Particle diameter, reducing

As in interactive modes of chromatography, reduction in particle diameter reduces mass transfer effects and improves column efficiency in SEC. Column packings with particle diameters of 10 to 12 pm are available for less demanding applications, whereas SEC packings with particle diameters of 4 to 5 pm can be used for applications requiring higher resolution. 

At this point, the stationary phase particle diameter is extremely important for the kinetic optimization of separations. A smaller particle diameter reduces the distance for the necessary radial diffusion of analyte molecules on the one hand, but increases the geometrical radial concentration gradient that drives the diffusion. Both effects are synergistic for an efficient analyte transport and this is the physicochemical foundation for the decrease of the C-term with the squared particle diameter (dp ). This will be used effectively in the speed optimization strategy. 

To halve the particle diameter reduces the slope of the C-term by factor 4. 

Another area where enhancement at the gas-liquid surface may become important is in viscous slurries where k is reduced due to the viscosity. A theory on possible enhancement assuming a uniform-distributed catalyst with particle diameter reduced to the molecular scale is straight forward e.g. for first order kinetics and c ... 

The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... 

It is seen from equation (26) that the optimum velocity is determined by the magnitude of the diffusion coefficient and is inversely related to the particle diameter. Unfortunately, in LC (where the mobile phase is a liquid as opposed to a gas), the diffusivity is four to five orders of magnitude less than in GC. Thus, to achieve comparable performance, the particle diameter must also be reduced (c./., 3-5 p)... 

The data shown in Figures 6 A-D indicate that while the smaller particles 85, 98 eind 109 nm are indistinguishable from the dissolved solute, sodium dichromate, in as far as detector behaviour is concerned, the detector response differs significantly for the larger diameter particles. The reduced peak area and hence t irbi-dity indicated for the larger particles is a direct result of the optical effects noted earlier. The observations are consistent with the findings of Heller and Tabibian that the corona effect... 

For catalysts reduced at 400°-500°C, average nickel particle diameters in the range 30-45 A (40,43), and 30-200 A (41) have been quoted. Coenen and Linsen (41) have assumed a roughly hemispherical shape for the nickel particles which expose (111), (100), and (110) planes, and this is at least consistent with the very limited electron microscopic evidence. On the whole, it appears to be more difficult to produce a very high degree of metal dispersion with nickel than with platinum, and it is very difficult to obtain an average nickel particle diameter <30 A, although not impossible. 

The attachment coefficient is a function of the aerosol particle diameter, d, and mean velocity, v, as well as the unattached progeny diameter, d, and its mean velocity v. Since in most situations d d and v v, equation (2) reduces to... 

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