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THE EFFECT OF SMALL TEMPERATURE GRADIENTS ON FLOW IN A CONTINUOUS FLOW ELECTROPHORESIS CHAMBER PERCY H. RHODES AND ROBERT S. SNYDER National Aeronautics and Space Administration, Marshall Space Flight Center, Huntsville, Alabama 35812 ABSTRACT Continuous flow electrophoresis employs an electric field to separate biological cells suspended in a flowing liquid buffer solution. Good separations based on differences in electrophoretic mobility are obtained only when a unidirectional flow is maintained. The desired flow has a parabolic structure in the narrow dimension of the chamber and is uniform across the width, except near the edges where the no-slip condition prevails. However, because of buoyancy, very small lateral or axial temperature gradients deform the flow significantly. The results of experiments conducted with a specially instrumented chamber show the origin and structure of the buoyancy-driven perturbations. It is found that very small temperature gradients can disturb the flow significantly, as was predicted by earlier theoretical work. INTRODUCTION Electrophoresis, a technique used to fractionate particulate and soluble materials based on their differential migration in an electric field, uses a fluid medium to provide the supporting environment. The fractionation of biological cells imposes specific ionic and biochemical requirements on this fluid medium which results in a high conductivity. This can cause significant Joule heating and thermal convection. Gels and density gradients are commonly used to minimize thermal convection by constraining the movement of the buoyancyinduced flow but, unfortunately, these methodologies have not been perfected for large scale separations which are possible with continuous flow electrophoresis. Continuous flow electrophoresis takes place within a flowing curtain of aqueous electrolyte in a long rectangular chamber of high aspect ratio. The narrow gap between the broad faces of the chamber confines the flow to a wide curtain. The sample is continuously injected into the curtain as a finely drawn stream and fractionated under the influence of a lateral electric field produced by flanking electrodes. Thermal convection is supressed in continuous flow electrophoresis devices by making the curtain as thin as possible. Of course this decreases the amount of sample that can be entrained in the curtain. As a result, the design and operation of a given system involves a series of compromises involving the properties of the fluid medium and chamber design. The present study was initiated to provide experimental and analytical data to support the rational selection of operating parameters. Fully developed laminar flow must be maintained in the electrophoresis chamber if true separations based on electrophoretic mobility are to be realized. While this base flow is parabolic in the narrow plane (thickness) of the

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COOLING: FRONT AND BACK. TEMPERATURE MEASUREMENT: MOVABLE THERMOCOUPLE PROBES MOUNTED IN REAR FACE FLOW VISUALIZATION: FLUORESCENCE OF FLUID MEDIUM.

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