Asymmetric flow field flow fractionation
Asymmetrical flow field-flow fractionation (AF4) is most versatile and most widely used sub-technique within the family of field flow fractionation (FFF) methods. AF4 can be used in aqueous and organic solvents and is able to characterize nanoparticles, polymers and proteins. The theory for AF4 was conceived in 1986 and was established in 1987 and first published by Wahlund and Giddings. AF4 is distinct from symmetrical Flow FFF because it contains only one permeable wall so the cross-flow is caused only by the carrier liquid. The cross-flow is induced by the carrier  liquid constantly exiting by way of the semi-permeable wall on the bottom of the channel. Applications and detection methods. Asymmetrical flow field flow fractionation (AF4) is nowadays a common and state-of-the art method for fractionation and separation of macromolecules and particles in a suspension. AF4 is an alternative to HPLC and SEC in cases where column chromatography is not suitable for the analyte. HPLC or SEC would be used for liquid separations for molecules up to 1000 kDa and nanoparticles up to 10 nm. As the size increases above 10 nm, AF4 becomes superior in terms of  resolution and recovery. AF4's applications are flexible for many analytical conditions where a column-based method would be unable to properly separate the desired particles. For macromolecules and nanoparticles AF4 is an alternative method especially when the stationary phase in columns interacts with the sample. AF4 is specifically powerful for inhomogeneous samples where it can separate soluble macromolecules from particles or aggregates. AF4 and other FFF methods have been extensively used in environmental research on the impact of nano materials and to characterize condensed tannins oxidation. For high molar mass and branched polymers, AF4 has been shown to achieve  good separation, whereas SEC fails, and AF4 has been applied to polyolefines at temperatures above 150 C Detection methods are the same as for FFF in general, UV is most popular as a concentration detector, but most AF4 systems include a multi-angle light scattering detector for direct measurement or size and molar mass. Operational procedures. The AF4 experiment can be separated into three stages: 1. Sample Injection 2. Sample focusing 3. Fractionation