Principle

Field-flow fractionation (FFF) is one of the important analytical methodologies, suitable for the separation and characterization of particles in the submicron and micron ranges. The effective Reld generates the flux of the separated particles and forms a concentration gradient of each particular species across the ribbon-shaped separation channel. The concentration gradients are counter-balanced by a diffusion flux. At equilibrium, a stable concentration distribution of each particular species is established in the direction across the channel. Simultaneously, a flow velocity proRle is formed across the channel due to the viscous

Figure 1 Schematic representation of the general principle and of the experimental arrangement of FFF: (1) carrier liquid reservoir; (2) pump; (3) injector; (4) separation channel; (5) detector; (6) computer; (7) external field; (8) hydrodynamic flow. Detail shows the schematic representation of two fundamental separation mechanisms: polarization FFF and focusing FFF.

the accumulation wall of the channel according to their sizes or focused at different levels across the channel according rather to an intensive property (see Figure 1).

The polarizing field force, F, and the velocity of the field-induced migration of the fractionated particles, U, are usually constant and independent of the position in the direction of the field action:

F O 0 and U O 0 within 0 < x < w where w is the thickness of the FFF channel in the direction of the field action (x-axis); x = 0 is situated at the accumulation wall of the channel. The steady-state concentration distributions of the sample components across the channel are exponential:

Figure 1 Schematic representation of the general principle and of the experimental arrangement of FFF: (1) carrier liquid reservoir; (2) pump; (3) injector; (4) separation channel; (5) detector; (6) computer; (7) external field; (8) hydrodynamic flow. Detail shows the schematic representation of two fundamental separation mechanisms: polarization FFF and focusing FFF.

drag in the longitudinal flow of the carrier liquid. As a result, each particle is carried along the channel with a velocity corresponding to an instantaneous position of the particle within the flow velocity profile. The carrier liquid thus elutes each species with a mean velocity which corresponds roughly to the position of the centre of gravity of the field-induced concentration distribution across the channel of that species. This principle is schematically demonstrated in Figure 1.

The separation is usually governed by the differences in size of the separated components of a polydisperse sample. If the appropriate relationship between the retention parameters and the size of the particles is known or found empirically by using a suitable calibration procedure, the fractograms can be used to calculate the particle size distribution (PSD) and the average values of the particle size of the fractionated species. However, the intensive properties (such as the electrical charge, density, etc.) can influence the separation based on size differences.

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Solar Panel Basics

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