Steric mode

Steric mode and Lift-hyperlayer mode are closely related... 

FIGURE 12.4 FFF particle equilibrium distribution in (a) Brownian (normal) mode, (b) steric mode, and (c) hyperlayer mode. 

As a general rule, species smaller than 1 pm are separated under Brownian mode, while larger species are separated in steric mode. The third mode—the hyperlayer—is achieved when the average flow rate is increased and secondary forces act signihcantly on the sample components. These limiting FFF modes are illustrated in Figure 12.4. 

The preferential attack at position 2 would to be due to the steric mode of adsorption of cis-decalin at the surface of Ti02-... 

The NaBH4 and NaBD4 reductions of thiohemiaminals carried out in ethanol by MacLean etal. (56) correspond only in part to the results obtained in methanol. Using 6,6 -dihydroxythiobinupharidine (54) and 6-hydroxythiobinupharidine (53) for the reduction, they concluded that at C-6 the reduction follows only one steric mode, introducing deuterium in an axial configuration (95% incorporation). This observation corresponds to results described earlier by LaLonde. However, the reduction at C-6 in ethanol as opposed to methanol follows two steric modes, introducing 60% of the deuterium in an axial fashion and 40% equatorially (95% incorporation of deuterium). Furthermore, the differences in the course of the reduction were also shown as a more rapid reduction at C-6 as compared with C-6 and 95% incorporation of deuterium in comparison with 70%... 

The NaBD4 reduction of sulfoxides of thiohemiaminals performed on 6-hydroxythiobinupharidine syn-sulfoxide (24), 6 -hydroxythiobinupharidine syn-sulfoxide (25), and 6,6 -dihydroxythiobinupharidine syn-sulfoxide (26) follows a single steric mode (90% of deuterium incorporation) and introduces axial deuterium at both C-6 and C-6. This reduction may not follow a mechanism with intermediate iminium salt formation (36). The presence of strong hydrogen bonding and the absence of a-iminium salts in the reacting mixture support this conclusion. 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

For samples with a broad size distribution in the micron range, it is important to avoid the transition region between the normal and the steric mode during the measurement. This can be achieved by proper adjustment of the channel thickness, channel flow and the strength of the applied field [69]. The transition region in Fig. 6 can be experimentally determined by plotting the retention ratio vs. the particle size, as illustrated in Fig. 7 for the example of flow-FFF. 

Fig. 7. Determination of the transition point between the normal and the steric mode for the example of S-F1-FFF. Reproduced from [70] with kind permission of VCH Verlagsges-ellschaft Weinheim...

Gr-FFF is based on the same principle as S-FFF. However, as the applied field is simply the Earth s gravitational field, it is clear the lower separable particle size is limited depending on the particle density. In fact, particles with sizes lower than 1 pm are usually not well retained so that, in Gr-FFF, particles usually elute only in the steric mode (see Fig. 6). 

There are, however, also some drawbacks to these techniques The inversion of elution from the normal to the steric mode complicates measurements in the particle size range around 1 pm and, although this transition region can be shifted by experimental conditions, serious interpretation errors can occur if the particle size distribution spans this transition region. 

If the acoustic contrast factor A, 0, then a conventional FFF channel will enable normal and steric mode FFF separations to be carried out (Fig. la). 

In 1978, Giddings and Myers described another elution mode for large particles under conditions when diffusion effects can be neglected [3]. The particles form a layer on the channel wall and, under the influence of the carrier liquid flow, they roll on the channel bottom to the channel outlet. This elution mode is referred as the steric mode [3]. 

In the majority of FEE techniques, the retention ratio is dependent on the analyte size. This dependence for Brownian and steric mode is described by Eq. (11), derived by Giddings [13] ... 

Fig. 2 Schematic representation of the dependence of the retention ratio R on the analyte size. Curve B corresponds to Brownian mode and the line S to the steric mode. The dashed part T denotes the transition between these two modes. The area F shows the range of applicability of the focusing mode. 

The method has been applied to the separation of hematite and titanium dioxide submicron spherical particles, to the separation of submicron hematite spherical particles with different sizes, to the separation of various mixed sulfides with supramicron poly-disperse irregular particles, and to the concentration of dilute colloidal samples, in both the normal and the steric modes of operation of sedimentation FFF. 

Sedimentation FFF. The two sedimentation FFF (or SdFFF) systems have similar specifications and design features that have evolved in two decades of work at the University of Utah. All have a horizontal rotation axis (radius 15.1 cm) that makes them suitable not only for the normal mode analysis of submicrometer-size particles, but also for the steric FFF of particles well above 1 /xm in diameter (see Figure 1). System Sed I is a research instrument constructed at the University of Utah, and system Sed II is a commercial instrument (model SI01) from FFFraction-ation. Although almost identical in design and performance, Sed I has been fitted with a special channel of reduced dimensions (see Table I). The reduced channel thickness (w = 127 /zm) makes this instrument particularly effective for the rapid analysis (in the steric mode) of particles ranging from 0.5 to 40 /xm in diameter (see Figure lb). (Steric FFF in Sed II will fractionate particles up to 80 /xm.)... 

Data Analysis. The computer program used for data analysis was developed at the Field-Flow Fractionation Research Center. The underlying theory is similar to that discussed by Giddings et al. (4). For normal mode characterizations, the fractograms are converted to particle size distributions by using developed theory. However, for steric mode analyses, calibration curves are required (15, 20). 

Caspi, E., T. Arunachalam, and P.A. Nelson (1983). Biosynthesis of estrogens The steric mode of the initial C-19 hydroxylation of androgens by human placental aromatase. J. Am. Chem. Soc. 105, 6987-6989. 

Perhaps the biggest issue that must be confronted in particle separation is the normal-steric transition that occurs at 1 pm. The problem arises because the elution order of particles in the normal and steric modes is opposite (increasing size with increasing elution time for normal mode, but the opposite trend in the steric mode). Thus for broad populations spanning the transition diameter it is possible for fractions eluted at a specific time (or volume) to contain two populations, submicrometer particles that have eluted in the normal mode and micrometer particles eluted in the steric mode. 

Any field force can be exploited to create conditions for effective action of the steric exclusion mechanism. The only condition is, as mentioned above, that the field strength be high enough to compress all retained species to the accumulation wall. In experimental practice, sedimentation FFF, flow FFF, and thermal FFF are the techniques actually applied in steric mode to separate effectively some particulate species. 

Field-flow fractionation is a family of high-resolution techniques capable of separating and characterizing colloids and macromolecules. In normal FFF, the particles form a Brownian-motion cloud that extends a short distance into the channel. Separation is possible because the solvent flows at different velocities at various points within the channel. The smaller particles, whose cloud protrudes out into the faster laminae, are transported more rapidly than the larger particles, so that the two populations are soon separated. In the steric mode of operation, which happens when the protrusion of particles into the flow stream is determined by their physical... 

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