Lowering the Steric Transition Diameter and Fast Separations in Flow Field-Flow Fractionation.
Keith D. Jensen, S. Kim Ratanathanawongs, and J. Calvin Giddings.
Seventh National Conference on Undergraduate Research, University of Utah, Salt Lake City, Utah, April 1993.


Field-flow fractionation (FFF) is a versatile analytical method for separating and characterizing a wide variety of samples ranging from humic acids (molecular weight 300) to silica particles in contact lens cleaners (up to 70 um large). Separations occurs due to differential flow through a thin open channel. There are several modes of separation in FFF, with the two most commonly employed being the normal and steric modes. In both modes, an external field (a crossflow in this work) drives an injected samples toward one of the channel walls. In the normal mode, diffusion of the sample away from the wall causes a sample concentration profile to be established. Smaller particles, which diffuse faster, have equilibrium positions further from the wall and are transported by higher velocity flow streams and elute before larger particles. In the steric mode, larger particles protrude further into the channel where they are carried by faster laminar flows. Thus, the elution order is opposite to that of the normal mode. The transition range between these two modes normally occurs around 1 um and is variable depending upon many factors. By using thinner channels and higher flow velocities, smaller particles are compressed into hyperlayers and elute in the steric mode. This allowed the inversion diameter to be lowered to approximately 0.24 um. these conditions also decreased analysis times. For example, a mixture of 0.040, 0.091, 0.135 and 0.300 um polystyrene latex particles was baseline separated in less than three minutes.

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