Diffusion column

Once the Pmi is very small, most reacting particles do not react. Even when inter-column diffusion is introduced, the reactants can reach the local minimum easily, but reactions seldom take place then and less active sites are foimd. From Figure 12(e), it ean be seen that the total number of non-active sites decreases with decrease in surface roughness. 

In practice, if the NMR acquisition times are longer than 30 min then on-column diffusion dilutes subsequent peaks, thus reducing the sensitivity [63], Typically, LC peaks will be broadened by 30% and 50% after the flow is stopped for 5 and 10 h, respectively [74], significantly reducing the LC-NMR sensitivity. Such peaks are more efficiently analysed by repeating the chromatography. 

If the retention times of the analytes are known, or there is an efficient method for their detection on-line, such as UV, MS or radioactivity, stop-flow HPLC-NMR becomes a viable option. In the stop-flow technique, all the usual techniques available for high-resolution NMR spectroscopy can be used. In particular, these include valuable techniques for structure determination such as 2-dimensional NMR experiments which provide correlation between NMR resonances based on mutual spin-spin coupling such as the well-known COSY or TOCSY techniques. In practice, it is possible to acquire NMR data on a number of peaks in a chromatogram by using a series of stops during elution without on-column diffusion causing an unacceptable loss of chromatographic resolution. 

Alternatives to batch testing include the use of diffusion cells or flowthrough columns. Diffusion cells are easier to operate, but are less representative of field conditions where some advection may occur. However, operation of columns at very low flow rates is difficult and subject to artifacts. To minimize possible wall effects associated with shrink/swell behavior of low-permeability clay materials, several researchers have utilized column devices that provide a confining pressure, such as flexible wall permeameters (e.g., Acar and Haider, 1990 Smith and Jaffe, 1994 Shackelford and Redmond, 1995 Khandelwal et al., 1998 Khandelwal and Rabideau, 2000). 

The two parameters in the rate equation that have been redefined are the column diffusivity H and the linear velocity u. The reduced parameters, which are dimensionless, are... 

The use of inverse gas chromatography (IGC) to study the properties of polymers has greatly increased in recent years (1,2). The shape and position of the elution peak contain information about all processes that occur in the column diffusion of the probe in the gas and the polymer phases, partitioning between phases, and adsorption on the surface of the polymer and the support. Traditional IGC experiments aim at obtaining symmetrical peaks, which can be analyzed using the van Deemter (3j or moments method (4). However, the behavior of the polymer-probe system is also reflected in the asymmetry of the peak and its tail. A method that could be used to analyze a peak of any shape, allowing elucidation of all the processes on the column, would be of great use. 

Theory and modelling of chemical partitioning and uptake from water phase to the SPMD has been described in detail by Huckins et al. (1990). Briefly, dissolved, nonionic lipophilic contaminants from water column diffuse across various rate-limiting barriers which consist of the water boundary layer, the biofilm, and the hydrophobic LDPE membrane prior to concentrating in the triolein. 

G5. Glenn, W. G., Some considerations in agar column diffusion analyses. J. Immunol. 82, 120-124 (1959). 

As solute bands move through a column, diffusion of molecules into the surrounding solvent occurs from the region of higher concentration to the region of lower concentration in proportion to the diffusion coefficient, Dm, according to Fick s law. The faster the mobile phase moves, the less time the zone is in the column, the less time there is for diffusion, and the lower the band broadening due to diffusion. 

This process is illustrated in Figure 1, which schematically shows how three proteins, marked (pli), (pis) and (pis), are focused in a column. Diffusion counteracts the electrofocusing. As the diffusion coefficient is inversely proportional to the molecular weight, it is understandable that better focusing is obtained with higher molecular weight proteins. 

In conventional GC a gaseous mobile phase flows in a defined direction over a stationary phase or packing, resulting in the selective retention of solute components. In reversed-flow gas chromatography (RF-GC) the system is modified another column (diffusion column) is placed perpendicularly in the center of the chromatographic... 

Despite the opposite effect of the flow rate on the diffusion along the column, diffusion within the column acts, of course, to broaden the sample zone, and it is higher at low flow rates. Small analytes diffuse more rapidly than large ones and are therefore more influenced by the diffusion phenomena. 

Remark in the case of diffusion.- When the rate-determining step for growth is diffusion, we do not have the functions e r, z), but we know the laws e(a). To use equations [10.19] and [10.20], it is necessary to associate with them the implicit functions a(t) gathered in the column diffusion of Tables A.3.1 to A.3.5 of Appendix 3, replacing t with t - z. We will discuss an example of this case in section 10.4.4.3. 

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