Overview of F cd Separations

In this section we briefly survey the F( + )cd methods. In Table 7.2 we listed among F( + )cd methods the techniques of field-flow fractionation, thermog- 

Flow is then instituted along the axis of the chamber and a differential flow pattern develops. This differential flow carries each component along in some uneven pattern. Different components distributed over the cross section with different concentration profiles will be carried downstream unequally because of the nonuniformity of flow. The net result is that the different components will be displaced at different average velocities in the tube and separation will be realized. 

The flow pattern itself can vary depending on the source of the flow. Ordinary pump-driven flow will create a parabolic (bullet-shaped) profile in narrow tubes, with a high velocity in the center and low velocities at the tube edges (see Section 4.4). Convective flow, on the other hand, is often countercurrent, with dense fluid sinking and less dense fluid rising (Section 4.8). 

Convective flow is used in both the thermogravitational (Clusius-Dickel) column and in electrodecantation. In the thermogravitational system, one wall of the channel is heated or, alternatively, a hot wire is placed along the axis of the channel. The fluid at the cold surface then tends to sink relative to that at the hot surface. Simultaneously, thermal diffusion (Section 8.8) causes different levels of enrichment in the hot and cold regions of the channel. The enriched solutes then move up and down the channel at a rate depending upon their distribution between hot and cold regions. In binary 

The perpendicular flow methods have a number of important similarities and differences [6]. Most significantly, both thermogravitational separation and electrodecantation are limited to the separation of only two fractions (i.e., peak capacity equals two). These methods are thus restricted to preparative applications. Field-flow fractionation is the only method in the /r(+)cd class designed specifically for multicomponent analytical separations. We describe this method in greater detail below. 

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