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Field-flow fractionation, abbreviated FFF, is a separation technique where a field is applied to a fluid suspension or solution pumped through a long and narrow channel, perpendicular to the direction of flow, to cause separation of the particles present in the fluid, depending on their differing "mobilities" under the force exerted by the field. It was invented and first reported by J. Calvin Giddings.〔Giddings, JC, FJ Yang, and MN Myers. "Flow Field-Flow Fractionation: a versatile new separation method.” Science 193.4259 (1976): 1244–1245.〕 The method of FFF is unique to other separation techniques due to the fact that it can separate materials over a wide colloidal size range while maintaining high resolution. Although FFF is an extremely versatile technique, there is no "one size fits all" method for all applications. In field-flow fractionation the ''field'' can be asymmetrical ''flow'' through a semi-permeable membrane, gravitational, centrifugal, thermal-gradient, electrical, magnetic etc. In all cases, the separation mechanism is born from differences in particle mobility (electrophoretic, when the field is a DC electric field causing a transverse electric current flow) under the forces of the field, ''in equilibrium with'' the forces of diffusion: an often-parabolic laminar-flow-velocity profile in the channel determines the velocity of a particular particle, based on its equilibrium position from the wall of the channel. The ratio of the velocity of a species of particle to the average velocity of the fluid is called the ''retention ratio''. ==Fundamental principles== Field flow fractionation is based on laminar flow of particles in a solution. These sample components will change levels and speed based on their size/mass. Since these components will be travelling at different speeds, separation occurs. A simplified explanation of the setup is as follows. The sample separation occurs in a thin, ribbon-like, channel in which there is an inlet flow and a perpendicular field flow. The inlet flow is where the carrier liquid is pumped into the channel and it creates a parabolic flow profile and it propels the sample towards the outlet of the channel. Relating force (F) to retention time (TR) The relationship between the separative force field and retention time can be illustrated from first principles. Consider two particle populations within the FFF channel. The cross field drives both particle clouds towards the bottom "accumulation" wall. Opposing this force field is the particles natural diffusion, or Brownian motion, which produces a counter acting motion. When these two transport process reach equilibrium the particle concentration c approaches the exponential function of elevation x above the accumulation wall as illustrated in equation 1. : l represents the characteristic elevation of the particle cloud. This relates to the height that the particle group can reach within the channel and only when the value for l is different for either group will separation occur. The l of each component can be related to the force applied on each individual particle. : Where k is the Boltzmann constant, T is absolute pressure and F is the force exerted on a single particle by the cross flow. This shows how the characteristic elevation value is inversely dependent to the Force applied. Therefore, F governs the separation process. Hence, by varying the field strength the separation can be controlled to achieve optimal levels. The velocity V of a cloud of molecules is simply the average velocity of an exponential distribution embedded in a parabolic flow profile. Retention time, tr can be written as: : Where L is the channel length. Subsequently, the retention time can be written as: tr/to = w/6l ⌊coth w/2l- 2l/w⌋−1 Where to is the void time (emergence of a non-retained tracer) and w is the sample thickness. Substituting in kT/F in place of l illustrates the retention time with respect to the cross force applied. tr/to = Fw/6kT ⌊coth Fw/2kT- 2kT/Fw⌋−1 For an efficient operation the channel thickness value w far exceeds l. When this is the case the term in the brackets approaches unity. Therefore, equation 5 can be approximated as: tr/to = w/6l = Fw/6kT Thus tr is roughly proportional to F. The separation of particle bands X and Y, represented by the finite increment ∆tr in their retention times, is achieved only if the force increment ∆F between them is sufficient. A differential in force of only 10–16 N is required for this to be the case. The magnitude of F and ∆F depend on particle properties, field strength and the type of field. This allows for variations and specialisations for the technique. From this basic principle many forms of FFF have evolved varying by the nature of the separative force applied and the range in molecule size to which they are targeted. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「field flow fractionation」の詳細全文を読む スポンサード リンク
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