Anal. Chem. 1994,66, 3047-3053
Enhancement of Performance in Sedimentation Field-Flow Fractionation by Temperature Elevation J. Calvln Glddlngs,' Yuehong Xu, and Marcus N. Myers Field-Flow Fractionation Research Center, Department of Chemistry, University of Utah, Salt Lake Cityl Utah 84 112 Nonequilibrium theory, combined with the principles of time optimization, show that the time necessary to achieve a given separation in FFF is scaled to q/r where q is viscosity and Tis absolute temperature. The q/Tratio for water (and other common liquids) decreasesseveral-foldfor modest temperature gains of -20-60 O C , implying a significant advantage for FFF operation at elevated temperatures. This concept was tested by modifying a standard sedimentation FFF apparatus with a heating system. The separation of 0.220-0.742 pm polystyrenelatex beads in aqueous carrier liquids was compared at room temperature and at elevated temperatures of 51 and 68 O C . Both separation power and speed were improved. In accordance with the predicted q / Tscaling, the separation time of five bead sizes at a given resolution level was reduced by a factor of 2.4 (from 29 to 12 min) in elevating the temperature from 25 to 68 OC. Some other potential benefits of temperature elevation in FFF are discussed.
in which Cis the traditional nonequilibrium plate height term descriptive of both chromatography and FFF. In common with optimization theories developed for chromatography, it is assumed that for FFF the magnitude of N is the minimum value adequate to achieve the desired resolution.' If now the diffusion coefficient D is expressed by the StokesEinstein equation19596 D = kT/3?rqd then eq 1 assumes the form 12nNI2d tdmin) = k T
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Field-flow fractionation (FFF) and chromatography are closely related elution-based methodologies; as such they share the common feature that analysis time is strongly influenced by nonequilibrium phenomena.' For both separation techniques, nonequilibrium band-broadening terms are inversely proportional to diffusivity. Diffusion coefficients are, in turn, inversely related to viscosity. As a consequence of these classical relationships, it has long been recognized that the speed of separation in both liquid chromatography and in field-flow fractionation can be enhanced by reducing the viscosity of the mobile phase or carrier liquid.2 The latter viscosity can be reduced by using a less viscous liquid, by elevating the temperature, or by When dilute aqueous solutions are used as carrier liquids, as in most cases of sedimentation FFF, temperature elevation is the only significant means for reducing viscosity. To gain appreciation of the possible degree of enhancement of separation speed in FFF by temperature elevation, we write the limiting equation for the time tN necessary to realize N theoretical plates for a well-retained component (Le., retention ratio R
