Field Flow Fractionation
Encyclopedia
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, in order to cause separation of the particles present in the fluid, dependent on their differing "mobilities" under the force exerted by the field. It was invented and first reported by J. Calvin Giddings
.
In field-flow fractionation the field can be electrical, transverse flow through a semi-permeable membrane, gravitational
, centrifugal
, magnetic
, thermal-gradient, 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 electrical 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.
process, in which various substances present in a fluid get separated based on their flow velocities under some applied external influence, such as an electric field.
Often these substances are various particles initially suspended in a small volume of a liquid buffer and pushed along a fractionation channel by more of the pure buffer. The varying velocities of a particular species of particles may be due to its size, its mass, and/or its distance from the walls of a channel with non-uniform flow-velocity. The presence of different species in a sample can thus be identified through detection of a common property at some distance down the long channel, and by the resulting fractogram indicating the presence of the various species by peaks, due to the different times of arrival characteristic of each species and its physical and chemical properties.
In electrical FFF, an electric field controls the velocity by controlling the lateral position of either a charged (having electrophoretic mobility) or polarized (being levitated in a non-uniform field) species in a capillary channel with a hydrodynamically parabolic flow-velocity profile, meaning that the velocity of the pumped fluid is highest midway between the walls of the channel and it monotonically decays to a minimum of zero at the wall surface.
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
or solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
pumped through a long and narrow channel, perpendicular to the direction of flow, in order to cause separation of the particles present in the fluid, dependent on their differing "mobilities" under the force exerted by the field. It was invented and first reported by J. Calvin Giddings
J. Calvin Giddings
J. Calvin Giddings was a professor of chemistry a the University of Utah.He is credited with the invention of field flow fractionation.-Selected Works:* J. Calvin Giddings Dynamics of Chromatography Marcel Decker, NY, 1965...
.
In field-flow fractionation the field can be electrical, transverse flow through a semi-permeable membrane, gravitational
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...
, centrifugal
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
, magnetic
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
, thermal-gradient, 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 electrical current flow) under the forces of the field, in equilibrium with the forces of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
: an often-parabolic
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...
laminar-flow
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
-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.
Fractogram
A graph of a detection signal vs. time, derived from an FFFField Flow Fractionation
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, in order to cause separation of the particles present in the fluid, dependent on their...
process, in which various substances present in a fluid get separated based on their flow velocities under some applied external influence, such as an electric field.
Often these substances are various particles initially suspended in a small volume of a liquid buffer and pushed along a fractionation channel by more of the pure buffer. The varying velocities of a particular species of particles may be due to its size, its mass, and/or its distance from the walls of a channel with non-uniform flow-velocity. The presence of different species in a sample can thus be identified through detection of a common property at some distance down the long channel, and by the resulting fractogram indicating the presence of the various species by peaks, due to the different times of arrival characteristic of each species and its physical and chemical properties.
In electrical FFF, an electric field controls the velocity by controlling the lateral position of either a charged (having electrophoretic mobility) or polarized (being levitated in a non-uniform field) species in a capillary channel with a hydrodynamically parabolic flow-velocity profile, meaning that the velocity of the pumped fluid is highest midway between the walls of the channel and it monotonically decays to a minimum of zero at the wall surface.
External links
- http://www.regional.org.au/au/asssi/supersoil2004/s4/oral/1458_chittleboroughd-1.gif
- Example