Field-flow Fractionation of Particles in the Steric Inversion Region.
Keith D. Jensen, S. Kim Ratanathanawongs, and J. Calvin Giddings.
Sixth National Conference on Undergraduate Research, University of Minnesota, Minneapolis, Minnesota, March 1992, Abstract #992.
Poster presented in the Chemistry section of Poster Session II, Thursday March 26, 1992.


Field-flow fractionation (FFF) is a relatively new analytical method for separating and characterizing polymers and particles. This method has been used to separate a wide variety of samples ranging from humic acids (300 Daltons) to silica particles in contact lens cleaners (100 um large). A parabolic flow profile is established in a thin channel. The flow velocity is fastest in the center of the channel and decreases to zero at the walls. An injected sample is driven towards one of the channel walls by one of many possible fields (e.g. sedimentation, crossflow, thermal gradient, magnetic, or electric). There are several modes of separation in FFF, with the two most commonly employed being the normal and steric modes. In the normal mode, an equilibrium position is established at the point where the diffusion velocity away from the wall balances the field induced velocity. Smaller particles, which diffuse faster, have equilibrium positions further from the wall and are transported by higher velocity flow streams and elute before later 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. The necessity of manipulating the steric transition point arises if one tries to separate a broad size range sample (e.g. 10 - 0.3 um). The present research deals with varying experimental conditions and monitoring how these effects change the steric transition point. Using thinner channels and appropriate flow rates, the steric transitions point has been lowered to 0.2 um.

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