Zonal separation

Fig. 11. (a) Differential centrifugation (pelleting), where time 1 < time 2 < time 3 < time, (b) Rate zonal separation in a swinging-bucket rotor, where tube 1 represents the density gradient solution, tube 2 the sample plus the gradient, and tube 3 the separation of sample particles under a centrifugal force, where... 

This equation shows that N increases with Xlcr, the ratio of the migration distance in which separation can occur to the zone width parameter zone overlap. The overall goodness of a separation obviously increases with this ratio, so N is clearly a valid index of separation power, as substantiated more fully elsewhere [8,15]. In chromatography, N is often taken as a measure of column efficiency, a definition that can be extended to other zonal separation methods as well. 

A Gaussian zone is the model around which most discussions of zonal separation methods revolve. However, there are frequent departures from... 

Reality, therefore, forces separation scientists to deal with various aspects of peak overlap. The subject is a complicated one. We present in this section a brief introduction to the nature, implications, and unexpectedly high frequency of overlap. Some theoretical methods for dealing with overlap are outlined in the following section. While this treatment is based on studies of peak overlap in chromatography [33, 34], the concepts are equally valid for electrophoresis and other zonal separation methods. 

Figure 8.1. Illustration of zonal separation executed by an applied field (e.g., electrical) without flow.

We found earlier that transient (nonsteady-state) sedimentation required density gradients to stabilize against convection. Isopycnic sedimentation relies on density gradients not only for anticonvective purposes but also as the secondary gradient needed to establish steady-state conditions. The difference in the two cases is found in the magnitude of the density gradient and in the degree to which components are allowed to approach their steady-state condition. The equipment is similar the zonal rotor developed by Anderson is used for isopycnic as well as transient zonal separations [45]. 

While the random walk model employed here is widely applicable to F(+) methods, it fails if the molecules do not transfer rapidly between velocity states, equivalent to many random steps. Such a limitation applies to electrodecantation (noted below), where the distances are too great for rapid diffusional exchange. The random walk model is most meaningful for zonal separation methods such as chromatography and field-flow fractionation. 

M. K. Brakke, Arch. Biochem. Biophys., 45 275 (1953). Zonal Separations by Density-Gradient Centrifugation. 

For rate—zonal separations, it is general rule that miniaturization of the dimension, in which the physical effective force is acting, brings along a... 

Rate-zonal separations It can be used to separate particles of similar density according to size or to separate particles of different density and size as a function of their sedimentation coefficient, s. In its simplest form, the sample mixture is layered in a narrow band on top of a preloaded, homogeneous medium as shown in Figure 3. 

Figure 3 Rate-zonal separations in a swinging-bucket rotor. (A) Centrifuge tube fiiied with density gradient soiution, (B) sample applied to top of gradient, and (C) under centrifugal force, particles move at different rates depending upon their mass. Courtesy of Beckman Coulter, Inc.)...

From Eq. (6) it is clear that the prerequisite for the enantioseparation in this case is the formation of the transient diastereomeric associates of both enantiomers with different mobilities Ci f C2 MCi I C2- The enantioseparations based on the mechanism described by Eq. (6) are very close to classical zonal separations (EKC mechanism is not directly involved). However, considering the lUPAC nomenclature, these separations may also be attributed to EKC. 

High resolution rate-zonal and iso-pycnic-zonal separation of purified membranes... 

With mica, Rhoades and Coleman [1967] zonal separation or random interstratification have been obtained with K replacing Mg and Na, respectively. 

---