressure driven flow within a broad and flat microchamber. While passing this extraction chamber, the species contained in the sample flow are deflected depending on their charge and thus exit the chamber through one of several outlets. 

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