Polarization FFF

In particle size separations by FFF, the nature of the applied field (physical or chemical forces) determines each particular method or technique of polarization FFF and, consequently, the appropriate instrumentation. The most important polarization FFF methods at the present time are:

• sedimentation FFF

The basic experimental devices as well as specific instrumentation are described here for each particular FFF method or technique.

Independent of the method or technique, all FFF apparatuses are composed of a system of solvent delivery (reservoir, pump), injector sample (injection valve, syringe-septum, etc.), separation channel (different construction for each method), detector (refractive index detector, spectrophotometer, molar mass detector, etc.) and a data acquisition and treatment system (computer). With the exception of the FFF separation channel, all other components, and the system as a whole, are practically the same as a conventional liquid chromatography system.

Schematic representation of the separation channel for sedimentation FFF is shown in Figure 3(A). The separation channel is coiled inside a centrifuge rotor. A delicate part of this separation unit is the rotating seal which must permit the flow-through of a carrier liquid and the connection to the injector at the entry to the channel proper and of a detector at the exit. However, this technical problem is solved and the rotors for sedimentation FFF are commercially available. On the other hand, a home-built solution is also possible providing that some technical skill is available.

If the particles to be separated are relatively large or dense and, consequently, the gravitational force is enough to generate the formation of sufficiently strong concentration gradients, the construction of the separation channel is much simpler, as shown in Figure 3(B). In this case, the channel is composed of two sandwiched glass plates, one of them is provided with holes and capillaries for carrier liquid entry and exit and a thin foil in which the channel proper is cut. The whole channel must be positioned horizontally to avoid casual parasite convections which could cause the separation to deteriorate.

The channel for flow FFF is schematically demonstrated in Figure 4(A). It is formed between two parallel, semipermeable membranes fixed on porous supports. The cross-flow of the carrier liquid is superposed perpendicularly to the flow of the carrier liquid in a longitudinal direction inside the channel. The cross-flow acts as an external field of hydrodynamic forces which generate a uniform flux of all particles.

Figure 3 Simplified schemes of the construction of the sedimentation FFF channels used in a centrifuge and in natural gravitational field. (A) Sedimentation FFF channel: (1) channel;

(2) direction of the flow; (3) rotation; (4) flow inlet; (5) flow outlet. (B) Gravitational FFF channel: (1) channel walls; (2) foil spacer;

(3) inlet and outlet.

The channel for electric FFF is usually formed by semipermeable membranes as in flow FFF (see Figure 5). The reason for such a solution is to decouple the separation channel proper from the electrode chambers and thus to avoid the contamination of the channel by products of electrolysis (gas bubbles). However, channels of simpler construction in which the metal or graphite electrodes form the channel walls and thus are not decoupled from the separation space have been constructed and work quite well under carefully chosen experimental conditions. The channel for thermal FFF is constructed in such a manner to allow a temperature difference between two metallic bar walls with highly polished surfaces. The walls are separated by a spacer in which the channel proper is cut. The upper bar is heated by using appropriate electrical cartridges and the lower bar is cooled by circulating water. Both bars should be equipped with several holes to accommodate the thermocouples for temperature control. Schematic representation of a channel for thermal FFF is shown in Figure 6. In some cases, when the temperature of

Figure 3 Simplified schemes of the construction of the sedimentation FFF channels used in a centrifuge and in natural gravitational field. (A) Sedimentation FFF channel: (1) channel;

(2) direction of the flow; (3) rotation; (4) flow inlet; (5) flow outlet. (B) Gravitational FFF channel: (1) channel walls; (2) foil spacer;

(3) inlet and outlet.

The carrier liquid passes through the membranes but the separated particles should not, due to the conveniently chosen porosity of the membranes. The uniformity of the cross-flow is, however, not necessary to achieve high performance separation. If only one of the main channel walls is semi-permeable, a nonuniform hydrodynamic Reld is generated in such an asymmetrical flow FFF channel. The dependence of the separation resolution on particle size in such a channel is different compared with a channel equipped with two semi-permeable walls, but high performance particle size separation is also achieved.

A classical type of rectangular cross-section channel has sometimes been substituted with a circular cross-section capillary with an overpressure applied inside or by applying an external cross-flow in a more standard manner, as shown in Figure 4(B). The simplicity of the construction of such a 'channel' is the main advantage of this conRguration. The theoretical description of the separation is complex, however, and, moreover, the probability of the formation of parasite flows degenerating the separation is higher.

1

Figure 4 (A) Construction of a rectangular cross-section channel for flow FFF: (1) porous supports; (2) cross-flow inlet and outlet; (3) membranes; (4) foil spacer; (5) longitudinal flow inlet; (6) longitudinal flow outlet. (B) Circular capillary for flow FFF with: (1) overpressure applied from the inside; (2) cross-flow applied externally.

4 5
Figure 5 Construction of a channel for electric FFF: (1) electrodes and electrolyte inlet and outlet; (2) membranes; (3) foil spacer; (4) longitudinal flow inlet; (5) longitudinal flow outlet.

the heated wall is above the boiling point of the carrier liquid used, the channel must be sealed so as to operate under high-pressure conditions. The thickness of the channel can be as low as few micrometers which permits performing high-speed and high-resolution fractionations. The separation can be accomplished in just a few seconds.

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