Field and flow programming and their combination, dual programming, are shown to extend the particle size range to which a single flow/hyperlayer field-flow fractionation (FFF) run is applicable to approximately 1-50 microns. The rationale for programming flow/hyperlayer FFF (or other forms of lift hyperlayer FFF) is to expand the diameter range of micron size particles that can be resolved in a single run. By contrast, the reason for programming normal-mode FFF, the only kind of programming previously realized in FFF, is to reduce the analysis time of submicron particle samples of considerable size variability. These differences are explained in detail in relationship to the basic mechanisms governing retention in normal, steric, and lift hyperlayer FFF. Experiments are described in which field, flow, and dual programming are used to expand the accessible diameter range of flow/hyperlayer FFF. An example is shown in which 11 sizes of latex microspheres in the 2-48-microns diameter range are separated by dual programming in 11 min.
This article provides an overview on the use of field-flow fractionation (FFF)for particle size analysis and for the characterization of other particle properties such as density, porosity, and the thickness of adsorbed layers. While FFF is a relatively new technology for particle characterization, it is one of the most versatile and powerful techniques now avaiable for characterizing particle populations. The unique features contributing to the effectiveness of FFF include high resolution, relatively high speed, adaptability to different types and sizes of particles, and the ability to collect narrow fractions for further characterization by microscopy and other techniques.For background, the mechanism of FFF is described in two parts, one applicable to particles over 1 !1-m diameter and the other relevant to submicron size particles ranging down to 1 nm size. It is shown how particle size distributions are obtained for a variety of particulate materials in both size ranges. The strategies needed for measuring particle properties other than size and size distribution are discussed.
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